A car battery is designed and intended primarily to generate a very high current for starting your engine. The secondary function is to provide low current power for lighting or accessory items for a short while when the engine may not be running to recharge the battery, or to provide power when the generator or alternator at a slow idle may not produce enough current to supply all the needs of the vehicle. When the engine is running (faster than a dead idle) the generator or alternator will provide sufficient power for all needs of the vehicle, and hopefully enough extra power to recharge the battery after the large current draw from starting.

When the automotive type battery is optimized for high cranking current, it has some limitations in other functions. You should keep the battery near full charge (between 90% and 100%) at all times. Allowing the charge level to drop below 90% of full capacity can reduce the ability to produce full voltage during high current cranking. It also promotes sulfation of the plates, which can shorten the life of the battery. Fully discharging can shorten the battery life severely, and leaving it fully discharged for even a short period of time may kill it completely. If you allow your car to sit for long periods of time unused, you might consider hooking up a regulated trickle charger to maintain full charge level in the battery. This device may be called a "battery maintainer".

For our older cars with no frills, when you switch off the engine there may be no remaining current draw from the battery. As such, you might be able to allow it to sit for months at a time and still be able to start the car.

Newer cars have a number of electronic devices that may draw a very small current from the battery while it is switched off. An engine control computer needs to maintain a memory of diagnostic codes and possible service intervals. A digital dial radio needs to maintain memory of the preset radio channels. Car alarms and remote controlled entry locks need to be minimally active at all times in order to receive signals from the remote control devices. These small current demands are called parasitic loads. A number of these small parasitic loads combined can conspire to run the battery down in a few weeks time. Batteries in newer cars are often sized according to the total parasitic draw in order to maintain sufficient charge for a minimum of 30 days so you can still start the engine. For many newer cars, if they are allowed to sit for more then 30 days they may not restart until the battery is recharged. Some new cars even have a second battery or a reserve section of the battery that cannot be discharged by the parasitic loads.

If you install some electronic devices on your older car which may have a parasitic current draw (such as a digital radio), then you should pay special attention to not allowing the battery to discharge when the car is idle for long periods. You might disconnect one battery cable or install a high current disconnect switch in one of the main battery cables. Or maybe use a battery maintainer regulated trickle charger. Dirt and acid on top of the battery can also discharge it slowly over time. One other occasional problem may come from a faulty diode in an alternator which can allow a small current to discharge the battery, but that's a repair problem, rather outside the scope of this discussion.