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Recurring Charge - March 2003

Based on some of the posts I've seen on the discussion boards and from some of the e-mail I've received, I think we need to start at the very beginning.

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Here we go on the second installment of an irregular series on batteries and chargers. I apologize for the long interval between the last installment and this one. Thanks to all of you who wrote me with encouragement and suggestions after the first column was posted on The E Zone. In response to some of that feedback, I think I'll go over some charger basics this time, with some emphasis on the different charging characteristics and requirements of the different battery chemistries we are using these days. Then, we'll take a look at the FMA Direct SuperNova 250S and the Kokam Lipo-402 chargers.

Charger Basics:

Based on some of the posts I've seen on the discussion boards and from some of the e-mail I've received, I think we need to start at the very beginning. My apologies for boring those of you who have been at this for a while, and know all of this stuff.

Of course, the first and primary task a charger has, is to charge our drive batteries safely and as quickly as practical. Up until three or four years ago, this meant charging batteries made from nickel-cadmium cells only. How to charge NiCds in 15 to 30 minutes and terminate the charge safely and automatically has been understood for some time, as I mentioned in the last installment. Just about all of our current chargers take advantage of a curious phenomenon of nickel-cadmium cells: While being fast charged, their voltage rises fairly quickly. When they are nearing 100% charged the voltage of the cells slows its rise, and finally stops rising altogether. Finally, if a charge current is still being applied, once the cells reach 100% charge, the voltage starts to DROP again, and simultaneously the cells start to heat up. Our automatic chargers then look for that maximum voltage (the "peak") then voltage drop - hence the term "peak detector charger". Then, once the voltage has dropped a small amount, they terminate the charge. There have been some more conservative chargers that instead look for the voltage to stop rising (the first derivative of the voltage curve goes to zero for those of you with a bit of calculus background) and stop charging at that point rather than wait for the voltage to actually start to decrease. There have also been a few chargers equipped with temperature probes that were supposed to be put in contact with the battery being charged so as to use the temperature increase as the signal to stop charging or as a safety backup in case the voltage peak and subsequent drop was somehow missed.

Chargers differ in the details, of course - some test the voltage of the battery while it is being charged, and some stop charging periodically (every few seconds) for a brief moment and measure the voltage during the pause in the charge. Some even pause periodically and DIScharge the battery for a short period while charging - these are the so-called "reflex chargers". Most, except the most sophisticated chargers (and then only in their "automatic" modes) otherwise charge at a constant current from the beginning of the charge until the end (or as close to that as they can muster as the voltage difference between the voltage of the source battery or power supply - or the output of their voltage booster circuit - and the voltage of the battery being charged decreases).

And, of course, there is a huge variation in how much information the charger gives to the user about what's going on during the charge, and how much control the user has over that process. The simplest have no adjustments and no indication of what's happening except perhaps an LED that comes on or goes out when the charge is complete. Others have multi-line LCD displays that show voltage, current, time of charge, the amount of energy put into the battery and more, and have many ways to adjust the charge rate and other charge parameters. In between are those with a meter, a current-adjust knob, and perhaps a "start" button, like the GWS MC2002, for example, or an older analog Astro Flight 110. This is one area that I'll be discussing in each of the reviews that come in this series as it progresses.

The introduction of a second nickel-based battery chemistry - nickel metal hydrides (NiMH) - brought with it some confusion. I will be the first to admit that I'm still confused about some aspects of what is the same and what is different with NiMHs vs. NiCds. One thing that seems to be fairly well agreed-to is that, while NiMHs also show a voltage peak when they are fully charged, it is less pronounced than that of a NiCd especially in smaller cell sizes, or the voltage should not be allowed to drop as much before the charge is terminated before damage from overcharge occurs. This has given rise to chargers with either a NiMH setting and a NiCd setting, or two (or more) peak detection sensitivity selections. Some chargers have neither adjustment but are sensitive enough for NiMH (but which may not put the last 1/2 of 1% of a charge into a NiCd pack). Personally, I have my Schulze chargers set to "sensitive" peak detection and leave them that way regardless of what I'm charging. On other chargers where there is a choice I try to make sure I've set it the correct direction, but not even having a choice suits me better, as a sport flyer, as it's one less thing to set incorrectly.

