Recurring Charge - February 2005

After a long hiatus, the charger column is back. This time: more on advances in lithium charging, and reviews of four small chargers.

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Finally, I’m getting back to working on my columns after a flurry of standalone reviews and other writing projects. Hopefully they won’t be so far apart in the future.

This time I’ll muse a little bit about the now full-blown lithium battery revolution, then take a close look at several small and relatively inexpensive dedicated lithium battery chargers – the Apache 2500 (which is an updated and less expensive successor to the Apache 1500 in my last Recurring Charge), the Thunderpower TP-425, the Aurora AC-3LP and the Hobbico ElectriFly PolyCharge.

In the fifteen (!!) months since my last installment the lithium polymer revolution has pretty well overtaken us. And my, how quickly things have changed. The batteries themselves are continuously improving so that what was state-of-the-art when I last wrote is now hopelessly outdated. In particular li-poly batteries can deliver much higher discharge currents without damage, and several battery makers have followed Thunderpower’s lead in making higher capacity cells that actually fit in our airplanes where the nickel-based ones did before. We also understand better how to treat them in the long term.

Lithium chargers are getting smarter

One happy development since my last column is that there have been a number of new lithium chargers released that do a good job of making sure the critical cell-count setting is correct, either by doing it for you or letting you know if you set it incorrectly. A number of these are less expensive than chargers that could do the same job a year ago. But one thing has not changed. I wrote this back in September of 2003:

From what I have read and heard, the most common cause of lithium fires is overcharging - and the most common cause of that is selecting an incorrect cell count - in particular setting the charger for one series cell more than the pack being charged actually has. So, GREAT CARE must be exercised EVERY TIME you go to charge a lithium battery to make sure that both the number of cells (where the user is required to set it) and the charge rate are correct."

This is still true. But as I just noted, more and more chargers are available that either double-check your manual cell-count setting or take care of it for you automatically and reliably. These features just have to help reduce the overcharge-related problems that are caused by incorrect cell-count settings. For up to 4s packs there are a number of choices now that make charging li-polys as simple or nearly as simple as charging nickel batteries with a good peak-detector charger. I’ll be taking a look at four of them in this column. For up to 9s packs, there is the Astro Flight 109.

There are also devices now available that go between the battery and the charger to prevent overcharging the pack. Some are built into the battery packs. Others are available as separate devices you add to your charger. These separate devices have to be selected to match the cell-count of the pack being charged/protected, so it seems to me they are subject to the same sorts of possible human errors as selecting the wrong cell count when setting up the charger itself. Consequently I have not yet started using these except for the one built into the one Electrifly pack I have. Instead I prefer to rely on a charger that won’t let me get the cell-count wrong at the charger in the first place. This is an area I plan to look into further in the coming months, though.

Still, charging issues remain

However, one other source of errors is still very much with us. Ironically it is found most often in higher-end chargers that are designed to handle all our charging needs. This is the very real possibility of charging a lithium battery while the charger is set for some other battery chemistry (usually nickel). There is no way (that I know of, anyway) that one of these multi-chemistry chargers can know that you’re charging a lithium battery on a nickel-battery setting. If you don’t notice it either, the charger will go its way looking for a voltage peak to appear in the battery. It will NEVER come. This situation, left alone, leads to the nearly inevitable result of “venting with flames”. Not good.

For this reason, I CANNOT recommend any of the multi-chemistry chargers currently available for the charging of lithium batteries unless you don’t need the multi-chemistry capability in the first place and will leave it set for “lithium” at all times. This includes everything from the Schulze isl-8 down to things like the Hobbico AccuCycle Elite. In fact, on this latter device, I very nearly had my first charging-destroyed li-poly battery because I started to charge one while the charger was set for a nickel pack on the output I was using. Fortunately I noticed after a short period of time and shut the charge down.

Another issue that I’ve not run into yet (except when I got one mismatched-capacity battery pack) but that is a source of much discussion, some of it rather lively, in the RC Groups Batteries and Chargers forum, is that of balancing and maintaining balance in battery packs and what happens when you charge packs that aren’t balanced. This becomes more critical as the number of cells in a pack goes up, as every charger I know of that is currently available charges to an ending voltage that is a multiple of the single-cell full-charge voltage. The more series cells there are, the smaller the variation between cells needs to be for a cell which has a higher voltage at the beginning of the charge to be overcharged as the pack as a whole reaches this target voltage.

