Remind me how the charging works again, please

[Dan Hunter]

Geez, it's been forever since I've been here largely because it's been forever since the truck misbehaved. In fact, going to work I thought to myself how reliable it's been. That was a mistake.

On the way home, the voltage was well below the normal indication and would dip when the turn signal operated.

I pulled over and checked the terminals giving them a bit of a twist. No help. I got home and cleaned the terminals even though they seemed serviceable. I put the charger to the passenger side battery and it would take no amps. The driver side pulled 6 amps. The interconnect NOW is showing continuity but of course that's no assurance that I had continuity earlier.

Can someone remind me how the charging works? My recollection is the driver side battery receives the charge and the passenger side provides the power. If that's correct, wouldn't a poorly charging driver's side and a fully topped off passenger side indicate a bad driver's side battery?

 

[WarWagon]

Sounds like you had a bad connection at the battery. Take them apart starting with both grounds first and clean them. I just had all 4 battery bolts come loose on me. (And I am still scratching my head over that one.)

Forget everything you are assuming about dual battery setups.

The batteries are both 12v and wired in parallel. This means one battery could burn open (fail) and have zero volts and infinite resistance and you could maybe start the truck off the one battery. Most times the battery sulfates and takes the other battery out with it.

Both batteries charge at the same time and both crank the truck. As the positive terminals are shorted together and so are the negative terminals. With that said how can the starter choose what battery to take current from and how can the alternator choose what battery to charge? Hint: the starter draws from both battries at the same time. The alternator charges both batteries at the same time. Only when a connection fails does one battery get isolated and this is a failure mode.

The grounds on the batteries are connected together through the engine block, a big chunk of iron. The positive cables are connected together with a large copper cable. This gives you a 1600 CCA 12v battery. To test the batteries you have to disconnect one of them to see what battery failed. Otherwise you are measuring both batteries. Only the most sensitive testers can sort of determine if a battery has failed by refusing to give a pass/fail reading at the dead battery when they are wired together.

If they made a common 1600 CCA battery we could get away with a single battery. But we spread the load over two batteries.

 

[orrum]

Thanks Warwagon I needed that info for my general understanding.

 

[Dan Hunter]

I'd previously stripped the housing on the passenger side terminal but not the driver's side. I did so while I charged the same side battery. It's surprising still how much that plastic hides. It showed a more nominal indication this morning.

Nice explanation, War Wagon. Thanks.

 

[JMJNet]

First thing to do is bring the batteries to be load tested.
Unless you have your own tester.

 

[WarWagon]

Disconnect the grounds first. Why? Well if you drop the ground wire and it hits the grounded sheet metal etc. - Nothing happens. If you disconnect the positive cable and it hits something - the other battery could be powering the disconnected cable and you have a problem that can roast marshmallows quick. (If not flat explode a battery - they hate shorts.) Remember everything is grounded so taking off both ground cables first will effectually remove the batteries from the system. After that you can safely take off the positive cables - because with the battery grounds disconnected a positive cable hitting something can't spark or burn because there is no current path back to the battery.