Lithium batteries and the BMS
Every lithium battery has a built-in BMS that protects it. When the BMS shuts the battery off it looks dead, but it usually isn't. Know what makes the BMS act before you condemn anything.
The short version
- The BMS opens the battery for low voltage, high voltage, too much current, and charging when it's too cold.
- A lithium battery reading 0V or very low may have been shut off by its BMS. Many wake up when a charger is connected. Follow that battery maker's wake-up steps.
- If the battery has an app or monitor, read its state of charge, temperature and any protection warnings before you test anything else.
- Charging from the engine alternator can overload the alternator. Many installs use a DC-DC charger to limit that current.
- The converter, solar controller and inverter-charger all need lithium settings. Check every charge source after a swap.
On the job
- 0 V at the posts: the BMS opened. Check the app or button, connect a lithium charger, give it a minute.
- Won't charge below about 32°F: normal. Warm the compartment or wait.
- Converter in lithium mode: 14.2 to 14.6 V at the battery during bulk. Lead-acid float at 13.2 never fills it.
- Dies suddenly under load: current limit (usually 100 A per battery). Parallel them or reduce the load.
Numbers
- Cold-charge cutoff: usually around freezing (32°F / 0°C); check the battery's spec
Shop tip
After a lithium swap, check the converter's output. One that only puts out about 13.6V can leave a lithium battery short of full.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.