Batteries, Charge Controllers and Energy Storage
Battery basics
Batteries, Charge Controllers and Energy Storage covers capacity, depth of discharge and charging. Capacity is quoted in ampere-hours (Ah) at a stated rate, often C10 or C20. Available energy depends on depth of discharge (DoD), temperature and age. Lead-acid types include flooded, AGM and gel; lithium iron phosphate (LFP) is common in newer small systems. Cycle life falls sharply with deep discharge and high temperature.
Charging and controllers
A charge controller prevents overcharge and, in off-grid systems, over-discharge. PWM controllers are simple and suit small systems where panel voltage is close to battery voltage. MPPT controllers convert excess panel voltage to extra current and are useful when the array voltage is well above the battery. Typical charging stages are bulk (constant current), absorption (constant voltage), float (lower maintenance voltage) and occasional equalisation for flooded lead-acid only.
Worked example
A 12 V, 100 Ah battery is at 50 % DoD, so about 50 Ah must be replaced. A 200 W array operating at 12 V gives about 16.7 A in theory. With 80 % overall efficiency, usable charging current is about 13.3 A. Time ≈ 50 / 13.3 = 3.8 h, but the last absorption phase is slower, so plan 5–6 h of good sun.
Traps
Do not equalise a sealed AGM or gel battery unless the manufacturer explicitly allows it. Do not size a battery only from its Ah label while ignoring DoD. A car battery is a starting battery, not a deep-cycle storage battery.