A question for guys who understand auto electrics UPDATE

700rpm

700rpm

Well-known member
A Model A friend has an electrical problem, and neither of us knows enough about auto electrics to figure it out. His alternator has been working fine on his 12v system. The battery is past its pull date by about two months, but the alt. seems to have been keeping it charged, until yesterday. The car wouldn't start till We jumped it. We drove it for a while thinking maybe the alt. would recharge it, but no go.

Does this indicate that the battery has just finally given up, or that the alternator has stopped charging? And can either of these conditions be checked without the car running? We would prefer diagnoses before just guessing and replacing unnecessarily, but we readily admit we don't know how to proceed.

If this was a stock 6v Model A, I would just get a new battery, but with this modern stuff I'm lost.
 
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Re: A question for guys who understand auto electrics

I would say the battery is shot. When you jumped it and then you said you drove it around, it would have been running off of the alternator (proving the alternator work) but not taking a charge at all, thus not starting again. Batteries seem to be real good now at failing right around their expiration dates. You could get the battery checked at an auto parts store, but pretty sure it has quit.
 
Re: A question for guys who understand auto electrics

Ray, it is not that much different with a 12V system.

Put a low-reading voltmeter across the alt. (one lead to ground, the other to the alt. output terminal. Ideally you would have a 0-20 V scale on your meter, which would be way more accurate than a 0-50 V scale).
For a 6V system, you want to see about 7.2-7.4 volts (engine on)
For a 12 V system, you want to see about 14.2-14.4 volts. (engine on)
If you are able to get those readings, then your alt is working fine.

In an ideal world, you would put a shop charger on the batt for 6 hrs (engine off) to freshen the charge on the battery before you do the voltage check above. If a battery is really really low, you can be fooled by the voltmeter test because even tho the alt is charging, under those conditions the voltage reading will be lower like maybe 13 volts.

It sounds like the battery can no longer hold the charge, which is what happens when they die.

We assume all your battery cable connections are good at both ends.

In the old days when they made GOOD batteries, a 12 V battery would just die without warning. A 6V battery would die slowly, giving you some warning. Today, they are all crap and seldom give a warning.

The reason the voltage from the alt. on a healthy system has to be slightly higher than the battery rated voltage is because if the alt were at 12V and the battery is at 12V then nothing would happen, no electricity would be forced into the battery. It is like a tug of war with 2 equally strong opponents; nothing happens. If the alt voltage is slightly higher, then it is able to force electricity into the battery. If the plates are shot, shorted, etc etc then the battery has lost the ability to store the charge. Dave
 
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Re: A question for guys who understand auto electrics

An inexpensive Volt-Ohm meter would tell you very quickly if the alt is working. With it running , the voltage should be over 13+ volts. Not running, the meter will tell the battery voltage. I agree on more likely the battery is bad. A good parts store might check it for you. Good luck.
 
Re: A question for guys who understand auto electrics

Static voltage (engine off and sitting for 2 or 3 hours) should be 12 or 12.5 volts.
Letting it sit for a while after charging will allow the float charge to dissipate and stabilize at 12 volts or so. Perhaps charge the battery at 2 amps overnight will bring it back up. Like the others say, it may be time to replace it.
 
Re: A question for guys who understand auto electrics

Get a small, cheap hydrometer at the auto parts store and check the charge in each cell by pulling the water up into the hydrometer. Most of the time if a battery is shot there will be one cell that does not have a full charge.
 
Re: A question for guys who understand auto electrics

Before you give up on the battery, disconnect one cable and charge the battery for several hours on a 6 amp charger. Let the battery set overnight, then connect the battery cable and try starting the engine. If it doesn't crank like it should, then the battery is junk.

IMPORTANT.........while charging the battery, feel the case. If it feels too warm, then you probably have a shorted cell, and should stop charging and trade it in. It's always safest to charge any questionable battery outside, where it won't damage something if it explodes.
 
Re: A question for guys who understand auto electrics

If it's charging by checking it with a volt meter (remember to rev it up to excite it) and you have no draw by cking it with a testlight replace the battery.... 5 years is the bench mark although many roll the dice longer.
 
Re: A question for guys who understand auto electrics

OK. Thanks everybody. We'll run some tests today and put a charger on it. But just to confirm I understand: a truly dead battery can not be charged with the alternator, nor will it take a charge from a charger. Is that correct?
 
