Why Batteries Die in Winter: Freezing, Watering and Storage for Flooded Lead-Acid Batteries

Which batteries actually need water?

Only flooded (wet) lead-acid batteries need water. These are the batteries with removable vent caps on top: golf-cart and floor-scrubber batteries, forklift batteries, off-grid solar banks, many marine and RV house batteries, and the batteries in older trucks, tractors and farm equipment.

If your battery is AGM, gel, or a sealed “maintenance-free” car battery, it does not take water, and you should not pry it open. Everything below about watering applies to flooded batteries only. The parts about cold, charge and storage apply to every lead-acid battery.

Why do batteries die in the cold?

Batteries die in the cold because the chemical reaction that produces current slows down as temperature drops, so the same battery delivers less of its stored energy. Trojan Battery’s user guide puts a number on it: a deep-cycle battery delivers 100% of its rated capacity at 80 °F and about 50% at 0 °F.

That is the part everyone notices: the golf cart that does 18 holes in July and quits on the 9th in January. The part people miss is what happens when the battery is sitting:

• Self-discharge never stops. Trojan says a flooded battery can self-discharge up to 15% a month at 77 °F. A cart parked in October is noticeably lower by Christmas, before it has done any work.
• Parasitic loads. Clocks, alarms, battery monitors and controllers draw small currents all winter. Trojan’s storage advice starts with disconnecting the battery from the equipment for exactly this reason.
• Low charge raises the freezing point. This is the one that kills batteries outright, and it gets its own section below.

A battery that was only “a bit low” in autumn can be both weaker and closer to freezing by January. The two problems feed each other.

Does battery water freeze?

Yes. The liquid in a battery freezes at a temperature set by how charged the battery is. In a fully charged flooded battery the electrolyte is a strong sulfuric acid solution and freezes far below any normal winter temperature. As the battery discharges, the acid moves into the plates as lead sulfate and the electrolyte becomes mostly water. Battery University puts it plainly: in the discharged state “the electrolyte becomes more water-like and freezes earlier.”

Published numbers vary a little between sources because they test slightly different electrolyte strengths, so here they are side by side:

Specific gravity 1.15 is about 40% charge on Trojan’s chart. The two sources land 11 degrees apart at roughly the same charge level, which is a useful reminder: don’t plan around the exact number. Plan around keeping the battery full. Both sources agree that a frozen lead-acid battery is permanently damaged.

How to tell how charged a flooded battery is

Use a hydrometer (specific gravity) or a voltmeter after the battery has rested off the charger. Trojan’s figures for a flooded battery:

The highlighted 70% row is Trojan’s line for stored flooded batteries: at 70% or lower, give it a boost charge. Trojan also notes hydrometer readings should be temperature-corrected: subtract 0.004 for every 10 °F below 80 °F.

Should you add water to a battery before or after charging?

After. Add water to a flooded battery after it has been fully charged. Electrolyte expands and rises during charging, so a cell topped up to the line beforehand can overflow acid out of the vents. The one exception, per Trojan: if the plates are exposed before charging, add just enough water to cover them, charge, then top up to the correct level.

Trojan’s fill level for standard flooded batteries is 1/8 inch (3 mm) below the bottom of the vent well. If the electrolyte is already well above the plates, you don’t need to add anything.

Why do flooded batteries use water at all?

Flooded batteries use water because charging splits some of it into hydrogen and oxygen gas, which escape through the vents. Near the end of a charge, when the plates can no longer absorb all the current, the extra energy goes into electrolysis instead. That bubbling is called gassing, and it is why a flooded battery slowly loses water but keeps its acid.

Three things follow from that:

• The acid stays, so you only ever replace water. The sulfuric acid is not used up by gassing; it just becomes more concentrated as the water level falls.
• Exposed plates are damaged plates. Any part of a plate above the electrolyte dries out and stops working. Checking levels regularly matters more than how much you add each time.
• Gassing is why charging rooms need ventilation. Hydrogen is flammable, which is exactly why the OSHA rule further down requires ventilation and bans smoking where batteries charge.

