PV Hybrid VRLA Battery Test Results from a Telecommunications Site Page: 4 of 6
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System Power Center
The PV power center manufactured by Digital Solar
Technologies model number MPR-9400 is microprocessor-
based and incorporates an integrated System Control and
Data Acquisition (SCADA) feature (see Fig. 2). The unit is
fully programmable by the user for on/off photovoltaic
battery charging, load control, generator battery charging,
and data logging. Sandia National Labs has worked
closely with Digital Solar to add the SCADA feature.
Extensive laboratory and field testing was conducted to
improve and debug battery charging algorithms using the
engine generator and Ah counting charge control methods
. Now all system control functions can be remotely
controlled through a modem connection. PV Battery
charging is accomplished with three subarrays switched on
and off with mercury displacement relays at the
temperature compensated voltages. Temperature
compensation is adjustable and in this case was set at -
0.128 V/*C or -0.0053 V/*C/cell. The setpoints for Mt.
Washington in Table 2 were chosen based on
recommendations from the battery manufacturer, array
size, system loads, battery size, and Sandia's previous
experience using on/off subarray switching. This was the
first use of these batteries, system controller, and system
design in the field; therefore, some uncertainty existed as
to the most desirable setpoints. Since The MPR-9400
controller allows these setpoints to be changed remotely,
regulation voltage optimization, especially the reconnect
voltage can be easily accomplished. The goal in setting
on/off regulation voltages is to provide maximum charge
without causing a loss in voltage control due to excessively
short on/off cycles. In this case, the MPR-9400 requires
on/off cycles in excess of 5 to 10 seconds.
Fig. 2. Digital Solar's MPR-9400 System Power Center,
Charge Controller, and SCADA system
Deka G-75 Gel VRLA Battery
The battery, a Deka 258 Ah G-75-7 VRLA gel, is an
industrial motive power battery configured in two parallel
strings for a total rated capacity of 516 Ah (see Fig. 3).
The system load profile is primarily a continuous 3 to 4-
amp load with intermittent load spikes up to about 15 amps
depending on the use of the radio transmitters and AC
cooling fan. Normally, the total daily load is less than 80
Ah, and the nighttime battery discharge is usually about 50
Ah or slightly less than 10% depth-of-discharge.
Fig. 3. Deka G-75 Gel VRLA Battery.
Battery capacity was initially measured in February of
1999 before the system was operational (see Table 1). It
was measured at 544 Ah to 1.85 vpc at a 25-amp rate.
This capacity was greater than the 516 Ah nameplate
capacity to 1.80 vpc at the 24-h rate. The initial battery
measurements included capacity in Ah, impedance in
micro-ohms, and cell voltage at the end of charge and at
the end of discharge. Operation began in March 1999, and
in October, a second test was conducted to obtain the
operational battery capacity. The results indicated that the
battery capacity was down by over 12%. Table 1 lists the
test results, showing that the battery capacity to 1.85 vpc
dropped from 544 Ah to 469 Ah, a 75 Ah loss in this eight-
month period. If battery capacity is temperature
compensated, then this initial capacity loss is considerably
more at about 18% based on the 40C lower battery
temperature of the initial capacity test. In addition, the
maximum cell impedance increased from 1,544 to 1,817
micro-ohms, and the range in cell voltages on discharge at
1.85 vpc increased from 0.09 volts to 0.16 volts. Both the
impedance increase and the cell voltage divergence are
indicators of battery capacity loss and degradation in cell
Cause of Battery Capacity Fade
A more detailed evaluation of battery charging
requirements from laboratory testing clearly indicated that
the battery was being under charged, and the cause was
attributed to a low on/off PV regulation voltage and a low
regulation voltage of 2.33 vpc in the engine generator
battery charger. The engine generator battery charger was
providing a biweekly "equalize" charge to the battery.
Laboratory PV hybrid battery cycle test results  in Figure
4 show that the same Deka battery will-.loose capacity
quickly if the time between finish-charges is too long (15 to
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HUND,THOMAS D. & STEVENS,JOHN W. PV Hybrid VRLA Battery Test Results from a Telecommunications Site, article, September 25, 2000; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc718238/m1/4/: accessed April 19, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.