RV Lithium Battery Not Charging: Why It Reads Wrong

A lithium iron phosphate (LiFePO4) house battery does the same job as a lead-acid one and follows the same rules about charging one source at a time, but almost every number a lead-acid test relies on changes. Two of those differences make a healthy lithium bank look broken, and one makes a broken one look fine.

The resting voltage is the first. Lead-acid sits near 12.7 volts at full charge and the common tables are built around that figure. Battle Born's voltage table for its 12-volt lithium battery puts a rested full battery at 13.6 volts, its own recharge threshold at 13.3 volts, and 0 percent at about 10 volts. So a full lithium battery reads where a lead-acid battery reads only while it is being charged, and a lithium battery reading 12.7 volts is roughly half empty.

The second is the shape of the curve. A lithium battery holds close to a flat voltage across the middle of its range and drops steeply at both ends, so 13.3 and 13.1 volts can be anything from a third to nearly full. Voltage is a rough guide here and a poor fuel gauge. Battle Born's own table shows the same thing: most of the steps between 90 and 40 percent are a tenth of a volt.

The third is the charger's voltage. The maker's charging figures for that battery are 14.2 to 14.6 volts for absorption and 13.4 to 13.8 for float, with a BMS cut-off at 14.7 volts, and Progressive Dynamics' lithium converter is nominally 14.6 volts where its lead-acid unit is 13.6. A lithium coach whose shore power reads a steady 13.6 is being floated, not fully charged, and one whose converter is still set to a lead-acid profile never reaches the last part of the range.

The test on the battery not charging guide still works: rest the battery, read it, switch on one source, read again, turn it off before the next. Only the expectation changes.

• A working converter or controller lifts the reading to about 14.2 to 14.6 volts, which is a rise of a volt or more from a rested 13.4 to 13.6. If it lifts to about 13.6 and stops, the charger is not on a lithium profile.
• A full battery will not rise much at all. Charging stops when the battery is full, so a bank that rests near 13.4 with everything off and does not move when a source comes on is doing its job. Do not run this test on a full bank and read the flat reading as a fault.
• Expect the reading to fall back once the source is off. A lithium battery sits at a higher voltage while it charges and drops back to its rested figure within an hour or so; a reading that fell is not a battery losing charge.

A lithium battery has a built-in battery management system (BMS) that disconnects the cells when it decides something is wrong: the voltage went too low, the current too high, or the cells got too hot or, on a heated model, too cold to charge. When it does that the whole coach goes dark at once and every source reads nothing at the terminals. It is not a flat battery and not a failed charger, and on a battery with a Bluetooth monitor you can often see the reason in the app.

The maker's figures say where those limits sit: a recommended low voltage disconnect at 10.5 volts, a discharge cut-off at 10 volts, and a high voltage cut-off at 14.7 volts. A bank that died overnight while the loads were small is more likely to have hit the low limit than to have been drained.

The test that separates a shutoff from a flat battery is the same one the lead-acid page uses, run from the other end: read the charger's output at its own terminals, then read the battery. If the charger is producing 14-plus volts and the battery reads close to 10, the battery has disconnected itself. If the charger produces nothing either, the fault is upstream of the battery. Many BMS units reconnect by themselves once the condition clears, so a bank that comes back after an hour on a charger has told you which one it was.

Shore power and the converter

A converter built for lithium has a mode switch: Progressive Dynamics' unit selects either a constant 14.6 volts for lithium or a three-stage profile for lead-acid. A converter left on the lead-acid profile still charges a lithium battery, slowly and incompletely, and the symptom is a bank that never gets past about 80 percent however long it stays plugged in.

The tow vehicle

The charge line through the seven-way connector is unchanged, and it matters more here, because a lithium bank can be pulled down much further than a lead-acid one without damage and then wants a much larger charge to come back. Battle Born's own isolation manager, fitted between the tow vehicle and the coach, connects for 15 minutes then disconnects for 20, repeats until the bank is charged, and disconnects when the alternator exceeds 14.4 volts or the coach bank rests above 13.4 volts. A reading taken at an instant during that cycle can show no charge while the system is working, so watch it over a few minutes.

Solar

A solar controller needs a lithium profile for the same reason the converter does: the absorption voltage it has to reach is higher than a lead-acid setting provides. The same test applies, with the same expected reading of about 14.2 to 14.6.

The cheap outcome here is a setting: a converter or controller switched from a lead-acid profile to its lithium profile, or a battery reconnected by its own BMS once the cause is cleared.

The next step up is a part that is doing its job badly rather than failing: a converter without a lithium mode, or a controller that cannot reach 14.2 volts. Those are replaced rather than repaired.

The expensive outcome is a battery replaced for a fault that was never in the battery. A lithium bank is the most costly component in the power system, so the readings above are worth taking before any part is bought.

The isolation test and the lead-acid numbers are on the battery not charging guide, and the converter's own modes are on the converter guide. For a solar bank, the solar guide covers the controller and the panels, and the solar sizing calculator works out how many panel watts a day's use needs. The battery runtime calculator shows how long a given bank carries a given load, and the battery storage guide covers what a lithium bank needs across a season.