A Case Study in NASA-DoD - The Black Vault

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each battery has its own charge control unit. The latter is frequently
considered a more reliable system in the event of a single point failure
and has been the system considered preferable by the Air Force. Replacement
of the HMS power system with one similar to that used on the
STPSS would require a substantial amount of redesign.
The PRU, however, has considerable redundancy: two peak power
tracking circuits, two bias supply circuits (bias converters with
separate fuses), three control logic circuits, and six switching regulators
(each rated for 600 W or 18 A maximum). With little additional
cost or time, it is possible to arrange two regulators in parallel to
supply each of three batteries, with separate logic control for each
pair of regulators. The battery outputs would be diode-isolated from
the load bus. These modifications would result in a battery charging
system more analogous to that of the STPSS.
The unregulated bus voltage (28 ±7 V) was selected to permit extraction
of the full Ah rating from the battery, even after several
years of aging when the discharge voltage may have decreased to as low
as 21 V. On the high side of the voltage range, the batteries require
a maximum of 33.4 V at the terminals under worst case charging conditions
(highest current level and a battery temperature of 0 0 C). Because
the PRU has a voltage clamp at 35 V, the tolerance was set at
±7 V for symmetry. The ±7 V tolerance requires that the experiments
incorporate a preregulator with a larger dynamic range than would be
required for the AEM or STPSS (±4 V and ±5 V, respectively). The PRU
locates the peak power point by hunting around the equilibrium value
at a 70 Hz rate. The resultant 0.5 V peak-to-peak 70 Hz ripple (at a
7 A load) that the PRU imposes on the bus also must be removed by the
preregulator at the input of each experiment (it is not practical to
filter out so low a frequency).
The PRU is a series regulating element and thus tends to provide
lower efficiency than the conventional shunt regulators, e.g., the
direct energy transfer systems used on the AEM and STPSS. At synchronous
altitudes, this shows up as about a 5 to 10 percent lower
efficiency for the PRU approach. In addition, the PRU approach may
be as much as 10 percent heavier than the direct energy transfer systems.
it has been claitied that in low earth orbits, e.g., altitudes