"0% battery", low voltage, sag, etc...

[Tremelune]

I converted a Rover Mini with Leaf batteries and I'm trying to figure out what my usable range is.

I've read that 3.6V per cell is a good number to use for considering when the batteries are at 0%. Thing is, when the cells are at 3.7-3.8V at rest, they can easily drop below 3.6V under load. I've read that's okay "for a few seconds". How low can I let it go? How many seconds is a few...?

How do I know when I'm "really" out of juice (within the safe/happy operating range)? When do these batteries really get stressed to the point that I should stop driving the car (or limit torque)?

I wish I knew battery meter Nissan's algorithm...

[Isaac96]

You really want an amp-hour counter for SOC measurements. Determining state of charge from voltage is a classic problem, and the algorithms for doing it are usually trade secrets.
A JLD404 should do the job well enough, it just needs a shunt attached.
Or an ISA shunt, but those are rather more expensive and CAN-only, a little annoying to deal with.

[johu]

You can find some real data pulled from the Nissan BMS at the bottom of this thread: viewtopic.php?f=4&t=1008

Bottom line: 3.6V is not 0% and like all NMC cells they are allowed to sag to 2.5V