Using my batteries wisely?

CVN-71CVN-71 Solar Expert Posts: 54 ✭✭
I have a off-grid PV system with 8, 200ah batteries. I have just been using it as a back-up system in case of grid power loss. The array is producing up to 5kw a day and I want to use it now to power a few small appliances which I will take off the grid, small fridge in garage and my aquarium. Given how the batteries degrade with each cycle, assuming I use 5% of the battery capacity each day, would the small savings off the electric bill offset this loss of life on the batteries? Any other ideas/tips how to use the wasted energy when the system is in float? Thanks.


  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Using my batteries wisely?

    You did not say what your battery bank voltage is (and the series/parallel nature of the battery wiring).

    Or if this was flooded cell or AGM.

    From what I understand, it may be even a better idea to take the batteries to 75% state of charge once in a while (once a week/once a month/etc.???)--The batteries actually can last longer with some deeper cycling.

    Also, what is the charging profile like (Absorb/Float charging voltages/time on absorb/etc.).

    Do you have a hydrometer (if flooded cell) or a Battery Monitor (almost a requirement for AGM/Sealed batteries).

    Less important to your question but perhaps helpful--Array specifications, wiring setup, and MPPT/PWM type controller (brand/model).

    Do you have a DC Current Meter to inspect parallel battery strings (if present) for proper current sharing during charging/discharging? Check battery (or cell) voltage during charging/discharging/rest to see all is "balanced".

    Checking the battery bank operation once a month or so to catch any problems before they damage the bank (poor connections, open/shorted cell, etc.

    If Flooded Cell, checking and logging all the cell specific gravity once a month. Logging maximum SG after equalization (2-6x per year typical for flooded cell).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,433 ✭✭✭✭
    Re: Using my batteries wisely?

    The batteries will degrade regardless. You mint just as well use them. Design a target loa of perhaps 20 percent daily draw. To produce 5 kwh and not use it is too bad.

  • CVN-71CVN-71 Solar Expert Posts: 54 ✭✭
    Re: Using my batteries wisely?

    Unfortunately, our electric company does not allow grid tie so I have just been storing energy in the batteries waiting for power outages. We are at work every day till 7:30pm and the only thing we really have on when we are gone is the aquarium, a samll Estar fridge in garage and a Estar fridge in house. I could certainly run all three of these off the 1.6kw panels daily while absorbing, and then the battery bank, (8 200ah 6v AGM's at 24v serial/parallel) for the 3-4 hrs without sun. I'm a newbee, so not sure how much the batteries would deplete at the same time they are in full absorb. Is there a way to calculate this? How much do they use when absorbing at the same time? Does this question make sense? How does this effect the number of cycles they will have over their lifetime/
  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Using my batteries wisely?

    go ahead and use some of it as i'm sure that that little bit of cycling will help battery life to a small degree. my older sunxtender does seem to like to be drained a bit and then brought back up rather than just float charged all of the time. incorporate this even once a week minimally to pose as a systems check. you can do this every day if you like too as whatever is powered will not be on your bill even if a very small reduction.
  • BB.BB. Super Moderators, Administrators Posts: 32,005 admin
    Re: Using my batteries wisely?

    As far as the batteries are concerned... During charging:
    • Battery current = Solar panel current - load current
    The interesting question is how do you configure the XW inverter and the XW charge controllers...

    Normally, the XW solar pv charge controller is set to charge the bank to ~29 volts, and the XW hybrid inverter shunts any power over ~26 VDC to the GT inverter function and pushes it off to the grid.

    In your case, do you configure the XW solar charger to maintain the charging voltage as:
    • absorb triggered when the batteries drop below ~25.4 volts--battery being discharged
    • when XW solar charger takes bank back to "optimum absorb voltage" of ~28.8 volts for 1-4 hours or so (timer based absorb timing--probably closer to 1-2 hours if you don't deeply cycle the batteries in summer, perhaps closer to 4 hours in winter and/or deeply cycle)
    • XW solar charger drops back to float per absorb timer (or other criteria) ~27 volts till night fall.
    • batteries discharge to support grid (as XW hybrid inverter programmed to do).
    • Cycle starts over again next day.
    Any way, that is roughly what I would be looking for in the manuals.

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