Momentary surge current for a starter

[memilanuk]

Looking at building my first DIY LFP battery, possibly a 12v 4S 280ah, to replace my pair of 12v FLA group 24s (guesstimate of 85ah each) in my slide-in truck camper.

One of the things I need to be able to support is starting current for the onboard generator. The factory-supplied wiring feeds the generator starter directly from the battery terminals using #4 wire. Last night I dusted off my old Fluke 36 clamp-on ammeter, put a fresh battery on it, set it to read max DC current, and put it on the positive cable going from the battery to the generator. Started the generator, and read a max instantaneous current of ~350 amps, for a second or two. Most of the rest of the time, the load will be pretty low - typical, from what I've read, for most solar systems.

I've started looking at the spec sheet for the Overkill BMS, and saw that it has a ~10 second delay for the over current trip, but I wasn't clear what level the instantaneous / short-circuit over current trip goes at? For that matter, I'm not particularly wedded to any one BMS at this point - would one of the Daly models be suitable for this use case?

 

[snoobler]

What was the OCV of the battery when discharging 350A? LFP has a higher nominal voltage and will have a less pronounced voltage drop due to lower internal resistance and higher capacity, and the current draw will be lower.

I would repeat the test and record minimum voltage and max current. From those two, you can calculate the power required. With the power required, you can estimate the current the LFP will need to supply. I suspect it's notably lower than 350A, but higher that what I would trust to a 100A BMS.

Typical 280ah cells are limited to a 1C discharge rate, and I would be hesitant to push them harder even if they have a surge rating.

Another option would be to get a small AGM used exclusively as a gen starter battery that is kept floated at 13.8V by the LFP bank with a 12-13.8V Dc-Dc converter.

 

[GSXR1000]

I start mine with a single 120a bms no issues

Thread 'Starting my Onan 4k with my LIfepo' https://diysolarforum.com/threads/starting-my-onan-4k-with-my-lifepo.15367/