openinverter forum

Hey all!

I'm looking for a suitable battery for my SDU, but I'm having trouble finding one that fits my unique requirements. I'm posting here as a last resort, as I'm all out of ideas. My needs are as follows:


~350-400V nominal
~250 kW peak discharge (10 seconds)
<300 lbs (no more than 20 kWh needed, in general the lighter the better)


It seems to me that this is simply impossible. I've looked far and wide and thought deeply, but I simply can't find a solution, here are some of my attempts:


1: Find an OEM battery
-No manufacturer (I can find) makes a suitable EV battery
-No manufacturer (I can find) makes a suitable battery for home storage or seemingly any other industry

2: Use a small number of OEM EV modules, then a DC-DC converter to get the required voltage
-DC/DC converter would need to be capable of ~96V to 400V and 250 kW peak
-No manufacturer makes such a product, and my custom fabrication inquiries have been rejected
-It seems to me like this is a market to be filled; if one could produce such purpose-built and efficient converters for power-heavy, light EV builds, it seems to me that this could become a viable method of EV conversion power

3: Manufacture a battery from OEM Cells/Pouches
-Running the numbers, no matter how I wire it, they always lack. It's either an insufficient discharge rate, insufficient voltage, or ridiculously excessive weight

4: Use an above solution (likely the manufacture custom battery) to hit all goals EXCEPT peak discharge rate, and then use bank of supercapacitors to be able to make up the total of 250 kW for 10 seconds that then recharge from the battery afterward
-The magnitude of size and weight of supercapacitors is absurd (and price too)
-Convoluted and difficult system



So, I'm out of ideas. If anyone has any, I'd be happy to hear em! And if it works out, I might just give some monetary thanks. And yes, I have tried my hand at finding anyone with a similar issue.

BMW PHEV.

As above, look for PHEV batteries which are power dense rather than BEV batteries which are energy dense.

I had indeed looked for PHEV batteries, and it's more of the same issues. For instance, the BMW batteries don't seem to be able to discharge more than 90 kW peak, which is significantly less than 250 kw. Thank you both for the replies!

Edudokram wrote: ↑Thu Apr 03, 2025 5:41 pm I had indeed looked for PHEV batteries, and it's more of the same issues. For instance, the BMW batteries don't seem to be able to discharge more than 90 kW peak, which is significantly less than 250 kw. Thank you both for the replies!

Then you fit 2x BMW PHEV batteries for short of 200kW.
Maybe you could go higher than 400V, 800V would double the power while actually keeping the same cables...

Use LiTO chemistry? They can handle 10C! More $$$ though...
That would actually be something!

https://www.ufinebattery.com/blog/lto-b ... omparison/

Edudokram wrote: ↑Thu Apr 03, 2025 5:41 pm don't seem to be able to discharge more than 90 kW

No they can easily do ALOT more, they are limited by BMW for warranty reasons and the loads of miles.

I have been involved with a build doing 1300A on from two parallel BMW packs.

tom91 wrote: ↑Thu Apr 03, 2025 6:58 pm I have been involved with a build doing 1300A on from two parallel BMW packs.

Holy crap. Was that a pair of 12kwh packs? Can you share what kind of cooling that took?


OP: Bumping your voltage by a factor of 4 would NOT be efficient. You'd suffer pretty decent losses in the converter and the cabling and contactors on the 96v side would have to be massive. 250kW @ 96v is 2600 amps. Better to use as high a voltage as you can to reduce losses and increase your constant torque band. Can you go any higher than 400v peak? Because if you could, you could theoretically put say 7 or 8 bmw modules in series, since I think 2 full packs in parallel would be over your weight limit?

Zieg wrote: ↑Thu Apr 03, 2025 7:33 pm Can you share what kind of cooling that took?

None. It was only peak outputs a few seconds at a time. Any pack needs to be monitored and derated based on temperature. If you need to extend the time to derate you then increase cooling.

Thank you all for the input! Zeig, I especially appreciate the insight

No worries, it's a challenge that has occupied my thoughts for some time now. Largely because I want more torque at higher speed, which I can get with voltage. You just need to make sure you are getting enough torque at lower speeds with whatever current output the batteries have. Not a factor in my build since it already has more torque than traction, lol.