3. Maintain high average
battery capacities
A high battery state-of-charge (SOC) is important
for battery health and for maintaining the
reserve storage capacity so critical for solar
system reliability. An FSEC Test Report (reference
6) noted that “the life of a lead-acid battery is
proportional to the average state-of-charge,” and
that a battery maintained above 90% SOC “can
provide two or three times more
charge/discharge cycles than a battery allowed to
reach 50% SOC before recharging.”
However, as noted in the previous section, many
solar controllers interfere with the recharging of
the battery. The FSEC study noted at the end of
the report that the “most significant conclusion
is that some controllers did not maintain the
battery SOC at a high level, even when loads
were disconnected.”
In addition, a comprehensive 23 month study
of SOC factors was reported by Sandia in 1994
(reference 7, page 940, attached). It was learned
that the regulation setpoint has little effect on
long-term SOC levels, but the reconnect voltage
is strongly correlated to SOC. Five on-off
regulators and two quasi constant voltage
regulators were tested (Morningstar controllers
were not developed when this test started). A
summary of the SOC results follows:
• 3 on-off regulators with typical hysteresis
averaged between 55% and 60% SOC over the
23 month period
• 2 on-off regulators with tighter hysteresis (risking
global instability) averaged about 70% SOC
• the 2 “constant voltage” controllers with
hysteresis of 0.3 and 0.1 volts averaged close
to 90% SOC (note that Morningstar controllers
have a “hysteresis” of about 0.020 volts)
Sandia concluded that the number of times
a system cycles off and on during a day in
regulation has a much stronger impact on
battery state-of-charge than other factors within
any one cycle. Morningstar’s PWM will “cycle”
in regulation 300 times per second.
It would be expected that batteries charged with
Morningstar’s PWM algorithm will maintain a
very high average battery state-of-charge in a
typical solar system. In addition to providing a
greater reserve capacity for the system, the life
of the battery will be significantly increased
according to many reports and studies.