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Bulk -vs Absorption charge stage - SOC, fixed current vs. voltage

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  • Bulk -vs Absorption charge stage - SOC, fixed current vs. voltage

    My TS-MPPT-45 controller manual says the following (summarized) about charge stages:

    (Vabs = 14.7v, Vfloat = 13.5v for my flooded LA):
    Bulk - full solar power until battery reaches Vabs
    Absorption - The battery is allowed to come to full state of charge at the Vabs
    Float - Vfloat low maintenance levels

    1) Is the controller discarding power during the absorption phase? I mean it can only control either I or V - and the other follows, right?

    2) Absorption confuses me a bit. Firstly because it just runs for a fixed time - not based on any sensors or anything. So how does it know it comes to a "full state of charge"? Is it that there's a typical SOC when bulk hits 14.7v and transitions to absorption? I realize it's triggered by 14.7v voltage, but is that usually 60% or 90% SOC?

  • #2
    Originally posted by chilly View Post
    My TS-MPPT-45 controller manual says the following (summarized) about charge stages:

    (Vabs = 14.7v, Vfloat = 13.5v for my flooded LA):
    Bulk - full solar power until battery reaches Vabs
    Absorption - The battery is allowed to come to full state of charge at the Vabs
    Float - Vfloat low maintenance levels

    1) Is the controller discarding power during the absorption phase? I mean it can only control either I or V - and the other follows, right?

    2) Absorption confuses me a bit. Firstly because it just runs for a fixed time - not based on any sensors or anything. So how does it know it comes to a "full state of charge"? Is it that there's a typical SOC when bulk hits 14.7v and transitions to absorption? I realize it's triggered by 14.7v voltage, but is that usually 60% or 90% SOC?
    1. You are absolutely right that going into Absorb during peak sun hours will not take the full potential power output from the panels.
    The CC is not discarding power in the sense that the extra is not going into heating anything up, but you are in a less than ideal situation in that your CC is not pulling as much power from the panels as they are able to produce at the time. Solar panels are perfectly happy being loaded to less than Imp.

    2. Actually, depending on the CC you use, Absorb may run for a fixed time, a learned time based on the length of the Bulk stage, or even run until the charging current falls below some configurable value (End Absorb).
    If you can measure the actual current into the battery rather than the current going to both battery and load (i.e. there is a battery shunt connected to the CC), the current based termination if probably the most satisfactory.
    If you were using a generator, whose fuel costs per watt are disproportionately high when running at partial load, you would not necessarily want an Absorb stage on the charger at all.
    For PV, especially as the panel output ramps down in the afternoon, the Absorb stage is relatively efficient.
    Dereck (Sunking) will be happy to tell you how to bypass Absorb completely if it is bothering you.
    SunnyBoy 3000 US, 18 BP Solar 175B panels.

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    • #3
      First understand what each Stage is and how it works.

      Bulk Mode is a constant current mode meaning it pumps a fixed amount of current until the battery voltage raises to the Bulk voltage set point. like 14.2 volts. In the solar world Constant Current is as much current as the panels can supply.

      Absorb or Boost Mode is the second stage and is a Constant Voltage with Current Taper for a specified about of time usually 2 hours. So when Bulk Terminates, the Absord Mode begins. Its voltage setpoint is higher than Bulk like 14.8 volts. When it first switches current will surge to Current Limit just like Bulk meaning the panels deliver as much as the can. As the voltage rises to the Absord set poin the current will begin to taper off toward 0 amps so at the end of the Absorb 2 hour cycle hopefully the current has tapers off to about C/33.

      Float Mode is just another Constant Voltage current taper. Float voltage is lower than both Bulk and Absorb usually around 13.2 volts.

