Sinrace Power Supply Technology (Shenzhen) Co., Ltd.

Step 1 Determine the voltage of your power source. DC power supplies vary in voltage. The two places where you usually have to use DC power are in a car and in a boat. A car alternator produces 12 V DC power. Marine batteries often use 24 V DC. USB also uses DC, but it gets that power from a wall socket anyway. If you want to charge batteries indoors where there are wall sockets available, it is more efficient to buy a conventional plug-in charger.

Step 2 Look at the voltage requirements for the battery you are charging. If you are charging the battery on a device that doesn't require you to remove the battery, such as a laptop or cell phone, the output voltage on the power cord is the voltage you need to charge the battery. If you are charging a removable battery, the necessary charge voltage will be written right on the battery itself.

Step 3 Find a charger specifically designed for using the DC power you have available to charge the batteries. Many laptop and cell-phone companies actually make laptop car chargers specifically designed for a particular model or series. In other cases, you can use a universal laptop charger such as the one linked to below. If you are charging your batteries with something other than 12 V or if you can't find a charger for your particular battery, continue to Step 4.

Step 4 Get a DC/AC inverter, which turns DC power into the same 115 to 120 volt AC current that comes out of wall plugs. Follow the links below for car and boat DC/AC inverters.

Step 5 Get a standard AC battery charger and put the battery in. Plug it into the wall and turn it on. Follow the charging directions that come with the charger.