"dumb" battery / temperature question

couchsachraga
couchsachraga Solar Expert Posts: 87 ✭✭
in Off Grid Solar & Battery Systems #1
I'm assuming this is a dumb question because i can't make the math work... and therefore I'm assuming I'm missing something!

As I've delved deeper in to my battery charging volts problem (charging at over 15.5v, at which point the inverter stops working shutting the heat loop down so it gets colder still...) I looked at the temperature compensation numbers.

The batteries (Trojan 1275) state a 5mv/ degree C temperature change (or .028v / 5.55C)(which is what i had programmed in; I've temporarily lowered it (figuring better than than turning it off...) while I keep looking for diodes). So, rather than 77 F (25 C) let's say it is zero C. I would assume a 125mv increase in the battery charging voltage to achieve full charge (so rather than 14.8v it would be 14.925). So given all the other parameters from the batteries, including 5mv / degree C, why does the Midnite Classic bump it up to 15.5? I've noticed other folks having similar problems, so I expect it is not the Midnite's problem but instead my understanding of it all....

Thank you for any light you can shed on this... it's driving me crazy not understanding it!!
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #2
    Re: "dumb" battery / temperature question

    Personally, I agree with the Classic.

    Most FLA batteries do have a temp compensation of --.005 V/CELL/C. So your 12 V battery is composed of SIX cells. So that is -- 0.030 V/C. So, at 0C this is 25 X 0.030 V, or 0.75 V. So the battery compensation would be your 14.8 + 0.75 V, nominal, or about 15.55 V nominal.

    One major disadvantages of 12 V inverters, in general, is that they shut down at far too higa a battery voltage, in cold conditions, IMO.

    Good Luck, 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.
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  • couchsachraga
    couchsachraga Solar Expert Posts: 87 ✭✭
    #3
    Re: "dumb" battery / temperature question

    Ah, that would do it! I can't see anywhere on the Trojan site that it talks about PER CELL, but that makes sense.

    And I know the classic is correct... I just couldn't figure out what was missing! Thanks!
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