: Battery Voltage When Cruising

09-05-11, 12:01 AM
I was wondering what other people are reading for battery voltage when cruising. I will start off around 13.9V, and then it will slowly drop off after about 10-15 miles to around 12.8-13.0V. To me this seems a little low for running. Could this be a sign of an alternator/voltage regulator or the battery on it's way out?



09-05-11, 12:58 AM
What you see is a voltage regulator compensating for the rise in temperature of the engine compartment and the battery becoming fully charged after the discharge of the last start; in other words it's completely normal.

09-05-11, 05:28 AM
As dk said, completely normal; this is actually saving a little gas (as the alternator isn't working as hard, after attaining battery full charge).

09-05-11, 12:04 PM
+1 on the first two replies.

The last thing you want, for battery longevity, is to overcharge it. This boils off the water (electrolyte), and even if the battery has caps, 99.9% of people don't check it, anymore.

Also, FWIW, even 'sealed' batteries, unless it's an Optima or the like, has fill caps, they're just hidden under the MAINTENANCE FREE sticker. Peel that sucker back, and check the level every few months. Distilled water ONLY.

Also, while it's not a problem on these cars, if you have a problem with battery terminal corrosion, the white fluffy stuff, add about 1 ounce of mineral oil to each battery cell. I'm not pulling anyone's leg. This really does work. Something about the oil traps the acidic gas during charging. It does not prevent water from vaporizing, but it'll stay clean!

Mineral oil you can get at the drugstore. Also, if you have a vehicle still using R-12 refrigerant in the A/C, this is the oil used. Can't find R-12? You can use propane...yes, like what is in the camping stove bottles at the grocery store. No, it doesn't explode if you hit something, hard. BTDT, unfortunately.

The batteries in my wife an I's large motorhome are going on ten years old using desulfation methods (sounds like trickery, but it's not), and regular maintenance.

Flooded-cell batteries should, with proper attention, last until there is a physical failure. IOW, in ideal physical conditions...forever. Cracked cases, broken terminals, and loose internal plates happen when they get moved/shaken about (like in a vehicle). The OEM battery in my wife's RX300 lasted 8 years, but it, strangely enough, had caps on it. Eventually lost a plate, shorting one cell out. Still managed to hold enough of a charge to start it...barely.

09-05-11, 02:46 PM
Thanks for the replies guys.

I'm no stranger to adding water to my batteries living in the Arizona Desert. My last car, I had a battery last Just over eight years. When I took it in for a new battery the guy at autozone was amazed. He said he had never seen a battery that had run out of the 7year pro-rated warranty :)

09-05-11, 03:14 PM

It's like the ultimate 'recycling', getting more use than expected.

I know how it is out in the desert. Phoenix makes Vegas, where we lived for a few years, look temperate.

09-05-11, 03:22 PM
It's actually pretty nice here today, I think it's only going to be 104 ha ha