8-1
Chapter 8 Battery Information
The standard battery charging programs in the SunStar controller, as described in Section
4.2, are typical charging algorithms for four battery types:
• sealed (VRLA)
• flooded (vented)
• L-16 group
• Nicad and NiFe
Other battery chemistries of special voltages such as 36V, can be charged using a custom
charging algorithm modified with the potentiometers VR2 and VR1. Only the standard
SunStar battery charging programs will be discussed here.
CAUTION: Never attempt to charge a primary (non-rechargeable) battery.
All charging voltages noted below will be for 12V batteries at 25°C.
8.1 Sealed Batteries
The general class of sealed batteries suitable for solar systems are called VRLA (Valve
Regulated Lead-Acid) batteries. The two main characteristics of VRLA batteries are
electrolyte immobilization and oxygen recombination. As the battery recharges, gassing is
limited and is recombined to minimize the loss of water.
The two types of VRLA batteries most often used in solar are AGM and Gel.
AGM:
Absorbed Glass Mat batteries are still considered to be a “wet cell” because the electrolyte
is retained in fiberglass mats between the plates. Some newer AGM battery designs
recommend constant voltage charging to 2.45 volts/cell (14.7V). For cycling applications,
charging to 14.4V or 14.5V is often recommended.
AGM batteries are better suited to low discharge applications than daily cycling. These
batteries should not be equalized since gassing can be vented which causes the battery to
dry out. There is also a potential for thermal runaway if the battery gets too hot, and this
will destroy the battery. AGM batteries are affected by heat, and can lose 50% of their
service life for every 8°C (15°F) over 25°C (77°F).
It is very important not to exceed the gas recombination capabilities of the
AGM. The optimum charging temperature range is from 5 to 35°C (40 to 95°F).
Gel:
Gel batteries have characteristics similar to AGM, except a silica additive immobilizes the
electrolyte to prevent leakage from the case. And like AGM, it is important to never exceed
the manufacturer’s maximum charging voltages. Typically, a gel battery is recharged in
cycling applications from 14.1V to 14.4V. The gel design is very sensitive to overcharging.
For both AGM and Gel batteries, the goal is for 100% recombination of gasses so that no
water is lost from the battery. True equalizations are never done, but a small boost charge
