I've had a continuous (15 year) battle with my batteries not lasting more than 3 years. I tried replacing the alternator, but it didn't change anything. The alternator puts out 14V with the engine running, so it and the voltage regulator are doing their job. Yet as soon as I turn off the engine, the battery voltage shows 12.1V, which tells me that the battery did not accept the charge from the alternator.

The battery has the proper water level. The only thing I can think of is that the wiring between the alternator and the battery is so corroded that it simply does not allow enough current flow to charge the battery. I'll check on it this weekend by putting a charger directly on the battery terminals and see if that brings it up to 12.6V. But I don't expect that to be the cause, because even after 4 hours of freeway driving, with the alternator putting out 13.7-14V, the voltage instantly drops to 12V when I shut off the engine.

I've pretty much accepted my fate of replacing the batteries every 3 years, but I do wonder, as a curiosity - what could be the real issue?

 

 

The car is 100% stock. In the past I've been worrying about parasitic drains, but they never were significant. Moreover, what I'm seeing now has nothing to do with parasitic drains - what I'm seeing is that the battery is not accepting the charge to begin with, not that it's discharging.

I just can't understand the physics behind why a battery would not charge above 12.1V despite being pumped with 14V over several hours.

Like yours, my batteries have a 3 year free replacement, so this has not been costing me anything other than increased price for the batteries every three years. But it bothers me to be wasting batteries like this.

 

That corroded wire can be exactly the problem. Corroded battery connections can prevent the charging system from topping off your battery when you are driving, thus creating a deep cycling issue which can shorten the life. Also, could be a minor parasitic drain causing the same issue I just mentioned. I say, replace that wire and/or do The Big Three. This should fix your issue.

Another cause could be a slipping belt not turning the alternator enough, however, I'm willing to put money on that corroded wire. Corroded terminals and loose terminals all cause the same issue.

 

Good points, thanks. I don't think it can be the belt slipping on the alternator, as it's putting out a constant 14V, so it's generating the voltage at the source.

The cable to the battery is definitely on my to-do list. The other test is like I said earlier, putting a charger directly on the battery, thus bypassing all of the vehicle's wiring and cabling. If that brings it to 12.6V, then I'll know that it's the cabling. Although like you said, it is possible that at this point the battery has been damaged and will never get to 12.6V. In that case I'll put in a new battery and see if it accepts charge, either from a charger or the alternator. I'll report back after i try charging the battery with the charger.

Thanks again.

 

I believe 12.4v is the magic number.

 

I've always seen 12.6 quoted as fully charged battery, but I'd accept 12.4 at this point .

 

Did you check the charge status of each cell with hydrometer? You are reading proper voltage from the alternator but how about charging current level?

 

Right, I didn't check the current actually flowing to the battery, I don't have the equipment to do that. That's why I have to guess as to whether the current is not flowing to the battery (i.e. bad cabling) or the battery is not accepting the charge (i.e. bad battery).

I can check it with a hydrometer, but it should be consistent with the resting voltage. The only thing it would tell me is if one particular cell was bad. But that still won't tell me why, which is what I'm really after.

 

I went ahead and checked each cell, they were all right around 1.150, i.e. a dead battery. So I replaced it with a new one, and will now keep an eye on the voltage to see if the battery gets/accepts a full charge or not. If not, then most likely it would be a wiring issue, since the alternator is putting out the correct voltage and the battery is now brand new..

 

The numbers I mentioned above are not voltage per cell, but specific gravity. 1.150 is about 25% charged.