Battery capacity cold weather and battery monitor

offgrid me
offgrid me Solar Expert Posts: 119 ✭✭
in Off Grid Solar & Battery Systems #1
I have a 450ahr 48v battery bank that is connected through a victron battery monitor. What is the best way to compensate for cold temps when looking at the state of charge indicated by the battery monitor. Should I reset the total amp hour capacity in the monitor to reflect the average battery temp I expect. IE 80% of 450 for an average temp of 50 degrees. I have not installed my insulated battery box yet so there have been some moderate swings in batt temp and I dont want to have to reset the monitor every week. Any thoughts.
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  • samuel
    samuel Solar Expert Posts: 80 ✭✭✭✭
    #2
    Re: Battery capacity cold weather and battery monitor

    I've also been pondering this same question. Summer to winter for me (batteries are in an insulated structure) brings a drop from 60-70°F to 30-40°F. This is the first year I've had a battery monitor (BMV-600s). I'm wondering if I allow a 50% SOC in summer if shooting for a low of 70% in winter is appropriate?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,613 admin
    #3
    Re: Battery capacity cold weather and battery monitor

    Good questions... Looking at a generic battery capacity vs temperature graph at Deep Cycle Battery FAQ, it appears near freezing, the battery capacity is about 80%.

    And if you plan on no more than 50% discharge, that would be a -10% or 40% state of charge... I probably would not worry about extra 10% discharge.

    First, because you are running the battery at -25C below room temperature, the rule of 10C vs 2x change in life, the battery will last >4x longer (longer storage life, and 1/4 the rate of sulfation...).

    So, I would not worry about it a lot. Most battery vendors consider a battery to be at end of life when it only has 80% capacity left... You would not be tanking it below 40% state of charge, so you still have reserve capacity anyway.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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