Why such large, heavy batteries?
 

Probably a stupid question to ask, but I wonder why these huge, expensive batteries are being used to push a car no more than 20 miles, when you can buy 10ah AA Li-on rechargeables online for like $10 a pack.

Because 1 kilowatt hour only moves a vehicle as much as 5 Mikes at low speed and as little as 2 miles at high way speeds.

If my maths are right, you would need 6666 of your 10ah 1.5v AA batteries, to store the same energy as a Tesla 100Kw battery pack.

Be aware that you don't need just amps, but also volts. Tesla uses a 375v nominal system, so you would need 250 AA cells in a series, plus a BMS connected to all of them, to get a 3.0Ah 375v battery pack. I'm seeing about $1.50 per cell, so you're looking at about $375 + BMS, wiring and casing per 3Ah "string" of batteries.

At ~18g per cell, you're looking at roughly 11lbs per string, + BMS, wiring and casing.

I can't find any published spec sheets on discharge rate and I'm no battery expert, but I think 10-15A would be a generous estimate as an upper limit to how fast you can drain these. 10A would give you about 3.7kW of power, or roughly 5HP. If you drew any more, you'd probably kill your battery. Charge current would be a small fraction of this - maybe 2-3 amps inefficiently, and 1-2 amps without turning most of it into heat.

3.0Ah * 375v gives us around 1.1kWh of capacity. You can chop 20% (or more) of that off to preserve battery longevity, so let's give an upper limit of around 0.9kWh per string. A Nissan Leaf is EPA rated to go around 100 miles on 30kWh, or ~3.3 miles per kWh. Therefore, each battery string would be good for around 3 miles in perfect conditions.

To get the same 100 mile range as a Nissan Leaf, you would therefore need 34 strings of batteries, totaling $12,500 in just cells. You could safely regen at around 25kW, or 34HP worth of braking - which seems reasonably useful, but not incredibly fast. It would weigh 375lbs in cells, plus probably another 75-100lbs in wiring/BMS/casing - which is a bit lighter than a Leaf's battery, at least, but it would probably have a fraction of the lifespan.

On the other hand, you could buy 3-4 used Nissan Leaf batteries for the price of a single set of cells for your proposed AA-based battery, and probably 5-6 after you consider wiring it up.

Battery Junction is selling 3.7v Li-on batteries with 0.7ah each for about $5 apiece.

To get to Tesla level you'd need 100 of those, which would produce 70ah at 370 volts for 26khw (about 80 miles).

Of course the super/composite battery is only guaranteed for 500 charges, but it is also very, very light. You'd save tremendous mpg on account of the lightness, or so I'd think. You'd be looking at $500 for the base cost (although you'd have benefit of volume so maybe a 30% discount), then the wiring/management and cooling (which I figure could be done with coolant and insulation).

Instead of what?

I was thinking 9 volts a battery would be easier to manage voltage-wise, but in rechargeable terms the 9-volts available have very low capacity, so I just killed that line of thought. I guess all the batteries are using the same technology, and have to distribute their power between voltage and capacity.

If your line of thinking had been valid I think the automotive industry would have adopted it a while back. What you have in an EV battery pack is already a collection of Lithium Ion CELLS, connected up to give the required voltage and capacity. A 375 volt EV battery is composed of multiple series sticks of 100 cells, connected in parallel, to give the required capacity. It is like a 12 volt lead acid battery, as used on most road vehicles. It is composed of six 2 volt lead acid CELLS connected in series.
To get 30 kWH of capacity from a Lithium Ion battery requires 8108 3.7 volt cells.
Kilo watt hour (kWh) to Ampere hour (Ah) Conversion | EverydayCalculation.com
So you would have 81 parallel sticks of 100 cells to achieve this. This is what the EV battery is composed of.

Jockot is correct.

3.7v per cell * 100 cells = 370v nominal

370v * 0.7Ah = 259Wh = 0.259kWh

Your math was off a little, the 0.7Ah cells would be 0.26kWh per string, not 26.0kWh. Each string would be $500, and you'd need $57,700 in cells to get the same capacity is a Leaf pack.

There is a flaw in your logic
 

You can make your pack in series or parallel, or some combination thereof.

A series string raises voltage. A parallel string raises current (amps).

If you make a string of 100 3.7v cells, you get 370 volts as you describe. This does NOT raise the amperage capacity. The resulting pack will deliver 370 volts at 0.7 amp-hours. You could ostensibly draw 0.7 amps for an hour, which would yield 0.26 kW-h. NOT the 26 kW-h you state. In order to generate sufficient power to move a car, you would have to pull power at a MUCH higher rate than that, on the order of five amps at minimum just to get the car moving. Most commercially available cells cannot sustain a draw that high for very long, and we already know what some of the risks are of overheating a Li-Ion cell. You would have to restrict its maximum draw to something less than its stated maximum in order to provide a bit of overhead for safety's sake. But all other things being equal, such a pack would provide sufficient power to move an idealized, featherweight car about three miles before gasping its last. I'm not pulling that guess out of thin air, I'm doing the math against the Delta 11 prototype in EM's own garage, consuming 95 w-h per mile - and that is a crazy good result. Compared to other, more realistic and yet still excellent cars like Planetaire's modified Prius (as little as 175 w-h/mile, an amazing result for a non-purpose built road legal car), you would get about a mile and a half (and not at any kind of high speed, either) before your batteries crapped out.

And of course, resistance being what it is, real world results will almost certainly be less. Non-idealized cars being what they are, real world results will be less again.

My dad once told me that you could start a car with a stick of flashlight batteries, but it would be the last thing those batteries ever did. The cells in EV and hybrid packs are serious, heavy duty hardware, designed with heavy load and recharge currents in mind. It doesn't matter how cheap consumer grade cells are, these are NOT consumer grade cells.

[edit]
Oops, I see others beat me to it.