connecting external SSR to BMS output for higher current capability

[sss]

hello guys
i use lifepo4 starter battery in my car (normal petrol car) without any balancing or bms for more then 2 years already .
recently i had 3 failed cells due to my wife leaving the lights and completely draining the battery overnight.

so far i was avoiding using a bms due to the high cost and size of high current capable bms . i also tried using active balancing devices , which were worse then without balancing at all .

so now i'm thinking getting a standard bms , but use its output to control high current dc ssr . i will lose output current limit and charge current limit and i can live with that .
the gains are low/high voltage protection and balancing .
what do you think ?

 

[fechter]

I think the SSR will constantly drain the battery (slowly). It would probably be better to use a large enough BMS or take a small one and build a new power stage for it (more/bigger FETs). My little battery spot welder uses FETs that are handling 3000A during the welding pulse. Normally a car starter shouldn't be running long enough to cause a heating problem.

 

[amberwolf]

Instead of SSR, how about a contactor? There are BMS already designed to use them, and a few threads around the forum discussing ways to use a standard BMS to drive a contactor instead, some of which have proposed wiring diagrams.

Some of the discussions also included thoughts about changing the current-shunt path to then retain current-limits.

 

[sss]

contactor is not an option , it will waste lots of power (it should be constantly on until there's an issue) .
@fechter
external power stage is the best solution , but i'll need to make pcb etc . i think modern dc-dc ssr's are basically a power stage

 

[harrisonpatm]

(it should be constantly on until there's an issue) .

Does it need to be on constantly? Or does it only need to be on while the car is on?

 

[Jrbe]

contactor is not an option , it will waste lots of power (it should be constantly on until there's an issue) .
@fechter
external power stage is the best solution , but i'll need to make pcb etc . i think modern dc-dc ssr's are basically a power stage

You might be able to find a latching contactor. But not sure any off the shelf BMS will be able to control it as is.
There are also circuits that can do full power then drop to a lower current output (commonly 1/4 the power) to hold the contactor on. Can search "peak and hold circuit" if interested.

A battery buddy / prioritystart! Should handle your dead battery issue without a bunch of extra stuff.

An SSR could work well depending both on how much current the SSR led consumes and how efficient it's drive circuit is.