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Discussion Starter · #1 ·
I'd like a quick sanity check from some of you smart guys:

I have a Tesla standard LDU (not Sport or Performance) for a swap and I'm currently shopping for batteries. I found a Rav4 EV pack, which is manufactured by Tesla and has similar modules to the Model S and Model X packs. It is a 42 KWH pack and I BELIEVE the cells are in a 48p configuration instead of the 74p that you see in the 85 KWH Tesla packs.

I'm confident that I can repackage the Rav4 pack to fit my vehicle, but I'm concerned about the power output of the smaller pack. The Model S85 pack can produce 1520 amps for 3 seconds at a 6.5C discharge rate, I'm using the S85 module specs from HSR Motors for reference:

85-type | Battery Modules | Products | HSR Motors

However, the quick math tells me that the Rav4 pack uses different cells: 85KWH at 364V for the S85 is 3.15Ah per cell, and 42KWH at 386V for the Rav4 is 2.26Ah per cell. Using the same 6.5C discharge rate, that puts the max output of the Rav4 pack at 705 amps.

So my question is this: is it possible that the 2.26Ah cells in the Rav4 can tolerate a higher discharge rate? To get 1000 amps out of them would be a 9.2C discharge rate. To me, 1000 amps is kind of the "magic number" to make sure I'll be getting the power I want from my LDU.

Of course, if I have fudged my math or if I'm off base with my assumptions then please correct me.
 

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I don't know what the p number is for the RAV4 battery, it will be different drom the S85 yes. But aren't they using the same cells?

But I think your number doesn't sound that strange. I mean 705 Amps compared to 1520 amps is the same as 42kWh compared to 85kWh.
If they are using the same cells but just less of them, you probably get half yes.
So 1000 amps is probably stretching it, and out of spec.

However, it may still be a good idea, because I don't think there are many alternatives that could produce that match power anyway without having to go to 80kWh or something.
 

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The RAV4 EV pack has a discharge limit of ~160kw. This is reported by the Tesla BMS inside it. Now, this is probably just to limit the motor's power (the RAV4 EV has a LDU in the front end!) but it might also represent a lower grade of cells, or simply the design of the pack.

LG Chem batteries are generally capable of high discharge levels. Chevy Volt batteries are widely available; many EV builders use two of those packs (16kWh each) to power a performance LDU. I do not know how safe that is.

Good luck!
 

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Yes. That is also way out of spec, but lots of people test the LG chem modules like that. So there is some experience. I'm not sure how much experience there is in overdischarging the Tesla modules out of spec.
 

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Discussion Starter · #5 ·
Exactly, I would love to know what the actual discharge limit is versus the factory "safe" limit. I've gotten my hopes up by looking at the discharge tests conducted by Idaho National Laboratory that show some pretty astonishing actual discharge data. The BMW i3 has an 18KWH pack and the Kia Soul has a 27KWH pack, but both are able to produce over 300 KW of power for short bursts:

Library - By Vehicle | Advanced Vehicle Testing Activity

Also tiger82's first iteration of their race car used a 27KWH Kia pack, and they were able to squeeze 220 KW out of it for a 20 minute race:

Tesla Powered Cobra Race Car

Of course, these are large format prismatic or pouch style cells, so the discharge characteristics are probably much different. But... the Tesla module has a much better cooling system. The vehicle I am planning will be a weekend cruiser and won't see many miles, so I'm not particularly concerned with shortening the lifespan of the battery by over-stressing it.
 

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@bronco_zed: As you say those are very different Samsung and LG Chem cells. The results are promising though.
I would think by now that somewhere on internet there is someone who has greatly exceeded the specced discharge rate of a Tesla module, and reported about it. Specifically for the RAV4 battery you will not find it, but something using the same cells would be great.
Still there is uncertainty about the used cell coupling and all, but I think it would be safe to assume that is very similar. Measuring the resistance of the module could perhaps offer some clues, comparing it to what you expect it to be.

The coolant system is not that important if it is only for short zips. I would worry about damaging the cells.
It will probably not be a big deal from 700 amps to 1000 amps, but you might also be damaging it. Especially if you find the power addictive :)
 
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