Nothing kills a trip faster than a battery that won't take a charge. The good news: "not charging" almost always traces back to one of a handful of faults, and most are cheap to fix. Work through these checks in order and you'll usually find it in under 30 minutes.
Start here: isolate the source
Your battery is filled by up to three independent sources — shore power (converter/charger), solar (controller), and the alternator (via isolator or DC-DC). Figure out which sources fail and which work. If it charges from solar but not shore power, the problem is in the converter path, not the battery. If nothing charges it, suspect the battery or its main connections first.
Step 1 — Measure the voltage
Before touching anything, read the resting voltage at the battery terminals with a multimeter (or a Bluetooth monitor). This single number tells you most of the story:
- Below ~10V (12V bank): deeply discharged or a failed cell — charge slowly and watch it.
- ~12.0–12.4V and not rising: not receiving charge current, or charger isn't triggering.
- ~13.0–14.6V at the terminals while "charging": the source is fine; the battery may be rejecting charge (BMS) or is full.
Compare the voltage at the terminals versus at the charger output. A big gap means a bad connection, fuse, or cable. See our voltage chart for the full state-of-charge map.
Step 2 — Shore power / converter
- Confirm the converter is actually on (fan running, indicator lit).
- Check the DC fuses and the reverse-polarity fuse near the converter.
- A "smart" converter should push 14.4–14.6V bulk to a lithium bank. If it's stuck at 13.6V float, it may be in AGM/lead-acid mode — many converters need a dip switch or button held to switch to lithium.
Step 3 — Solar not charging
- Look at the controller display: no panel voltage means a broken panel, shaded string, or a tripped breaker between panels and controller.
- MPPT controllers need ~5V more on the panel side than the battery to start. In low light they may sit idle — normal.
- Corroded MC4 connectors are the #1 solar culprit. Unplug, clean, reseat.
Step 4 — Alternator / isolator
- A simple isolator drops ~0.6V; if your lithium needs 14.4V, the alternator may top out too low. A DC-DC charger fixes this by boosting and protecting both batteries.
- Check the engine-battery fuse and the ignition signal wire on a DC-DC unit.
Step 5 — Lithium BMS cut-off
A LiFePO4 battery management system will refuse charge to protect the cells: over-temperature, an over-voltage cell, or (on cold-weather models) temperature below the charge limit. If the battery reads fine but won't accept current, let it warm up and do a BMS reset — disconnect all cables for a minute, then reconnect.
| Symptom | Likely cause | Fix |
|---|---|---|
| Charges on solar, not shore | Converter in lead-acid mode | Switch converter to lithium |
| Nothing charges it | Loose main terminal / blown fuse | Clean, torque, replace fuse |
| Charges then suddenly stops | BMS over-temp / cold cut-off | Cool/warm bank, reset BMS |
| Alternator won't fill it | No DC-DC boost | Install DC-DC charger |
Picking a charger that actually charges
If your converter only does lead-acid, a lithium bank will sit perpetually undercharged. Match the charger profile to your chemistry:
| Charger | Best for | Note |
|---|---|---|
| Upgraded converter (lithium mode) | Shore-power charging | Cheapest fix if your converter supports it |
| MPPT solar controller | Solar input | ~20–30% more yield than PWM |
| DC-DC charger | Alternator charging | Boosts + protects both batteries |
Batteries we recommend
314Ah LiFePO4 — best for full-timing
Stores for panels, controllers, and combos:
Visit HumsiENK Store Visit Wattcycle Store
Frequently asked questions
Why does my RV battery charge on shore power but not from solar?
That usually points at the solar side: a blocked PWM/MPPT controller, shaded or faulty panels, or a tripped breaker between panels and controller. Shore power and solar charge through completely separate paths, so one working doesn't confirm the other.
Can a lithium BMS stop my battery from charging?
Yes. A LiFePO4 battery management system will cut charge if the cells overheat, if a single cell drifts too high, or — in cold-weather models — if the temperature drops below the charging threshold. Let the bank warm up and reset the BMS by disconnecting for a minute.
My converter only shows 13.6V — is that enough?
For a lithium bank you generally want a 14.4–14.6V bulk/absorption stage. A steady 13.6V float means the converter is in lead-acid mode and will never fully charge lithium. Switch it to lithium mode or add a dedicated lithium charger.
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