Another area where NiMH cells are different from NiCds is in the way their internal resistance changes as they are charged. This is why chargers that have some sort of automatic charge-rate setting ability, such as the FMA SuperNova and Schulzes, should NOT be used in automatic mode on NiMH batteries. I do not know the specifics here, but I can say from my own experiments that while charging a NiCd battery such chargers start at a slow charge rate, increase it in the middle of the charge, then reduce the rate again as the battery reaches full charge, with a NiMH battery, once the charge rate starts to increase it generally goes to the maximum rate the charger can deliver and stays there until the peak is detected. This can lead to very hot and perhaps damaged NiMH batteries.

I will admit to using my Schulze isl 6-330d in automatic mode (with sensitive peak detection selected) on 3000 mAh NiMH packs, as its maximum charge rate is 5.6A, or a bit under 2C for those cells, which they seem to tolerate OK. I did, for a time, let my isl 6-636+ charge some 3000s this way, which meant rates of up to 8.2A. While those packs performed well off of a charge like that, their capacity is down quite a bit now after a relatively small number of cycles, so I don't recommend doing this. On the SuperNova I only use a NiMH preset to charge such cells, though I have been known to tell the charger the cells had a higher capacity than they did in order to get it to allow a charge rate of greater than 1C. More on that in the SuperNova review further down in this column.

Even with the differences, properly charging NiMH cells is more like properly charging NiCd cells than it is different, and the same basic approach, with some care, works for both.

Enter Lithium Batteries

But now we also have a completely different family of battery chemistries to deal with. These are batteries based on lithium, rather than nickel. Lithium batteries are attractive because they have a much higher energy density than nickel batteries - more energy per unit weight. And with the recent availability of lithium-polymer cells that can deliver currents that are useful for higher-power motors than those used in purely indoor flyers, interest in these cells has really taken off.

But lithium-based batteries are different than the nickel-based ones we've been using for years in several key ways. One of them is how they are safely charged. Properly charging lithium-based cells - Li-Ion and the newer Lithium Polymer types (which are also lithium ion in chemical principle) - is done COMPLETELY DIFFERENTLY from nickel-based cells. Trying to charge a lithium battery with a peak detector charger will, if you are lucky, only make a chemical mess. If you're not so lucky you'll start a fire. Lithium batteries do NOT have a voltage peak to detect, and further, charging them past a particular voltage (which varies depending on the particular lithium formulation/construction) will damage them. If continued overcharge will eventually cause them to disassemble themselves, perhaps forcefully. Of course, one of the advantages of lithium-polymer construction is that because of their "soft pack" construction they swell, then if pushed even further, will vent, but they can't throw fragments of metal like a hard-cased lithium-ion battery could.

The charge process for Li-ion and Li-polymer goes something like this:  Initially, a constant charge current is fed to the battery, and the voltage is monitored. This charge rate is recommended to be 1C or less, though there is some experimenting going on with Li-poly cells right now at higher initial charge rates. I, myself, have been charging Kokam 1020 mAh cells at initial rates of 2A with no signs (yet, anyway) of battery distress. Fred Marks tells me that in his tests charging at 2C reduces the useful life of the batteries by about 20%, so instead of 600 cycles they will last about 480.

Once the voltage rises to a certain level - for example about 4.2V per cell for Li-poly cells - the charge current must be reduced in order to prevent the voltage from going higher. This is the constant voltage phase of the charge. The current is then continuously reduced to hold the voltage until it reaches some low level - the Kokam Li-poly applications manual on the FMA Direct site says full charge is reached when the charge current falls to 0.1C or less. The charge is then terminated. The voltage must not be allowed to rise above 4.23V per cell or the cells may be damaged. I should note that the four lithium-capable chargers I've tried all will continue to charge in constant-voltage mode until the charge rate goes to zero.

Chargers that have the right software to do lithium-based cells with high enough charge rates and cell counts to be of real use to electric-powered model flyers are just becoming available. Some of the more expensive German chargers add lithium charging capabilities to the others that they have. I've used my schulzes to charge two cell Li-ion and Li-Poly packs from the outlet that's intended for receiver batteries. But these are limited to low currents. More recently I've been using a pre-release version of the Bishop Power Products SC-2 which is really two chargers in one box, each able to do up to 4.2A into up to three lithium cells (with a 13.8V power supply as input). I've also just begun to use the Kokam 402 that is sold by FMA Direct. This one has a voltage booster inside so that it can charge up to four lithium cells at up to 1.5A initial rate, from a 12V supply. More on this one at the end of this column.

