Why can’t I charge my battery?

A problem can arise when you battery is extremely discharged, say a light is left on in the vehicle and the battery voltage drops below 7 volts. Some chargers will not charge a battery when the voltage is that low and you will have to take it to a professional to charge it for you. Chargers are getting better and there are some on the market that will start at 3 or even 0 volts. The problem is that the battery has developed an extremely high internal resistance (caused by sulfation) and the charger is not strong enough to push current through the battery.