      The real problem for Solar is the Absorb phase. Commercial 3-stage battery chargers that use commercial power work differently. The three stages work the same with two exceptions. Bulk is a fixed Constant Current usually of C/10 because there are no power fluctuations with a hard source like the grid. Like its Solar cousin Bulk terminates the same of a voltage set point. However Absorb works differently. It is still Constant Voltage current taper like it solar cousin, but it terminates when the charge current tapers off to C/33 or 3% of C where C = the battery AH specification @ 20 hours. Time is irrelevant and the Absorb phase normally take 4 to 6 hours. A solar system cannot do that because there are not enough hours in a day with Sun.
      MSEE, PE

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      • #4
        Originally posted by Sunking View Post
        Absorb or Boost Mode is the second stage and is a Constant Voltage with Current Taper for a specified about of time usually 2 hours <snip> As the voltage rises to the Absord set poin the current will begin to taper off toward 0 amps so at the end of the Absorb 2 hour cycle hopefully the current has tapers off to about C/33.
        So this tapering is a result of battery behavior at higher SOC levels - it inherently absorbs less current, right? That is to say - it's not the controller actively reducing current - it's still just doing what it needs to to maintain the voltage.

        Originally posted by Sunking View Post
        The real problem for Solar is the Absorb phase. Commercial 3-stage battery chargers that use commercial power work differently. ...<snip>... Time is irrelevant and the Absorb phase normally take 4 to 6 hours. A solar system cannot do that because there are not enough hours in a day with Sun.
        I think this was actually part of my confusion - my A/C inverter on my R/V does have an absorb phase that (they claim in the docs) charges from 90% to 100% over many hours at a lower 13.x voltage similar to what you mention.

        Thanks for the clarifications. I think I understand reasonably well now. It sounds like absorption stage might be the right time to charge laptops and other electronics - as long as the controller can maintain Vabs, it should be able to provide the excess solar to charge the load devices. @inetdog, I get your point about monitoring current - although it doesn't look like my controller can be configured to look at that. I'll have a shunt/meter on the battery bank though, so I can monitor it and maybe adjust the time if it seems warranted.

        I also found the controller has a configurable overall total current limit (for bulk phase) - one thing I was concerned about not exceeding the charge rate since I may oversize the array slightly for my current batteries to have more headroom in non-ideal conditions and times of year.

        Comment


        • #5
          Originally posted by chilly View Post
          So this tapering is a result of battery behavior at higher SOC levels - it inherently absorbs less current, right? That is to say - it's not the controller actively reducing current - it's still just doing what it needs to to maintain the voltage.
          Yes that is basically correct in thought. I real life the current is determined by the battery internal resistance, source voltage, and battery voltage. For example let's assume the chargers voltage is 14 volts with 30 amp limit, battery resistance is 10 milli-ohms, and the resting battery voltage is 12 volts. Initially the charger goes into current limit at 30 amps, the charge voltage folds back to battery voltage of 12 volts plus [30 amps x 10 milli-ohms) = 12.03 volts. as the battery voltage rises when being charge it will get to 13.97 volts. At this point the current will begin to taper to 0 amps as the battery voltage approaches 14 volt. Say at 13.99 volts the current tapers off to [14 volts - 13.99 volts] / .01 Ohms = 1 amp.


          Originally posted by chilly View Post
          I think this was actually part of my confusion - my A/C inverter on my R/V does have an absorb phase that (they claim in the docs) charges from 90% to 100% over many hours at a lower 13.x voltage similar to what you mention.

          Thanks for the clarifications. I think I understand reasonably well now. It sounds like absorption stage might be the right time to charge laptops and other electronics - as long as the controller can maintain Vabs, it should be able to provide the excess solar to charge the load devices. @inetdog, I get your point about monitoring current - although it doesn't look like my controller can be configured to look at that. I'll have a shunt/meter on the battery bank though, so I can monitor it and maybe adjust the time if it seems warranted.

