Q –  I left the key in the ignition of my Porsche and my battery went dead. So I put it on charge and waited. Now three days later, it is still dead.   Should I buy a new battery or a new battery charger? And why would leaving the key in the ignition make the battery go dead? Nothing was left on. – Todd

A –  My hunch is that you used an “automatic battery charger”. Oddly, in another of those Madison Avenue oxymoronic terms, an automatic battery charger may not actually charge a stone dead battery. Surprise! I am not knocking automatic chargers or trickle chargers. In fact, they are one of the best inventions for batteries ever created, but they just don’t work well on dead batteries.

The automatic “brain” of the charger regulates the amount of power that the battery will receive. To know how much power it needs, it has to have some voltage before it can even decide to charge the battery. The brain in the charger may not know that there is a dead battery in the circuit that needs a charge. So it just goes to sleep and does nothing…while you wait.

If your battery is totally dead, you can connect it to another car with jumper cables or to another charged battery. After a short time, usually fifteen minutes to an hour, the dead battery will have absorbed enough energy from the good battery to wake up the automatic battery charger. You can then disconnect the jumper cables, connect your automatic charger to the (mostly) dead battery and after that it should work fine.

Don’t rush the process though. Porsche’s have large batteries and it may take two or three days to fully charge the battery. A safe charging rate on an automatic charger is about 15 amps.  Quick charging a battery at a higher rate can damage the battery or your car. And, I have warned about this before, don’t charge the battery by running the car or you can damage the car.

Another method, but a potentially dangerous one, is the tried and true “manual battery charger”, designed at the turn of the century. There is nothing automatic about it. If you put a manual charger on a dead battery it will charge till the cows come home. The problem is that it doesn’t know when to stop and the cows may never come home.

In the old days, charging batteries wasn’t so simple. You had to determine how low on charge the battery was by testing voltage and the “specific gravity” of the electrolyte. Then depending on the size of the battery, your had to figure out how many amps that to apply, and for how long. Last but not least, you had to remember to turn it off! A miscalculation could lead to a boiling battery, acid all over the floor (or in your trunk) and in extreme cases, a fire! Now aren’t you glad that YOU don’t have a manual charger?

Now that you have heard the warnings, with a manual charger, an overnight 2 amp charge will bring a dead battery to life enough to top it off with an automatic battery charger. And last but not least, if you leave the key in the ignition, on many cars, circuits will be powered up, you won’t even know it, and your battery will go dead.  –MC