Battery bank reaction

stmarstmar Solar Expert Posts: 362 ✭✭✭
I think this is a good sign but wanted to confirm. The grid went down and by the time I realized it the bank had dropped to 25.4 volts with a 4 amp AC load. It was getting late and raining so I thought it would be a good time to test "load shedding". I disconnected the frig & freezer, tv & sat. I left the phones and clocks on and the AC load dropped, going between 0 - 1 amp. In a very short time the voltage went up to 25.7 and maintained that until the grid came back up, about 2 hours. Am I correct that it is a good sign for the bank to increase with no charging once I shed the load? And it did not take the full bulk time to get it to float, I have it set on 2 hours but it went to float after about an hour. I think the SW4024 must have a sensor that tells it when the bank is optimal, my old bank took the full 2 hours when it went to bulk. Any SW4024 users have a similar experience with a new battery bank??


  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery bank reaction

    That is perfectly normal. It is the inverse of the Voltage sag seen when batteries are heavily loaded.
  • waynefromnscanadawaynefromnscanada Solar Expert Posts: 3,009 ✭✭✭✭
    Re: Battery bank reaction

    Both discharging and charging will throw off the voltage reading of a battery bank. It is recommended that the battery be left alone to rest, neither being charged or discharged for aprox 2 hours before voltage readings can be used as a rough state of charge indication. The heavier the discharge at any one instant, the lower the voltage reading will be regardless of battery state of charge, likewise the harder the battery is being charged, the higher the voltage will be during that charging. It would be totally normal for battery voltage to rise when a heavy load is removed.
  • stmarstmar Solar Expert Posts: 362 ✭✭✭
    Re: Battery bank reaction

    Thanks, that is what I thought but it is good to get confirmation. I did not get a chance to use my generator, raining too much and the grid came back before it was necessary. A steady 25.7 seemed okay for my battery bank with a light load of less than 1 amp. These batteries are not abused, stay in float most of the time, and the rare times they are discharged it is not for a great duration. I appreciate all the input and information on the forum, it helps make decisions about these systems.
  • BB.BB. Super Moderators, Administrators Posts: 31,160 admin
    Re: Battery bank reaction

    This will give you a rough idea about how battery voltage varies under state charge and various loads/charging currents from the Working FAQ thread:

    Working Thread for Solar Beginner Post/FAQ

    New poster "leaf" has a really nice set of charts that compare battery voltage against different rates of discharging and charging (as well as resting voltage readings).
    leaf wrote: »
    Am trying to upload the charts I am using...

    Attachment not found.Attachment not found.



    [note charts are from: I believe those charts are from Home Power #36, August- September 1993. Lead-Acid Battery State of Charge vs. Voltage ©1993 Richard Perez.
    Here is a link:


    I don't quite a agree with the resting voltage line (I think the voltage is a bit low)--But it shows how to estimate a battery's state of charge while operating.

    Note, where the charts "flatten out"--the room for error estimating state of charge is pretty high.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • PhotowhitPhotowhit Solar Expert Posts: 5,416 ✭✭✭✭✭
    Re: Battery bank reaction

    I would hazard to say the charts Bill posted will draw more questions than they will answer, unless you understand that the curves represent different rates of discharge as a percent of the battery capacity, so 1/5C would be 1/5 of the battery capacity in amps.
    Home system 4000 watt (Evergreen) array standing, with 2 Midnite Classic Lites,  Midnite E-panel, Prosine 1800 and Exeltech 1100, 660 ah 24v ForkLift battery. Off grid for @16 of last 17 years. Assorted other systems, and to many panels in the closet to not do
  • westbranchwestbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: Battery bank reaction

    A better test, next time you have an outage will be to let 'er rip and not load shed so you can see how your batteries react to a 'full house' load, ie normal usage for 3 hrs or more. Better yet shut off the grid and simulate an outage, monitoring every 1/2 hr and log it all. Also do SGs at each logging interval.

    Have fun.
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • inetdoginetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Battery bank reaction
    westbranch wrote: »
    Better yet shut off the grid and simulate an outage, monitoring every 1/2 hr and log it all. Also do SGs at each logging interval.
    Just do not do what a building engineer I know did and run a test in advance of a scheduled outage and not allow enough time for the batteries to charge back up.:cry:
    SMA SB 3000, old BP panels.
  • stmarstmar Solar Expert Posts: 362 ✭✭✭
    Re: Battery bank reaction

    As the summer, very busy summer, winds down I may take your advice and do some "experimenting" before the winter snows fly. SG is NA since they are AGMs. My old bank ran the essentials; heat, refrig, etc. for 3 days 5 years back and the voltage dropped to 23 and they rebounded and lasted 5 more years. These are Lifeline AGMs and the old ones were Concord Sunextenders same manufacturer. From all of my research these were the best for my application and I went from 12 volt to 6 volt to simplify the system, one string, although I did drop the amp hours by 100. Since I am using it as a UPS for short periods of time I think that 400 amp hours is adequate, plus I now have my trusty Honda to charge if there is a long outage and the sun is not cooperating.
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