          I also found the controller has a configurable overall total current limit (for bulk phase) - one thing I was concerned about not exceeding the charge rate since I may oversize the array slightly for my current batteries to have more headroom in non-ideal conditions and times of year.
          OK you do not have to wait to absorb, nor can you control where the current is going. For example lets say the batteries are taking full power from the panels at 30 amps during Bulk mode. Initially no load devices are turned on demanding power so the battery sinks or uses the full 30 amps. Now you turn on your laptop and it draws 1 amp. Now they batteries are only taking 29 amps, and the other 1 amp is your laptop.

          Controllers do not control the current directly. Maximum current is is designed by panel wattage. Example a 1000 watt panel with MPPT on a 12 volt battery will have a bulk charge current of 40 amps. So with 40 amps you would need a 320 to 480 AH batteries to safely take full current from the panels/controller. It is all taken care of with proper design.
          MSEE, PE

          Comment


          • #6
            Originally posted by Sunking View Post
            Controllers do not control the current directly. Maximum current is is designed by panel wattage. Example a 1000 watt panel with MPPT on a 12 volt battery will have a bulk charge current of 40 amps. So with 40 amps you would need a 320 to 480 AH batteries to safely take full current from the panels/controller. It is all taken care of with proper design.
            Wouldn't a 1000 watt panel @12v be closer to 80amps? I was planning 400W panels to 12v batteries and thinking 30A on a 225Ah batteries. The controller (Morningstar TS-MPPT-45) has settings:
            Enable battery current limit:Y/N
            Battery current limit:

            Since as you say they cant control current directly, maybe they can do it indirectly by throttling back the normal voltage settings for the given charge stage to reduce the net current. Or maybe I'm interpreting the setting wrong and should check with the mfg? Was only concerned if I were to add 200W panels (all that will fit - while I have tools and a good work area - not full time RV) - that could overcharge rate my current batteries.

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            • #7
              Originally posted by chilly View Post
              .. I think this was actually part of my confusion - my A/C inverter on my R/V does have an absorb phase that (they claim in the docs) charges from 90% to 100% over many hours at a lower 13.x voltage similar to what you mention.
              Just be careful knowing that many non-battery manufacturers may actually use the wrong terms as we generally know them. What you describe above is actually a "float" mode, and not a true absorb. Absorb just means that the charge controller only limits the maximum voltage the terminals can reach, and as the battery voltage rises, when it meets that CC voltage limitation, the *battery* is actually the one cutting down on current all by itself. When the terminal voltage meets the CC voltage, no current flows, and falls back to the float mode, although the controller may just simply count time in absorb, and not measure actual charge current to determine the switch to float.

              Deciphering these terms can cause a lot of initial confusion, but now you know.

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              • #8
                Originally posted by chilly View Post
                Wouldn't a 1000 watt panel @12v be closer to 80amps?
                Yes my bad, sorry bout that.
                MSEE, PE

                Comment


                • #9
                  Originally posted by chilly View Post
                  Wouldn't a 1000 watt panel @12v be closer to 80amps? I was planning 400W panels to 12v batteries and thinking 30A on a 225Ah batteries. The controller (Morningstar TS-MPPT-45) has settings:
                  Enable battery current limit:Y/N
                  Battery current limit:

                  Since as you say they cant control current directly, maybe they can do it indirectly by throttling back the normal voltage settings for the given charge stage to reduce the net current. Or maybe I'm interpreting the setting wrong and should check with the mfg? Was only concerned if I were to add 200W panels (all that will fit - while I have tools and a good work area - not full time RV) - that could overcharge rate my current batteries.
                  I forgot Morningstar has the ability to limit current. Not really sure why a manufacture would do that. But with that said if you have 225 AH of batteries with 400 watt panels, there is no need to limit current (33 amps), all that will do is slow things down. I bet anything those are golf cart batteries you have. If so they are hybrid type batteries and can take up to 60 to 70 amps with no problems. 33 amps on a 225 AH battery is C/7 rate which is OK. Just keep an eye on water levels.
                  MSEE, PE

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