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Note: this is for deep cycle lead chemistries only.! Lithium chemistries 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


The 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 will require more frequent watering, due to increased outgassing
  • calcium-enhanced batteries (typically starter batteries and some hybrid “marine” batteries) can be damaged3) 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.
electrical/12v/eternal_absorption.1594234948.txt.gz · Last modified: 2020/10/11 19:48 (external edit)