Battery, Alternator, or Something Else?

Okay, here’s the deal. I left the interior lights on all night so the car did not start the next morning. It jumped fine and ran fine. The next morning it would not start again, but I am 100% certain that every interior light / appliance was completely off. So I replaced the battery with a brand new one, and the car started fine. The next morning, the car started fine and I drove it downtown (about 20 miles) with no problem at all (no battery light, no rough engine, etc.). When I got to the parking garage, I got out of the car and tried to lock the doors with the keyless fob, however it did not work. In fact, nothing electrical worked! Here is a potential clue- I had this almost identical problem about 3 years ago and I replaced the OEM alternator at that time with a refurbished alternator and haven’t had this problem since. Is this a repeat event? Question I am pondering- is it possible to have just enough power from the alternator to keep the sparks going, but not enough to maintain charge???

Have the alternator tested. Most parts stores will do the testing free. Don’t guess, test and find out.

The charging system should be tested to determine if the alternator was damaged after jump starting the vehicle and driving it. Alternators aren’t designed to recharge a dead battery.


Alternators have a set of rectifier diodes built into them. If the battery jumpers are even momentarily reversed when you jump start the car, you can “blow out” two of these diodes. Then, instead of getting full wave rectification, you will only get half wave rectification, which will give you the same voltage output from the alternator but only halve the current flow. Thus your battery will not be recharged by the alternator as you drive. I read somewhere that it takes aprox. 13 miles at high way speed for the charging system to recover from one engine start, but that may be an urban myth. Anyway, it sounds like you blew the diodes and you need to have the alternator checked out. While you are at it, just because you have a new battery does not mean the lugs on the battery leads are clean and making good contact to the battery, you should have that checked out too.

update- i had the battery and alternator tested, both are fine. the next part is strange- after the car died downtown, i walked away and came back 6 hours later. the car started just fine as though nothing had happened… but the clock had reset. which means the entire car had lost power and regained it spontaneously!!! i will check the battery connections again but they were greased so i doubt that is it. is there a master relay or master fuse somewhere that would shut down power to all electrical appliances?

This is an odd ball problem, but, and this is a guess not a diagnosis, you may have a faulty ignition sw. If it is cracked internally it will be intermittent, and it controls all the power on power off funcitons. One other thought, and most unlikely but possible, is that the battery ground strap is not making good consistent contact with the body/engine due to rust or a loose bolt, or a frayed lead.

I’m assuming when you say “nothing electrical worked”, that the battery was dead and you couldn’t start the car either.

Have the alternator tested. You can do this yourself if you have a voltmeter. Put the meter on the battery–you should have over 12V. Start the car. You should normally have 13-14.5V with the engine running. If less, the alternator is bad. If the voltage is actually lower than what you measured before starting the car, the alternator isn’t doing anything at all.

Check it soon. A couple of deep discharges is all it usually takes to damage even a brand-new battery.