Classic/Antique Car Repair: electric failure, electric failure, gentle stroke

Question
Dear Vern,
Last week, when driving my MkVI Bentley (1951), I noticed that the battery was not charging at all (ammeter on 0). As the electrical consumption was very low, I assumed the battery was close to full, and the needle of the ammeter did not show that small variation. On the way back, the car started again quite normally. But it quickly appeared that the headlamps were weak. This time the battery was loosing amps, which indicated that the ammeter was working properly.

I got back safely to the garage, and after stopping the engine, attempted to
start her again: the battery was too low. So I put it on charge: it did not seem
to take it: the indications on the charger were: connection to 220v ok –
connection to battery ok – battery full. I measured the voltage: it was
just above12v. Therefore, albeit not being full, it was not completely empty.
Indeed, the headlamps could still be put on. So I decided to leave the battery
(disconnected from the car) on charge all night.

Approx 18 hrs later it was however still low, unable to start the car. I started the engine manually: she started first go after a ‘gentle stroke’ on the crankshaft. And off we went.But there was no electricity in the car (incl. no headlamps).

That is the second time my battery is put into what I assume is a short-circuit: won’t take the
charge even when empty: the previous time occurred a few months earlier. At that time
it was practically new, and was replaced by the one I used this week-end, therefore also new (75Amps). I thought the fault came from the battery.

The odds that I got two faulty batteries in a row seem very low. Therefore, I suspect something / some connexion must be faulty on the car itself. The question is therefore obvious: what should I check out on the car that can damage a new battery like that?  As you know, these cars were fitted with dynamos and not alternators.

Thanks for your time... and suggestions!

Frederick  

Answer
Hello Frederick:
The problem most likely is in the fan belt. Check the belt and see if it is loose. If the belt is slipping, it will just drain the battery.

Another possibility is a loose or faulty battery cable. Check to make sure you have good connections on both ends and there is no corrosion.

However, a loose belt or faulty cable would only drain the battery. If your new batteries won't hold a charge, check the commutator brushes. They could be worn to a point where no electric is being generated.

The commutator definitely could be shorting out the battery if it is bad.

Hope this helps.