Quote:
Originally Posted by Hybrid-Battery-Repair
I'm sorry, but these calculations are not bourne out by my experience.
A pack left to cycle at 10 amps with the fan accidentally left off will reach an unhealthy 130 degrees farenheit or so (I haven't made that mistake in more than a year), but I've left full packs soaking on a 0.5A trickle for up to six one-hour periods in a row with the fan on and there has been no discernable heat rise in the air being exhausted. My chargers run to delta V and then kick over to trickle for an hour. 20 chargers at once make the air perhaps 1 degree above ambient.
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Fair enough, and thanks for the input. The biggest limitation of the above calculation is that it guesses at a lot of the numbers, and I have no ability to verify it experimentally.
Packs soaking on a 0.5A trickle charge: >90% of the energy going in is converted to heat, right? They should be giving off >80W each at equillibrium, which is a moderate amount. My computer (built for efficiency) gives off 60W, and I can sometimes tell the difference in ambient temperature with it on vs. off.
Quote:
Originally Posted by jonnyvtec
I dont think battery efficiency is anywhere near that personally.
Next issue is that NiMH uses around 6kg of Nickel per kWh so thats alot less, I guess its the film and electrolyte along with the metal housing that form your main heat sinks.
Uniform temperature distribution is not going to happen either.
Natural convection relying on the bouyoncy of air is a good cooling method on many things, gues it doesnt help a big deal on the insight though as the cells are all higher than any of the air ducts to the IPU!
Personally I dont see why fan control with a grid charge is discussed I think it should be included?
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There are only two fan control schemes worth considering. Fan always on is one. It's easy, foolproof, safe, and cheap. The other is temperature-based fan control. It's less of all the above things. But it allows you to start with a warmer pack, which I expect will be nice come wintertime. I'm doing it, but I wouldn't recommend it to a friend.
Regarding the uniform temperature distribution assumption: a "lumped capacitance" thermal analysis is valid if heat is generated and removed much slower than it's conducted through the body. I think that's fair.
Anyway, don't place too much stake in the analysis; it's got its flaws.