Re: A question for guys who understand auto electrics

700rpm said:
OK. Thanks everybody. We'll run some tests today and put a charger on it. But just to confirm I understand: a truly dead battery can not be charged with the alternator, nor will it take a charge from a charger. Is that correct?
Click to expand...

an alt or charger can bring a low / good battery back.. It's best to use a charger to do it as solely relying on the alt it can smoke it...
 
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Re: A question for guys who understand auto electrics

yep if the battery is good, but drained an alt or shop charger will bring it back.
But if truly dead (ie shorted etc) nothing will get it to hold a charge
 
Re: A question for guys who understand auto electrics

700rpm said:
OK. Thanks everybody. We'll run some tests today and put a charger on it. But just to confirm I understand: a truly dead battery can not be charged with the alternator, nor will it take a charge from a charger. Is that correct?
Click to expand...




Thats correct. A dead battery will not be charged by an alternator. It needs a charger and it may or may not come back to life. However a jumper battery could be used to help.

A generator will generate on its own, just spin it. An alternator needs current to produce current.
 
Re: A question for guys who understand auto electrics

A bit beyond the casual D.I.Y.'er, but I'd scope the alternator output to assess the diode condition, especially if it is one of those popular ancient tech 10/12SI diode trio jobs. Then load test the battery. Store will do it free if you do not have a load tester. Last, assess the overall electrical system for parasitic drain while off.
 
Re: A question for guys who understand auto electrics

Just to throw into the mix, charge the battery. Take a voltage reading. Turn the headlights on for a few seconds to take the surface charge off....or just leave the lights on. Watch the voltage. If it quickly drops to say 11.75 volts quickly the battery is suspect. Think about how a flashlight with bad batteries behave.
You can do the inverse. If you have a battery charger that displays amperage charge the "dead" battery and watch both voltage and amps. If the voltage rises quickly and the amperage drops off, the battery is suspect.
These are little things you can do w/o a lot of specialized tools. Not definitive but gets you close.
And as Tom suggest, check for a drain.
 
Re: A question for guys who understand auto electrics

I also suspect that the battery is the problem. As others have stated, the alternator will charge up a low battery, but if it is shorted between plates, no amount of charging will correct this. With most 12V batteries today are sealed and you cannot remove the caps to replace the water that boils off from heat, low humidity and charging. Here in Arizona batteries do not last even the warranty period due to the extreme heat. Just think how hot they get when the outside air temp is greater than 115, then bring air around the battery in an engine compartment with the additional heat from the air conditioner and radiator.
If you can get the caps off each cell and use a hydrometer, you can check each cell for its charge. If you have a bad cell, the specific gravity of that cell will be low.
 
Well, she starts and runs. First we hooked up a good battery (one we knew was good from another car), and that didn't help. We went through every connection, and found some rusty and corroded junctions, loose connection at the ammeter, and a loose wire on the alternator that didn't look loose. Then we started on diagnosis, and after each adjustment, cleaning, or any other step, we made a try to see if we could get the readings we expected. We got it to turn over nicely, and run OK, but couldn't get a good charge reading. After tightening that one loose wire, bingo.

The lesson here for my friend is, keep everything clean and tight. The lesson for me is, 12v isn't that much more complicated than 6v on an A, and I learned a few things about how to check charging systems.

We have the old battery on an overnight charge, and will reinstall tomorrow, then see if we really caught the problem.

A big thank you to all who gave their suggestions.
 
Re: A question for guys who understand auto electrics

Tom Wesenberg said:
Before you give up on the battery, disconnect one cable and charge the battery for several hours on a 6 amp charger. Let the battery set overnight, then connect the battery cable and try starting the engine. If it doesn't crank like it should, then the battery is junk.

IMPORTANT.........while charging the battery, feel the case. If it feels too warm, then you probably have a shorted cell, and should stop charging and trade it in. It's always safest to charge any questionable battery outside, where it won't damage something if it explodes.
Click to expand...


Listen to Tom.... I had 1 year old battery that I suspect a trickle charge might have actually slowly discharged the battery. Using another non-tickle charger on my 6V battery at 6A, I let it change for 12hrs, let it rest over night and did the same the next day.... That was Oct 2015 and that was the last time I attached a charger at all.. Worth a try... oh I removed the battery from the car... --call me chicken.. :)
 
just pull the battery and have a parts store ck it they will let you know if the battery is bad if not the ck the charging system
kevin
 
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