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I can get used prius battery pack out of salvaged cars for about $400. Would this be a good investment?

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I can get used prius battery pack out of salvaged cars ...
Well, NiMH are supposed to have a good life if not used too hard.

But these are the most difficult to charge properly; they need equalisation charges, careful monitoring, etc. Even Toyota didn't do such a great job there, IMHO. But it's harder equalising when you don't know how long your power source (gas engine) will be switched on, I guess.

You'd need some sort of fancy charger that had a very good NiMH algorithm, and use the temperature sensors and run the fans at appropriate times (charge and discharge). It's possible you could adapt the Toyota battery ECU to do this for an EV, but it might be tricky.

So the issues would be:
1) Finding enough hybrid packs to make a suitable EV pack from.
2) Rearranging the cells to a more convenient voltage; 200-240 VDC is a little high for DC conversions, and a little low for AC. Also, you need many hybrid packs in parallel; 6.5 Ah is way too low for an EV.
2a) Decide whether to parallel at the cell, 6-cell module, or pack level (each has advantages and disadvantages; NiMH cells should probably not be paralleled at the cell level)
3) Finding and replacing the bad modules; any random set of hybrid batteries of unknown age and history will likely have some bad cells or 6-cell modules
4) Sort out the charging issue.

Two things you can't to much about:
1) Slightly lower energy density than LiFe
2) Higher self discharge than LiFe or PbSO4.

A Prius 201.6 V nominal pack has the equivalent of 70 LiFe cells in it, but at 6.5 Ah. So the equivalent cost for LiFe is about 70 * 6.5 Ah* 1.25 US$/Ah (?) ~= $570. A Prius pack at $400, while cheaper for the bare cells, isn't that much cheaper that you'd say it's definitely worth the hassle of solving all the above issues. If you can get LiFe for 1.1 $/Ah, then each Prius hybrid pack is $500 equivalent. 20% difference in price.

If you're talking Gen 1, then they have more cells (19 modules compared to 14 from memory), so if they are $400, they would be better value by some 35%.
 
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