As I say, I’ve not run into a problem here yet. I am flying mostly 3s packs of various capacities and check the cell-to-cell balance from time to time. I have yet to find any pack with more than a 0.01V difference between adjacent cells in the pack and 0.02V across the pack, even in packs I have used quite a bit and are approaching two years old. Nevertheless the potential is there and much activity is underway by various battery vendors right now.

This is another area where I will be trying to learn more and get some experience which I can then put into a later edition of A Recurring Charge.

OK, let’s look at some chargers!

Thunderpower TP-425


Type:Constant current/constant voltage charger for lithium polymer batteries
Cell count range:1-4 cells, cell count automatically set
Charge current range:300 mA to 2.5A in six steps
Input power:12 volts DC nominal (11 to 15V), 4A maximum
Input Lead:Coaxial jack. Supplied lead 30 inches (76 cm) long ending in insulated alligator clips
Output Lead:Standard banana jacks. Supplied lead 7.5 inches (19 cm) long ending in JST connnector
Additonal Features:Auto-cell-count check/setting
Pushbutton operation - no jumpers to set or lose
Reverse polarity protection on both input and output
Compact and rugged design
Manufacturer:Thunderpower Batteries
Available From:Several Ezone sponsors
Price:$54.95 US

Thunderpower took rather longer than some of the other lithium battery makers (such as Kokam and E-Tec) to bring a small charger to market, but that gave them a chance to improve upon the others in several areas. Like a number of other small chargers, it is for 1 to 4 series cells. Its maximum charge rate of 2.5A is equal to or greater than the other small 4s chargers. Where it really differs from the others is in its much better user interface.

Physical description

The first thing I noticed in getting it out of the box is that it’s TINY – even smaller than the Kokam chargers, and much smaller than the Apaches of similar capability. It is in a very rugged extruded aluminum case and as-received there is nothing sticking out of that case – no shorting jumpers and no leads. Both the input and output leads plug into jacks on the left end of the charger. The input lead ends in a pair of the usual alligator clips. It plugs into a coaxial barrel-type connector on the charger, similar to most transmitter charge connectors.

The output lead plugs into a standard set of banana jacks and ends in a JST connector. These jacks let you interchange charge leads with other chargers or make up your own using readily available plugs if a short lead with a JST on it doesn’t suit you.

Note: the first batch of these chargers used a smaller set of jacks/plugs for the output leads and came with two of them – one with a JST connector on it and one ending in small alligator clips.

The second thing I noticed was there were no little shorting jumpers like those normally used to set cell count and charge rate on small chargers. Instead, on the right end are three pushbuttons – one to select the cell count, one to select the charge rate, and one to start or stop a charge. Once the charger is powered up (which it annunciates with a short little beep), you select the cell count and the desired charge current by pushing the appropriate button as many times as needed.

Also in the box is a single A4 sized sheet of instructions that gives the basics of how to run the charger and its specifications, with only a few amusing translation errors.

There are quite a few LEDs visible through slots in the front face of the charger. A row of them is used to indicate the cell count and is to the left of the “cell” selection button. Another indicates the charge rate and is to the left of the “current” selection button. Each of these rows has green, yellow and red LEDs in it spread from the low end to the high end. This makes it easier to tell them apart.

These LEDs also tell you what’s going on during the charge cycle. The charge rate LED flashes slowly while the charger is in the constant current portion of the charge, and the cell count LED flashes slowly when the charger is in the constant voltage portion of the charge.

There are two more LEDs to the left of the start/stop button. These give you an idea of what’s going on once a charge has started but are both off until then. Once a charge is started, the blue LED flashes fairly rapidly until the charge is completed, then it goes out and the green one comes on, indicating “full charge”. They will both flash, accompanied by a warning sound from the charger, if one of several errors occurs, including reversed polarity on the output or the input power is out of spec (below 11 or above 15V). The instructions indicate it will give an error signal also if the auto-cell-count function times out, indicating the battery is over discharged or there is some other problem. I haven’t seen this indication yet. By the way, if you hook up the input power backwards, the charger simply will not “wake up” – no LEDs, no chirp.

I took a look inside and found neat board layout and good quality looking construction.

