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.
Individual battery cells may increasingly vary in charge resistance over time. An uneven acceptance of charge can lead to significant capacity deterioration in weaker cells. Equalization is a process to overcome such unbalanced cells.
The increased charge acceptance and capacity recovery capabilities of PWM pulse charging will also occur at lower charging voltages. Morningstar's PWM pulse charging will hold the individual battery cells in better balance where equalization charges are not practical in a solar system.
More testing will be done to study the potential benefits is this area.
Clearly battery heating/gassing and charge efficiency go hand in hand. A reduction in transient gassing is a characteristic of pulse charging. PWM will complete the recharging job more quickly and more efficiently, thereby minimizing heating and gassing.
The ionic transport in the battery electrolyte is more efficient with PWM. After a charge pulse, some areas at the plates become nearly depleted of ions, whereas other areas are at a surplus. During the off-time between charge pulses, the ionic diffusion continues to equalize the concentration for the next charge pulse.
In addition, because the pulse is so short, there is less time for a gas bubble to build up. The gassing is even less likely to occur with the down pulse, since this pulse apparently helps to break up the precursor to a gas bubble which is likely a cluster of ions.
As batteries cycle and get older, they become more resistant to recharging. This is primarily due to the sulfate crystals that make the plates less conductive and slow the electro-chemical conversion. However, age does not affect PWM constant voltage charging.
The PWM constant voltage charging will always adjust in regulation to the battery's needs. The battery will optimize the current tapering according to its internal resistance, recharging needs, and age. The only net effect of age with PWM charging is that gassing may begin earlier.
Self-regulate for voltage drops and temperature effects
With PWM constant voltage charging, the critical finishing charge will taper per the equation I = Ae-t. This provides a self-regulating final charge that follows the general shape of this equation.
As such, external system factors such as voltage drops in the system wires will not distort the critical final charging stage. The voltage drop with tapered charging current will be small fractions of a volt. In contrast, an on-off regulator will turn on full current with the full voltage drop throughout the recharging cycle (one reason for the very poor charge efficiency common to on-off regulators).
Because Morningstar controllers are all series designs, the FET switches are mostly off during the final charging stages. This minimizes heating effects from the controller, such as when they are placed inside enclosures. In contrast, the shunt designs will reach maximum heating in the final charging stage since the shunt FETs are switching almost fully on.
In summary, the PWM constant voltage series charge controller will provide the recharging current according to what the battery needs and takes from the controller. This is in contrast to simple on-off regulators that impose an external control of the recharging process which is generally not responsive to the battery's particular needs.