Twenty-four-volt motors are common in golf carts, boats, RVs, and powered handicap scooters because of their greater efficiency in continuous-duty applications. Twelve-volt batteries are far more common and economical, though, so the most practical route to 24 volts in 2010 is still to connect two 12-volt batteries in series. This poses a challenge when trying to charge the two 12-volt batteries when they are used to provide 24 volts. The safest, most straightforward way is with a 24-volt charger.
Use a 24-volt charger with an amp-hour rating of about one-tenth of one battery's normal continuous amp-hour rating to obtain an overnight charge. This number should be clearly printed on the label of each battery, near its voltage rating. Two small 12-volt scooter batteries connected in series to provide a 24-volt output, each with a 12-amp-hour rating, for example, would therefore require a 24-volt charger with at least a 1.2 amp-hour rating. Two large 12-volt RV batteries connected in series each with a 250-amp-hour rating would require a 24-volt charger with a 25-amp-hour rating.
Disconnect the scooter's, RV's, or other device's red positive and black negative connections from the series-connected batteries before charging. This prevents damage to the device's electronics.
Connect the red positive cable clip of the 24-volt charger to the positive terminal of the battery with a plus sign (+) which supplies the positive voltage to the device.
Connect the black negative cable clip to the negative terminal with a negative sign (-) of the battery supplying the negative voltage to the device. The cable between the negative post of the first battery to the positive post of the second battery, called the series-connect cable, should be left in place.
Plug in the charger to a grounded 115-volt AC wall receptacle (normal house current) and check for safe operation. Most chargers have reverse-hookup detection and green and red lights to advise correct or incorrect hookup respectively.
Unplug and disconnect the charger when the battery is charged. Most chargers will stop flashing their green charging light, or the light will go from yellow to green. Individual charger instructions will contain this detail. As a rule of thumb, 10 hours or overnight is usually enough. Reconnect the device's power leads, red to positive and black to negative, of the series-connected battery set, and test the device.
Always consult the user manual for the device for specific recharging information. Disconnect both of the scooter's or RV's leads (or other device's leads) from the battery while charging to prevent damage to the device's electronics.
Never use a 24-volt charger to charge a single 12-volt battery; nor attempt to use a 12-volt charger to charge a 24-volt battery series (two 12-volt batteries connected in series).