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Note: this is for deep cycle lead chemistries only! Lithium batteries should not be held at full voltage.

Words of wisdom:

“As long as… you cycle the battery daily, you could set float voltage to [Vabs] too without worry. Only when you stop cycling the battery do you need to return float voltage to more regular 13.2v levels.” – sternwake1)

dealing with insufficient Absorption

In off-grid solar charging scenarios batteries are most often damaged by chronic undercharging. Specifically, not holding the manufacturer's recommended Absorption voltage (Vabs), and not holding that voltage long enough2) for acceptance to drop to endAmps.

Causes of insufficient Absorption:

  • configuration error - controller set to an insufficient duration
  • controller limitation - max duration is insufficient. Example: Tracer-style controllers typically have a 3-hr max.
  • solar conditions - not enough good sun long enough to finish Absorption


While you may not be able to increase the actual duration,3) you can achieve a similar effect with one or more of these techniques:

  • if duration setpoint is maxxed then set Vfloat == Vabs. Absorption duration will effectively be “until the sun goes down”.
  • charge at a lower current (charging minimum). While overall charging will take about the same amount of time, Bulk's share of that time will increase and Absorption will take somewhat less time.
  • Absorb at a higher4) voltage (Vabs), this will decrease Absorption duration. Keep a close watch on water levels for flooded batts, and also mind the max allowable voltage for AGM/Gel.
  • Float at a higher5) voltage (Vfloat).
  • supplement solar charging with another charging source during Bulk, allowing more time for solar to finish absorption.

Some controllers/chargers have a “restart Absorption” option allows one to override Float and get more time at Vabs. Best use of this feature would be to look at the controller display a few minutes before the Absorption timer is due to expire. If acceptance is good let it go. If acceptance is too high give the bank another round of Absorption.

If those approaches are insufficient, Eternal Absorption approach completely forgoes Float stage, forcing the controller to stay in Absorption by setting Vfloat to Vabs. This addresses the controller issues above. It will not address the insolation problem on a given day, but extended Absorption will help reduce damage from previous days of poor harvest.

side benefits

PWM (and shunt) controllers make more power from panels when battery voltage (Vbatt) is higher.

Eternal Absorption may make timing opportunity loads simpler.


  • increased Vabs duration may require more frequent watering, due to increased outgassing.6)
  • calcium-enhanced batteries (typically starter batteries and some hybrid “marine” batteries) can be damaged7) by long periods of excess voltage. The question is which is worse for them: chronic undercharging or grid corrosion from higher voltage. Pick your poison.

For these reasons, it would be optimal to watch the bank's tail current (endAmps) to see when he bank is fully charged. If this occurs shortly before sundown (an hour, for example) then it's nearly optimal. If endAmps is reached hours before sundown you likely don't need Eternal Absorption anymore and can return to normal charging.

as when the controller setpoint is maxxed out
4) , 5)
but still manufacturer-sanctioned
but see this blog post which suggests no dramatic increase
positive grid corrosion
electrical/12v/eternal_absorption.txt · Last modified: 2021/05/05 11:04 by frater_secessus