Batteries: lead-acid vs lithium
Lead-acid voltage tells you roughly how full it is. Lithium voltage barely moves until it's almost empty, so check its monitor or app.
The short version
- Read lead-acid voltage after it's sat a few hours with no charger and no loads.
- A lead-acid that reads fine but dies under load needs a load test.
- Lithium (LiFePO4) has a BMS that can shut the battery off for cold, low or high voltage. A battery reading 0V may just be asleep.
- Lithium needs a charger set to a lithium profile. A lead-acid profile won't fully charge it.
On the job
- Resting voltage after 15 minutes with no charge and no load. 12.6 to 12.7 V is full lead-acid.
- Load test at half the CCA for 15 s: hold above 9.6 V.
- Count the overnight amp-hours: furnace blower 7 A for 6 h is 42 Ah. One lead-acid gives about 50 usable.
- Lithium: 13.0 to 13.3 V most of the way down. Use a shunt monitor for state of charge.
Numbers
- Lead-acid full: 12.6–12.7V at rest
- Lead-acid half: ~12.2V
- Lead-acid flat: 11.9V and below
- Lithium (most of its range): 13.0–13.3V
Shop tip
Lithium won't take a charge below freezing on most batteries. The BMS blocks it on purpose.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.
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
- Fuses, breakers and grounds
- Converter / charger
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