LFP as a starting battery – two concerns
Question: I want to use the LFP battery (4x 3.2V cells) for starting of my car. What SBM should I use to protect the battery?
You cannot use a SBM to protect the starting battery. For starting the car, the current will be very high, hundreds or even nearly 1000 Amps, it is not possible to disconnect such high currents.
WARNING: Additionally it is not possible to disconnect the battery from vehicle circuits. The car electronics may burn or become faulty of the battery is suddenly removed from the circuit, as a result of disconnection, during the operation (when the engine is running).
To install LFP battery, the battery must be hard wired to the car, the same way as the lead acid battery is. This brings some risks and problems:
1) No disconnection, no fuse. Note that the car battery is wired without any fuse or disconnection ! ! ! This is a huge risk that exists in all cars. It is crazy, but this is the reality. Yes, this is the most crazy thing about all cars you see in the streets — no high power fusing or disconnection of the 12V battery. This is also one of the main reasons for the many car fires – the fire starts from short circuit on the battery that cannot be disconnected from the car…… so the car will burn completely.
There is less of a issue with the Lead Acid battery, as the Lead Acid battery can give the power for a short time, and it will stat loosing the peak power rating, after longer time of high current discharge.
However for LFP battery, in case of short circuit, the battery will release all power suddenly, constantly and continuously. The LFP battery has much more power that the lead acid battery.
ADVICE: Our definite suggestion is to install a high-power rating fuse for the LFP battery. The rating of the fuse should be 300Amp or even more depending on the starting currents for your application.
2) Deep discharge. If you left your car for long time without care, the on-board electronics (e.g. alarm, security systems, car radio, etc….) will drain the battery to completely empty – the deep discharge. In some cars the battery will be depleted as quickly as in two weeks.
This is a lesser issue for the Lead Acid battery, as most Lead Acid batteries can survive the deep discharge. They can be recharged and they will perform in a reasonable way again. However this is not possible with the LFP battery. If you deep discharge the LFP battery, it is damaged for ever and there is no way to restore it to function.
ADVICE: Our suggestion is that the LFP battery should be fully disconnected from the car in case you park the car and the car will not be used for a longer period of time.
Photos from Compass Marine Inc. Thank you