Lead-acid batteries are the most common type of battery used in PV off-grid systems, for cost reasons. There are many different types of lead-acid batteries with different characteristics. Some are sealed (so-called ‘maintenance-free’). Others are what is called ‘flooded’, and require regular topping up with de-ionised (distilled) water. The best-performing and longest lasting are ‘deep cycle’ flooded batteries.
Many locally available (either locally manufactured or imported) lead-acid batteries which are used for other purposes, such as in telecom systems, can also be used in off-grid solar systems. Importing batteries is expensive, and very often impractical except for larger systems/projects.
Lead-acid batteries are the weakest point in off-grid PV systems – they are usually the first system component to fail, and if systems are not designed/managed properly they can fail very quickly.
Left: 12 V lead-acid battery designed for use in small off-grid systems.
Right: 2 V deep-cycle flooded battery cell, used in larger systems.
Batteries are usually connected in series rather than parallel.
Left: 2 x 12 V batteries connected in series. Right: 6 x 2 V batteries connected in series
Overall battery voltage is the sum of the voltages of all series-connected batteries or. Overall battery Ah capacity is the same as that of the sum of the individual batteries/cells.
Lead-acid batteries should never be completely discharged. For a good quality ‘deep cycle’ battery, the maximum depth of discharge (DoD) permitted is 80%. The deeper a lead-acid battery is regularly discharged, the shorter its service life will be. High quality lead-acid batteries come with detailed installation, commissioning and maintenance manuals.