Using it

Using the TP-425 is very straightforward. Connect the charger to your DC power source (recommended to be good for at least 4A) of between 11 and 15V with the input lead. The charger will chirp once briefly, and the cell count and charge rate LEDs will indicate the current settings (which will be whatever you set them to last time you used the charger).The other two LEDs will both be off. If necessary, change the charge rate to one appropriate for the battery you’re about to charge by pushing the “current” pushbutton as many times as needed to select it. You’ll get a very short beep with each button press as well as seeing the current indicating LEDs cycle through the choices.

Once the charger is set, connect the battery to be charged to the output lead. Push the “start/stop” button. There will be a chirp, the bright blue “charge” LED will start to flash fairly rapidly and the charger will go into the first stage of charge. During this time it puts out about a quarter of the selected charge current, and evaluates and resets the cell count as needed. Because of this, I tend to just leave mine set to one cell and let the charger set the correct number itself. This auto-detect function is very reliable. I have tried 1s, 2s and 3s batteries on the charger with the initial setting anywhere from 1 to 4s and it has set itself correctly in virtually every case.

There is ONE scenario where it can be “fooled” into setting the cell count too high, and it’s the same one that fools the Astro Flight 109 into self-setting the cell count high. This is attempting to charge a small-capacity pack that is already full or nearly fully charged on a much-too-high current setting. For example, if you put a fully charged 2s E-Tec 250 pack on the charger and set the charge rate to 2.5A, it will likely set itself to 3s. Note that this ONLY happens with a severe mismatch of battery capacity and selected charge rate and NOT when you set the charge rate appropriately (that is no greater than 1C). The first stage of the charge, during which the charger checks and resets the cell count, takes exactly one minute from the time you push the “start” button. Keep an eye on it for that first minute, verify the cell count and all will be well.

Once a charge has started, you can no longer change either the charge rate or the cell count with their respective pushbuttons. If you discover you’ve set the current incorrectly just push “start/stop” again to stop the charge, change it, and restart.

After the first minute, presuming the battery isn’t full enough to have risen to 4.2V per cell, the charger goes into a constant-current charge. I found the actual currents to be exactly as set or very slightly lower. During the constant-current portion of the charge, the LED indicating the charge current will flash very slowly and the blue “charge” LED continues its rapid flashing.

Once the battery has reached 4.2V per cell, the charger switches to constant-voltage mode and the charge rate first drops to about half the set charge rate, then slowly tapers down from there while the battery voltage maintains itself very near 4.2V per cell. You can see this has taken place as the current LED stops flashing and the cell count LED starts to flash slowly. As before, the blue LED keeps on flashing rapidly.

Finally, when the charge current has dropped to about 1/10 of the initial setting, the blue LED goes out, the green “full” LED comes on steadily, and the charger signals “charge complete” with a series of six chirps. It will repeat these six chirps once per minute for 5 minutes in case you missed it the first time. After that it stays quiet. During this time the charger continues to put in a very low rate pulsed charge, which continues indefinitely so far as I can tell. This pulsed charge seems to be low enough rate to not drive the battery voltage any higher.

You can stop the charge at any time by pushing the “start/stop” button. This will also silence the “charge complete” alarm or any of the error indications.

Here’s a look at the current during a charge of a partially depleted 3s Tanic 1500 pack, captured with the Medusa Power Analyzer Plus:

The bottom line

I like the Thunderpower TP-425. It’s small, light and ruggedly built. It has a nice user interface between the buttons, the LEDs and the audible signals (something I really liked about the now-discontinued Apache 1500). As long as I don’t try to charge tiny batteries at outrageous rates it gets the cell count right with no help from me. And at about $50 from several E Zone sponsors it’s a good value. It has become my new favorite small lithium charger. Highly recommended.

ElectriFly PolyCharge

 Out of the box
Out of the box
Type:Constant current/constant voltage charger for lithium polymer batteries
Cell count range:1-3 cells, cell count automatically set
Charge current range:250 ma, 500 ma, or 1000 mA (1A), selected by switch
Input power:12 volts DC nominal (10.5 to 15V)
Input Lead:28 inches (71 cm) long ending in insulated alligator clips, permanently attached
Output Lead:4 inches (10 cm) long ending in JST connnector, permanently attached
Additonal Features:Compact design
Auto-start
Reverse polarity and overcurrent protection on both input and output
Manufacturer:Hobbico
Available From:Your local hobby shop, Tower Hobbies
Price:USD

After my generally positive experience with the Hobbico AccuCycle Elite (review in the E Zone’s sister glow-modeling publication, R/C Power, here) the announcement of a lithium-polymer charger from Hobbico at the $25 price point caught my attention. It is designed to do 1-3 cells at up to 1A charge rate, and has automatic cell-count detection. While that means it’s not for very large power systems, it still covers indoor models and smaller parkflyer applications pretty well. As soon as I saw that it was in stock, I asked AnnMarie Cross if she could get me one. I got lucky – she’d already just gotten one for review. Soon, I had it in hand. And, it turns out to be a good little charger. Here’s a closer look.

