Glossary

Absorption phase

Also Constant-voltage phase

What is Absorption phase?

The middle phase of a multi-stage charging process: the voltage is held constant at the end-of-charge voltage while the current falls by itself.

What matters about Absorption phase in practice?

In this phase the last few percent go into the battery — and it takes disproportionately long, because the battery bank only accepts little current. Anyone who stops the engine as soon as the charging current drops breaks off exactly here.

A multi-stage regulator ends the phase when the current falls below a threshold or a time has elapsed. A simple fixed-voltage regulator does not know this phase at all.

How long does the absorption stage last?

With lead batteries often several hours, because the last percent are taken up only slowly. With LiFePO4 batteries it is much shorter, because the cells accept almost full current until shortly before full.

When does it end?

  • When the current falls below a tail current, with lead often a few percent of the capacity expressed as current.
  • After a fixed maximum time or one calculated by the controller.
  • With lithium usually when the end-of-charge voltage is reached and the current has become small.

Why is it often too short with alternator and solar?

If the engine is switched off or the sun goes down, charging ends in the middle of this stage. Lead batteries then stay permanently below full. Regular full charging on shore power helps, or a regulator that also runs the absorption stage properly via the alternator, see boat alternator.

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