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Satellite Battery Capacity Problem

Batteries make up a significant part of the in-orbit weight of a communications satellite but are needed to keep the communications system operating during eclipses. This question looks at some trade-offs in battery capacity. Therefore, given a satellite carrying batteries that weigh 65kg of a total in-orbit weight of 1130kg at start of life; the communication system weighs 167kg, and the station keeping fuel and propellant weigh 189kg. The solar array and power system weigh 77kg. The satellite requires 140 watts of housekeeping power at all times, and the communication system draws 875 watts of electrical power. During an eclipse of 70 minuets, the batteries discharge to 50 percent of their capacity.

If a power system operates at 50 volts, what is the capacity of the batteries in ampere-hours? Note, one ampere-hour means the battery supplies a current of one ampere for one hour.

Solution: The total power demand of the spacecraft is equal to: 875w + 140w = 1015w

With a 50V supply, the current drawn is equal to: 1015w / 50V = 20.3A (Ohms Law)

The battery must supply 20.3A for 70 minuets and discharge to 50 percent of itís capacity, so:

Power Supplied = 20.3A (70 minuets / 60 minuets) = 23.68Ahrs

Therefore, the battery capacity must be: 2 (23.68) = 47.4Ahrs

Where 1Ahrs means the battery can supply a current of one amp for one hour.

Satellite key words: electrical battery power, battery capacity, discharge rate, battery life, housekeeping, current draw, ampere, amps, communication system, supplied power, spacecraft batteries, weight and configuration, solar power, DC generation, SATCOM, power consumption.

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