Physical description

The Electrifly PolyCharge is a compact plastic-cased charger. The case has a sturdy feel to it, as if this unit is designed to survive rattling around in your field box. Simplicity of operation was clearly a major design objective as the front the charger has only one control – a three-position charge rate switch - and one indicator - a very bright blue LED. Emerging from the right side of the case is the output lead, which ends in a JST connector. On the left side is a 7 1/2 amp ATO-type automotive fuse and the input lead, which ends in the usual insulated alligator clips. According to the instruction sheet (which is online here ) the fuse is for reverse-polarity protection on the input side and overcurrent protection. There are ventilation grilles on two sides as well.

A look inside showed good quality construction, at least to my eye, with a mix of surface-mount and more standard PC-board components. There’s strain relief on both the input and output leads – a good thing.

Using it

With the only control being the current selection switch, using the PolyCharge is about as simple as it gets. Hook it up to your input power supply (between 11 and 15V DC). The blue LED will start to flash slowly once the charger wakes up, telling you it’s ready. Select the charge current appropriate for your battery with the three-position switch, then plug the battery into the output connector. Charging starts automatically, and the blue LED goes to solid on. That blue LED, by the way, is bright enough to be easily seen under daylight conditions and really lights up a darkened room!

I found the charge rates were just below the indicated 250, 500, or 1000 mA. Also, I found that once the charge has started, moving the current select switch has no effect. If you decide you need to change the current setting, stop the charge by just unplugging the battery from the output lead, reset it, then plug the battery back in.

The PolyCharge takes 5 minutes to be sure it has the correct cell count selected during which it sends pulses of varying currents into the battery. At the 5-minute mark it ramps up to the selected charge current quickly. From there the charge cycle is a standard constant-current/constant voltage charge. The selected charge rate is maintained until the battery reaches 4.2V per cell then the current ramps down gently to maintain that voltage. Once the current decreases to about 1/10 the set current, the charger signals it’s done with 15 beeps, each one second apart. The blue LED goes back to flashing, but at a faster rate than when the charger is idle and not charging.

The charger continues to charge and decrease the current while maintaining the appropriate voltage and if left for some time, the current does finally go to zero. In the graph below, I just disconnected things about six minutes after the charger signaled “finished”.

If you manage to hook the battery backwards the charger signals the error insistently with rapid tones from the built-in beeper and double flashes from the blue LED. You’ll also get an alarm if you hook a Whattmeter-type device up to the output without a battery to be charged attached – the meter draws enough current to start the charger and it soon sees the output voltage go too high. Triple flashes from the LED indicate this error. Single flashes indicate the input voltage is too low or too high, and quadruple ones an internal circuitry error. I have only verified the reverse polarity on output and output overvoltage errors. In any case, the alarm will stop a few seconds after the problem is remedied.

While charging it runs fairly coolly. It gets warm to the touch, especially on the bottom, when charging at the full 1A for awhile, but only warm, never hot. Also, I’ve found that it is tolerant enough of voltage variations on the input to be able to charge in my vehicle while both starting it and driving. I have a heavy-duty lead that is wired directly to the battery in the cab of my little pickup that I sometimes use for on-the-go charging. Yes, I know it’s not recommended, but sometimes I do it anyway.

The charger itself works well, but there are a couple of things in the instructions that I have a quibble with. In particular is the comment that li-poly batteries “becoming slightly warm” while being charged is considered normal. So far as I know, a Li-poly battery should NEVER increase in temperature above ambient if it is being properly charged. None of my batteries ever have, anyway. There is also one place where it says disconnecting the battery being charged will cause an error warning to be sounded, yet just before that it says to disconnect if you want to stop a charge that is underway. Disconnecting the battery being charged does not cause an error indication. And my charger doesn’t beep when being powered up or when the charge starts as the instructions say it should, yet it otherwise is working exactly as described. But it’s so simple to use, you don’t really need the manual anyway, so none of this is a big deal.

