Q: I just went out to start my car and the battery is dead. If I can start it with a jump, is it OK to let the car run for a while and charge up? – Justin
A: It is a widely held belief that the car’s charging system is for charging the battery. Oddly, the folks who designed, built and supplied your alternator do not think so. I know it doesn’t make sense but Bosch warns against trying to charge a dead battery with the car’s charging system.
The alternator’s mission is to replenish the electricity lost during starting and maintain the charge of the battery by powering all the vehicle accessories. A dead battery can send the alternator into thermal overload by delivering too much electricity. Bosch clearly states this on the installation instructions provided with every alternator. It certainly doesn’t seem intuitive does it?
Alternators are designed to produce their rated load for relatively short periods and then taper off. Alternators that produce maximum output continuously are used on commercial vehicles such as police cars, ambulances, etc.
If you have the time to wait, your best bet is to charge the battery at a low rate to charge it up fully. I use a 2 amp manual charger for the first 24 hours then an automatic charger at the 10 to15 amp setting for another 24 hours. To fully charge a stone dead battery may take a couple days and it is best not to rush the process. If you are going to try this however, please note the following paragraph.
If you notice the odor of rotten eggs or see liquid bubbling out of the battery, unplug the charger at the wall socket and sprinkle a little baking soda on that bubbling liquid (which is probably acid). Leaking battery acid will damage the trunk of your Porsche and that nasty odor indicates the presence of an explosive gas (hydrogen). A spark can ignite the hydrogen which is why it is wise not to simply unclip the battery clamps.
If you are in a hurry and you can’t wait to drive your car, connect the jumper cables and let the running car send power to the dead battery for about 15 minutes before you even try to start your Porsche. Once your car is running, rather than letting it idle, drive the car to charge it up. At higher rpm’s, the alternator runs cooler as it charges.
When you park the car that night, put the battery charger on at the 10 to 15 amp automatic setting. Usually one overnight charge will bring it back up to snuff if the battery is healthy. Please note: If you have a charger with both manual and automatic modes, I suggest that you only use the automatic setting. Keeping the battery in a proper state of charge will save your alternator and your battery tray. – MC