Charging Questions and Answers

I have a 1965 Bug, all original 40 horse 6 volts. I had the generator light come on a week ago, so I ran all the tips on your fault-finding page as well as the Compleat Idiot book.

The generator was not putting out at all. I purchased a Bosch premium rebuilt unit and installed last Saturday. I drove the car very little Sunday, some on Monday and Tuesday. Wednesday morning I crank it up and the generator light is glowing dimly again... Something seems to have happened over night. By the end of the day it was on fully, and all of the system checks indicated the generator was not charging at all. The only thing we found remotely wrong with the generator was that the shaft had a slight bend in it on the fan end. We considered that at high RPMs the wobble may have caused harmonics that damaged the generator. Hard to swallow that one, but again, the only thing we could find.

I installed another generator last night and all seems OK. I was seeing ~6.9V off idle. I had forgotten to polarize the unit, so this morning I did so and the charge went up to ~7.2ish. I drove the car maybe 10 miles and came home to adjust the brakes. I rechecked the voltage and now off-idle is back to around 6.85V, no higher. I have installed a new battery, cables and transmission ground strap. The generator came with a new regulator mounted on it. I read that 7.0 - 7.2 is what it should be. Any idea what may be wrong here? Or is that optimal and 6.8 - 6.9 is fine? The car is all original except for a hard start relay that was on it before my purchase. I have cleaned all cable ends everywhere.

Could something be killing the polarization of the generator? The only thing I did not do to the original unit and first replacement was try to re-polarize them. I really do not want to change this thing again. Any thoughts?

There is probably nothing wrong. When first charging, battery voltage may go to 7.2V. As the battery becomes charged, the voltage tapers off to around 6.7V and stays there. Sounds like the regulator is doing its job properly.

I went back out about 1/2 hour from sending the email and checked everything over. Cranked her up, came off idle and heard the generator kick in and she went right to 7.15V. Several tests throughout the day have shown consistent 6.9VDC off idle, occasionally topping 7 volts. I finished replacing the battery cables and rewiring the hard start solenoid, cleaning the relay's contacts with a Dremel and sand disk. I'll keep and eye on the system for a while . . .

As "Speedy Jim" says, your charging system is probably OK now.

A 6v battery does in fact make slightly more than 6v (and a 12v battery is slightly more than 12v) so the generator and regulator have to maintain a voltage above the max battery voltage to charge it (force power back through the battery).

So anything above about 6.7v will charge the battery, although 7-7.2 is the ideal. Voltages under 7 will take a little longer to bring a flat battery up to speed but a fresh one will be maintained OK by 6.9v or so.

The regulator voltage is set by compromise - if it's not much above the best battery voltage it will be slow to charge, but if it's too high the battery will bubble and loose water. For the lead acid battery the best compromise charge voltage is about 2.3-2.4v per cell - 7.2v for a 6v system, 14.5 for a 12v system and 28v for a 24v system.

The real test will come after a week or so of driving including using the lights - if the battery stays charged then the system is working fine. You may just have a regulator with a fractionally low setting - the generator itself makes about 17 volts max and it's the regulator's job to keep it at a steady 7-7.2v for the car's electrical systems.