Dual Battery voltage controller?

[BobEdwards]

Trying to decide between a 96V or 144V system. Then thought "why not both"?

If a controller (OpenRevolt?) could be modded to allow "normal" motor operation at (say) 96V (30 x LiFePO4 cells, say 90AH), then, when the pedal is to the floor, switch in an additional 48V (15 x LiFePO4 cells, say 40AH) that would give a power advantage whilst saving some battery cost, some weight saving and some space saving.

Would require 2 sets of MOSFETs and drivers, but only one (modified) controller. Would also probably need two separate chargers.

As a bonus, if you are out and about and run out of "main" battery, a contactor could be used to switch over to the bank of "reserve" cells so that you can "limp home" (or to the nearest charging place).

Or am I missing something?

[Ryland]

more connections are more point that can fail so you want as few contacters and as few battery cells as you can get away with, lower battery pack voltages like the 48v pack is also going to require larger gauge cable if you want to get much work out of them, each lug, each crimp and each run of cable between batteries is also a cost you have to factor in, found some tinned 2/0 lugs for $1.30 each after shipping if you buy 100 and crimping them on can cost you $5 per crimp, welding cable looks to be priced around $2.50 to $3 per foot and it's amazing how much you can end up using when you are connecting multiple packs together!
I would opt for a battery gauge that tells you the state of charge so you know what kind of range you have left, that way you don't destroy your batteries either, a limp home mode can be programmed in to a low voltage cut off that cuts the controller back to half power when the voltage drops far enough and a higher voltage pack will allow you to use smaller gauge battery cable that is both lighter weight and cheaper while getting the same amount of energy out of the battery pack to the motor.

[BobEdwards]

Thanks, Ryland, for your prompt reply and useful thoughts on my query.

As I see it, if I want 144V, I am committed to 45 LiFePO4 cells in any case, so my idea doesn't increase the number of connections between cells, just adds one extra connection to the alternative driver board.

Thinking this through a bit further, my 96V @ 90AH battery will give a 10C current of 900A. My motor can take up to 200A continuous (@144V) or peak at 600A. 600A @ 96V gives me up to 59.6kW (for short periods).

The series-connected 48V @ 40AH battery will give a 10C current of 400A. When this is in use, the 40AH cells will limit the motor current, so I can get up to 400A @ 144V or 57.6kW (almost the same), but the extra volts will give me more RPMs, so will only be needed when already at speed.

What does this gain? I save almost A$1000 in LiFePO4 cells, over 22kg in weight and some space savings. The cost is the development of a controller that can drive two sets of driver boards (but only ever one at a time!).

Worth doing? Am I missing something important?

[Ryland]

The higher voltage will give you less line loss as well, to have a split system you are going to need a good contactor, so that will also cost you $200 or so and will have more connections on it, then you need two battery chargers and battery gauges, it makes the whole system complex!

[lesliebr]

How will you control two batteries? What are the components used to control battery packs. I can help you in the design stage.

smt assembly