Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Absorption Timing
Currently I'm not using my batteries much as I have full hook-ups but I leave Thursday and will be relying on my 600W of solar panels a lot more for at least the next month or more.

My current controller is on mostly default settings for the type (AGM) and is set for 120 minutes for Absorption. I read that this isn't enough if you regularly deeply discharge your batteries to the 50% mark. 

So, do I change this? I didn't initially realize that Absorption time was limited to a set time...just thought that it automatically switched from Absorption to Bulk or Float (or whatever it is that comes next) when it reached a specific voltage. I also wonder if this time limit is why my batteries (400AH) didn't get topped off in September after taking them to 70% despite a full day of sun and not using anything (I only had 400W of solar then).
It can be enough;  depends on how much current the battery is still accepting at the 2 hour mark (subtracting any loads).   The important idea here is that the battery is accepting the current it wants;  the controller isn't forcing it into the battery.*  

The flow looks like this:
  • Bulk charging continues until the Absorption voltage setpoint (Vabs) is reached.
  • then Absorption stage holds Vabs until either the time is up (2 hours in your case), or until the current into the battery drops sufficiently, or some automagic combination of the two.
  • then Float stage takes over and holds the Float voltage setpoint (Vfloat)
Sufficient current drop is usually described as falling to something like C/200:  (the bank's capacity in Ah divided by 200).

So let's say your battery manufacturer specifies C/200.  In that case 400Ah/200 = 2A.  So when current accepted by the battery during Absorption drops to 2A, absorption is done.  I will use this pretend number from here on.

Examples using C/200 and 400Ah:
  • if the controller drops to Float when current is still falling toward 2A it is floating prematurely and the batts aren't getting tippy-topped off.  This is what you suspect your system is experiencing.
  • if the controller drops to Float fairly quickly after current stabilizes at 2A then it's on the money.  
  • If the controller drops to Float a long time after current has stabilized at 2A then it is holding Vabs longer than it needs to.  In a flooded battery this could accellerate water loss and therefore require more maintenance.  I don't know the effects on AGM but probably minimal since AGM Vabs are already set lower than flooded..  My guess is it's better to run Absorption a too long rather than too short.
So your homework Smile  is to turn off as many loads as you can at about 1.75hrs into Absorption and see how much current (in amps) is flowing into the battery.   If the battery is still accepting** > C/200*** then the bank might benefit from a longer Absorption duration.  I don't think you mentioned what controller you have, but should you need to change that value hopefully another owner can chime in.

more info if everyone hasn't already hung themselves:

* if the controller tried to force in more than the battery wanted then the battery voltage (Vbatt) would rise.  But the controller is set to a particular Absorption voltage setpoint (Vabs) so it has to back off...
** for this reason sometimes Absorption is called Acceptance
*** or whatever the manufacturer says.  Remember my number is fake.
frater/jason - FT 2018.  Retired/boondocking 2020.  
159" Promaster, 570w:220Ah, 35gal fresh
twitter | RVwiki
Good info.

Some controllers will allow a set time, AND have the amperage threshold which will end absorption and drop to float.

If one stops cycling their batteries, then they do not require 3.5 hours of absorption each day.

If the controller uses an amperage threshold to initiate float, well each morning the solar will get the batteries to absorption quickly, and that amperage threshold will be crossed soo after and then it is float all day from there.

it really depends on your controller as to what it will do. I;ve reset mine for 0.5 hours absorption before leaving my rig, and watched it climb to 14.7 in a half hour then take about 3 more minutes for amps to taper to less than 0.45( mine is a single 90AH AGM battery, 0.45amps is 0.5% of capacity) and it went into float.

Your charge controller instructions should say what triggers float voltage, whereer simply time based only, or both time and amperage based float triggering.

An amperage based float trigger controller would also want to know the size of the battery bank in Amp hours. if fit does not ask for total capacity size then set absorption duration to 15 to 30 minutes when not cycling the batteries at all overnight.

I usually leave my fans running at some speed when gone, and this last Xmas, forgot to defrost my fridge and was running late for my flight, so I just left the fridge running for 2 weeks while I was gone, and did not touch any Solar settings.
I just set up Victron controllers on my trailer. The absorption time is self setting, depending on the good morning system voltage:
<11.9v, 6 hrs of absorption time
11.9-12.2v, 4hrs
12.2-12.6v, 2 hrs
>12.6v, 1 hr absorption time

But if the smaller charge controllers (my smaller ones are 75/15) hit <1 amp, it flips into float.
The larger charge controllers (the one I have is 100/30) will flip into float when it goes <2amps. All the voltages, even equalization (which I wouldnt use on my AGMs) are adjustable down to 0.01volts.

