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Recurring Charge - October 2002

Battery chargers have come a long way in the past 20 years or so, and this column highlights those changes, as well as reviews a few of the current chargers on the market.

Splash
Battery chargers seem to be one of the hotter topics lately on the RC Groups discussion boards, and in the last year or so, I have had the opportunity to take a look at a number of chargers. Therefore, it occurred to me to suggest to our esteemed Editor that a recurring series on battery chargers, including reviews, would be a good addition to the E-Zone. He agreed, so here we are. In keeping with the financially metaphorical name of my column on electronic speed controls, "A Controlling Interest", we'll call this one "Recurring Charge".
I'm not going to make any promises about the regularity of this series, but I do have a handful of chargers to write about and there are several charging-related topics I could cover along with the reviews. There is enough potential material for several installments, and as with A Controlling Interest, what I write about from one installment to the next can be guided by reader input.

Battery Charger History and "The 7 Cell Trap"

I think I'll start out this series with a look at how far the technology has come, just as I did with the A Controlling Interest series. Here is a short history of electric flight battery charging over the last 20-odd years.
In the beginning, there were eight cell NiCad batteries, 12V car or motorcycle batteries, and a length of "zip cord", which was the stuff normally used to wire a floor lamp to the receptacle in the wall. By connecting a 12V lead-acid battery to an 8-cell NiCad pack through this length of wire, whose length was chosen to act as a current limiting resistor, you could "safely" charge the NiCads without overcharging them. The trick was that you had to use ratios of four NiCad and three lead-acid cells, so the earliest AstroFlight systems required drive batteries in multiples of four or eight cells. To charge a 12-cell NiCad pack you put one 12V and one 6V motorcycle battery in series. To charge 16 NiCad cells, you used two 12V motorcycle batteries in series. By the way, these cells were General Electric 550 mAh 1/2sub-C cells that were the same size as those we now know as Sanyo CP-1300s.
Charging this way, the charge current started out very high (limited by the resistance of the charge cord) and tapered down to zero when the voltage of the source battery and the NiCad pack reached equilibrium.
The next level of sophistication, in the early 1980s, was to put a current-limiting resistor or two (with a switch to select between the two for charging either six or seven cells) in a box along with a 15-minute timer, and maybe an ammeter. With chargers like this, you generally ran the motor battery down, selected six or seven cells, and then set the timer for 15-minutes. If the battery were run down, this would give you a moderate charge. If not, or if you forgot which batteries were charged and which weren't, and you tried to charge one that was already full, you could easily toast a battery pack. Many chargers made during this time were fixed at about a 5A charge rate, which was 4C for the sub-C-sized cells (1200SCRs!) of the time. This was also the beginning of the seven-cell trap. You could only charge up to seven cells on a 12V source and maintain anywhere near a 4C charge rate. The more sophisticated chargers of the time added a variable resistor instead of the 6/7-cell switch. This allowed the actual setting of a charge rate and thus the ability charge other than sub-C cells. Depending on how much heat the charger could withstand, you might be able to charge fewer than six cell packs. Several flight systems, such as the AstroFlight ferrite 020 and 035 systems, used four and five cell packs.
You could charge more than seven cells by running a timer charger on an 18 or 24V input by putting source batteries in series. AstroFlight produced a device called the Voltage Booster about this time. It was just that, a voltage booster circuit that was a forerunner of the circuit found inside just about any of today's chargers capable of charging more than seven cells. It was the same size as the chargers of the time, and made an annoying high-pitched whine. I could barely stand to use it.
Then we learned about the effect that a NiCd battery's voltage would drop once it was fully charged, so people started hooking voltmeters up to their charges so they could stand there and watch for the peak and subsequent voltage drop! The drill went something like the following. Run the battery down and then do a 15-minute charge. If the battery wasn't warm at the end of the 15 minutes, restart the timer for another five minutes and start watching the battery voltage. After the voltage dropped a bit, stop the charge and go fly. This was really a very dangerous way to charge packs. If you were distracted while charging, you ran the risk of missing the peak on the voltmeter, and then it was "goodbye battery pack". I have seen packs literally explode from this process.
