Ideally, there would be enough energy put back into the battery to bring it up to 100% SOC each day. Sounds simple, but loads and available charging vary almost constantly. An amp-hour meter can help, since it keeps track of the net energy in the batteries, but many charge controllers only have an absorb “time limit” function.
Manufacturers have a formula that can estimate this time limit based on expected SOC, available charging current, and battery capacity—but it is still just an estimate. Since PV systems often have relatively low charging current, erring on the longer side for PV charger absorption times is appropriate for most off-grid systems.
For example, Surrette Battery Co. provides the following formula:
Absorb time = (0.42 × battery’s 20 hr. capacity) ÷ charge current
If there’s 800 Ah capacity, and 75 A of charge current:
(0.42 × 800 Ah) ÷ 75 A = 4.5 hrs. absorb time
More sophisticated charge controllers also have a current trigger to end an absorb cycle. The higher the SOC, the less current is needed to keep the battery at absorb voltage. Since most PV charge controllers start a new bulk/absorb cycle each day, this is a great way to ensure batteries are getting just enough of a charge—even when they start at different SOCs. For example, a vacant off-grid cabin will not need much PV absorb time, since the batteries will be relatively full each morning. When the cabin is being used and the batteries are being discharged more deeply, PV absorption should be maximized.