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

  1. #1
    Join Date
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    Default When is a proper volage for a fully charged 12v battery?

    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.

  2. #2

    Default 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.
    1220 Watts of PV, OB MX60, 232 Amp hrs, OB 3524, Honda eu2000.

    Ohm's Law: Amps = Volts / Ohms
    Power Formula: Watts = Volts * Amps

  3. #3

    Default 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/show...ng-the-battery
    12 kw Generac, Honda EU3000is, 4.2 kw APC UPS powered either by battery or a Prius. Really.

  4. #4

    Default 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"

  5. #5
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    Default 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
    100% Off Grid @ 51* 46' N lat 124* 44' W long

    New House system: coming - 2 arrays @ 2240W
    CL150&Epanel 2 @140 W 12v PVs , 24V C&D AT-15P AGM @ 950Ah
    + CL150 2@120W 12 V panels, 24V C&D AT-15P AGM @ 950Ah
    Cotek ST1500W inverter, TBS 30a-24v Omni-charger, Honda Eu3000is,

    Guest cabin system: 3 - 70W panels to SB 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter

  6. #6
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    Default 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.
    NIEL

  7. #7
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    Default 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. :(

  8. #8
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    Default 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.
    100% Off Grid @ 51* 46' N lat 124* 44' W long

    New House system: coming - 2 arrays @ 2240W
    CL150&Epanel 2 @140 W 12v PVs , 24V C&D AT-15P AGM @ 950Ah
    + CL150 2@120W 12 V panels, 24V C&D AT-15P AGM @ 950Ah
    Cotek ST1500W inverter, TBS 30a-24v Omni-charger, Honda Eu3000is,

    Guest cabin system: 3 - 70W panels to SB 2000e CC, with 2 - 100 ah 12v SAFT wet NiCd's , 600W TSW Inverter

  9. #9
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    Default 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).

    -Bill
    20x BP 4175B panels (replacement) + Xantrex GT 3.3 inverter for 3kW Grid Tied system + Honda eu2000i Inverter/Generator for emergency backup.

  10. #10
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    Default 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. :)

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