Lets see if this garage can help you....with the engine running, your alternator should be charging your battery at around 14.4 v or thereabouts. With the lights on, the voltage should remain about 13.8 to 14 v if the alternator is ok. However, with the engine off, the alternator can quickly discharge the battery if its diode pack is faulty - this is like an electrical valve that 1) Changes AC into DC 2) Prevents the alternator from becoming like an electric motor, running your battery back down. So, to check this - once you have stopped the engine after a run, disconnect one battery terminal. If the battery retains its charge then I'm right. If it doesn't, the battery itself is goosed.
John
May 2009
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