I will have more to say on lithiums in the next installment. Now, on to the featured chargers.

 

FMA Direct SuperNova 250S

The SuperNova has been available for some time now - at two and a half years. It was introduced with some seemingly astonishing claims for a charger priced at US $129 - including features that were then only available on high-end European chargers that cost several times as much. Since then there have been some shifts in the market so that there are several other full-featured charger/dischargers available for less than 200 USD. However, the SuperNova 250S, even though it was the first in this class, is still among the best.

Description

For those of you not familiar with the SuperNova, it is a fairly compact microprocessor controlled peak-detector fast charger, rated to charge from one to 25 cells, at rates from 100 mA to 5A. It also can discharge the same number of cells at rates between 0.5A and 3A. It runs from a 12V source. The currents it can charge or discharge at are lower at both ends of the cell-count range. It is packaged in a rugged aluminum case with cooling ribs on the bottom and back. It has a two-line LCD display for the menus and charge/discharge information and is controlled through three buttons labeled "discharge/down", "set stop", and "charge/up". Pressing both "up" and "down" buttons simultaneously during or after a charge or discharge brings up additional data.

Overall view of the SuperNova. The display alternates between these two views when in manual charge mode with no charge or discharge underway. The settings are for the battery for my Multiplex Cockpit MM transmitter.

Power input is through a three-foot lead ending in small alligator clamps which comes out of the left end of the charger. I cut the alligator clamps off about six inches from the end and put a pair of Anderson Powerpoles on the input cable to interface the SuperNova with both my field batteries and my under-bench power supply. I installed a mating pair of Powerpoles on the short clamp lead and keep it in the field box in case I need it. Output is through two standard "banana" jacks on the right end. No mating plugs are supplied, but a good selection of charge leads are also available from FMA Direct. Banana plugs are also widely available at electronics stores including Radio Shack.

There is a fan in the unit, with the intake on the front to the left of the LCD display and the exhaust adjacent to the output banana jacks on the left end. The fan runs whenever the charger is either actively charging or discharging a battery. It also has an end-of-charge signal which sounds just like my wife's electronic oven timer and a couple of electronic alarm clocks in the house. You can hear this end-of-charge signal from quite a distance at the field. It (and the individual beeps that accompany pressing the buttons) can be shut off as one of the menu choices if you want it to be a little less obnoxious late at night in the house<G>.

Operation

Charging and discharging can be done in either "automatic" mode, which is the default when the charger is powered up, or by using stored battery profiles. These profiles, which include cell type (NiCd, NiMH or lead-acid), the number of cells and the capacity, can be set and saved by the user. You can also define a maximum charge and discharge rate, within limits the charger sets for you, which are, in turn, based on the battery profile you create. The SuperNova has ten such presets, numbered, interestingly enough, zero through nine. If you have, as I do, a mixture of NiCd and NiMH batteries for your fleet, the ability to define and recall the charge/discharge presets is very useful. I also use one preset to charge the NiCd battery in my Multiplex transmitter at a relatively conservative rate, and I even use one preset for AA sized NiMHs I use in my digital cameras.

  Charging batteries for my digital camera with the SuperNova.

As I noted above, charging NiMH batteries using the "automatic" setting on chargers that have this capability can lead to charge rates that are too high for the batteries being charged.

Using these presets admittedly is still more complicated than simply dialing the desired charge current up with a knob as with, say, the Astro Flight chargers (such my 110D) or the GWS or WattAge PF-12. But none of the knob-adjusted chargers I have can also do discharging or cycling of packs. (By the way, yes I know that the recently introduced Astro Flight 110Deluxe and 112Deluxe do have discharge capability.)

Speaking of discharge capability, my SuperNova was the first charger I have owned that could discharge and cycle my drive batteries. For years I thought that I didn't really need this ability. After all, the best way to discharge a drive battery is to fly it, right? Maybe that is true, but there are times when you can't do that, and, perhaps more importantly, there are times when you want to measure the capacity of your drive battery, and the SuperNova does that task for you automatically. It retains the information until you disconnect the battery you were discharging from the charger.

The SuperNova will automatically recharge a pack after you've discharged it, after a few minutes' pause to let the battery cool off. If you only want to discharge the pack (as to prepare it for long-term storage), you have to stop the charger or disconnect the battery before it starts to recharge. One way to catch it is to be near enough to hear the fan stop as it does when the charger pauses between discharge and charge.

