Battery-powered lawnmowers that operate on 24-V batteries are a fairly recent addition to the world of lawn mowing, and the batteries that come with the various models require a special charger that comes with the unit. Riding mowers that employ 12-V batteries to power their ignition systems have been around much longer. The batteries are similar to the ones you find in motorcycles, and like any vehicle batteries, they can discharge after a prolonged period of disuse. To charge one, you need a battery charger like the one you use for your car.
24-V Push Mower Batteries
Pull the battery out of its harness in the lawn mower. You usually have to press a button to release a lever while you pull.
Insert the battery in the charger that came with the lawnmower. Most manufacturers design the batteries to only into their brand of charger; if you don't have one of these, you'll have to buy one.
Plug in the charger and verify that the charging light illuminates. Often, a red light flashes and turns to flashing green to indicate that charging has started. If the light remains solid red, the battery is probably too warm to charge. Remove it and wait for 30 minutes before trying again.
Leave the battery in the charger and charger plugged in until the flashing green light turns to solid green. At this point, charging is completed and the battery is ready to use.
Push Mower Self Chargers
Some push mowers come with a plug-in charger. To charge the battery, plug the transformer into the wall, remove the safety key if applicable and insert the charger plug into the receptacle on the mower; consult your owner's manual for the location of the receptacle -- it's often on the mower handle. A red light will illuminate, indicating the battery is charging. Charging is complete when the light changes to green. It can take up to 12 hours or more to charge a fully discharged battery.
12-V Riding Mower Batteries
Open the battery compartment. You may need a screwdriver to do this, depending on the design of your mower.
Loosen the battery cables with a wrench and pull them off the battery terminals. Remove the cable from the positive terminal first. You can leave the battery in the mower while charging it, but if you may prefer to take it out, you can do that, too.
Set the toggle on your battery charger to 12 V. Choose a charging current of 2 amps or, if the charger has an "Automatic" current setting, choose that.
Connect the positive lead on the charger -- it's usually the red one -- to the positive battery terminal and connect the other lead to the negative battery terminal. Most chargers have alligator clips that securely latch onto the terminals.
Plug in the charger and turn it on, if it has an on/off lever. Leave it connected and plugged in for at least five hours. Many chargers turn themselves off when the battery is fully charged; if yours doesn't have that feature, don't leave it on for more than eight hours.
Unplug the charger, then disconnect the leads from the battery terminals. Replace the cables and tighten them onto the battery lugs with a wrench.