Have you ever had a customer state that their batteries will not take a charge even though the charger was plugged in overnight? When you checked their batteries, you found that they both read 9 volts? This is usually due to a light or a brake being left on for an extended period of time, which drains the batteries.

The reason why the charger is not working is that most wheelchair battery chargers need to read at least 21-22 volts in order to begin charging. This is how the polarity protection system of many chargers works. If the user were to hook up the positive and negative backwards, nothing would happen to the charger or the batteries because the batteries never read any voltage so it never started.

The drawback to this polarity protection design is when a user over-discharges their batteries below the 21-22 volt cutoff. Although the charger is connected, it does not receive the signal to begin the charging process so the batteries never get charged.

The best way to solve this problem is to remove the batteries from the wheelchair and charge each battery separately with a 12-volt battery charger. When each battery is fully charged, they can then be reinstalled in the chair and returned to service. Note: It may take the batteries up to 15 cycles to return to their former capacity if they have been severely discharged.