Don talks about Charging LI-POLY batteries.

From : Don Stackhouse

Actually, we think things are already just about "there". We've been using the Plantraco LPC-400 charger with excellent results. It has two rotary switches on the front, one for setting it for one, two or three cells, and the other for 100, 200 or 400 milliamps charge rate. There's also a button to tell it to start the charge, and two LED's to tell you what's going on during the charge. The instructions are printed on the case, and it's powered from an AC adapter that's included with it. Price is under $60 retail. We've charged everything on it from 145 mah single cells to 3300 mah 3-cell packs with no trouble. It checks to make sure you have everything set and connected properly before it will start the charge. It initially starts with the preset charge rate, then switches to a slow "topping off" rate at the end of the charge. It does a beautiful job, does not require a degree in rocket science to use, and doesn't cost much.

The availability of chargers like that was one of the things that had been holding us back from endorsing Li-poly batteries. Based on our experience so far, it looks like that problem has been solved. We have also found that it doesn't seem to be necessary to charge cells individually in the applications we're using. So far we've charged them as a complete series-wired pack, and when we check the individual cell voltages after the charge they're all within about 0.01 volt. Altogether charging does not seem to be a problem any more. Also, we've heard that E-tec is coming out with a charger within a week or two, and it can charge at rates up to about 1.5 amps. It's supposedly only $15-$20 more than the Plantraco. We have some on order to try out as soon as they're available.

As far as monitoring the state of charge, we've found that a simple digital voltmeter is adequate. The other factor seems to be the match between the plane and the battery. In our experience so far, the drop in power as the voltage decreases is enough to make one of our Roadkill Series models sluggish comfortably before the cells reach their minimum safe voltage. An ESC with a good cutoff system like the Castle Creations Pixie 7-P is a great backup, but from what we've seen it's highly unlikely that you'll ever fly long enough to actually trigger the low-voltage cutoff in the ESC.

The best tool for monitoring the performance of any cells, regardless of whether they're lithium or nickel based, is a good optical tachometer. Power absorbed by the prop is proportional to the square of the rpm, so any abnormal drop in static rpm could indicate sick cells or an incomplete charge. My tachometer is one of the most useful diagnostic tools in my flight box.

Don Stackhouse
DJ Aerotech