LM Electrical Power: Silver-Zinc Batteries and the Art of the Power Budget
How the Lunar Module ran on batteries instead of fuel cells—and how every watt was rationed across systems that all wanted more than the budget allowed
The Command/Service Module ran on fuel cells—hydrogen-oxygen electrochemical power plants that generated electricity continuously for the entire mission duration, producing water as a byproduct. The Lunar Module had no fuel cells. It ran on batteries. Non-rechargeable, one-shot, silver-zinc batteries that were fully charged before launch and steadily depleted throughout the LM’s operational life. When the batteries were dead, the LM was dead. Every system, every light, every heater, every actuator, every computation the AGC performed drew from a finite reservoir of stored electrical energy that could not be replenished.
This made power management on the LM a zero-sum game. Every watt allocated to one system was a watt unavailable to another. The power budget—a meticulously maintained accounting document that tracked every electrical load on the vehicle—was one of the most critical mission planning artifacts. Going over budget didn’t mean a higher electric bill. It meant running out of power before the mission was complete.
Why Batteries Instead of Fuel Cells
The decision to power the LM with batteries rather than fuel cells was driven by weight, simplicity, and mission duration. Fuel cells were superior for long-duration power generation—the CSM’s fuel cells could run for two weeks with adequate hydrogen and oxygen supplies. But fuel cells were heavy, complex, and required cryogenic reactant storage systems. The hydrogen and oxygen tanks, the fuel cell stacks, the water separator, the thermal management system—the total weight of a fuel cell power system for the LM would have exceeded 400 pounds.
The LM’s powered operational life was short—typically 48 to 75 hours from activation to jettison, depending on the mission. For a power system that only needed to operate for a few days, batteries offered a better power-to-weight ratio than fuel cells. Silver-zinc batteries had the highest energy density of any rechargeable (or primary) battery chemistry available in the 1960s, storing approximately 60-80 watt-hours per pound. The LM’s battery complement weighed roughly 300 pounds and stored enough energy for the planned mission with reserves.
The simplicity argument was also compelling. Batteries had no moving parts, no plumbing, no cryogenic storage, no startup sequence. They were electrochemical devices that produced electricity when a load was connected. No warmup time, no steady-state operating temperature to maintain, no reactant flow control. In a vehicle where every mechanism was a potential failure mode, the passivity of batteries was a feature.
Battery Architecture: Descent and Ascent
The LM’s electrical power system was divided between the descent stage and the ascent stage, mirroring the vehicle’s two-stage architecture.
Descent stage batteries: Four silver-zinc batteries, designated ED batteries, were mounted in the descent stage equipment bay. Each battery had a nominal voltage of 28 volts DC and a rated capacity of approximately 400 ampere-hours. The four descent batteries were the primary power source from LM activation through the lunar surface stay. They powered the descent engine’s control electronics, the landing radar, the guidance computer, the communications systems, the environmental control system, and the cabin lighting during the entire descent and surface operations phase.
The descent batteries also powered the LM during the undocking and separation sequence, the descent orbit insertion burn, and the powered descent itself—the most power-demanding phase of the descent stage’s operational life. The AGC, the Digital Autopilot, the descent engine gimbal actuators, the landing radar, the DSKY, the communications systems, and the crew’s suit fans all drew from the descent batteries simultaneously during the 12-minute powered descent.
Ascent stage batteries: Two silver-zinc batteries, designated EA batteries, were mounted in the ascent stage. These batteries had similar voltage and capacity specifications to the descent batteries and powered all ascent stage systems from staging through rendezvous and docking. After the ascent stage separated from the descent stage, the ascent batteries were the sole power source—the descent batteries, still sitting on the lunar surface, were no longer connected.
A power transfer switch allowed the descent batteries to supplement the ascent batteries before staging. During the surface stay, when power demand was relatively low, the descent batteries carried most of the load while the ascent batteries were reserved for the ascent phase. This cross-feeding optimized battery life by ensuring that the descent batteries—which would be left on the Moon anyway—were depleted before the ascent batteries were drawn down.
The Electrical Power Distribution System
Power from the batteries was distributed through two main DC buses: DC Bus A and DC Bus B. Both buses were nominally connected to the battery complement, and most critical systems were powered from both buses through diode isolation circuits that prevented a failure on one bus from propagating to the other.
Critical systems—the AGC, the primary communications transponder, the ECS fans—were powered from both buses so that the loss of either single bus would not cause the loss of the system. Less critical systems—certain lighting circuits, non-essential heaters, test equipment—were powered from only one bus.
The buses supplied 28 volts DC, which was the standard power supply voltage for Apollo avionics. Inverters converted DC to 400 Hz AC for systems that required alternating current—primarily the IMU’s gyroscope spin motors and certain instrument panel indicators. The inverters were solid-state devices (no rotating machinery) and were themselves powered from the DC buses.
