Charge rate

Will2020Will2020 Solar Expert Posts: 62 ✭✭✭✭
What would be the minimum charge rate on 315 ah worth of batteries?

I have a 135 watt panel. Is this enough to cover the minimum?

I highly doubt I will use more than 20 amp hours per day, and seriously doubt I will ever go below 80 %.

Also, will it hurt the batteries to charge them with a converter while the sun is out?


  • BB.BB. Super Moderators, Administrators Posts: 31,884 admin
    Re: Charge rate

    Depends on the type of batteries--but 5% of the 20 Hour AH rating is a good minimum. 10% or so is good nominal for large flooded cells.

    One way to calculate the charge rate (assuming 12 volt bank):
    • 315 AH * 12 volts * 1/0.77 panel+charger derating * 0.05 rate of charge = 245 watts of solar for 5% rate of charge
    AGM's have a lower self discharge rate, and don't need the bubbling of charging to mix stratified acid--So you could go lower than 5%...

    But, most AGM's still have the issue of sulphation (crystallization of sulfates below ~75% state of charge for days/weeks/etc. at a time).

    A large bank with small solar array taken well below 75% state of charge is going to take a long time to recharge without firing up a genset or other alternative means of recharging. That is why I, personally, don't really like to see very large battery banks with small loads and/or solar arrays.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Charge rate

    you might get away with it, but you would need to stir the electrolyte periodically (maybe once every week or 2) and you'd need in the area of about 10% to do that or roughly 31a for your battery bank. if you can't do that then get some more pvs if you want to keep that battery for a good lifetime..
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Charge rate

    It might keep the battery in float, but if you are loading it, you are going to be way short. Too much battery, not enough PV.

    135 watts might put out ~6-8 amps on a perfect day. At a bare minimum you would like to see at least 15 consistently,,, and ideally more.

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