Recharging dead battery using engine

[petershen1984]

My 960 has had the battery run down by the electric cooling fan that wouldn't shut off. This happened on a Saturday night so I didn't have ready access to a mechanic who could help me replace the cheap relay stuff. Anyway...my insurance company provided roadside assistance and jump-started my car, and I was shown how to unscrew the battery terminal so that I still have juice in the morning.

I have a voltimeter that measures system voltage at the cigarette lighter, which would normally register 13.6 V with the engine running and battery in good condition. Today, with the flattened battery, the system voltage was a low 12.0 V at idling engine speed. This is when many people might rev the engine and attempt to charge the battery this way - and many rev high rpms which uses more fuel than necessary. Here's why - the alternator output is regulated and in otherwords limited in spite of higher engine speeds. With my inline-6, a light foot on the throttle, maintaining an unloaded 1,500 rpms is sufficient to have the system register 12.9V ~ 13.0V, and that's as high as it will go, even if you rev high.

So if you're going to temporarily charge the battery this way, there's no need to rev the engine too high for the alternator to deliver juice. However, service the car asap and get the battery properly charged.

[billofdurham]

With a good alternator and a receptive battery, i.e. one that is not on its last legs, the quickest way to recharge the battery is to go for a 20 mile drive. Might as well use the fuel instead of just idling it away.

Bill.