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Old 01-20-2012, 10:48 PM   #5
nulluser
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Joined: Nov 2008
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Quote:
Originally Posted by _cy_ View Post
hmmmm ... was not planning on getting Peukert law, but was going to experiment with effects of temperature on lithium iron phosphate battery once I get them in. it's well known li-ion cells can drop dramatically under cold temps. the same with AGM which could loose about 45% of rated capacity at 4f degrees.

what Peukert law basically states is high amp draws will use up more capacity than slower amp draws. efficiency loss could be 85% vs lower current drain rates will achieve close to full rated capacity.

which leads us up the some of the mumbo jumbo used by some folks trying to sell these new li-ion batteries. we get to hear about effective amp hour rates. vs actual AH rates. marketing strikes.... 6 AH becomes 18AH for an effective increase of 3x.

let's cover one important aspect of li-ion cells... which is it's ability to discharge at an extremely high rate. this is why CCA (cold crank amps) rating for li-ion batteries means very little. Li-ion cells will deliver full power, then fall flat. little to no warning battery is running low.

different li-ion chemistry deliver different voltages and capacities. some are more stable then others. lithium iron phosphate is the most stable of all the li-ion chemistry so far. li-ion cells is a dynamic field with improvements being discovered constantly.

for instance...Aleees claims new double-layer nano-carbon coating technology enhances battery cycle life 2.7 times ... if this claim pans out... implications could be HUGE for electric auto mfg.
A big problem I had was estimating the 45 minute rate from the usual two hour advertised rate. In particular, my batteries are rated at 55 amp-hour (2 hour), and that ends up being about 25 amp-hour for 45 minutes. Combined with keeping the pack above 50 percent depth of discharge, I was left with around 13 amp-hour at 72 v, (6 batteries) which was around 1000 watt-hours. Well bellow the 4000 watt-hours I imagined from the raw numbers.

At a 50 percent depth of discharge, the AGM batteries were good for around 500 charge-discharge cycles, which I consider poor. My pack would have cost around 1200 full retail when I bought it, and a two year lifespan is not good enough in my opinion.

Now, consider modem lithium cells for a 4000 watt-hour pack. 14 of the 3.2v 90 Ah cells would run around $2000. I wonder if the run-time would be longer because of different internal resistance due to the battery chemistry.

It was hard for me to make an apples to apples comparison because of the lack of manufacture data. What wold be very useful is a graph of capacity vs discharge rate, and a graph of capacity vs cycles for a modern cell chemistry.

nulluser screwed with this post 01-20-2012 at 10:55 PM
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