The bottom line

If you fly fairly small models for which a 1A charge rate and a maximum of three cells is adequate, then this little charger, especially at $25, is well worth a look. It couldn’t be simpler to operate, and it does its job with no fuss. It is also well worth picking up just to have a second charger for small lithium-polymer batteries handy for those situations when you really want to charge another pack and your main charger is already busy. I’ll be keeping this one for just that reason. Highly recommended.

Aurora AC-3LP

  What's in the box: The charger and a couple of spare jumpers
What's in the box: The charger and a couple of spare jumpers
Type:Constant current/constant voltage charger for lithium polymer batteries
Cell count range:1-3 cells, cell count set via shorting jumper
Charge current range:100 mA to 2A in five steps, set via shorting jumper
Input power:12 volts DC nominal (10.5 to 15V), 3A maximum
Input Lead:24 inches (61 cm) long ending in insulated alligator clips
Output Lead:12 inches (30 cm) long ending in JST connnector
Additonal Features:Auto-cell-count check - charger will not start if cell count in incorrect
Reverse polarity protection on both input and output
Temperature compensated for all-weather operation
Distributor:Sureflite/GWS-online.com
Available From:The distributor
Price:$41.95 USD

The folks in the SureFlite/GWS-online.com booth at the AMA Convention Show (Mike Heer's show report here) in January were showing a new small lithium charger called the Aurora AC-3LP. This one is designed for one to three cells, with charge rates ranging from 100 to 2000 mA in five steps. This puts it between the other two chargers I’ve covered above in capability, with current approaching that of the TP-425, but with the three-cell limitation of the PolyCharge. In any event, at the end of the show they handed one to Mike Heer and that unit has subsequently found its way to me to be included in this small-charger roundup.

Rather than auto-cell-count setting, this one operates more like the Apache 1500 (and later Apaches) in that you set the cell count with a shorting jumper and the charger will give you an error signal – in this case a flashing red LED – and refuse to start to charge if what you have set doesn’t match the battery.

It also has very benign reverse-polarity protection on both the input and the output, and has temperature compensation so that it delivers the expected charge rates from -10 to +45 degrees Celsius (14 to 113 degrees Farenheit).

Physical description

The AC-3LP is fairly compact in a light aluminum case. Out of the left end of the case emerges the input lead of 0.5 sq. mm silicone-jacketed wire which ends in the usual insulated alligator clips. Out of the right end emerges the output lead (labeled “battery”). It is of the same wire but about half as long, ending in a JST connector. This is still longer than the output leads of other small chargers. Both leads come out though rubber grommets in the case and are soldered to the circuit board just inside.

Also on the right end are two sets of jumpers. One gives five choices of charge current ranging from 100 to 2000 mA. The second set selects one, two or three cells. On the face of the charger near the jumpers is two LEDs. The red one is labeled “chg/err”. The green one is labeled “full”.

Included in the box is a little plastic bag with two spare jumpers, but no instructions are provided except on the back of the charger itself. Using this charger is pretty straightforward, but the lack of any more detailed directions would be a liability for newcomers to charging lithium batteries. I've seen a draft of the full instruction sheet, so subsequent units should come with them.

A look inside shows a clean layout. Like the PolyCharge, there is a mixture of surface mount and more “traditional” PC board-mount components. Unlike the PolyCharge there is no strain relief on the wires except for the grommets. They ARE a pretty tight fit so do prevent a wire pull from being “felt” at the circuit board.

Using It

To use the Aurora AC-3LP, first select the desired charge rate and cell count by moving the jumpers to the appropriate pairs of pins. Then, supply input power (nominally 12V). After a moment the green LED will start to flash slowly showing the charger is ready to go. Plug in the battery to be charged, and the charge will start automatically. The red LED lights steadily indicating the charge is underway.

If the cell count is incorrectly selected, the red LED will blink and the charge will not start until you set it correctly. It only takes a moment for the error to be detected. You will also see this error indication if either of the jumpers is missing.

If the input polarity is reversed, the charger simply will not power up (no slow green LED flash). If the output polarity is reversed the charger just ignores it – acting as if the battery to be charged is not connected at all. Both of these are safe situations. So if the AC-3LP doesn’t power up or start to charge as you expect, check your connections for proper polarity.

