Zener diode limits solar cell voltage used to charge battery. System improves life and reliability of solar cells

Eliason, J. T.

English

NASA Technical Reports Server

August 1975	 .	 B75-10162 
NASA TECH BRIEF 
Marshall Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Zener-Regulated Solar Array/Battery Power System 
A new zener-regulated solar array/battery system 
has been proposed to improve the life and reliability of 
the solar cells. The system shown in the simplified 
diagram includes a series of solar array modules (one 
shown). Both zener and blocking diodes are mounted 
on 4-cm by 4-cm thermal radiators (radiating from 
both sides) embedded in the flexible array. These 
radiators can be reduced in area (to provide better 
heat distribution) to the size of either 2-cm by 2-cm or 
2-cm by 4-cm solar cells by the proper choice of zener 
diodes connected in series. The outputs of the solar 
array are bused to the batteries. 
Each solar cell string is shunted by a zener diode 
which limits the voltage used to charge the battery. 
Because the load bus is tapped from these strings of 
solar cells several cells down, it also has the
characteristic of zener regulation. An open circuit 
failure of a zener in this application is not 
catastrophic since many must fail in order to raise the 
bus voltage. Similarly, the blocking diodes prevent 
low-voltage (possibly shorted) strings from reducing 
the bus voltage. In addition, zener currents are 
naturally limited by the output of the solar cells. The 
extreme ranges of voltage from full load at -140° C to 
open circuit at 800 C fall within 12.0 to 16.5 volts, 
respectively. Load regulation at any array tempera-
ture is expected to be within 2.5 volts from no load to 
full load. 
Under open circuit conditions, each zener is 
capable of thermally dissipating the output power of 
one string of solar cells. For flexible arrays, a zener is 
suitably bonded and heat sunk with the isolation 
One of Five Solar Array Module	 Charger! Regulator Assemblies 
(Typical I of n)	 16.5 Vdc to 12.0 Vdc Load Bus 
118.0 Vdc to 13.5 Vdc Battery Bus 
1 N4245
Charge!
Discharge
Control
I
' 1
4ells 
1N4963
I
Cells 
2 by 4 cm I I	 Each
Return Bus L- 6- - 6 
63 parallel strings of solar cells 
required per battery.
Power	
-•.. To Load Distributor 
T 12-Cell 
33-Ampere-Hour 
I	 NiCd Battery 
Note: 125 watts average continuous output power 
in 94 minutes per 63 parallel solar-cell 
string/battery combination. 
Zener-Regulated Solar Array/ Battery System
(continued overleaf) 
This document was prepared under the sponsorship of the National
	
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https://ntrs.nasa.gov/search.jsp?R=19750000162 2020-03-19T20:26:05+00:00Z
diodes to approximately solar cell-sized printed-
circuit cards embedded in the array. An alternative is 
to use a stud-mounted zener heat sunk to a rigid 
honeycomb substrate. Approximately 756 zeners are 
used to regulate a 1,500-waft system. 
In the absence of solar radiation, the battery 
characteristic determines system regulation. The 
no-load to full-load voltage swing during the Sun-lit 
period is approximately 2.0 to 2.5 volts (zener range 
plus diode drop). For a change from 10 percent load 
to full load, the regulation is approximately 1.1 volts. 
During the dark period, the battery voltage can 
change from 16.5 volts at full-charge open circuit to 
12.0 volts at low-voltage cutoff (1.00 volt/cell). 
Because the battery can discharge through a series 
regulator, the normal voltage swing is approximately 
16.0 to 15.0 volts with 15 percent depth-of-discharge 
cycle. This estimate, 16 to 15 volts, assumes a voltage 
swing from no load to full load and a good 
low-impedance 12-cell battery. 
Negative pulses generated by loads are prevented by 
the zener during the Sun-lit hours and by the battery 
during the night. Positive-going pulses are clamped by 
diodes from the load bus to the battery output located 
in the battery/charger modules. Since the battery/ 
charger module is a series-dissipative type, minimum 
switching noise is generated by the system. Because of 
these general system design features, it is anticipated 
that pulse noise generated externally to the system 
should be limited to within ±3 volts from the bus
voltage. Therefore no internal system requirement for 
bulky and troublesome noise filters should exist. 
Moreover, one battery/charger/regulator in each 
assembly can operate from either the 28-volt or the 
14-volt bus to balance the system in the event of 
module failure. - 
Note: 
Requests for furhter information may be directed 
to:
Technology Utilization Officer 
Marshall Space Flight Center 
Code ATO1 
Marshall Space Flight Center, Alabama 35812 
Reference: B75-10162 
Patent status: 
Inquiries concerning rights for the commercial use 
of this invention should be addressed to: 
Patent Counsel 
Marshall Space Flight Center 
Code CCO1 
Marshall Space Flight Center, Alabama 35812 
Source: J. T. Eliason of

Sperry Rand Corp.

under contract to

Marshall Space Flight Center 
(MFS-23195)
0 
. 
B75-10162	 Category: 02 (Electronics Systems)
	 S


Zener-regulated solar array/battery power system

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