Circuit breakers on the instrument panel allowed the crew to disconnect individual systems from the buses. The LM’s circuit breaker panels were dense—dozens of breakers, each labeled with the system it protected, arranged in rows that the crew memorized. Powering systems down to conserve battery life was a routine procedure, and the crew’s familiarity with the circuit breaker layout allowed them to reconfigure the electrical system quickly in response to changing mission requirements or anomalies.
The Power Budget: Accounting for Every Watt
The power budget was developed months before each mission by the Electrical Power System engineers at Grumman and the Mission Planning and Analysis Division at the Manned Spacecraft Center. The budget listed every electrical load on the vehicle, its power consumption, and the timeline during which it would be active. The sum of all loads at any point in the timeline gave the instantaneous power demand; the integral of demand over the entire mission gave the total energy consumption, which had to fit within the battery capacity.
A typical LM power budget during powered descent—the highest-demand phase—looked something like:
- AGC and DSKY: ~70 watts
- IMU: ~100 watts
- Communications (S-band): ~80 watts
- Environmental Control System: ~150 watts (fans, pumps, valves)
- Landing Radar: ~110 watts
- Descent engine electronics: ~50 watts
- Instrument panel displays: ~40 watts
- Lighting: ~20 watts
- Miscellaneous: ~80 watts
Total: approximately 700 watts during powered descent. At 28 volts, this was about 25 amperes drawn from the descent batteries. Over the 12-minute descent, the energy consumed was roughly 140 watt-hours—a modest fraction of the total battery capacity, but one that had to be budgeted against all the other mission phases.
The surface stay was the longest phase and consumed the most total energy. Systems that were non-essential during the surface stay—the landing radar, the descent engine electronics, the rendezvous radar—were powered down. But the AGC continued running (maintaining the navigation state and providing timing for the crew’s EVA activities), the communications systems remained active (providing voice, telemetry, and television), and the ECS ran continuously. The surface stay power demand was lower than the descent phase but persisted for 24 to 75 hours depending on the mission.
Apollo 13: The LM as Lifeboat
The LM’s electrical power system was never designed to support three crew members for four days—but that’s exactly what it did on Apollo 13. After the oxygen tank explosion in the Service Module knocked out the CSM’s fuel cells, the LM became the crew’s lifeboat. Its batteries and oxygen became the only power and life support available for the return to Earth.
The power budget for the lifeboat configuration was desperate. The LM’s batteries had been sized for a nominal two-day mission with two crew members. Now they needed to last four days with three crew members and a completely different usage profile—no landing, no surface stay, but continuous operation during the transearth coast.
Mission Control’s electrical power engineers worked through the night to develop a minimum-power configuration. Every non-essential system was shut down. The AGC was powered down except during critical navigation updates and the midcourse correction burns. The communications system was reduced to a single low-power mode. The environmental control system was cut to minimum—one fan, reduced water flow, no cabin heating. The cabin temperature dropped to about 38°F as the crew huddled in a darkened, cold spacecraft.
The power consumption was reduced from the nominal 700+ watts to approximately 250 watts—barely enough to keep the life support running and the communications link alive. The crew switched between descent and ascent batteries to distribute the drain. Ground controllers tracked the battery voltage and current draw in real time, adjusting the power plan as the remaining capacity was consumed.
The batteries held. When the crew powered up the Command Module for reentry—using the LM’s batteries to supplement the CM’s reentry batteries through a power-transfer procedure that had never been tested in flight—there was enough energy remaining to complete the startup sequence and separate the vehicles. The LM’s power budget, designed for a Moon landing, had been replanned in flight for survival, and it worked with almost no margin to spare.
Finite Energy, Finite Time
Every LM mission was a countdown. Not just the clocks on the instrument panel, but the unseen countdown in the batteries—the steady, irreversible conversion of chemical potential energy into electrical current, each ampere-hour consumed bringing the vehicle closer to the moment when the lights would go out. The crew never saw the countdown, but the ground controllers watched it constantly, tracking battery voltage and amp-hours remaining against the power budget’s predictions.
The silver-zinc batteries performed reliably on every mission. No battery failed in flight. No mission was cut short by power depletion. No crew was ever in danger from a power system malfunction. The engineering was sound, the budgets were accurate, and the margins—thin on some missions, comfortable on others—held.
But the finite nature of battery power shaped every aspect of the LM’s mission planning. Surface stay times were limited not just by oxygen and consumables but by battery capacity. EVA durations were constrained by the power needed to keep the cabin systems running while the crew was outside. Every new instrument or experiment that mission planners wanted to add to the LM required a line item in the power budget, and if the budget was full, something else had to be cut or the addition couldn’t fly.
The LM ran on batteries because batteries were lighter than fuel cells for a short mission. The trade-off was that the mission had to stay short—every hour on the lunar surface was an hour of battery life consumed, and the batteries had no way to recharge. The power budget was the LM’s true operational clock, counting down in ampere-hours instead of minutes, from fully charged to the last watt that would carry the ascent stage back to the Command Module and home.