In the mid 1980s, there were a couple of timer-chargers that had built-in voltmeters to help you manually peak a pack. The top of the line was the Leisure model 109 Digital Charger, which had a big LCD display that was switchable between current and voltage. It sold for about $100. The problem with this was that moving this sort of electronics into the charger box meant that you couldn't just feed the charger a higher voltage if you wanted to charge more cells. As a result, people would split their batteries into groups of six or seven cells and charge them independently. Some folks still do this.
About this time, AstroFlight introduced the model 102 charger that had a voltage booster built in. It was a timer-charger, and if I remember correctly, rated for up to 24 cells.
As you might expect, the next development was early peak-detectors. Development of this charging technology was complicated by the need for the charge current going into the NiCad battery to be fairly constant, and at a high enough rate that a reliable voltage peak could be detected. Since the chargers of that era didn't really try to maintain a particular set current into the battery during charging, more electronic wizardry was needed.
The first US peak detector that seemed to give consistent results under varying conditions was put out by Leisure Electronics. It was their model 100 (if I remember correctly) and was a fixed-rate charger for six or seven cell sub-C packs. AstroFlight soon followed with the first Model 110 peak detector in early 1989.
AstroFlight also introduced an update to the model 102, the model 112. This was still a timer-based charger, but was redesigned to produce a constant output current under most conditions, rather than a continually tapering charge. It also had voltmeter jacks so that you could manually monitor the voltage peak. This charger became the "large system" charger standard at the time, though many (including mine) were soon modified with cooling vents and the addition of a small fan so that they would run cooler.
In the early 1990s an electronics whiz named Brad Baylor produced a microprocessor-based peak detector add-on for the AstroFlight 112, called the BEP MicroPeaker. I remember receiving mine in early 1993 and examining it by candlelight. (The power was out at our house and many other homes in the Seattle area due to a great big windstorm that came to be known as the Inauguration Day Storm, as it happened on the day of Bill Clinton's first Inauguration as President of the USA.) This add-on device provided reliable peak detection, and had two preset timeouts for safety under microprocessor control. You still had to set the current with the knob and meter on the face of the charger, and could still start a charge with the timer if you wanted. This combination worked very well. In some ways, it was ahead of its time, since it had features that would not be incorporated directly into production chargers for some years to come. I still have my AstroFlight 112 charger coupled with a BEP MicroPeaker (and the added cooling fan), and it is still in working order, but it has been a while since it's been used.
This is my AstroFlight 112 with the BEP MicroPeaker add-on, a cooling fan installed, and the input fuse holder replaced with a circuit breaker. Note the voltmeter jacks just to the right of the meter.
The BEP MicroPeaker foreshadowed the next development, the assumption of the peak detection or slope detection chores by a microprocessor and software. This advent allowed for much more information provided to the user, such as how much energy was put back into the battery. The AstroFlight 110D and 112D chargers were for many years the standard chargers for up to 16 and up to 32 cells, respectively.
Later, through the use of more sophisticated software, chargers gained the ability to select their own charge rate. These very chargers would even ramp the rate up at the beginning of the charge, then drop down again near the end so as to charge as fast as or faster than the constant-rate units, all the while being kinder to the battery being charged. Schulze led the way here, but now, there are many chargers available that can do this. In addition, of course, there is now the added complication of different battery chemistries, which is addressed in various ways by the software of various chargers.
Even so, especially at the lower-cost end of the spectrum, many analog and digital peak-detectors charge at a constant rate and do a good job of putting a full charge into a NiCad or NiMH battery safely. The price for such tools is far less now than it was 10 years ago. As with many other aspects of electric flight, we've come a long way.
I think this may even be the beginning of the end of the "7 cell trap". Until very recently, the cost of a charger with a voltage booster inside that could handle more than seven or eight cells was at least double that of one which did not. Usually, the higher-cell-count chargers started at about $100 US and went up from there. Just in the last few months or so, this has changed dramatically. In particular, there are two chargers that can charge 12 cells that cost less than $50. The first is the GWS MC2002, which is a DC only charger, which can charge at up to a 6A rate. It is available from several outlets for around $40. The second is this installment's featured unit, the AC/DC WattAge PF-12.

