RV battery winter storage: Lead-acid and lithium rules

Flooded lead-acid batteries: Charge to 100%, then disconnect

For lead-acid, your state of charge is your freeze protection, and Trojan's storage guidance is the short version of everything below: charge it fully, disconnect it, keep it cool and dry, and check the voltage partway through the winter. The electrolyte freezing point depends on how charged the battery is, and the difference is enormous. A fully charged lead-acid cell does not freeze until roughly -85°F, and a fully discharged one freezes just below +32°F: those are the figures Rolls Battery publishes, and Rolls adds the consequence in its own words, that damage from freezing is typically not recoverable and will require replacement. So a flat battery is the one that cracks, at temperatures an unheated garage reaches easily. This is why a battery that survives a Colorado winter can die in a warm garage: it was stored half-empty, and the weak electrolyte froze.

Charge first, then water. Trojan is explicit about the order: "Charge the battery first and then add water if needed. Adding water to a battery before charging may result in overflow of the electrolyte." Add distilled water, never acid, and stop about an eighth of an inch below the bottom of the vent well, because an overfilled cell pushes electrolyte out as it charges.

Charge fully before storage. A partially charged lead-acid battery will sulfate, forming hard crystals on the plates that permanently reduce capacity. Charge to 100 percent, then disconnect the negative terminal so no parasitic load can drain it.

Check voltage every 4 to 6 weeks. Lead-acid self-discharges in storage, and how fast depends on the plates: Victron publishes 2 percent per month for gel, 3 for AGM, 4 for spiral-cell pure lead, and 6 to 12 for the antimony and traction types. Sealed batteries sit at the low end of that, which is why they hold their charge longer. Trojan's state-of-charge table puts a flooded cell at 12.50 volts at 80 percent charge and 12.10 volts at 50 percent, so treat anything below about 12.4 volts as a battery that wants charging before it sits. Clean the terminals before storage and coat them lightly with dielectric grease or petroleum jelly to keep corrosion out. (While the battery is out, it is the perfect moment to verify your RV weight limits against what you actually carry.)

LiFePO4 lithium batteries: Store at partial charge, never charge near freezing

Lithium plays by the opposite rules, and getting them wrong is expensive because the damage is permanent and cumulative. Charging a lithium cell near freezing causes lithium plating, and the makers put the floor above the freezing point rather than at it: Victron specifies 5°C (41°F) as the lowest temperature its cells may be charged at, and says charging below that causes permanent damage. In Fahrenheit terms: the lithium ions cannot absorb into the anode quickly enough, so they deposit as solid metal on its surface instead. Each cold charge steals capacity that never returns, and repeated plating can eventually form internal structures that compromise the cell. This is not a gentle caution, it is a hard electrical rule, and it is why a lithium bank with no low-temperature cutoff must not be connected to solar or a charger during a cold winter.

Store at least 50 percent. Battle Born, which makes these cells, says to store them with at least a 50 percent charge, and notes that a fully charged bank in good conditions can last a year; 40 to 60 percent is the band to aim for unless your BMS or vendor manual lists a specific figure. Storing lithium at 100 percent for months causes slightly faster degradation than storing at 50 percent, and storing near zero risks the BMS entering a protection mode that is annoying to exit in spring.

Disconnect the loads, and do not leave a trickle charger on the pack. Lithium holds its charge far longer than lead-acid does, but a detector, monitor or inverter standby still drains it. The maintenance-charge advice that keeps lead-acid alive is wrong for lithium: a float charger left on a lithium pack all winter is the wrong tool.

Never charge below freezing. If the bank stays where it can freeze, physically disconnect the solar panels, the charger, and shore power. If your BMS has a low-temperature charge cutoff, confirm it is actually enabled rather than assuming it is.

AGM and gel batteries: The lead-acid rules, minus the watering

Sealed lead-acid batteries follow the flooded rules: full charge, disconnect, re-check every 4 to 6 weeks, keep a smart maintainer on them if they sit connected. East Penn, which builds both AGM and gel types, publishes a state-of-charge table with a column for each, and neither needs the watering a flooded cell does. They never need watering, and they tolerate cold the same way, with the same freeze protection from a full charge.

Disconnect the battery, and you assume the drain stops. It does not always. Many RVs keep some circuits live even when the coach disconnect is off, and the list of quiet consumers is longer than it looks. Recognizing them is the difference between a battery that greets spring at 80 percent and one that greets it at zero.