It IS possible to define a preset to discharge at, say 3A, then recharge at the lowest rate (100 mA) which would give you quite awhile to take the battery off the charger before it was charged very much. It is also possible to modify a preset temporarily to accomplish this.

There is a good review of this charger on Red Scholefield's R/C Battery Clinic site (www.rcbatteryclinic.com). This is an annotated version of the review Bob Aberle wrote for Model Airplane News with additional info by Red himself. The review itself is at http://www.rcbatteryclinic.com/nova.htm.  Take a look at that, then come back here.

To Bob and Red's review I will add that since I have a mixture of NiCd and NiMH batteries in use, the ten pre-configured profiles are very handy, especially when it comes to cycling NiMH packs. It is much easier to call up one of the presets than it is to reprogram, say, a schulze each time I want to cycle a NiMH pack that would be treated too harshly by the automatic settings.

Miscellaneous Comments

Here are a few other hints and thoughts from using this charger for quite a few months now.

The charger comes with a pair of plastic brackets or feet that are intended to get the charger off the table or floor to allow air circulation around it, and also set it at a convenient angle for reading the display. But, these brackets fit rather loosely into the cooling ribs on the bottom of the charger and if you move it or bump it they just fold to the side, dropping the charger. So, most folks I know of just ignore them. I've found, though, that if you run a dowel from one bracket to the other as shown in the picture below, the feet stay pretty much aligned rather than flopping over and actually become quite useful.

The back of the SuperNova and a closeup of the dowel I use to stabilize the feet.

As mentioned in the review on R/C Battery Clinic site, the instructions that are supplied with the charger are dense and kind of hard to follow. When I e-mailed FMA about this I got the same answer as Bob Aberle reported in his review - the suggestion just start using it and it will soon become easy to use. To some extent I agree with this advice. I find that if I am usingi it frequently operation seems straightforward, though I won't ever say it gets to be intuitive. But if I've not used if for awhile (I do have several other chargers) when start using it again it is sometimes frustrating. In particular I find the button presses necessary to call up a preset then having to go through two other choices to "exit" before I can then use the preset kind of irritating. I almost always wind up going the long way around in the menu to get there.

Fred Marks has prepared a flowchart and a quick reference guide to help sort this out. You can print or download a copy of it here. Also, there is another diagram that I found thanks to posts in the Ezone discussion boards, also in the form of a sort of flowchart. It, too, is a handy reference for navigating the menus. This chart is here .

One feature to be aware of, is that when the charger is first powered-up it always comes up in automatic mode, even if you were using it in manual mode last time (unlike some others). This makes sense, really, as that's the easiest way to use it. Just connect the battery to be charged and press "charge" or "discharge" to get the process started. The particular preset you were using last time you used manual mode will be the one that shows first when you call up manual mode again. In general the automatic mode is pretty conservative, at least for NiCd cells of, say, 600 mAh capacity or higher. The charge rate is increased slowly (mine doesn't even attempt to ramp up the charge rate until a little after one minute of charging) and ramps it back down near the end of the charge - again when charging NiCd batteries.

Default charge mode upon power-up

When you start a charge in manual mode - that is using one of the presets - the charger will sometimes go through a "battery check" function and so state on the display. If you are in a hurry and want to bypass this, just press charge again. This checking function is supposed to be validating the condition of the battery and the selected cell count. I've asked why it only happens some of the time, but the answer seems to only be known to the folks in Korea who actually make the unit.

Also, I, like many others, have had the switches seem to get harder to press with long-term use (over a year). I found that realigning the plastic buttons you actually press (which in turn press on the actual switches inside) so that they're not hanging up on their cutouts in the faceplate help with that some, but that it still takes more force now to press any of the three buttons than it did when the charger was new. They are, however, still working just fine. FMA tells me that they have a policy of replacing failing switches in SuperNovas at no charge if the owner wishes them to.

Summary

Despite its quirks (and there are a few others) the SuperNova has proven to be a reliable and effective charger and discharger in the many monthts I've had mine in use and still represents a very good value in NiCd and NiMH charging. In particular it is the charger I reach for from my collection whenever I have NiMHs to charge and it usually goes with me to the field whenever I'm flying anything bigger than my parkflyer types (most of which are flying excusively on lithium polymer cells now anyway). If you're in the market for a charger/discharger with its capabilities, the SuperNova is well worth your consideration.