Featured Charger - WattAge PF-12 AC/DC "Park Flyer" Charger

Specifications
  • Type: Delta peak detection charger for NiCad and NiMH batteries
  • Fast charge current range: 250 mA to 2.0A
  • Trickle charge current range: 25 to 200 mA (10% of selected fast charge current)
  • Cell count range: 4 to 12
  • Input power: 12 V DC or 110 V AC 60 Hz
  • Input lead lengths: DC - 36 in (90 cm), AC - 55 in (140 cm)
  • Output connections: Speaker terminal style spring clips, no output leads provided
  • Dimensions: 7 7/16 in. W x 5 1/2 in. D x 3 1/2 in. H (18.9 x 14 x 8.9 cm)
  • Weight: 3 lb. 5 oz. (1.50 kg)
  • Distributed by: Global Hobby Distributors/Hobby People
  • Additional features: It has a 90-minute timeout on charging (both fast and trickle), cooling fan
One of the new chargers on the market that will put an end to the "7 cell trap", at least from cost of the charger standpoint, is the new WattAge PF-12. This $50 (US) unit can handle packs of up to 12 cells. It can also can be run off a 12-volt source or plugged into an AC outlet in your home or indoor flying site. This is a smart feature for a charger aimed at the "park flyer" market. I'm sure there are many folks who will appreciate bringing it to their indoor flying session and just plugging it in, rather than lugging around a charging battery too.
Incidentally, the WattAge PF-12 is only available for 110 VAC systems (as we use here in North America), but I have heard that a 220V version is winding its way through the CE certification process for our friends in other parts of the world.
With the ability to charge 4 to 12 NiCad or NiMH cells, at from 250 mA to 2.0A, the PF-12 covers a wide range of batteries used in slow and park flyer-type models, as well as larger packs. This flexibility is only limited by your patience and the 90-minute safety time-out on charging. This charge rate range covers NiCads of 75 mAh and up, and NiMH batteries from 150 mAh and up, though a 3000 mAh NiMH pack might not be finished when the timeout kicks in. This charger has a constant current trickle charge rate that is approximately 1/10 of the selected fast charge rate, which is also subject to the 90-minute timeout. In other words, if the charger is left alone, all charging including the trickle charge is terminated 90 minutes after the "start" button is pushed.
Physically, it's a fairly large and heavy unit as chargers go today, but it has a rather hefty transformer inside, which is necessary for AC operation. The case is all plastic, but it doesn't feel flimsy. Emerging from the back are both the AC and DC input leads. The DC lead is supplied with small, insulated alligator clips for connection to your source battery, and this lead is a generous 36 inches (90 cm) long. The AC input lead is about 55 inches (140 cm) long. On the front panel are a NiCad/NiMH selector switch, an output current adjustment knob marked in mA, a red LED indicator, and a "start" pushbutton switch. A pair of spring terminals similar to those used for stereo speakers provides output connections.
Here is a front view of the PF-12, with three charge leads. These leads are not supplied with the charger.
The PF-12 also has a small cooling fan in the bottom of the case that runs whenever the unit has power applied (either AC or DC). The instructions caution against setting the unit on a soft surface, such as carpeting, and blocking the fan.
Here is the bottom view of the charger. Note the cooling fan vent in the upper center of the picture.
In terms of user instructions, early units shipped with only a partial set. Even the full set of instructions, which is available on the Hobby People web site (http://media.hobbypeople.net/manual/130108.pdf), suffers from awkward translations to English and some non-standard terminology. In particular, it refers to capacity-based charge rates with the "C" ahead of the factor (for example "C2") where the more common terminology is to do it the other way around, as it would be in an algebraic expression (for example, "2C"). I hope this doesn't confuse newcomers who might be seeing this sort of thing for the first time. This complaint aside, the instructions cover the operation of the charger, its limitations, and give some good, if conservative, advice for the handling of NiMH batteries.

