Partial State of Charge (PSoC) refers to the state of a lead-chemistry battery when it is left below 100% State of Charge (partially discharged), which lead-chemistry batteries do not like. (Nomads who expect PSoC conditions may get better results from battery chemistries that don't mind PSoC, like Lithium or carbon-doped lead.)
Since State of Charge is measured "on the way down" from full charging, PSoC can result from:
- failure to fully charge lead batteries (commonly failure to hold Vabs for sufficient duration); or
- fully and correctly charging but then allowing the battery to rest without a Float (maintenance) charge (less common)
Both long periods of PSoC and incomplete charging are damaging to lead acid batteries (ie, battery murder), usually due to "hard sulfation".
Optimal deep cycling
In a perfect world, lead-acid batteries (both flooded and AGM) would be discharged no more than 20% (80% remaining) and then fully charged again (back to 100% State of Charge) each day. In some studies this 20% depletion yields the most Ah/$. The downside is carrying around a lot of "dead lead", the remaining 80% capacity that is never used.
In actual usage it is more common to discharge to 50% SoC. This allows 2.5x the power to be used overnight at the cost of longevity.
It is easier to get the batteries fully charged each day when they are overpaneled, but that trades battery health for less reserve for days of poor insolation.
Practical deep cycling
In practice, it may be "good enough" to get back 100% SoC every other day or even every third day. One should expect to get fewer duty cycles when getting back to 100% SoC less frequently.
Some premium AGM like Lifeline, Northstar and Odyssey may be able to tolerate longer stretches of PSoC although capacity will be reduced up to 30% during those periods[1]. The only lead-acid battery proven to handle PSoC with minimal harm is the carbon-foam Oasis Firefly AGM.
Sulfation