Can I charge a 12 volt battery with a 9 volt charger?

Answer 1

Unfortunately No. Voltage potential is such that electric current will flow from a higher potential towards a lower potential in much the same way that water will flow from a high level to a lower level until it reaches equilibrium (balances out on both sides).

If the battery is completely discharged then a 9 Volt charger may bring the battery up to around 9 Volts but it will never go above equilibrium.

Connecting a fully charged 12 Volt battery to a 9 Volt charger will cause current to attempt to flow "backwards" into the charger. If the charger has (as many modern ones do) overload and other protection devices then the charger will simply shut down but if the charger has no such protection, it could cause permanent damage to the charger and, depending on the rate that the battery discharges, the battery could explode and one or both devices could in fact set on fire.

Electricity can be potentially very dangerous and you should always ensure that you are using the correct equipment designed for the job. If in doubt, always seek professional advice.