Using It

So, how does the PF-12 work? I've had my test unit (provided by Hobby People from their first batch) for about six weeks now, and have used it at two weekend flying meets as well as at home. I have found that it does exactly as it says it will do.
Using it is simple. First, hook it up to source battery or plug it into a regular AC outlet. (The instructions caution you against doing both, though when hooked to DC, you can hear a relay inside switch, which may well protect it from simultaneous connection to both types of power). The LED on the front panel will flash rapidly a few times, and then begin to blink slowly, indicating it is ready to go. Attach the appropriate output lead to the output terminals on the front (if you haven't already), and then connect the battery to be charged. Check the NiCad/NiMH switch and make sure it's set correctly. Set the current knob to an appropriate charge rate for the battery to be charged, and then push the start button.
The LED will glow solid red while the unit is in fast-charge mode, then switch back to flashing when the battery has peaked and the charger goes into trickle-charge mode. In trickle mode, the flashing is slower and the flashes themselves shorter than the "ready" flash rate. This continues until you either disconnect the battery being charged, or about 90 minutes elapses. After the 90-minute time limit, the charger goes back to the "ready" state and stops charging altogether. I admit that I haven't actually timed this to see how close it is to 90 minutes, but I have left the charger for over 90 minutes. When checked later, I found that the charging process had ended and the charger returned to "ready" status.
The PF-12 charging one of my 720 mAh NiMH Switchback packs. The current is set to just over 600 mA.
While checking out the PF-12, I measured the output currents into a couple of different batteries, and found the measured currents to be within about 10% of the markings around the current-adjust knob, which I consider to be close enough. (Of course, adding a meter of some kind would make the unit more expensive, and even if there was one, it might not be any more accurate.) I have also found that packs are warm, but not hot after the fast charge was completed, as long as the battery type select switch is set correctly. I asked Hobby People if the switch was even necessary, and the answer I got was that the peak detection is gentle enough on the NiMH setting that a NiCad battery might not be fully charged when the charger stops fast charging. I have not charged a NiMH pack on the NiCad setting to see how hot it gets if the switch is in the wrong position, however...
I did look inside just to see how well the PF-12 was built and was satisfied with the simple and clean construction that I saw. There are fuses on both the AC and DC inputs (even though these are not mentioned in the instructions), so there is protection against shorts or failures on the input side. Hobby People is planning to carry spare fuses. The charger is also supposed to be protected from an output short circuit.
I asked Hobby People about reverse polarity protection on the output. I was told that the charger would detect this and simply refuse to start running with no damage. I conducted a brief test of this and found that if you try to charge a battery backwards, it does appear to start the charge, but it stops charging again a few seconds with no ill effect to the charger. This is a good feature because it is fairly easy to hook something up wrong with the spring-terminal output connections.

Summary

There are many chargers on the market today, but the WattAge PF-12 is unique in its ability to charge from 4 to 12 NiCad or NiMH cells at up to 2 Amps, and do it on either AC or DC source power. It is simple to use and is well protected from the mistakes in hookups that we sometimes make. The AC/DC capability makes it equally useful at home, at the indoor flying site, or outdoors at the park or flying field. It seems to be well made, and at around $50, it's a good deal as well. For someone whose interests center around park flyer-type airplanes, it seems to cover all the bases. It would be a good first (or only) charger. It's also a great supplementary charger for just about any electric flyer except those who exclusively fly very small models with NiCad cells of 110 mAh capacity or less, or NiMH packs smaller than 175 mAh, or those individuals that exclusively fly larger models powered with sub-C sized cells. I recommend it (and have to a few people already).

What's Next?

I have several other chargers that I need to write about, including the FMA SuperNova 250S (yes, I know they've been around for a while), and the Schulze isl 6-636+RS. I'm sure that I will think of some aspects of charger behavior to discuss in future installments.
Here is my charging station at the spring Chilliwack electric meet in British Columbia. From top to bottom is the WattAge PF-12 charging a Switchback pack, the GWS MC2002 charging a 10 cell P3000 pack, the schulze isl 6-636+ charging a 14 cell CP-1700 pack, and the schulze isl 6-330d, which had just been disconnected from another 10 cell P3000 pack.
As with "A Controlling Interest", if you have comments or a suggestion of what you might like to recommend that I write about in a future article, drop me an email. (Click on my name at the top of this article.)

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