[Murphree]

I'm thinking about experimented with DIY DC fast charging. Imagine a situation with no BMS, limited (like six) charge cycles, and the only requirements are quick charging and quick discharging. Used automotive quality 32kw battery pack. Think 25kw use and immediately 15-30kw DC fast charging after use. Battery cooling done during charging.

Here are some of the challenges I've found:
Transforming 3 phase power into DC in a 15kw+ format at a desirable voltage (200-500vdc)
Managing the saturation charge above 4.0v/cell for lithium (nonlinear current)

I guess my real question is: can the core 3.2-4.0vdc charge be done in a relatively crude manner so long as voltage and amperage are supplied until the cells reach 4.0vdc? Then move them to a smarter charger to correctly provide current along the ideal curve for the saturation to 4.2vdc?

 

[GreenGyver]

Define "crude manner". Bottom line, you need to balance the cells or you risk depleting them past terminal voltage or overcharging them.

 

[Murphree]

Define "crude manner". Bottom line, you need to balance the cells or you risk depleting them past terminal voltage or overcharging them.

Instead of charging out of a chademo or factory dc fast charger, it would be directly out of a large DC generator for the 80% to 4.0vdc.
Basically, can the diy dc fast charge just be linear power out of power supply and then the topping up saturation charge on the exponential curve be done by a Brusa nlg513 or similar charger?

Packs would be manually checked for balance between events after six charge cycles. Looks like 400 cells per pack would be incredibly difficult and expensive to monitor via bms.

 

[GreenGyver]

Instead of charging out of a chademo or factory dc fast charger, it would be directly out of a large DC generator for the 80% to 4.0vdc.
Basically, can the diy dc fast charge just be linear power out of power supply and then the topping up saturation charge on the exponential curve be done by a Brusa nlg513 or similar charger?

Packs would be manually checked for balance between events after six charge cycles. Looks like 400 cells per pack would be incredibly difficult and expensive to monitor via bms.

What cells do you have that allow charging to 4VDC? I wouldn't advise fast charging anything past the nominal voltage marker if you try. What do you mean by "Large DC generator"? Sounds like you want to use a mobile generator that has DC outputs on it. Generators will have frequency fluctuations when under load.....especially if that load is over 80% of it's load max. Supplying a fluctuating DC voltage to anything is never a good idea. Most, not all, but most devices have basic circuitry to buffer incoming power supply fluctuations, but not to this magnitude.

Something to keep in mind....you will see a reduction in output from any manner of fast charging. Your cells (dependent upon what they are) will lose capacity if fast charging/depletion cycles are repeated. Unless you have individual module battery balancing boards connected to each module that are connected via CAN with your Brusa charger and your ChargeStar software is configured for CAN (which I believe it can be).

If you can completely reconfigure your battery pack to a centralized distributed pack or individual linear packs....(each of your packs/modules was completely paralleled acting as 1 cell) so that it can be removed and placed on your charger parallel to other packs so all packs are charged at the same voltage (linear charging), then this can be done. But there's a reason why packs, chargers, and generators aren't set-up to do this......the higher the current, the higher the heat generated, the larger the gauge wiring that is needed to carry this current safely.

 

[Murphree]

I have not purchased cells yet but planning on two chevy volt packs used together at ~200vdc. Max cell voltage 4.15 min 3.0. Something along the lines of 50s8p would be ideal. In that instance I think I could get away with a 50 cell bms if it was distributed and well labeled. Unusually high capacity loss is acceptable, so long as it's not all during the first charge! I am tempted to purchase a used $1000 load bank to test the generator first before hooking to an expensive battery bank.

I know there would need to be a diode between the charger and battery. The particular generator is a twin motor, induction and brushed commutator same shaft setup. In this instance I believe it is "self-rectifying" and it's 20+ phases based on size, but surely there would be a high level of noise compared to other methods. Do you know of any modern and efficient ways to clean this current?

 

[brian_]

The particular generator is a twin motor, induction and brushed commutator same shaft setup. In this instance I believe it is "self-rectifying" and it's 20+ phases based on size, but surely there would be a high level of noise compared to other methods. Do you know of any modern and efficient ways to clean this current?

Do you mean that your actual power source is AC, and you are using an ad-hoc rotary converter consisting of an induction motor driving a brushed DC generator to convert that to the needed DC? If so, the obvious solution to all aspects of this problem would be a properly managed AC-to-DC charger... with no mechanical parts involved. I realize that would be expensive...

 

[Murphree]

This is the DC shunt generator. My plan is to test voltage and overload condition at ~1900rpm eventually. Initial test will just be 540rpm with the tractor PTO and improvised load bank. Ultimately I'll replace the 25hp 3ph AC motor with a gasoline or diesel power unit.