Power: the batteries
The fuel cells were not the only source of power in the spacecraft. A collection of batteries were included and. despite their weight, there were good reasons for their inclusion. The fuel cells had a limited range of output power. They could not deliver more than 1.4 kilowatts at any one time, yet their power output had to be maintained above 400 watts at all times. The spacecraft’s requirements were much more variable, especially when motors had to operate. For example, the SPS engine had sizeable motors that gimballed its nozzle from side to side during a burn, and these placed heavy drains on the spacecraft’s electrical system. Batteries w ere a way of smoothing out the load on the fuel cells because they could supply extra power during the peaks in demand. At other times, their need to be recharged provided a convenient load for the fuel cells. At the end of the mission, after the fuel cells had departed along with the rest of the service module, the batteries were all that remained to power the command module as it streaked through the atmosphere during re-entry. They were therefore essential!
The CM carried a total of five silver oxide-zinc batteries mounted in the lower equipment bay below the navigation instruments. This battery technology had the highest energy to weight ratio at the time. Two of them were never recharged after launch. Their only use was to provide energy for the various pyrotechnic devices around the spacecraft. These devices performed a range of critical tasks; they separated the launch escape tower, the S-IVB and. at the end of its mission, jettisoned the lunar module. At re-entry, they separated the CM and the SM, jettisoned the upper heat shield from around the spacecraft’s apex and deployed the parachutes. Their health w’as checked regularly throughout the mission.
A further three batteries, each rated at 40 amp-hours, provided supplementary power during busy periods, but they became the sole source of power through reentry, splashdown and post-landing operations. It was these that were recharged at times when the load on the fuel cells was low’. All five batteries were installed in separate pressure cases which, in case the batteries were to emit gases through failure or improper operation, could be vented to space to ensure that the gases did not enter the cabin. An additional 400 amp-hour non-recharged battery was added to the service module after the Apollo 13 incident.
In general, the batteries gave very little trouble. Only once, during the Apollo 7 Earth-orbital flight, were problems encountered when Walt Cunningham discovered that the batteries were recharging more slowly than expected. Then when the CM separated from the SM for re-entry, the voltage delivered by the batteries fell low’ enough to make the caution and warning lights "glow: yellow’ the rest of the w ay", as Cunningham put it. “This was a slightly traumatic experience at this point because we hadn’t expected anything like it,” he added. How’ever. the spacecraft’s systems operated satisfactorily with the slightly low operating voltage. Conversely, Apollo 12
relied on the CM batteries when its lightning strike incident took the fuel cells offline for most of the ascent to orbit.