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Old 06-26-2009, 06:23 AM   #2 (permalink)
roflwaffle
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The Wh value of a pack is what you should check out if you're interested in energy available, and as a result, range. Calculating energy is easy, just multiply volts w/ amp-hours. The Odyssey batts would then be 12V*13ah=156Wh, and at $100 each, that's ~$.64/Wh, or $640/kWh. The marathon batts are about 384Wh, so at $75 each that's about $195/kWh, and the last poll option is about $1094/kWh.

The problem with using those ah figures, is that they are probably the 20 hour rates, if the battery was discharged completely over 20 hours. Most EVs are discharged much quicker, over a couple hours or so, so because of Peukert's law, the available energy will be much less than whatever the 20hr ah rating is, probably about half of what it is give or take IME, although this depends on the battery's Peukert exponent, which is a pain to find as far as battery specs go. Generally ~1.2-1.5 is the range lead acid batteries fall under, and the effective ah rating drops by ~35-65% compared to the 20hr ah rating, depending of how high the exponent is (higher is worse). The LiFePO4 batteries mentioned have a fairly low exponent, usually about 1.05 or less, so with more current, as is seen w/ an EV, capacity only drops by ~10% or so.

Anyway, in terms of usable capacity, because of the drop in energy w/ higher current draws, for lead acid, halving the ah rating, and as a result doubling the cost per kWh, gives a good idea as to what the usable energy will cost, so for the above batteries that's ~$1300, ~$400, and ~$2200 per kWh respectively.

Course, this isn't the whole story. Lifespan also matters. A battery that can be cycled twice as much will cost the same for storage as one that can be cycled half of much even if it's twice the price. Lead acid batteries tend to be in the 500 cycle range before they effectively go flat, so dividing the cost per kWh by the number of cycles at whatever percent depth of discharge will give the cost per kWh stored.

For instance, if I have a lead acid battery that's $400/kWh, and it can be cycled 500 times at 80% depth of discharge, then that's 500*.8=400 equivalent full cycles, so the cost is about $400 for 400 equivalent full cycles, $400/400 kWh cycles, and ~$1/kWh in terms of storage costs.

Compared to current LFPs (same as LiFePO4 batteries, and that site has the cheapest I've seen), that are ~$.10-.20/kWh stored, depending on how much the owner can let capacity drop, we're looking at a battery that's five to ten times cheaper to use than the cheapest battery in your poll. LFPs are also lighter, so that helps out w/ acceleration, but on the downside current draw tends to be limited to about 5C, so the most power you can get out is about five times the pack size in kW. Eg a 15kWh pack like the Volt has would only output about 75kW, possibly less if the pack isn't warm enough.

That said, as always, YMMV. If possible, the best course it to build a test bench that will mimic the current draw you'll see when you're driving, so you can see how much capacity drops as the different types of batteries are cycled and ultimately which battery lasts the longest for the least amount of money, which is what most EV builders are shooting for IMO.
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