Safety detectors and memory drains

Propane and CO detectors draw continuously by design, and they usually stay wired directly to the battery because they are safety equipment. Stereo memory, clocks, and appliance control boards draw a trickle each. Small alone, and real in aggregate over a four-month season.

Inverter standby: The biggest hidden drain

A pure sine inverter left on with nothing plugged into it still draws power, continuously, because it is holding an AC waveform ready for a load that never comes. This is often the surprise that explains a battery much flatter than the detectors and clocks would suggest. Turn the inverter fully off, not just the appliance that was plugged into it.

Charge controllers and monitors in standby

The solar charge controller draws a little simply by being on, and a covered panel does not cancel what the controller consumes, which is why an RV with covered panels can still lose charge over a cloudy week. Battery monitors, tank monitors, and communication modules each add a small ongoing draw of their own.

How to test for an unexpected draw

With the disconnect off, measure the current at the battery with a multimeter wired in series. A normal low standby draw reads in the tens of milliamps. If it reads in the hundreds of milliamps, something is pulling real current, and you should find it before storage, not after. If the drain is unexpected, unsafe, or excessive, fix it. If it is simply normal, isolate the loads that do not need to stay powered for the season.

The box between your solar panels and your battery, the component that turns raw panel voltage into a safe charging current, is the solar charge controller. It is the piece of the system that decides whether winter storage helps or hurts your battery.

What a solar charge controller does

A solar panel in full sun puts out far more voltage than a battery can accept, up to 18 to 22 volts from a panel labeled 12 volt. Feeding that straight into a battery would overcharge and damage it. The charge controller sits between them, regulates how much current reaches the battery, and stops charging entirely once the battery is full. It is the battery's traffic light, and every RV solar system needs one.

MPPT vs PWM: Which charge controller is best for winter?

Two kinds of controllers exist, and the difference matters most in cold weather.

PWM (pulse width modulation) is a smart switch. It drags the panel voltage down to match the battery voltage, which wastes whatever the panel produces above that level. It is simple and cheap, and it works fine in warm climates with closely matched panels, but in cold weather it gives up meaningful harvest.

MPPT (maximum power point tracking) is a converter with a brain. It lets the panel run at its most efficient voltage, then converts the excess into extra charging current. That captures more power than PWM from the same panels, with the largest gains in winter, because cold panels produce higher voltage and MPPT is the only type that can use it. For an RV that sees winter at all, MPPT is the right choice.

The winter hazard: A sunny day can charge a frozen lithium battery

This is the trap that catches owners who leave solar connected for winter. The sun comes out on a cold morning, the panels light up, and the charge controller wakes up and starts charging. The battery is still below freezing from the night before. If the bank is lead-acid, that charging is a harmless maintenance top-up. If it is lithium, that single cold charge does permanent damage through lithium plating, and a whole season of sunny winter days repeats it, stealing capacity that never returns.

The rule for a lithium bank in a freezing climate: disconnect the solar panels, disable the charge path, or confirm the BMS low-temperature charge cutoff is actually enabled rather than assuming it is. For lead-acid, solar through a working controller behaves like a smart maintainer, but only if the controller is rated for that chemistry.

The safe shutdown order for solar in storage

The controller needs the battery connected to configure itself, and some units are damaged by connecting panels first. The correct order: connect the battery before the panels, and when shutting down for storage, disconnect the panels before the battery. Use a breaker or disconnect on the panel side so you can kill the PV input for service, and fuse both the panel and battery legs.

The converter and charger chemistry check

If the RV sits plugged into shore power, check what the converter or charger is programmed for. Many factory converters only know lead-acid charge profiles, and leaving a lithium bank on an incompatible converter all winter stresses the battery management system. The safe pattern for lithium is partial charge, disconnect, no maintainer. For lead-acid, a smart maintainer rated for the chemistry is the tool, and a dumb charger left on the bank is not.

• Charge to the target for your chemistry: lead-acid to 100%, lithium to 40 to 60 percent unless the battery's own manual says otherwise.
• Charge the bank, then top up flooded cells with distilled water to about an eighth of an inch below the vent well.
• Turn the inverter fully off, not just the appliances.
• Disconnect solar panels from the controller, and shore power from the converter.
• Disconnect the negative battery cable, and label the cables before you pull them.
• Clean the terminals and protect them with dielectric grease.
• Record the date. Re-check voltage every 4-6 weeks; lead-acid below about 12.4V gets recharged.
• If you use a maintainer on lead-acid, use a smart one. Never leave a float charger on a lithium pack.