How to Charge a 12V Battery With 4V Output

Charging a 12v battery with a 4-volt output will more than likely result in charging the battery to 4-volts. You should use a 12-volt charge source for a 12-volt battery. In the event that this is not possible, you may try creating a charge pump circuit, linking 4, 4-volt capacitors in order to create enough capacity to charge the 12-volt battery fully.

Be aware that the steps described in this article may discharge your battery if it is already charged or damage the 4-volt capacitors of they are a lesser quality. This process should be conducted with a near dead or empty 12-volt battery.

Step 1: Create a Charge Pump Circuit

Link the 4-volt capacitors by wiring them in a parallel circuit. This will create a 4-volt charge for the battery. Connect the 12-volt to the pump circuit in order to accept the charge. You will then need to change the parallel circuit into a serial (cross) circuit in order to complete the charge. Disconnect the 12-volt battery at the end of this process.

Step 2: Test the Battery

Use a 12-volt battery tester in order to determine how much of a charge was accepted by the charge pump circuit.  It may be possible that through all of your efforts, only 4-volts of charge was completed.  Repeat the charge process with the 4-volt capacitors and test until an acceptable level of charge goes into the 12-volt battery.

Please note that information provided online overwhelmingly suggests that this process cannot be done and that the best solution for charging a 12-volt battery is to purchase a recommended 12-volt charger.  If you wish to pursue this do-it-yourself process, this article gives no assurance that it will work as intended or that the 12-volt battery will be charged.