Kalyan Jana, development support manager (specialty markets) for EnerSys, sent us his four-step guide to winter battery storage.

1. Testing

Use a reliable digital voltmeter to measure the open circuit voltage (OCV) of the battery and determine its state of charge (SOC). Measure the OCV no sooner than 6 to 8 hours after charging the battery so that the chemical reactions inside the battery can reach a state of equilibrium. The battery manufacturer’s specifications will indicate the OCV that corresponds to 100 percent SOC. Batteries must be stored fully charged.

Marine batteries can be tested using the half-CCA method. A fully charged battery at room temperature (75 F to 80 F) is load tested at half its CCA rating. If the battery is good, its terminal voltage at the end of 15 seconds will be at least 9.6 volts (for a 12-volt battery).

2. Storage

I’ll say it again: batteries must be charged fully before storage. The electrolyte in a lead-acid battery can freeze if the temperature gets too low. The manufacturer’s specifications will indicate the lowest temperature at which the battery should be stored. At 77 F or cooler, absorption glass mat, pure lead valve-regulated lead-acid (AGM-VRLA) batteries can be stored for two years, or until the OCV drops to 12 volts. The lower the temperature, the longer the storage time, as long it does not drop below the manufacturer’s specifications, which, for example, is minus 40F for Odyssey batteries. Approximately every 18F increase in temperature reduces storage time by half. If the temperature rises to 95 F, the battery may be stored for only one year. Regardless of the storage temperature, batteries must be recharged before the OCV drops to the manufacturer’s recommended minimum value.

If the boat is left idle for extended periods, you should connect the batteries to a trickle charger. Parasitic loads on the battery created by on-board electronics, such as communications and instrumentation systems, will drain the battery to discharge if left idle long enough. If the battery discharges to less than 70 percent of its capacity, it is unlikely that it will be able to turn the engine .

Verify that the charger output voltage conforms to the battery manufacturer’s specifications before storing with a trickle charger. The alternative to using a trickle charger is to disconnect the negative battery cable.

3. Removal

If the battery needs to be recharged, consult the manufacturer for specific recommended procedures. If the battery won’t recharge and needs to be replaced, observe these guidelines:

4. Maintenance

Proper maintenance of the battery, when both in use and in storage, will not only extend its service life, but also will ensure that the warranty is enforceable.

When refilling a conventional flooded battery, use only distilled water to avoid contaminating the battery. AGM-VRLA batteries, however, are sealed, and will not need water or electrolyte added, which also prevents corrosion of the positive terminal and its surrounding area.

Again, maintaining the battery near full charge (12.8 volts or whatever the battery manufacturer specifies) ensures that it will last for its entire intended life.