Lithium battery
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- Lithium complaint? LiFePO4 sits at 13.0 to 13.3 V across most of its range, so voltage alone tells you little. The BMS inside disconnects on low voltage, high voltage, low temperature charge, high temperature, and over-current. Expect: 13.2 to 13.4 V resting, 14.2 to 14.6 V while charging, BMS connected
Causes and repairs
BMS has disconnected
0 V at the posts with a lithium battery is the BMS protecting it: over-discharged, over-current, or over-temperature. Some wake when a charger is connected, some need a reset button or an app, a few need a small 'wake' voltage.
Tools: multimeter
Check the app or button. Connect a charger with a lithium profile and watch for it to come alive. If it never does, the BMS or a cell failed: warranty.
Charging blocked below freezing
Normal protection. The BMS won't accept charge below about 32°F. Heated models run their pad from the charger first, then charge.
Tools: thermometer
Warm the battery compartment, or wait. Tell the owner. No fault.
Charger profile
A lead-acid converter floats at 13.2 to 13.6 V and never fills a lithium bank. Equalize mode (15 V+) can trip the BMS high voltage cutoff. Lithium wants about 14.4 to 14.6 V absorption and little or no float.
Tools: multimeter
Set the converter to lithium, or replace it with one that has a profile. Verify charge voltage at the battery during bulk: 14.2 to 14.6 V.
Over-current or low-voltage cutoff
A single 100 Ah lithium is usually limited to 100 A continuous. An inverter surge or a big slide draw trips it. Low voltage cutoff around 10 to 11 V also drops it, and it looks sudden because lithium holds voltage until the end.
Tools: clamp meter
Check the battery's current rating against the loads. Parallel batteries to share current. Charge it if it was simply empty.
Capacity or monitor
Voltage-based monitors can't read lithium state of charge. A shunt-based monitor can. A battery that truly lost capacity has a bad cell.
Tools: shunt monitor
Install a shunt monitor. Do a capacity test: charge full, discharge at a known rate to cutoff, compare to the rating.
Mixing chemistries or old equipment
Lithium and lead-acid in parallel don't share well; one ends up doing all the work. An old converter with no lithium mode undercharges. Alternator charging without a DC-DC charger can overwork the alternator.
Tools: none
One chemistry per bank. Lithium-capable converter. DC-DC charger for alternator charging.
Keep going on 12V and batteries
Learn it
- Volts, amps and ohms, the simple way
- Voltage drop: the test that finds bad grounds
- Practice: find the bad connection
- Batteries: lead-acid vs lithium
- Fuses, breakers and grounds
Prove it
- Back lights are dim
- Battery dies even on shore power
- Lithium won't charge on cold mornings
- Coach is dead, battery is fine
- Batteries dead after storage