Why Does a Riding Lawn Mower's Battery Drain?

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Your lawn mower battery should maintain enough charge to start the mower reliably.
Your lawn mower battery should maintain enough charge to start the mower reliably. (Image: Comstock/Comstock/Getty Images)

Most riding lawn mowers are equipped with an electric starter and with a lead-acid battery. If the lawn mower will not start, one of the things that may be causing this problem is a drained battery. A lawn mower should recharge the battery while the lawn mower is in use; however, if the battery is drained every time you attempt to start the lawn mower, it may be due to one of several reasons.

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The Function of a Lead-acid Battery in a Lawn Mower

A lead-acid battery serves two major functions in a lawn mower: providing start-up electrical power, and serving as the reference point (also known as “electrical ground”) for the lawn mower electrical system. If the electrical system and the battery are both functioning correctly, the battery will provide enough power to start the lawn mower, and the lawn mower’s electrical system will recharge the battery during operation. If one – or both – systems are not functioning properly, the battery will become discharged.

Proper Battery Maintenance

A lead-acid battery provides electricity by means of an electrochemical reaction between the battery’s electrolyte and the lead plates within the battery. Over time, the electrolyte will begin to combine with the lead plates to form lead oxide crystals. These crystals impede the electrochemical reaction and reduce the power available from the battery. To dissolve these crystals and maintain the proper electrical balance, the battery must be periodically refilled with distilled water and recharged. If the battery electrolyte level is low, or if the battery has not been recharged for more than one month, the battery may be too discharged to start the lawn mower.

Excessive Power Drain during Mower Operations

Some riding lawn mowers are designed to be used as lawn tractors and as small snow plows. During these operations, you may have to use headlamps, trailer lighting, or other electronic accessories. If the lawn mower electrical system produces only enough energy to operate the engine and charge the battery, these devices may rob the battery of charging current, or may even tax the battery if the expended power is greater than the lawn mower electrical system can produce.

Parasitic Electrical Drain

There may be a problem with the lawn mower electrical system that drains the battery while the lawn mower is turned off. The electrical system may continue to draw electric current from the battery if there is a short-circuit in the system. A parasitic drain may also occur when certain devices that are supposed to shut off–like an electrical relay, for example–remain on after the engine has been turned off. One way to test the lawn mower for this condition is to check for a current draw while the engine is turned off. Disconnect the positive battery cable from the battery. Connect an ammeter between the positive battery terminal and the battery cable. If the ammeter shows that there is a current draw of more than 1 milliamp, the electrical system is drawing power while the engine is off.

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