Battery Absorb Time

fireflyfirefly Solar Expert Posts: 66 ✭✭
Looking for recommendations, I am shuting down my system for the winter, only the MX will be left on to maintain the batteries. Should the MX be set for a short or long absorb time? In the past I left it set on default, 2 hours, but seemed to use more water then expected, the MX has a battery temp. All advise is appreciated.



  • RCinFLARCinFLA Solar Expert Posts: 1,420 ✭✭✭✭
    Re: Battery Absorb Time

    If there is no significant consumption to discharge the batteries then set the absorb time short to avoid water usages.

    Other question is how long does it take to get from float to bulk voltage. This depends on solar charge current against battery size. If that is taking a long time then it will all cause increased water consumption.

    You would be fine if all you do is maintain float level through the unused period.
  • fireflyfirefly Solar Expert Posts: 66 ✭✭
    Re: Battery Absorb Time

    Thanks for the info, if batteries are at approx. 60% first thing in the mourning and it is a sunny day and minimual load, they will go to Float by approx. 1PM and say that way for the rest of the day. Most of our consumption is at night. Very bad weather now, batteries will be approx. 60% when we leave and the forecast is no sun for days, not sure how long it would take to bring up to 100%, just left in Float.

  • BB.BB. Super Moderators, Administrators Posts: 32,216 admin
    Re: Battery Absorb Time

    Ideally, get a genset on the batteries and charge to at least 90% state of charge... Sitting below ~75% State of Charge will encourage sulfation and early loss of battery capacity.

    Placing the charge controller on float (which depending on the set point voltage) is not really intended to recharge a battery bank--may extend the time the batteries will set below 75% of charge.

    I would either leave the system as is (assuming you don't boil batteries dry in a month) or set the absorb time down to the minimum recommended.

    You are heading into winter where the total amount of sunlight may be severely limited (in your area?) and there just is not that much power to pump into the battery bank anyway.

    Will you be visiting the setup once per month (check water levels and such) during the winter--or will it be left alone... Also, how cold will the batteries get (i.e., near Zero F or below?--undercharged batteries freeze at warmer temperatures)?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • Dave AngeliniDave Angelini Solar Expert Posts: 6,076 ✭✭✭✭✭
    Re: Battery Absorb Time

    The Surrettes you have a huge resevoir of water. The downside I suspect of winter where you live is the battery could sit close to absorb voltage for too long (low sunlight) and use up your water. I would set for float only if I was not going to be back for 4 months and a normal absorb time if I could get back monthly.

    Do a nice 2 hour EQ at 15.5V before you leave. Good Luck!
    "we go where power lines don't" Sierra Mountains near Mariposa/Yosemite CA
    E-mail [email protected]

  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery Absorb Time


    I have a couple of off grid battery systems that sit for 10 months at a time, especially over the winter. I just top up the water before I leave them, and leave the system configuration as normal. The batteries go through bulk, to absorb and then they bounce back and forth between absorb and float most afternoons. With the battery temp sensor on board the batteries just chug along just fine. When I open up the battery shed, the controller is usually in float, the water levels are fine (since the batteries never get a big charge current since the voltage is always quite high).

    Since you have no winter loads on the batteries (I assume) then all you are doing is overcoming self discharge. I assume that the MX series is "smart" enough to keep the batteries healthy, and if they are just floating, they should use just a tiny amount of water over the winter. As for freezing, a fully charged battery won't freeze until it is way colder than -40. Even a 1/2 charged battery won't freeze at 0f/-20C. So if there is some charge going into the batteries they won't freeze. (I have left vehicle batteries fully charged in the shed, off any charge source for 6 months and they were fine. (Not recommended however!)

    So as you close up the cabin, get the batteries as full as you can before you shut down. They will come up again in the next few weeks (you will get some sun) so if they are ~80-90% I wouldn't worry. You have enough Pv so that even with dark grey days, you will get SOME charge into the batteries. ( My little 300 watt system puts out ~1.5- 2 amps on an overcast day for ~4 hours, not an insignificant amount of power. We haven't had sun in 3 weeks! I have had to run the genny three times in the last couple of weeks! A first!)

    Good luck,

  • CariboocootCariboocoot Banned Posts: 17,615 ✭✭
    Re: Battery Absorb Time

    Just a comment based on my experiences:

    Absorb time may not be the 'culprit' in battery water usage; Absorb Voltage could be too high.

    Also, a battery temp sensor is an absolute must for 'left over the winter' systems.
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: Battery Absorb Time

    Just to add what Marc says. If you have a cheap controller with no BTS, set the absorb voltage a little bit lower, or like the little Sun Saver CCs have, set the unattended battery to "Warm battery" voltage. That way it keeps the absorb voltage down a bit so that the battery doesn't use excessive water, but keeps the charge level high enough to prevent sulphation.

    Personally, I have seen more batteries damaged due to chronic overcharging than due to SLIGHT under charging. I don't think that batteries left 98% charged aren't a problem, but consistently charging to 105% would use excessive water!

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