LiFePo4 charge current limits

[leoms]

My bike has a battery pack with sets of 3 LiFePo4 cells in parallel. Each has 3.8Ah and recommends max charge current of 1C=3.8A. All well and good, I try to charge the bike even slower than that. But I'm interested in running my regen brake as hot as possible, because (at least for now) it's my only rear brake and it's not powerful enough as a safety backup if capped at 3x3.8A. And also just because it's nice to have more regen, and I'm interested in finding out whether I can get away with it.

So my question is: What happens if you exceed LiFePo4 charge limits? Are the limits based on thermal dissippation, in which case it would be OK to way exceed them as long as the average over time is not too high and the temperature doesn't climb too much? I'm sure at some point you chemically cause problems, build dendrites, or something, but how worried do I need to be for a brief pulse? Put another way, if my battery's max rated charge current is 3.8A, how do I determine the max safe 10-second charging current? I am fine with modest battery damage, I plan on avoiding using the brake this hard very often.

Thanks!

 

[amberwolf]

it's a chemistry thing--different cells are "built" differently, so they each have different limits based on what happens to the chemistry during recharge. Specific failure modes could be different for different cells, so if the datasheet doesn't specify anything for your specific cell, and no data is available from the manufacturer of that cell about this, then you'll be generating the data experimentally as you go.

Heat isn't really the issue; it'll heat about the same resistively from either charge or discharge, and discharge currents are usually a lot higher than charge.

Many systems with regen exceed the charge limits of the pack they use, and most of them don't explode because they don't do this for more than a couple of seconds, and not very often.

If you're using regen for 10 seconds at a time, that's very long for a high-current braking event, and you might want to beef up your mechanical brakes, tires, wheels, etc., to reduce the need for the regen, because it is not just hard on the battery, it is also often hard on the controller and motor, and if you have a clutchless/nonfreewheeling middrive it's hard on all the drivetrain parts between wheel and motor.

 

[leoms]

Thank you! I'll play around with it, and try to find info for similar cells.