REFERENCES TO WHY PWM? Preprint from the 14th NREL Photovoltaic Program Review, November 1996 PV CHARGE CONTROLLER AND FIELD TESTING Sandia is now working with Momingstar and Digital Solar Technologies to test lower cost, improved performance, and higher reliability charge controllers such as the SunSaver PWM charge controller and the MPR-9400 microprocessor based PV charge controller. Some examples of this work are identified below. Morningstar SunSaverTM The development of this charge controller was partially funded by Sandia as part of DOE's Photovoltaic BOS Program. The SunSaverTM has been in field tests for over six months. The SunSaverTM has demonstrated exceptional performance and reliability for a small «10 amps) low cost PV charge controller. It increases charge acceptance in VRLA batteries which should improve VRLA battery cycle-life. The increased charge acceptance is due to the PWM charge algorithm. Previous workJI has indicated that improved charge acceptance is possible ". ..by discharging prior to charging or during the charging process". Figure 8 is an example of the PWM charging algorithm in the SunSaverTM. The charge and discharge current pulses may be responsible for the improved charge acceptance seen when used with VRLA batteries. Field tests have shown that the SunSaverTM does provide VRLA batteries with a much higher overcharge or charge acceptance. Figure 9 is an example of how the PV Battery Cycle-Life Test can be used to evaluate battery charge acceptance. In this test the Momingstar SunSaverTM was placed in series with the Firing Circuits automated battery tester to make a direct comparison between DC constant voltage charging and PWM charging using the same VRLA battery and test program for both charge algorithms. This battery survived over 200 cycles of the PV Battery Cycle-Life Test before it "pemanently " lost over 20% of its capacity (99 to 73 Ah) .The battery capacity could not be recovered with constant voltage charging using recommended charging procedures. SunSaverTM PV Battery Cycle-Life Test results now indicate that much of the battery capacity has been recovered. This is illustrated by the deficit charge cycle which removed 74 Ah at 11.46 volts. Previous capacity tests on this battery resulted in about 10 Ah of battery capacity between 11.5 and 10.5 volts. This indicates that an increase in battery capacity of about 11 Ah has occurred from the SunSaverTM PWM PV charge controller.