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). Both long periods of PSoC and lower voltage are damaging to lead acid batteries (ie, battery murder), usually due to “hard sulfation” (see below).

In a perfect world, lead-acid batteries (both flooded and AGM) would be discharged no further than 50% then fully charged again each day.

It is easier to get the batteries fullly charged each day when they are overpaneled, but that trades battery health for less reserve for days of poor insolation.

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⁾²⁾ The only lead-acid battery proven to handle PSoC with minimal harm is the carbon-foam Oasis Firefly AGM.³⁾

Sulfation is the conversion