Sealed AGM and gel batteries are built to recombine most of that gas back into water inside the case, which is why they never need topping up and should never be opened.

What kind of water should you put in a battery?

Distilled or deionized water. Trojan’s instruction is direct: “Use only distilled or deionized water. Tap water can contain contaminants that will damage the battery.” If tap water is used anyway, Trojan says it must be tested against their impurity limits first. A few rows from that table show why:

Notice the iron row. Extra self-discharge is the last thing a battery needs going into a winter of sitting still. Every time a flooded battery is topped up, whatever is dissolved in the water stays behind and builds up, because only the water gets used up.

Our Battery Water is deionized, mineral-free water made for exactly this job. For how deionized and distilled water differ in general, see our distilled vs. deionized water guide. For what is inside the battery, see battery acid vs. battery water.

How do you winterize a golf cart, RV, boat or solar battery?

To winterize a flooded battery, fully charge it, top it up with distilled or deionized water, disconnect it from the equipment, store it cool and dry, and boost-charge it whenever it drops to about 70%. Step by step, following Trojan’s storage guidance:

1. Clean the tops and terminals. Dirt and acid residue on the case can carry small currents between terminals and drain the battery. A baking-soda-and-water wipe neutralises acid residue; keep it out of the cells.
2. Charge fully. Never put a battery into storage partly charged.
3. Check levels and top up after charging with distilled or deionized water, to 1/8 inch below the vent well.
4. Disconnect it from the cart, RV, boat or inverter so parasitic loads can’t drain it.
5. Store it cool and dry, out of the weather. Cool slows self-discharge; just avoid places that freeze if you can.
6. Check every 4–6 weeks with a hydrometer or voltmeter, and boost-charge at 70% (12.37 V on a 12 V battery, specific gravity 1.217).
7. Recharge before first use in spring.

A battery maintainer or smart charger with temperature compensation makes step 6 automatic. Battery University notes the recommended compensation is about 3 mV per cell per °C, which means a cold battery should be charged at a slightly higher voltage than a warm one; a charger without it tends to undercharge in winter.

What about forklift and floor-scrubber fleets in winter?

Fleets have the same chemistry at a bigger scale, plus a regulation. OSHA’s powered industrial truck standard, 29 CFR 1910.178(g), requires battery charging in designated areas with facilities for “flushing and neutralizing spilled electrolyte” and “adequate ventilation for dispersal of fumes from gassing batteries,” and prohibits smoking in the charging area.

Winter adds two fleet-specific problems:

• Cold docks and freezer work. Trucks working in unheated docks or cold storage deliver less runtime, so operators opportunity-charge more. Watering still happens after a full charge, not after every top-up charge.
• Water use is a line item. A warehouse watering dozens of batteries uses enough water that buying it by the gallon jug gets expensive fast. Single-point watering systems and a drum or tote of battery water near the charging room solve both the labour and the cost.

What are the most common winter battery mistakes?

• Storing it “mostly charged.” 70% in October can be 40% by February, which is freezing territory on a cold night.
• Topping up before charging. Acid boils over the vents, corrodes the tray, and the cell loses acid.
• Using tap or filtered drinking water. Minerals don’t evaporate. They build up with every top-up.
• Adding acid instead of water. The battery lost water, not acid.
• Leaving it connected. Controllers, clocks and monitors drain it slowly all winter.
• Opening a sealed battery. AGM, gel and maintenance-free batteries are not meant to be watered.
• Charging a frozen battery. Thaw it and have it tested first.

How much does battery water cost in bulk?

Battery water costs less per gallon the more you buy at once. These are Alliance Chemical’s current prices for Battery Water:

A single golf cart or RV bank is a few-gallon job. A fleet, a golf course or an off-grid site watering through the winter is where a drum or tote makes sense.

What we do for you: tell us what you’re watering (forklift fleet, golf-cart barn, solar bank) and roughly how many batteries, and we’ll tell you which pack size makes sense. That way you’re not paying jug prices for a drum’s worth of water, or storing a tote you’ll never empty. A certificate of analysis is available on request, and recurring orders and bulk quotes are straightforward.