Generator Battery Maintenance: Care and Charging Tips

Why the starting battery matters

Electric-start generators use a 12V battery to crank the engine, just like a car. If the battery is weak or dead, the electric starter cannot spin the engine fast enough to start it, even if the generator is otherwise in good condition. A generator that only has a recoil starter does not rely on a battery at all, though some inverter models use a battery to power the control panel and electric start. See how electric start compares to recoil for the differences.

Portable and standby generators both need a healthy battery. Standby generators typically have a battery heater and a charging circuit that keeps the battery topped up while the unit sits between outages. Portable generators often have no charger at all, so the battery can slowly discharge during storage. Understanding how your generator charges its battery is the first step in keeping it ready. If you are shopping for a new unit, check the best electric start generators for models with reliable battery systems.

• Check the owner's manual for the battery type and charging recommendation.
• Find where the battery is located and whether it is easy to access.
• Confirm whether the generator charges the battery while running or only through an external charger.

How the generator charges the battery

Most portable generators with electric start have a small charging circuit that sends current to the battery while the engine is running. This is enough to replace the energy used during starting, but it is not a full battery charger. If the generator sits for months, the battery self-discharges and can sulfate, which permanently reduces capacity.

Standby generators usually have a more robust charging system, sometimes with a battery warmer for cold climates. Even so, the battery should be tested periodically. A battery that is never exercised or charged can fail when you need it most, usually during a power outage.

If your generator doesn't charge the battery, or if you store the generator, use a battery maintainer designed for 12V batteries. A maintainer supplies a low current and automatically stops when the battery is full. Overcharging a battery can cause it to gas and lose electrolyte, so do not leave a shop charger connected indefinitely.

Inspecting the battery and terminals

Check the battery terminals every few months for corrosion. Corrosion appears as a white or blue-green crust on the posts and cable ends. It can create resistance, which means less current reaches the starter. Clean the terminals with a wire brush and a solution of baking soda and water, then rinse with clean water and dry thoroughly.

Inspect the battery case for cracks, bulges, or leaks. A swollen case usually means the battery was overcharged or deep-cycled too hard. A cracked or leaking battery should be replaced immediately. Also check that the battery is held down securely; vibration from the generator engine can loosen connections and damage the plates inside the battery.

If the battery has removable caps, check the electrolyte level and top off with distilled water, not tap water. Many sealed batteries do not require this, but the manual will say which type you have. Low electrolyte can expose the plates and ruin the battery.

Charging and testing the battery

The best way to keep a battery healthy during long storage is to connect it to a battery maintainer, not a full charger. A maintainer keeps the battery at its optimal voltage without overcharging. Disconnect the battery from the generator or use the maintainer's harness if the generator provides one. Always connect the charger to the battery before plugging it into power, and disconnect the charger before unplugging from the battery.

If the generator has been sitting for months, charge the battery fully before the first start attempt. A battery that is only partially charged can still spin the starter slowly and may make the engine hard to start. If the battery will not accept a charge, or the engine still cranks slowly after a full charge, the battery likely needs replacement.

To test the battery without special equipment, check that the terminals are clean, then try the electric start. If the starter seems weak, the battery may be failing. A multimeter is the more reliable way: a healthy 12V battery reads around 12.6V when fully charged and at rest. If it reads below about 12.4V, charge it and recheck. See generator troubleshooting when it won't start for other causes of hard starting.

When to replace the battery

A lead-acid battery in a generator that is maintained properly can last several years, but it will eventually lose capacity. The most common signs are slow cranking, the need to recharge after every session, or a battery that will not hold a charge for more than a week or two.

When replacing the battery, use the group size and type specified in the owner's manual. Do not substitute a smaller battery that has less cranking capacity. If the generator has a dedicated battery compartment, confirm that the replacement fits and that the vents are not blocked. A sealed AGM battery is a common upgrade for generators because it cannot spill acid and tolerates vibration well, but check the manual for compatibility.

Dispose of the old battery properly. Most auto parts stores and recycling centers accept lead-acid batteries. Never put a battery in the trash.

Storage and cold weather care

If you store your generator for the off-season, remove the battery or connect it to a maintainer. A battery that is left connected will slowly drain through the generator's control circuits. Store the battery in a cool, dry place that stays above freezing when possible. A fully charged battery is much less likely to freeze than a discharged one.

In cold weather, a battery's cranking power drops, so the generator may be harder to start. This is normal. If the generator is installed in a cold climate with a battery warmer, use it. For portable generators, bring the battery indoors to warm it up before starting if you can. See generator operation in cold weather for more on winter starting.

Whenever you handle the battery, remember that generators produce carbon monoxide. The CPSC warns that CO from a generator can kill in minutes, and never use a generator indoors or in a garage, even with doors open. Charge the battery outside the living space and away from running equipment.

What to do with your generator battery

Questions

How often should I check the battery on my generator?

Check the terminals for corrosion and the battery case for damage every couple of months. Test the battery before the start of the season or after long storage.

Do I need to disconnect the battery when storing the generator?

Yes, if you do not have a battery maintainer connected. The generator's control circuits can draw a small current even when the engine is off, which will slowly drain the battery.

Can I use a car battery charger on my generator battery?

You can use a 12V charger rated for the battery type, but use a maintainer for long-term storage. A full car charger may overcharge and damage the battery if left connected for days.

How do I know when to replace the generator battery?

Replace the battery if the starter cranks slowly after a full charge, if the battery will not hold a charge, or if the case is swollen or cracked.

Why is carbon monoxide safety important when working on the battery?

Running a generator indoors exposes you to deadly carbon monoxide. Never run a generator inside a home, garage, or shed. Charge the battery away from the exhaust and never start the generator indoors.