Uneven Battery Voltage

[97XJSP]

I just put two new batteries in my truck, but one was put in about 3 months earlier and run maybe four times for a short period, with a not-so-great battery. I know it's good practice to change them in pairs, but I just couldn't afford two at the time. When checking them with a volt meter, I noticed while running that one was at 13.6 (the one that was put in first) or so and the other was at about 14.4(the newest of the two). The voltage gauge is dead-on the tick mark too. I would imagine that they should be the same, any ideas on this? Will they even out eventually? I'm not very familiar with how batteries behave in series. I'm thinking that since both are close to acceptable voltages it won't be an immediate issue, but it doesn't seem right to me. I can't think of any reason for it other than differences in batteries...

[97XJSP]

I should add that both batteries are the same brand/ type etc.

[Barrman]

I would suggest checking all the connections and cables just to make sure it is actually getting all the charge it can. I would also suggest you charge each one individually to get them both topped off. Then try your running voltage test again. Should be even with the truck running by the way. Preferrably 14.4 on each.

[rboltz]

I would also suggest you charge each one individually to get them both topped off. Then try your running voltage test again. Should be even with the truck running by the way. Preferrably 14.4 on each.

I was to suggest the same. I still do that from time to time as with everything, batteries will always have slight differences in manufacturing, even the same brand/size. We always ran equalization cycles on large serial connected battery banks in substations and the like. One or more will charge ever so slightly faster then the other and you need to top them off occasionally.

[jeeper]

We series batteries regurlary in the security industry. It is not uncommon for two batteries to have different voltages. If one battery has 13.6 and one has 14.4 you should end up with 13.6+14.4 which would equal 28 volts. (unless you use kentucky math as in the other post on here today). There really isn't too much concern for the lower voltage battery draining the higher voltage when the batteries are at rest. They will even out after a little load has been placed on them in series.

Batteries in parrallel would be where you would need to be concerned if you didn't have some way to isolate them from each other while they are not in use.

[Barrman]

Basically, figure out why one is lower than it should be. 28.8 volts running is what the alternator should be putting out. Is there a 12V plug attached to the low battery for cell phone charging? Does that battery have dirty terminals? Are the wires all corroded inside the insulation? Stuff like that should be checked. Load testing both batteries by themselves would also be a good idea.

If everything checks out, switch the batteries and see if the low voltage stays with that battery.

The alternator has an adjustment on it which will allow you to raise the charging voltage up to 28.8. Just be careful because a little turning goes a long way.

[97XJSP]

Thanks guys. I'll have to check it out later, things have been pretty crazy at school lately so I haven't had much time... I'll let you know what I find out when I finally get a chance.