Once the charge starts, the unit does a low-current charge for one minute, then ramps up over the next minute or so to the selected charge current. Like the other chargers in this group the output currents I measured were just below the indicated setting. Conservatism here is a good thing.

From here on the charge is pretty much a standard constant current/constant voltage cycle, as can be seen in the plot below. You’ll see on this plot an area where the current varied widely as it was tapering down. The voltage plot looks similar at this point as well. I can’t explain why this happens, but it did on both charge cycles from this unit I captured with the Medusa Power Analyzer Plus. I don’t think this is a problem really, though the voltages spiked to as high as 13.1V for this 3s pack for fractions of a second each time. Once it got past this point, the voltage settled down to where it should be and the current continued to taper as it should. Once the current drops to about 1/10 the input setting, the green LED is turned on. There is no other signal the charge is complete as it has no sound capability.

Like the other chargers in this group, however, a small current continues to flow. It does continue to taper and I expect if left along long enough it would eventually go to zero. The battery voltage is held at 4.23V per cell through this process.

I noticed that it gets noticeably warm to the touch while charging at 1200 mA or 2000 mA on 3s batteries.

One other odd thing: I like to listen to the FM radio while in my workroom, and I’m often tuned to a station that’s not very strong. When the Aurora charger’s output rate ramps up past about 600 mA it throws off enough RF noise to pretty well wipe out my radio listening. This continues until it ramps back down on the CV portion of the charge below that current. I noticed this even when I was running the test to get the graph just above in one room and had the FM radio on in the next room.

Bottom Line

The Aurora AC-LP does what it says it will do well. Since it will not start to charge if the number of cells is set incorrectly (or current or cell count jumpers are missing), it prevents the most common cause of overcharge. It does a good job of protecting itself and the battery you’re charging from other errors. The presentation could be a little nicer in some areas, but the problems are generally minor. As long as you already know what to expect (and therefore the minimal instructions on the back of the unit are sufficient), and you don’t need the ability to charge 4s packs (and you don't listen to a weak FM station while charging), it should serve its purpose well. Recommended.

Apache Smart Charger 2500

  Label-side up view
Label-side up view
Type:Constant current/constant voltage charger for lithium polymer batteries
Cell count range:1-4 cells, cell count set by shorting jumper
Charge current range:250 mA to 2.5A in six steps, set by shorting jumper
Input power:12 volts DC nominal (10.5 to 15V), 4A maximum
Input Lead:21 inches (35 cm) long ending in insulated alligator clips
Output Lead:3.5 inches (9 cm) long ending in JST connnector
Additonal Features:Auto-cell-count check - charger will not start if cell count in incorrect
Manufacturer:AnyRC
Available From:Several Ezone sponsors
Price:$49.95 US

The Apache Smart Charger 2500 is a reduced-cost successor to the Apache Smart Charger 1500 I wrote about away back in November of 2003 in my last Recurring Charge with the ability to charge at higher currents than its predecessor – up to 2.5A as compared to 1.5A of the prior charger. There are now six current choices, with the 110 ma option of the older charger being deleted, and two new ones, 2000 ma and 2500 ma (or 2.0 and 2.5A) being added. It is still a one to four cell charger.

Physical description

In order to reduce the cost the maker eliminated the beeper of the 1500 and the full case. This leads to a rather interesting physical arrangement with the surface-mount-component side of the circuit board protected only with a layer of blue heat-shrink tubing. The LEDs indicating the charger’s status are on this side of the unit (the opposite side from the label), yet the information labeling the LEDs and their function are on the label/heat sink side.

Looking at it label-side up, the input lead of blue and pink silicone-jacketed wire (the same as on factory-made E-Tec batteries) emerges from the left end. These end in typical alligator clips. On the right end are the output lead of the same wire ending in the obligatory JST connector and the two groups of selection jumpers. The upper one is for cell count, the lower for charge rate.

These jumpers project out past the edge of the heat sink and circuit board and so are vulnerable to damage if you drop the unit (I write from experience). Also both the input and output leads are soldered right to the board with no support or strain relief of any kind, as you can see in this picture. Both of these concern me for the long-time durability of this charger. That said, I've been using it for about a year and haven't had a problem with it yet....

