C3 Alternator question

[MAJohnson]

Hello, I have a 1976 and have been having problems with battery power draining within about 3 weeks if the cars sits. I replaced the battery and bought a battery disconnect switch. If I disconnect the battery then the problem does not occur, I tested for any electrical drains and the test came out ok. I am told though that occasionally the alternator can have a minor short that could cause this problem, any else ran into anything like this or have any input as to where the problem might lie?
Thanks,
Michael Johnson

[Jim T.]

Re: C3 Alternator question

Did have a problem with the middle compartment light. The door was not closing tight enough to turn off the light and it stayed on all the time. Real simple solution since I never used the light anyway. Just disconnected it. When my 68 and 70 are not driven much, I just disconnect the ground battery cable where it is connected to the frame. Very quick and very easy. If you think your alternator is the problem, disconnect the alternator and see what happens? I have had a alternator do this in another car. The red light on the dash would immuminate when the ignition was turned off. Just disconnected the connection at the alternator for a couple days until I removed it for repair.

[Jack H.]

Re: C3 Alternator question

Every battery has a self-discharge profile. Cars driven on a daily basis don't 'push the envelope' so we're not used to seeing their our battery run down when not driven and re-charged.

Yes, it IS possible for an alternator to have a 'flakey' diode (or more) in its 3-phase rectifier bridge and be the cause of battery drain during disuse. But you said you 'checked' for abnormal electrical drain(s) so, if we take you at your word, this isn't your problem. A good auto electric shop can test the alternator and tell you about it's 'reverse leakage' profile if you're concerned however.

When the car sits, there are still active drain circuits (E.g. clock). On newer cars, there are even more drain circuits like the engine control processor and it's memory storage (position of seats, mirrors, dynamic engine control coefficients, Etc.). That's why a battery cutoff switch is always a good idea to install/use on a 'weekend warrior' car. Plus, it helps to periodically recharge the battery even when it's disconnected since it WILL self discharge all by itself.

One technique for 'tending' your battery is to have a 'smart' battery charger (most are today) and connect it once a month or so to keep your battery charged up. Also, there are intelligent forms of trickle chargers (low output battery chargers) that 'profile' your battery, learn about it's specific discharge characteristics, then match their output to the battery keeping it 'topped up' without the risk of damage due to overcharging. These are neat devices ($100 range) aimed at long storage interval situations like our summer time Corvettes, boats, Etc. The beauty is you hook 'em up and walk away....