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Rechargeable batteries for solar systems - overview

The most suitable batteries for solar systems: Lead-Acid-GEL and LiFePO4

The battery is a critical component of all solar products. Some battery types have a shorter life than others, but even good batteries’ life span depends on correct usage. Good batteries can last longer than 5 years with proper use and as such support the potential of solar energy as a durable technology.
Most of the solar products use batteries with a lead, nickel, or lithium base. Which battery is the right type is dependent not only on technical details but also on its intended use: a permanently installed solar home system (SHS) presents different requirements than a mobile flashlight.

The most important criteria
a) For Solar Home Systems (SHS)
In a SHS, the solar module is permanently connected to a battery, which secures the daily charge. The battery is therefore seldom completely discharged; there is usually a little charge left. With a SHS, batteries are used that enable a high number of charging processes.
Lithium and sealed lead acid batteries meet these requirements: the less they are discharged completely before recharging, the higher their life expectancy. For example, a high-value gel battery specifies:

Capacity Withdrawn    charging procedures (Lifetime)

With NiMeH and NiCd batteries, the battery life is shortened, if it is charged frequently before being completely discharged.
Start batteries are completely unsuitable for a SHS: They are made to deliver high currents for short periods. If they are more than 10% discharged, the structure of their lead electrodes changes, and sulfation starts. Also starter batteries lose charge capacity quicker than solar batteries.
With a SHS, however, it is still expected to provide a reliable source of energy over a longer bad-weather period. Therefore, the battery capacity requirements are much greater than with mobile flashlights. Today, the standard is based on a capacity that is three times the daily usage time. A SHS can thus be used three days without the battery having to be recharged.

Our recommendation for a SHS battery: lead acid gel battery.
High potential for future: LiFePO4 (still too expensive for three-day runtime capacity).

b) For Solar Flashlights
Unlike a SHS battery, the battery for mobile solar flashlights is not permanently attached to the solar module. Experience shows that the user charges the battery in a similar way to those in their cell phones: they aren’t charged every day, but rather first when they are completely or mostly discharged.
Implementing a gel battery therefore clearly requires user training, since without daily charging the life of the battery is reduced.
Or, one can just choose a different battery type: the greatest hopes today lie with the LiFePo4 (lithium iron phosphate) battery. Because of its robustness, energy density, and the possibility of multiple charging processes, it offers the best features for solar flashlights. The price for good LiFePO4 batteries has come down far enough that the current cost for the size required of a flashlight is very reasonable.

Our recommendation for mobile usage: LiFePO4.