Question about battery selection for EV build
 

I know I sound like a broken record but I need to ask some more questions about batteries before I buy them.

I understand the voltage is dependent on the motor and controller I use (plan on using a 9-11" DC @ 144v)

Then I also understand that the aH (amp hour) rating will tell me how much work the battery can do over a given period of time.

HEre is my question. I've located a company that sells replacement PRius battery packs (LiFEPO4) for $900/each. They are rated at 48v/10aH. He states that 5 of these are needed to replace the entire prius battery pack. That would mean that the Prius battery is only rated at 50aH but higher voltage.

I have since found several other types of batteries of varying aH ratings and voltages. I do not know what to buy.

I've found 8v@ 3.2aH batteries VERY cheap and they only weigh 3lbs. I could put 36+ of these in my vehicle and save a TON of space and weight. My voltage would be in line but I'd only have 110aH or so. That's still double the Prius' so is that good? Do I need less Volts and more amps or more amps and less volts?

I've been wanting to order (12) Odyssey batteries but they have very low aH ratings (12v@13aH). I've heard these batteries are good starter batteries but don't have the depth of power to run an EV for more than a few minutes.


Someone in the know....TELL ME WHAT TO BUY!! (I'm working on a budget but don't want to buy crap.)

Toyota Prius Hybrid PHEV Lithium Ion LiFePO4 batteries :eBay Motors (item 280351225061 end time Jun-27-09 00:33:25 PDT)

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.

I agree with roflwaffle, you want the largest amp hour battery that you can fit if you go with lead acid, or you will tend to fry your batteries and replace them by the end of the summer, lithium you want the highest discharge you can get, lead acid you want that too but you also want capacity unless you are drag racing.
something that is rated as a traction battery is going to be the best way to go.

http://www.evcomponents.com/SearchResults.asp?Cat=33 Was the cheapest place I've found batteries, but they seem to have changed things around when I wasn't looking...Their 200ah Blue Sky LFP4 pack was like $200 or so, and since I'll be throwing the numbers around for the 3+ years it'll take me to save up, lol...it would put your battery pack cost up around $7k or so, for like 72V @540 amp draw, and double that for 144V, but you would also have insane range and reasonable (possibly great) acceleration, depending on the rest of your setup. I'll be watching these prices anyway, and have been digging around some of the more interesting places for info/parts given some possible project ideas I have. Seriously, though, 144v + 540 amp (c3 draw on these) would be 77kw for an OK draw...hehe, plus 2000 charge cycles is like 5.5 years, if your run it every day! Lead acid is only cheaper initially, but since I have to save up to begin with, I'm not going to bother unless I fall into like half the pieces lol!

I voted (for the smallest initial expense) before I saw the comment about "those in the know". I'm cheap, so I voted as I did. I am not really someone in the know about EVs, though, so take my opinion with a grain of salt...

-soD

Err, not really. 5 of those packs are going to give you 240V/10ah OR 48V/50ah, depending on how you wire it.


Those would be almost useless for anything larger than a go-kart. You would have less than 1KWh of usable energy. That, if I recall is about half the capacity of the Prius pack. You would actually only have 288V/3.2ah if you wired the batteries in series.

Honestly, I would not get anything in that poll for an electric car.

Remember this simple rule of thumb: If you are wiring cells in series, multiply voltage, and if wiring in parallel, multiply amp-hours.

The problem with the poll is that none of the choices are suitable for anything but a go-kart. So none of it is really cost effective, in the short or long terms.

In the other posts I feel like I'm being talked AT. The above post makes sense but leads me to more questions...

Why should I wire my batteries in series or parallel? What is the physical difference in wiring them up this way? I only know of one way to tie the batteries together.

If I'm using a 9" DC motor which type of battery connection should I be using and would I need to focus more on kW ratings then??

Wiring in series increases the voltage, which is almost universally necessary, as you won't run across many batteries over 12V, and you need higher voltages to achieve higher speeds. I think 48V is good for 35-40 mph, but I could be wrong. I think the rule of thumb for highway speeds is a minimum of 96V.

On the other hand is wiring in parallel, which increases amperage, an example being if you have a pair of 100ah batteries, in parallel, they become a single 200ah battery. It's generally not useful unless you simply can't find high-enough capacity batteries.

You can run combination's of the two, but generally if you can find batteries with a high enough capacity, getting them and wiring in series is the better bet.

The physical difference is more easily explained by the pictures on this page.

As for what configuration of battery to use, that will be dictated by what voltages your controller is capable of handling, and how much weight your car can handle, or you've allotted towards the batteries.

KW simply give an idea of how much power potential is in the pack, and it's found from pack voltage multiplied by AH rating, and your poll choices give the following:

Odyssey: 1.872 KWh
Marathon: 4.608 KWh
8V: 0.921 KWh

The closest to a usable pack comes from the Marathon's.

For a comparative reference, the Zap! electric car and truck have 6, 12V, 100ah batteries, giving them a 20-mile range on a good day with a careful driver, and the pack comes out to 7.2KWh. This in a car that weighs maybe 1,800lbs curb...

GREAT LINK!! So it seems that a series/parallel is the best configuration as it increases both voltage AND amperage. Those are very disappointing figures you posted for my battery choice vs the Zaps...

The WarP9 motor I plan on using is rated for no more than 156v (or so). I plan on running 144v but all of the performance charts only show power usage at 72v. Can I just double (or halve) those numbers to get ratings at 144v?