A portable battery jumper works by providing the car's starter motor with the necessary power to start the engine. If used properly, it allows the you to jump start your car without another vehicle.
Most portable battery jumpers only have enough power for one or two jumps before they need recharging, so it is important to make sure you know how to properly use it. Most importantly, keep it charged. Every jumper has some way of indicating it is fully charged. Second, ensure there is a good connection between the jumper and the battery when using the jumper. Clean any corrosion or debris from the battery terminals, and clamp the alligator clips fully when attaching them to the terminals.
Like most tools, battery jumpers come in a range of prices and features, some of which may not be necessary depending on your needs. The most important number to look for is the peak-amp rating; this is how much current the unit is capable of delivering. The next thing to look at is the gauge of the wire; a smaller gauge indicates a larger wire that can handle more current. These numbers are especially important if you are counting on the jumper to start a large truck or van, which requires more power.