14
10ºC / 50ºF
+ 0.45 V
+ 0.90 V
+ 1.80 V
5ºC / 41ºF
+ 0.60 V
+ 1.20 V
+ 2.40 V
0ºC / 32ºF
+ 0.75 V
+ 1.50 V
+ 3.00 V
Temperature Compensation
Equalization
Routine equalization cycles are often vital to the performance and life of a battery —
particularly in a solar system. During battery discharge, sulfuric acid is consumed and
soft lead sulfate crystals form on the plates. If the battery remains in a partially
discharged condition, the soft crystals will turn into hard crystals over time. This process,
called “lead sulfation,” causes the crystals to become harder over time and more difficult
to convert back to soft active materials.
Sulfation from chronic undercharging of the battery is the leading cause of battery
failures in solar systems. In addition to reducing the battery capacity, sulfate build-up is
the most common cause of buckling plates and cracked grids. Deep cycle batteries are
particularly susceptible to lead sulfation.
Normal charging of the battery can convert the sulfate back to the soft active material if
the battery is fully recharged. However, a solar battery is seldom completely recharged,
so the soft lead sulfate crystals harden over a period of time. Only a long controlled
overcharge, or equalization, at a higher voltage can reverse the hardening sulfate crystals.
In addition to slowing or preventing lead sulfation, there are also other benefits from
equalizations of the solar system battery. These include:
Balance the individual cell voltages.
Over time, individual cell voltages can drift apart due to slight differences in the cells.
For example, in a 12 cell (24V) battery, one cell is less efficient in recharging to a final
battery voltage of 28.8 volts (2.4 V/c). Over time, that cell only reaches 1.85 volts, while
the other 11 cells charge to 2.45 volts per cell. The overall battery voltage is 28.8V, but
the individual cells are higher or lower due to cell drift. Equalization cycles help to bring
all the cells to the same voltage.
Mix the electrolyte.
In flooded batteries, especially tall cells, the heavier acid will fall to the bottom of the cell
over time. This stratification of the electrolyte causes loss of capacity and corrosion of the
lower portion of the plates. Gassing of the electrolyte from a controlled overcharging
(equalization) will stir and remix the acid into the battery electrolyte.
NOTE: Excessive overcharging and gassing too vigorously can damage the battery
plates and cause shedding of active material from the plates. An equalization that is
too high or for too long can be damaging. Review the requirements for the
particular battery being used in your system.