Battery Pack Step Down

[Scrappyjoe]

Hello,

I am seeing more and more 800v architecture EVs come on the market, and, as they start to end up in salvage yards, we will start to think about using them in our conversions. Unfortunately, I am also seeing that many of these new models have moved from a Cell - Module - Pack architecture to a 'cell to pack' architecture, where breaking up the pack is quite difficult and, sometimes, unfeasible without having to deal with lots of glue and laser welds. So, it looks to me that we might see more and more complete packs used in EV conversions.

As of right now, most reverse engineered components seem to operate in the sub 450v range. Reverse engineering stuff that is in the 800v range is not very far along, and (my opinion), since these components are becoming more tightly integrated (see the BYD all in one component approach), probably harder to reverse engineer.

Putting these two together, I can see the desire to mate a high voltage pack (~800v) with lower voltage components (~400v). So, my question to the community is - is there any prior art here? Is it strictly a bad idea?

I can't see why it is a bad idea; one could use a Battery-Emulator device to manage the battery and a Zombieverter to manage the drive train. BUT one would need some sort of buck convertor in the middle, and perhaps to modify the Battery-Emulator device to apply some sort of scaling math when passing CAN messages from the Zombie to the BMS.

Ok, questions :

1. Is bucking an 800v battery to a 400v drive train an inherently bad idea?
2. Is it easy to implement?
3. Are there any reference implementations in the community?
4. Is my rough understanding of the challenges complete?

[johu]

It is hard to find a powerful synchronous converter (i.e. one step steps down battery -> consumers and steps up regen -> battery) at reasonable cost. See this one project with the 800V motor.
It would be a whole lot easier if this would only concern the auxiliaries like charger, heater, DC/DC and so on. Then a Prius converter would easily suffice.
Speaking of Prius, that one can take 650V anyway and some "800V" architectures are more like 600V.

Not sure how straight forward it would be to parallel the Prius (or any other) converter. You could set them to the top end voltage of common 400V kit (say 420 or even 450V). Then the 100A capacity would yield around 80 kW when two are paralleled.

CCS would be 800V anyway.

[Scrappyjoe]

I never thought of the Prius converter as a step down convertor (i.e battery voltage higher than the components) since in the OEM application it's the other way around, but I suppose current doesn't care about direction.

Ok, so it sounds like, actually, a fair number of traction motors which came in ~350v architectures are good till maybe double that (I recall the Outlander 400v cutoff is via CAN, but with the OE board it could go up to 650v), so one could maybe run the traction motors on the ~650v side and the auxiliaries on the ~350v side.

The other idea I had was to implement a buffer of some sort. 100A @350V is 35Kw which is actually sufficient for most continuous driving applications, so one could perhaps address it by putting a small ~350v buffer pack on the 350v side (say, an LTO custom pack, or perhaps even just an old 200v Pruis pack), and let that take the power spikes above 35kw.

But this all adds complexity, I suppose, and the more complex it is the easier it is for failure to occur.

[tom91]

2. Is it easy to implement?

No and very costly in Time, Costs and Expertise.

3. Are there any reference implementations in the community?

Not that im aware of

[arber333]

Hm... i would advise you not to put this into a working everyday car. Besides adding complexity it creates weak points in your build.
This would be a good experimental device with one motor acting in regen and other in power mode both using the same HV battery.
Maybe you could split the battery in halfh even though it would not equate to full 96S 2P...

Outlander rear motor with OEM control actually does adapt to the voltage up to 400Vdc. When applying more voltage amps are adapted to keep the power at the same level - 70kW. Funny thing is i feel the motor more responsive and smooth at 340Vdc than at 380Vdc.
At higher voltage motor seems more sluggish from start but can drive at higher speed with more continuity.

[Scrappyjoe]

I think you misunderstood me arber, I did not mean send the current via a motor on the 650v side to the 350v side. I just meant put the stuff that can handle 650v on one side and the stuff that caps out at 400v on the 350v side.

Anyway, I think I got my answer to this, which is, basically, don't do it, it's probably not going to go well. Rather get a 350v pack, or wait until the community has figured out 800v component control.