winter battery performance

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  • samuel
    samuel Solar Expert Posts: 80 ✭✭✭✭
    #62
    Re: winter battery performance

    Of course cold batteries will not meet their ah rating of 100% soc - this was never the question.

    If 1.277 is achieved after equalization at 80°F and
    if 1.264 is achieved after eq. at 45°F
    are the batteries considered to be at full charge in each scenario despite decreased capacity due to the cold?

    Is it normal to have lower specific gravity in winter?
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  • Volvo Farmer
    Volvo Farmer Solar Expert Posts: 209 ✭✭✭✭✭
    #63
    Re: winter battery performance

    I looked at that data, perhaps I am missing something.

    Are you only really testing the SG of a few individual cells and assuming that the rest are OK? I only saw the same 4 or 5 cells tested, out of 42 cells in your bank over the last 60 days.

    Also, when you equalize your batteries, are you equalizing all those parallel strings together?
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  • samuel
    samuel Solar Expert Posts: 80 ✭✭✭✭
    #64
    Re: winter battery performance
    Are you only really testing the SG of a few individual cells and assuming that the rest are OK?

    Yes. Last time I went through all the cells was about 5 months ago and everything was OK. Going through every single cell didn't seem necessary at this time since I had this same issue last winter - I just didn't have the battery monitor to back up the observation.

    For better or worse, the batteries are equalized all together.
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  • Vic
    Vic Solar Expert Posts: 3,208 ✭✭✭✭
    #65
    Re: winter battery performance

    samuel,

    First, SGs need to be temp corrected, because float-type Hydrometers measure density of liquids. The acid in the electrolyte is more dense than water. The more acid (higher SG) in the electrolyte, the more dense it is. The density of fluids like the electrolyte changes with temp changes. Therefore, there needs to be a temp compensation. This is not a huge difference for reasonable changes in temps, but should be accounted for.

    And, you are seeing one of the many, many reasons that running very many parallel strings (to get desired AH capacity) is a bad idea. ALL of those darned cells to water and measure the SG and so on.

    One thing that makes things a bit more difficult for those who are trying to help you, is that there you make a statement about all of the SGs being "OK" 5 months ago. However, just what OK means is subject to intrepretation ... one person's OK might be almost all cells at or above 1.240 mabe a 25 point variation. To another, 1.260 minimum and 10 point variation.

    One reason that your winter SG readings are lower in the Winter than Summer, might be that the battery bank is not getting fully charged in the Winter .. a common problem ... less sun.

    Also, this might have been noted before, many inverter/chargers and CCs do not temp compensate EQ voltages. THe EQ voltage on your chargers might need to be manually compensated by YOU when doing an EQ.

    And so on. None of this is a slam-dunk. Some of the fine details matter, some of them matter a lot.
    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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  • samuel
    samuel Solar Expert Posts: 80 ✭✭✭✭
    #66
    Re: winter battery performance

    I noted that the EQ this time around was hitting 15.8 / 15.9V instead of the usual 15.5V for summer. The batteries were all within 0.005 of each other last time I went through each cell.

    I EQ'd for 1 hour, then attempted a second EQ after a few minutes because the SG was only about 1.264 (1.275 was achieved in summer / 80°F) - which cut out after 18 minutes. I attempted to restart the EQ but the inverter/charger would not allow me to. I took this to mean that I couldn't put any more current in to the batteries.

    I'm finding mixed information on how to temperature correct SG readings. As far as density is concerned, water increases in density as temperature drops (only .004 increase between 32°F and 80°F). The electrolyte should be something like 36% acid, the remainder water when fully charged. Correcting a 1.264 to a 1.250 or so does not make sense at (42°F) when I get 1.275 at 82°F. This suggests that I should be correcting up for cold weather, and 1.264 should be adjusted to 1.278 or so.

    So what is going on?

    Do I add 0.003-0.004 for each 10°F below 80°F (1.264 adjusts to 1.278 ) or
    do I subtract 0.003-0.004 for each 10°F above 80°F (1.264 adjusts to 1.250)?

    And most importantly - has anyone measured an SG at full charge that is HIGHER in cold weather than in warm?
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