Many modern 12V battery chargers will not turn on when an attempt is made to charge any type of 12V battery that has a very low open circuit voltage (OCV). For example, a charger set for 12V charging connected to a 12V battery that has an OCV less than 4 or 5 Volts, the charger senses it is connected to a 6V battery (which it is not) and therefore will not start a charge because it is set for 12V charging.
Most lead acid batteries are good enough to accept a charge even when its voltage is less than 5.0 volts. As long as the charger's output voltage does not rise above 15.0 volts. If your charger refuses to charge your battery the following procedure should allow you to bypass the charger's safety circuit, and safely attempt to recover the battery.
Note; batteries that have been operated over a prolonged period of time, and have not routinely been charged back to near or full charge, or have not been used for a long time will have developed sulfated oxide on the plates, and will take a number of charge discharge cycles to recover. In some cases, if the sulfation condition is well developed especially over time, it may not be possible to achieve 100% capacity.
You will need a healthy 12 volt battery of a similar capacity as the dead battery, and a Battery Extra EX01 12-150 or EX01 12-24-200 battery desulfator
1.Disconnect the battery.
2.Connect the Battery Extra to the positive and negative terminals of the dead battery. Make sure the Battery Extra low voltage switch is in the off position.
3.Using jumper cables connect the positive terminal of a healthy battery to the positive terminal of the dead battery; then connect the negative terminal of the healthy battery to the negative terminal of the dead battery.
4.Connect your charger to the dead battery and switch on.
5.Monitor the voltage of the dead battery with the Battery Extra voltmeter until it reads more than 11.5 volts.
6.Disconnect the jumper cables from the good battery.
7.Continue charging until the charger switches off, make sure the charge voltage reading does not exceed 15.0V or that does not get hot to the touch (over 40c). If the charge voltage goes over 15 volts or the battery gets hot discontinue the charge.
8.Disconnect the charger and allow the battery to sit for 4 hours, check the voltage. A fully charged battery will read around 12.8 volts indicating a full charge.
9.Leave the Battery Extra connected and use the battery in the normal way.
10.Sulfated batteries may take anything from 3 to 20+ cycles to recover to their full available capacity. On old batteries this may not be 100% of the original rated capacity.