Mine came with a separate one-page instruction sheet prepared by AirCraft World that covered the basics.

Using it

Operationally the Apache 2500 is just like a voiceless version of its predecessor the 1500. Select the cell count and charge rate with the jumpers, then supply 10.5 to 15V input power to it. The 2500 is much more tolerant of input voltage variations in both directions from 12V nominal than the 1500 is, as I have proven to myself by using it (against recommendations) in my vehicle in the same way as I described above in the PolyCharge review.

Once power is applied, the green “power” LED, which is close to where the input lead attaches to the board, will light up steadily indicating the charger is ready to go. Then plug in the battery to be charged. The red “chg/err” LED (as marked on the label side of the unit) will light steadily indicating the charge is started. Unlike the other chargers in this group, the Apache goes right to the selected charge current when the charge begins – there is no delay.

If the cell count selected doesn’t match the battery you’ve just plugged in the “charge/err” LED will flash immediately and the charge will not start. I have tried to fool it many times and have never succeeded. This is the main reason that I have been recommending Apache chargers ever since I got my 1500 some time ago. By the way, you will also get this error indication if either jumper is missing. Since there is no sound capability, the only error signal is the flashing LED.

Once the charge is underway it is a standard constant current, then constant voltage charge cycle as you can see in the graph below. Once the battery’s voltage gets to 4.2V per cell, the current starts down while maintaining the voltage. I thought it odd that the Apache, too, exhibits some fairly wide fluctuations in current and voltage at 1/4 the set charge rate as it ramps down, just like the Aurora. Again, I have no explanation for this but it seems to do no harm. I’ve been using this particular Apache 2500 for a number of months now and had no problems with any of the batteries I’ve charged with it (at least so far as I know).

Once the current falls to the typical 1/10 of the original set current, the green “chg full” LED will come on. Like the others, current is still flowing at this point, but it continues to taper off. For a short period of time both the “chg full” and “chg/err” LEDs will be lit, then the red one goes off.

There is little noticeable heating while the charger is running. That big heat-sink plate must do a good job.

Bottom line

As I said, I’ve been using this one for several months and I trust it to not let me goof on cell count selection. It has always done exactly what it’s supposed to do despite once-bent cell-count-selection jumper pins and unsupported wiring. Until the release of the similarly priced Thunderpower TP-425, which has similar capabilities, but is better packaged and has a better and more informative user interface, I have been recommending it whenever anyone asks for me about a charger within its limits of up to 4s and 2.5A charge rate. It is still a very good charger and still worthy of recommendation.

A look ahead

I’m sure I’ll have more to say about lithium charging developments in the next installment, which I sincerely hope will be much sooner than another 15 months from now. As I mentioned at the beginning, I plan to delve into taps, balancing circuits, and upcoming battery-balancing chargers in the near future. I probably should also give a long-term test update on the Astro Flight 109 (I now have two of them) and perhaps an update on some of the four chargers in this column as I continue to use them day-to-day.

Thanks for making it this far!

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Feb 07, 2005, 10:28 PM
Leave me alone!
Martin Hunter's Avatar
Nice summary, Bernard. I wasn't aware of a few of those chargers. Thanks for overviewing the options in the parkflyer realm

Martin
Feb 07, 2005, 10:36 PM
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Thanks for the good words. I wasn't aware of the Aurora until I saw Mike Heer's AMA show report myself.

I have fun visiting with Pandi from Sureflite at the NW Model Expo this past weekend. He's a fellow who really wants to have good quality but affordable stuff. I came away with an ESC that I will be writing about soon.....
Feb 07, 2005, 10:45 PM
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Great Review Bernard! You were indeed the right man for the review. Mike
Feb 08, 2005, 01:55 AM
It'll work this time!
The Commodore's Avatar
Just a quick question - I might have missed this, but how do these chargers rate in terms of getting the battery pack full. I currently have the Hobbico Mk II, and its performance on 3S packs is ... underwhelming. I'd like something that can get me the extra mAH out of my 1500 mAH packs. Any thoughts?

Thanks, and great article,
Chris
Feb 08, 2005, 02:00 AM
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Chris,

All of them charge the batteries to at least 4.2v per cell. The Aurora goes to 4.23. That's as full as I dare go.

Thanks for the good words.

I was watching for incomplete on both the AccuCycle Elite review I linked to in this one and the little PolyCharge. Both go right to the recommended 4.2V/cell.