I'm really happy with them. I can adjust them and have lots of data to look at on Bluetooth. Better than TV! I haven't hooked up my battery monitor yet, a BMV- 700, but can't wait.
Good morning System voltage. Interesting approach.

One thing about amperage based float triggering by charge controllers, is that DC loads on the system during absorption, could keep it in absorption long after amps into the battery have tapered to the float trigger level.

It would have to have a battery monitor or a separate shunt, or perhaps hall effect sensor on/over a battery cable, to determine that the battery is only accepting less than X.xx amps.

Sometimes other charging sources screw with my battery monitor. If I have my absorption on solar controller set to 14.6, and I allow my alternator to take battery to 14.7, then the BlueSky SB2512i solar controller gives up, and the battery monitor(Blue Sky IPN Pro Remote) will display 100% State of chargeand 0 amp hours from full, even though the battery is no where near fully charged. When i shut the engine off, if the solar can maintain float voltage then it simply stays there. only when the solar cannot maintain float voltage will the controller go back into bulk and seek absorption v, but by then the sun might be too low to actually achieve it.

These components I am using, have been in operation 10+ years now. At first i was pretty unaware and just put a foolish blind faith in the reading of the battery monitor, and had prematurely dying batteries as a result. Part of that was my Vehicles vltage regulator would goto 14.9v each time the engine was started. Second was the Float Current. Initially I though one could command it to float at 13.2 volts and deliver as high as 6 amps current. i did not realize that the 'float current' as my charge controller calls it, was a trigger to switch from Absorption to float.( hangs head in shame and embarrassment)

Had I not been so ignorant of this, my first set of batteries would have likely lasted many years, as my overnight discharges were perhaps 35Ah for 230 or 345Ah of capacity. Yet the first battery failed at 13 months, the second at 23, and the third, relegated to engine starting duty at 23 months never to be cycled to less than 99.8% again, made it 7+ years total, in that duty.

So, while drifting off topic, this is one of the reasons I have harped on holding Absorption voltage as long as required to reach a true full charge via solar. And determining full charge on flooded batteries should be done with a hydrometer, AGM batteries an Ammeter at absorption voltage. it need not be done every time, but after some hard usage, should be.

battery Monitors should not be considered 100% accurate. They are most accurate the day after being reset/rezeroed when a hydrometer and or Ammeter and a human with a brain and the interest to measure, have determined that the batteries are indeed full.

Sometimes an Equalization charge is required to max out specific gravity, for the true full charge on flooded/wet batteries.

I have found my Northstar AGM, basically requires a high amp recharge from a well depleted state, before i can consider it truly fully charged. A bunch of shallower cycles and being held at absorption until amps taper to 0.45 or less at 14.7v, does not always cut it. For several days after the high amp recharge, the voltages held during discharge would be higher for the same loads and AH removed.

My point is that sometimes a human needs to double check even the most detailed and capable systems.

Also one needs to be realistic about the programmed battery capacity, as the batteries age and lose capacity., as what the battery monitor considers 50% State of charge when the batteries are new and healthy, is likely closer to 40% after a few hundred cycles.

Absorption times for amps to taper to the float threshold, also takes longer and longer as the batteries age/ accumulate cycles.

The Victron battery monitor also factors in the peukert effect, from what I have read. Mine does not.

peukerts law basically says a 100 Ah battery can provide 5 amps for 20 hours, but not 20 amps for 5 hours. Large loads on lead acid batteries significantly reduces their available capacity. As batteries age this peukert component is also affected and the battery has less initial capacity to begin with too.

So A human needs to monitor the battery monitor, sometimes, to insure the batteries are indeed full when it says they are. Accepting too much amperage at absorption voltage when battery monitor says 1Ah from full, is a big red flashing light to me. But no red flashing light actually exists. instead the green light 100% all is well 0Ah from full happily flashes away with a green thumbs up, while the battery is being teased with too short an Absorption duration and perhaps too low an absorption voltage too.

My Sb2512i solar controller does not have temperature compensation.
[-] The following 1 user says Thank You to sternwake for this post:
  • rvpopeye (01-28-2018)

Forum Jump:

Users browsing this thread: 1 Guest(s)