The Going-Ons @ Pacific Electric Vehicles

but not much for sale yet.

Pacific EV Shop

For now, we’re just selling a good power steering pump. We have a few extra, so act quickly!

We’ll  be adding our BMS to the page once it is in stock.

I had an e-mail exchange with a fellow I met at our local Seattle Electric Vehicle Association about my company’s battery management system, which prevents every cell from ever going above or below programmable setpoints.

Also, reader, please do not forget PEV offers electric vehicle consulting services for your own project.

I guess what I’m asking is, when a battery cell is in circuit, its voltage and the current going through it are interdependent.  Under heavy load, a cell’s voltage will sag, but remove the load, and the open cell voltage will jump back to a quiescent level.  At November’s SEVA meeting, Stephen Johnsen[sic] showed a video of dragging a TS cell’s voltage down to 0.5V under an 800 amp load for 20 seconds, but when when he switched the circuit OFF, the voltage jumped right back up to 3.2V.  If you had a BMS that was just looking at voltage, then I think 0.5V would have tripped alarms in anybody’s system, even though the cell wasn’t anywhere close to dead.

Ah, I see where you’re coming from, but there are several reasons why you don’t want nor need to do the above.
The reasons you don’t need to are
1. 800A loads the cell to 0.5V, but 150A might load the cell only down to 2.0V (I would need the cell size to give you a more exact number) The power increase is only 33%, while efficiency drops 75%.
2. Once you have an EV-sized pack, you don’t need more than 3C for very quick acceleration, so there is no need to go below safe voltage. (unless you have a very heavy vehicle w/ short range, but then you may as well use flooded LA)
The reasons you don’t want to are:
1. Probable cell damage — until shown otherwise, I’m going to assume drawing a cell below its recommended voltage damages it like NiMH and LA and unlike NiCd (which will take anything so long as you don’t reverse them)
2. Greatly reduced efficiency — drawing at that rate versus a 1C draw (@3.2V) requires 6.4 times more Ah used per kilowatt-hour delivered

No one has shown it is safe to take the cells below 2.0V, but there are multiple independent sources of test data showing if you never allow the cells to go above 4.25V nor below 2.0V at any time, they last 3000 cycles. I would not risk cutting the lifetime of $10000 batteries for a small increase in peak power, but I’m happy to help others do the testing on their dime :).  So, reader, if you wish to sponsor this test, e-mail me!