Truck starts, and voltage goes to 14.6. As I'm driving around, the voltage slowly drops. After driving for 10 minutes, it's down to 13.2 or so. After 15 minutes, it's down to 12.8. Shut off engine and it drops to right around 12. A couple hours later it's 11.8.

Revving the engine makes no difference.

The Battery light hasn't come on while driving.

Left to sit for 3 days, the battery won't be strong enough to start it.

Measuring amps between the negative battery post and the negative battery cable reads .01A.

With the battery removed from the truck, and fully charged on a trickle charger, it slowly drops down to 12.05 vdc, and stays there.

The battery is a sealed gell-cell type battery.

Any thoughts?

 

maybe I missed something but THE BATTERY IS SHOT

 

Hay your the guy that's posed to give out the answers. This is a test right? I'd borrow or swap battery temporary. Always a chance of two problems.

 

Battery passes a load test.



I'm just curious to see what kind of replies I get.

 

The thing that throws me is a gel cell. It's per cell voltage may be 2.0 vdc instead of 2.1 vdc like a wet cell. I've got that info over in the pile. If so the battery would be OK. Passed static and load test. But for kicks I would still swap battery if I could. If you cover everything you don't get bit

I did a quick lookup and read that a gel cell battery would read 12.8 vdc or 12.9 vdc at full charge. Better wack that meter. Or the battery.

Maybe bad diodes in Alt or bad connection on alt.

 

I forgot about this thread. The least I can do is post an update.


You were right on the money. The battery was fine, it's just that the charge drops to just over 12v, but because it's a gel cell, this is not unusual.

The drain was being caused by a short in the alarm module, but I wasn't able to identify it because the fuse in my Fluke 88 was burned out, and on my Fluke 88, when the fuse is burned out, it reads 1mA, so I thought the draw was never more than that. When I tested with a different meter, I found the problem right away.

Everything is fine now.

 

You need to put a bigger fuse in that meter. I always forget to take the meters off ohms when checking voltage. So I never trust my meters. Best 2 out of 3.

 

First fault I see in your oniginal post is the charge voltage value after starting.
It should have gone to at least 15+ volts for short time then slowly reduce to about 13.5 +/- and remain there while powering the truck needs.
I would think the regulator has a problem.
After starting, a large drain has been imposed. The voltage has to go high to force current into the battery to restore the charge level back to normal.
Anything less than this operation will let you set with a slow or no crank.
It makes little difference what the cell voltage is because the regulator is the regulating element. It has a fixed internal reference circuit set to about 13 volts +/-any tolerence.
A battery that has a lower cell voltage will just be charged more and may or may not shorten it's life.
6 cells times 2.17 = 13.02 volts.
6 cells times 2.1 = 12.6.
Most auto charge systems are set up for lead acid battery use. If some other cell config is used there is now a difference.

 

maybe alternator isn't charging it up? if not, you have the chance to get an optima battery now

 

You know, it's kinda funny; one of the first things I did after buying this truck in '96 was install 4 different AutoMeter Sport Comp mechanical gauges, including one for Volts, yet I never really paid that close attention to it, or so it seems. It may have always acted the way it does now, and I just never noticed until I started having other battery issues.

I plugged my DMM into the cigarette lighter socket for a more accurate measurement while driving, and I am yet to see it surpass 14.6 after starting up, but since finding the short, it never drops and remains below 13.2 while running.


The per cell voltage was important for understanding the voltage after being charged out of the vehicle. A fully charged battery should maintain 12.6 volts, and this one consistantly dropped to 12.1, but gel cells often will only maintain 2.0 volts per cell on the shelf.

 

There is consideration for how far the voltage should rise after a starting load.
It depends on the battery capacity, it's age/health and the regulator performance and in cases where the battery is not lead acid type.
The readout gauge accuracy is also a factor.
An over capacity battery that cranks the engine well will not lose so much capacity at starting thus requireing less recharge and recovers much faster.