15.2 volts at the cigarette lighter.

[Andy D]

Measured with an old Fluke meter. Wow. Alternator is singing at high idle. Poor old Valuecraft battery was stone dead. The hood had 2 1/2 feet of snow on it. I'm in no hurry, I cleared the snow away last weekend. When I got the door open last Sunday afternoon, the dome light light didn't work. Oops, I did it again. I rigged the charger to the lighter leads, set it on 2 amps, and went in. On my way to work Monday, I tried it. It started right up. Friday, I took the rear end off the stands, and backed it up onto the drive way after a 2 month vacation. I let it run for awhile. The tires are full of summer air and need pumping up. I put the meter across the leads where I hooked the charger. 15.2 volts ! Is this over charging? Am I hurting the alternator with a bad battery? How much?

 

[RonD]

Yes you are not doing any good for the battery or electronics above 15volts.

A temporary spike above 15v may happen just after startup as alternator recharges battery after the drain of starting but it should drop to the standard 13.6v-13.8v in less than 5 minutes.
The alternators output voltage is controlled by the voltage regulator in the alternator, it feeds from 5 to 9volts to the rotor, the spin provided by the engine increases that voltage, so your voltage regulator is sending max voltage to the rotor if you are seeing 15volts.
That could be because the battery is bad or because the voltage regulator is.

While this is not an approved testing method, i.e. tester beware, with engine running and voltage at battery showing 15v, disconnect negative battery cable, and test voltage between negative Cable and positive cable, if it has dropped to 13.7v(approx.) then yes battery is bad, if it stays at 15v then regulator is bad.
Shut off engine BEFORE reconnecting negative cable, in either case.

You can also just test the battery voltage after it has been sitting a few hours
12.7v is a new battery
12.4v is a 3-4 year old battery
12.2v is 5+ years old and time to shop for a battery sale, it will die in cold weather

 

[Andy D]

Thanks Ron D. I'll take a reading tomorrow and see how good the battry is.

 

[kimcrwbr1]

It is never good on a charging system to charge a dead battery. Fully charge the battery then test the system. For a alternator with a internal regulator it should not be 2.5 volts over battery voltage. and a external regulator it should not be 2volts over battery voltage.
After a full charge measure battery voltage say 12.7 volts. Start the motor and run at high idle about 2000 rpms for a few minutes and measure battery voltage it should not be over 15.2 volts. Then turn all the lights, A/C, accys on and measure the voltage at 1500 rpms it should be no less than .5 volts over battery voltage or 13.2 volts. If you pass those two tests the charging system is working properly.

 

[Andy D]

I charged the battery for 16 hrs at 2amps. It started the truck easily. It is a small cheap battery that is 4 yrs old. The battery slowly goes soft because it sits. I have to charge the battery because the truck sits for weeks at a time. Imagine my surprise when I checked the voltage, this morning, 12.6. Something is gonna have to break before I fix it

 

[4x4junkie]

If it's very cold out, 15.2V (or even higher) at startup is actually quite normal. However as said, it should slowly drop down a bit to maybe around 14.6-14.8 after the engine is warmed up (but while still cold outside).
The VR is temperature-compensated to facilitate better charging of the battery because the chemistry of lead-acid batteries as a general rule requires higher charge voltage when cold than it does when warmer (15.2V on a 85°F day would be more cause for concern).

Also, FWIW, the battery's resting voltage correlates much more with it's state-of-charge, not it's age. A battery that's 6 years old (but otherwise healthy) can just as easily have a 12.7V resting voltage as one a year old.

A better test of a battery's health is to load-test it (or do a capacity test if it's a deep-cycle type). If the battery has removable caps you can also get an excellent idea of it's health by checking the specific gravity of each cell with a hydrometer.


Something you might consider getting is a Battery Tender or a similar "maintenance charger" if the truck sits unused a lot (a solar one is fine if there's no AC power nearby). This will help the battery last much longer, as sitting at anything less than full charge for an extended period greatly shortens the life of any lead-acid type battery.

 

[kimcrwbr1]

Clock/radio memory and computer memory provide a constant battery draw you can put a disconnect switch for the battery if a trickle charge option not available.