Today I was testing a new power setup for an old sailplane. The results were strange and disturbing.
The sailplane is one that I used to fly with my boys when they were entering their teenage years. And I’m thinking that even back then it was old – acquired second hand. We would either launch it from a bungee, or use an .049 on a pod to get it up in the air. I’m now putting an electric motor with a folding prop on the nose. It’s a Shenzhen D2836 1500 kv.
Anyhow, as I like to do, I was testing the power system that I plan to use. I decided to use my little Ovonic 3s 2200 mAh batteries for it to keep the weight down. Getting one of the batteries out of storage I hooked it up using a 60A ESC – I wanted to check the amps before using a smaller one. 935g of thrust, pulling 29 Amps. I’m figuring that’s OK, but after a few short bursts I noticed I was only getting about 400 g of thrust. Checking the battery it was now down to about 14%, so I figured that was the issue. I didn’t think too much about the fact that I store my batteries at around a 50% charge.
So, I put both Ovonics on the charger. With a full charge on the one I had just been using, I hooked it up and got a reasonable 985g thrust, pulling 32A. Running it for a while I got it down to 3.93 V per cell resting. That’s well over 50%. But now it would only generate 445g thrust – and a short while later, 430g thrust pulling 10A.
Next, I grabbed the other battery. It wasn’t quite fully charged, but close, at 91%. It produced 1100g thrust, pulling 32A. Bringing it down to 3.86 V per cell resting (52%) it still generated 1040g thrust, pulling 31A.
I note that in my previous battery testing, these same two batteries were able to deliver 71% and 77% of their stated 2200 mAh to full operational depletion. In checking them out today, the most poorly performing one had 12 mOhm resistance in each of its three cells, and the other one was 7 mOhms in each cell. That doesn’t seem to be terrible.
Any idea about what’s happening other than somebody making crummy batteries?
By the way, after the fact I looked up the specs on the motor. I’ve gone a bit big on the prop. While they expect it to be able to deliver 1150g of thrust and a 7 to 9 inch prop is recommended, the maximum current is supposed to be 25.4 Amps. But, amazingly enough, it is supposed to be able to handle a 4s battery!
RC Jim 