# When is a proper volage for a fully charged 12v battery?

Solar Expert Posts: 395 ✭✭
I've been reading the FAQ and doing some digging online, and the answers to this question vary from 12.6 to as high as 14.5v. I ask because I decided to test my batteries to see how much charge they had on them once the system said they were fully charged (I disconnected each before testing to ensure proper data) and the batteries are running between 13v and 13.5v each. I suspect that the minor variance in voltages is just the batteries displaying their individual personalities, so that doesn't bother me. What does grab me is their charge level. I'm wanting to ensure that they all are charging to the correct level. Especially since I can't seem to find an authoritative answer on what's the right level. I've even seen some people on here say that 14.5v is fully charged and 15.5+ is considered overcharging. Now if anything 12.6v-14.5v is considered a safe "full" charge, then I'm golden.

EDIT: I forgot to mention that I'm using AGM batteries.

• Banned Posts: 17,615 ✭✭
Re: When is a proper volage for a fully charged 12v battery?

I think you're getting the Voltage confused with Voltage set points.

Normally a 12 Volt battery is fully charge with an at rest Voltage of around 12.7 (+/- depending on manufacturer's specs).

To get there, deep cycle batteries need to be taken up to 14.2 to 14.8 (again depending on the particular battery) Absorb Voltage and held there to complete the charge.

Then there's the Float Voltage which is a level that is maintained as long as possible to keep the system working well. This is usually around 13.8 Volts and can be as low as the resting Voltage. Again it depends on the battery maker's numbers.

So it all depends on the particular battery. And how accurate your meter is. And the temperature. And the phase of the moon.
• Solar Expert Posts: 1,366 ✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

And according to the pdf recently posted elsewhere on this forum regarding lab tests on battery capacity, even the manufacturer's charging specs may be incorrect. http://forum.solar-electric.com/showthread.php?15475-Burping-the-battery
4.5 kw APC UPS powered by a Prius, 12 kw Generac, Honda EU3000is
• Registered Users Posts: 11
Re: When is a proper volage for a fully charged 12v battery?

Also, keep in mind that the results you get right after charging the batteries will be different than if you let them sit for a few hours and then check them. They need time to "settle"
• Solar Expert Posts: 4,858 ✭✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

Here is the manual for my 'new' batteries, currently enroute. There is some interesting stuff in here, like try to find a Bulk or Absorb voltage, just float and EQ values and max Amps ??? So different from my old AGM.
http://www.cdtechno.com/pdf/ref/rs_02044_0508.pdf

hth

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
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Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

steven lake,
i'm not sure what it is you are doing there. did you fully charge them and then immediately disconnect them and measure them individually? it would seem to me you shouldn't have that much of a variance, but just to be sure fully charge them while also keeping them disconnected from any loads and wait about 3hrs or so and measure them again and let us know what you see.
• Solar Expert Posts: 395 ✭✭
Re: When is a proper volage for a fully charged 12v battery?

Well, we had to move the rig outside (long story, so I'll spare you the details.) so I figured that while the batteries were disconnected (good lord those things are heavy!) I'd test them with a multimeter to see how much charge was in them and ended up with . Thus what spawned the question. I guess I understood 14.5v as "fully charged", and not the "Absorb Voltage". So with that being said, then I know that the batteries are properly charged. Now why they've got varying degrees of charge (yes, it's only 0.5v, but still) I don't know. But so far it hasn't affected things any. The only thing I have to do now is figure out what's sucking the soul out of them whenever the power goes...out, just like it did now. Argh. I so need to get this solar setup done so I can tell the power company to kiss off.
• Solar Expert Posts: 4,858 ✭✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

Steve, since you have them all separated and have got a good > 72 hr static readings on record, I would number them if not already done and then you can compare with later readings as you mention you expect, in trying to locate the parasite.

KID #51B  4s 140W to 24V 900Ah C&D AGM
CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM
Cotek ST1500W 24V Inverter,OmniCharge 3024,
2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge, Hughes1100 Sat Modem
Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
• Super Moderators Posts: 26,837 admin
Re: When is a proper volage for a fully charged 12v battery?

If you have a single shorted cell among parallel connected strings--it can discharge the entire bank (another shameless plug for the \$60 dc current clamp meter--wish I was making some money on these:p).

-Bill
Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
• Solar Expert Posts: 395 ✭✭
Re: When is a proper volage for a fully charged 12v battery?

Oh, no, no, no. I'm not looking for a parasitic battery. All of my batteries are fine. I was only testing the voltages to make sure they were charging right as I wasn't 100% sure I'd gotten the charge setup right, hence why I put a multimeter to the batteries while they were disconnected. So far everything on the DC side is 5x5. I was really just asking to be sure that my batteries were in fact charging properly. The issue with the power leech at this point appears to be something on the AC side. The thing is, I can't seem to nail down what it is. I've found the circuit its on, so now I've just gotta figure out what all is on that particular circuit and start eliminating things until I have the culprit. I've got a sneaking suspicion what it is, but I want to be totally sure before I go ripping apart anything.
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

14.5v is not an at rest voltage. i am concerned if you are charging them correctly or not or if a battery may be going bad. if the batteries were fully charged and disconnected then after letting them rest for about 3 hrs you can take another measurement letting us know what you get.

phantom loads can be a pain to trace sometimes and could contribute to your batteries going bad.
• Solar Expert Posts: 395 ✭✭
Re: When is a proper volage for a fully charged 12v battery?

The resting voltages on them were between 13v and 13.5 across all the batteries after resting. I didn't test them until at least 3 hours after they were disconnected because it took us at least that long to get them unhooked, carried up the stairs and delivered to the shed. As I remember correctly, we also took several breaks so as not to give ourselves a stroke from all the lifting. lol. Wow, lead weighs a lot. If I ever need to swap out the batteries at any time, someone else is gonna do it. :P
• Solar Expert Posts: 10,311 ✭✭✭✭
Re: When is a proper volage for a fully charged 12v battery?

that seems a tad on the high side for at rest, but the battery bank sounds like it may be out of balance. can you charge up individual batteries to get them even?
• Registered Users Posts: 1
My batteries read 13.5v but only 80 % charged in my motorhome why?
• Solar Expert Posts: 2,815 ✭✭✭✭

Hi Dav,  Welcome to the Forum.

There are several things that one needs to consider when trying to use battery voltage as an indicator of the State Of Charge (SOC) of a battery (this has probably been covered in this discussion - have not re-read it);

The battery must be Rested for a number of hours (Resting is where there has been NO charge,  and NO Discharge for hours).

The battery temperature has a fairly large effect on battery voltage,  and must be considered.

And,  battery monitoring devices are generally not very accurate in showing actual SOC of a battery  --  just too many variables that are not accounted for,  or even known.

My opinions,   Vic

Off Grid - Two systems -- 4 SW+ 5548 Inverters, Surrette 4KS25 1280 AH X2@48V, 11.1 KW STC PV, 4X MidNite Classic 150 w/ WBjrs, Beta KID on S-530s, MX-60s, MN Bkrs/Boxes.  25 KVA Polyphase Kubota diesel,  Honda Eu6500isa,  Eu3000is-es, Eu2000,  Eu1000 gensets.  Thanks Wind-Sun for this great Forum.