Preview: Kokam Lipo-402 Lithium Polymer Charger

Speaking (again) of charging lithium polymer batteries, one of the limitations on faster adoption of this battery technology is the dearth of suitable chargers. Kokam, who makes LiPoly cells, has created a compact charger for one to four LiPoly cells in series and charge rates from 100 mA to 1.5A to help fill this gap. I'll have a more to say about it next time since I've just received my unit and haven't had a chance to use it much yet, but here's a start:

The Lipo-402 is about 4 3/8 by 1 5/8 by a little less than 1/2 inch. The case is a rectangular aluminum extrusion. The input cable (which is very similar to that of the SuperNova) and the ouput jacks (also banana jacks) are on the left end of the charger. On the right end are two sets of jumpers. The upper one selects one of six charge rates from 100 mA to 1.5A. The lower one (labeled "voltage select") is for setting the number of cells to be charged, from one to four. Supplied along with the charger is an instruction pamphlet, a pair of banana jacks for your output lead, and four jumper plugs for the current and voltage select jumpers (two for the charger and two spares). These jumper plugs, by the way, are bright red rather than the usual black so will be a little easier to find if you happen to drop one on the floor or in the grass at the field.

 
This is what's in the Kokam Lipo-402 package - Very compact unit.

Visible through small holes in the face of the charger are three mode LEDs, labeled "constant current", "constant voltage" and "full". These indicate what stage of the charge process the battery being charged is in (refer to the description above of the lithium charging process). There is a portion of time during the charge when both the "constant current" and "constant voltage" LEDs are illuminated. During this time the charge rate is being reduced from that selected by the jumper to a very low level. By the time the charge is terminated the "constant current" LED goes out and the "full" LED comes on.

This is how the LED indications chage from the beginning of the charge to the end. The jumpers are set for 750 mA and two cells

The charger has a voltage booster built in, so can it charge three and four-cell LiPoly packs from a 12V source. This, I am sure, accounts for some of the seeming high price of $99 for the charger.

In the testing I've done so far, I've found the output currents to be close to the jumper selections, and the critical cutoff voltages (for the constant voltage portion of the charge) to be exactly right. Running at 1.5A and charging a two-cell ET 1200 pack it gets just warm to the touch.

The only concern I have right now, and it's a small one, is that if there is NO jumper on the current select, the charger defaults to the maximum charge rate. I would think the minimum charge rate or even zero would be safer. The voltage default with no jumper installed is the safest one cell choice.

I'll be using the Kokam Lipo-402 as much as I can in the coming weeks and report on it some more in the next Recurring Charge. So far it looks like it works very well. It is also very ruggedly made and compact.

FMA Direct SuperNova 250S
Distributed by: FMA Direct
Price:  $128.50

  • Type: Microprocessor controlled charger/discharger/cycler for NiCd, NiMH, and lead-acid batteries
  • automatic and programmable peak detection for NiCd and NiMH
  • Fast charge current range: 100 mA to 5.0A
  • Discharge current range: 500 mA to 3.0A
  • Cell count range: 1 to 25
  • Input power:  12 V DC
  • Input lead lengths:  approx. 36 inches (0.9m)
  • Output connections:  Banana jacks, no output leads provided
  • Dimensions: 5.7 x 3.6 x 1.6 in. (14.5 x 9.2 x 4.0 cm)
  • Weight:  19 ounces (540g)
  • Additional features:

    • Ten user-programmable preset charging profiles
    • two-line LCD display
    • audible end of charge/discharge signal (which may be turned off)
    • cooling fan

    Kokam Lipo-402 
    Distributed by FMA Direct
    Price $99.95

  • Type:  Constant current/constant voltage charger for lithium-polymer batteries
  • Charge current range: 100 mA to 1.5A in six steps
  • Cell count range: 1 to 4
  • Input power:  12 V DC
  • Input lead lengths:  Approximately 32 inches (80 cm)
  • Output connections:  Banana jacks. Mating plugs supplied
  • Dimensions:  4.4 .W X 1.6 H X .5 in.D  (10.7 x 4.1 x 1.3 cm)
  • Weight:  3.5 ounces  (100g)
  • Additional features:

    • three LED status indication
    • two spare jumper plugs

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