|Aug 02, 2004, 10:05 AM|
Can LiPo batteries take overdischarge for short time periods?
I recently converted a BOT to electric and was running a 7 cell Nicad pack. Rate of climb is very respectable, ( I would guess around 900 ft/min). The batteries weigh 15 oz and I had to add 2 oz to the tail to balance the plane. I ordered one of the new 11.1 volt 20C batteries from FMA Direct which only weighs 8 oz. That allows me to remove the weight from the tail and saves me a total of 9 oz of weight. It also gains me about 3 volts, (11.1 verses 8.4 volts).
Now for my problem;
The battery is rated for 42 amps continuous discharge. I am pulling 54 amps. Can LiPo batteries take the higher discharge for the 30 to 45 seconds it takes to get to altitude without damage?
I can use a smaller prop to get the amperage down. My climb rate will still be higher and my plane will still be lighter. I figure I can always add ballast if I need to.
|Aug 02, 2004, 03:45 PM|
I copied this data directly from the FMAdirect.com site. They do not give the "peak" rating so I will contact them about it. While I am at it I will ask them what their feelings are on my question and what 20C / 90% means.
Kokam Super High Discharge LiPo Electric Pack
2100 mAh, 3 cell series (11.1V), heatshrunk with DNS connector, super high discharge (20C / 90%)
Size: 108mmH x 63mmW x 23mmT
Weight: 254 grams
Ratings: 20C / 90%
Outputs: 11.1V Nominal, 2100 mAh
Applications: Supports up to 42A continuous discharge
|Aug 02, 2004, 03:48 PM|
I think peak is defined as 5-10 sec. The time is limited by the rapidly increasing pack temperature at high C rates. Even the 42amp discharge rate may overheat the pack. That's the problem you'll have to look out for. I haven't seen any constant current discharge graphs on these packs so all you have to go on is the manufacturers data. Sounds like you might want to make some test runs and measure the amps and temps.
High discharge rate = heat = cell deterioration or destruction.
|Aug 02, 2004, 04:17 PM|
You could get away with it for a few times but, you are risking a U$100 battery pack and the plane. With the added 3volts, your amp draw will increase to 60+. The easiest and cheapest option would be to prop the motor to get around 40 amps static. Why only 40 amps? Because FMA/Kokam is pretty optimistic with their current ratings. You can also sell the battery and get two packs of the 2000mah 15c batteries @ U$70 each. Two of those weigh ~10 oz and will give 55 amps. Besides, the two packs wil give 4000mah capacity so they will last twice as long.
|Aug 03, 2004, 02:01 AM|
If you were to draw a graph of current versus time for a given temperature rise you find that it starts at a very large value (short cuircuit current) and rapidly tails off to 'steady state discharge' current at about 5 minutes or so.
I have briefly sparked a cell when building a pack on full short. It survived.
So the answer is always 'it depends how long you do it'
At very high currents the foils will melt after a second or so: at lower currents you will get hot spots in the pack after a few seconds, at lower currents yet you get overall pack heating and delamination, and at 'rated currents' teh pack survives but lifetime is overall lower due to pack temps being a bot too close to maximum.
If you like at say 100C pack life is less than a second, at 20C or so maybe 20 seconds, at 10C maybe 20 cycles, at 5C maybe 200 cycles....etc etc.
I am trying to make sure NONE of my packs dioes more than 8C on WOT static, and fly at 5C or less for cruise.
|Aug 03, 2004, 12:49 PM|
Here are the FMA answers to my questions:
To answer your questions:
What does 20C / 90% mean? 20C = 20 times the rated capacity. If you are discharging at 20C, you will have 90% of the rated capacity.
What is the "peak" discharge rate of this battery if 42 amps is the "continuous" discharge rate? Not rated.
Can this battery sustain a discharge of a higher amperage for short periods of time without being damaged? (I have an electric powered glider that will draw about 54 amps with this battery unless I prop it down. Typical power on cycle is about 30 to 45 seconds.) No, the maximum continuous discharge is as stated.
Howard G. Matos
FMA Direct, Inc.
5716A Industry Lane
Frederick, MD 21704
|Aug 03, 2004, 06:24 PM|
Joined Oct 2001
It most likely is because of the tabs on the cell. 42 amps is a lot of amps to put through those tabs. Hopefully they're a lot wider and thicker than the tabs we usually see on lipos to allow them to carry that much current. 54 amps may cause them to glow orange with heat, ruin the seal, and fail.
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