Mike, thanks!
Feb 09, 2005, 02:24 PM
Rocket Scientist
dtknowles's Avatar
Just wanted to add my thanks and compliments on a nice review. Tim
Feb 09, 2005, 10:08 PM
It'll work this time!
The Commodore's Avatar
Okay - thanks for the info on the charging. I just don't want to drop more $$$ on a charger that only does 90%.

Thanks!
Chris
Feb 10, 2005, 03:44 AM
Always right
Hovertime's Avatar
Nice review, thanks!

Please include Graupner automatic lipo charger in the next review.
Feb 10, 2005, 10:27 AM
Mitch G's Avatar
Great review.

Here's an idea for a future column:
Trying some of the different chargers out there with some of the different safety/balance modules out there.

The reason I bring it up, is that I am working with some Duralite cells and their balancing module with an Apache 2500 and noticed some strange behavior near the end of the charge when I had the balance module in line between the charger and battery pack.
From talking to Duralite, the solution is to connect the balance module to the pack but charge the pack through the pack's "normal" connector.

Anyway, it might be interesting to get some side-by-side analyses of how modules like Duralite's Stay Balance module or Polyquest's PCM guard interact with different chargers like the Apache or Astroflite, etc.

This should keep you busy for the next 15 months.

Again, great column.


Mitch
Feb 10, 2005, 12:44 PM
Registered User
Bernard,

Thanks for the thoughtful review. I have two questions:

1. You said: "We also understand better how to treat them in the long term. " Would you care to elaborate on that a bit?

2. Maybe related to that (e.g. charge at 1C or less): These chargers have rather coarse rate settings. I have a 480 mah pack, but my charger has closest settings of 240 and 560 ma, the latter measured to be ~ 600 ma actual. If I choose 240 ma, it takes 2 hr to charge. Is 600 ma (1.25C) really harmful?

Thanks,

Gary
Feb 10, 2005, 02:23 PM
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Hovertime,

Thanks. If I can get my hands on one of those chargers, I will.

Mitch,

As I mentioned in the piece, balance/protection modules and how they work (or don't) and how they interact with chargers (or don't) is an area I need to do lots of research/testing on. Right now I have exactly ONE protection module - built into a 2s 1500 Electrifly pack. Sorting through all that may indeed take another 15 months.

Gary,

I was thinking in terms of my own learning that the fastest way to kill a lithium pack (besides overcharge or crash damage) is to pull more current from it than it wants to deliver, for example. In the two years or so they've been available we've gone from "I have no idea how well this is going to work or how long" to having a fair idea, at least in some areas, of what really works and what doesn't

On charge rates - when you charge at only a bit more than 1C what happens is the battery gets to the constant-voltage portion of the charge faster and spends more time there. In my limited experiments it doesn't really shave off that much time. In your particular case, you're probably fine and will save some time going to the higher selection.

I would love to see one of these small chargers with dial-it-yourself charge rates like some of the inexpensive nickel chargers (like the GWS MC2002 or WattAge PF-12). Right now they seem to choose ratings close to the batteries that particular vendor sells, which may or may not match others. Of course the AF 109 is dial-a-rate, and that works well.....
Feb 14, 2005, 09:08 AM
Walt
I just made the "LiPo Leap." Now I'm reviewing as much as I can on LiPos (I have tried to keep up with developments even though I wasn't using them as I write an introductory e=power column for my wet power club). Your article succinctly sumarized the whole charging issue. (I have a 109)

I do have one question, I think I know the answer, but we know what assumptions do don't we? The charger is only interested in the number of cells in series, not the number paralleled (plus individual cell capacity for the rate). I will be flying 4S2P (two series of four packs paralleled). So I would set the 109 for 4 cells?
Thanks
Walt,
Illinois
Feb 14, 2005, 11:11 AM
BEC
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Yes.... you are correct. And you can set it for the capacity of the parallel set. For example, if it's 2P of 2000 mAh cells, the resulting capacity is 4000 mAh, and you can set the 109 for 4A to charge that pack.

"Li-Po leap". I like that.... I'll have to steal it for future columns .
Feb 15, 2005, 11:02 PM
Always right
Hovertime's Avatar
BEC - could you - please post a link here when (and hopefully soon ) your new "Recurring Charge" gets published? This way we would not miss it. (I do not check articles that often )


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