A man perched on a riding lawn mower mows his lawn.

When to Change a Lawn Mower Battery

Electric-start lawn mowers and other types of power equipment sure are convenient, and they’ve saved untold numbers of shoulders from rope-starting. But what if you can’t start your lawn mower?

Maybe the culprit is your lawn mower battery — or maybe it’s not. Before you condemn the battery, take a voltage reading with a quality digital multimeter (DMM). A fully charged 12V battery should read at least 12.6 volts, while a moderately discharged battery might only read 10.5 volts.

Here are a few ways to tell whether it’s time to change the battery or look further into your lawn mower’s electrical system for the source of the problem.

Interlock Problems

At the beginning of the mowing season, and after sitting for months in winter storage, lawn mower batteries are often found to be dead, but that’s not the only reason for a no-start condition. If the battery is good, something else might be interrupting the starter signal, like the starter interlock.

Most starter interlock switches on lawn mowers are located under the seat or in the handle. If the contacts are bent, broken or corroded, or if mice have chewed through the wiring, the interlock won’t enable the starter motor — no matter how good the battery is. Use your DMM to trace the interlock circuit for repair or adjustment.

Resistance Problems

Corrosion is a Great Insulator!If the interlock is good and the battery is good, getting enough juice to the starter motor can be a problem. If you hear a depressing click or buzz when trying to start the lawn mower engine, this might be due to a couple of problems.

Corrosion occurs in the presence of dissimilar metals, sulfuric acid, electricity and water. The byproducts of corrosion, anhydrous copper sulfate and lead sulfate, are also great insulators. Fortunately, a wire brush and some baking soda solution can neutralize battery acid and eliminate resistance.

Pitting in the starter relay is also common, created when the sudden circuit break arcs and burns away the contacts. With enough pitting, the flow of electricity to the starter motor could completely stop. Depending on the setup, you might be able to bypass the relay or solenoid to get the engine started.

Battery Problems

Of course, everything else being in good condition, the battery could be weak or dead, which could cause any of these symptoms. Considering lawn mowers tend to sit for months during the wintertime, it’s normal to expect a lawn mower battery to last three or four years.

If you’ve confirmed with your DMM that the battery and connections are weak, then it’s likely time for a new lawn mower battery. You might be able to revive it with a charger, but the damage might already be irreversible if the voltage is less than 7.5 volts. If it still won’t hold a charge after a few hours on a slow charge, replace it.

The best way to help the battery last longer is to keep everything clean and dry and protect the battery terminals from corrosion during the mowing season. When wintering over your lawn mower, disconnect the battery and store it in a clean, dry spot, preferably where it won’t freeze. Consider using a 12V float charger to keep it in optimum condition for the next season.

Check out all the lawn and garden products available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on the right lawn mower battery for your equipment, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Flickr.

about author

Benjamin Jerew

Ben has been taking things apart since he was 5, and putting them back together again since he was 8. After dabbling in DIY repairs at home and on the farm, he found his calling in the CGCC Automobile Repair program. After he held his ASE CMAT for 10 years, Ben decided he needed a change. Now, he writes on automotive topics across the web and around the world, including new automotive technology, transportation legislation, emissions, fuel economy and auto repair.

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