In Earth's shadow its solar panels produce nothing, so the satellite runs on batteries charged in sunlight until it comes out again.
An eclipse, for a satellite, is the time it spends in Earth's shadow. Its solar cells stop producing power, as NASA's small-spacecraft review points out. For those minutes the battery carries the whole load.
So every satellite carries a battery big enough to last through its longest eclipse, with power to spare.
A satellite in low Earth orbit dips into shadow on most laps. The ISS circles Earth 16 times in 24 hours, according to NASA, so its batteries are charged and drained many times every day.
Worked example: at about 408 km, the ISS can spend up to roughly 35 minutes of each 90-minute lap in shadow (see the calculation below). Sixteen laps a day means nearly 6,000 charge–discharge cycles a year.
Kalpana-1 (2002) used an 18 Ah nickel-cadmium battery. Newer Indian satellites use lithium-ion: IRNSS-1A carries one 90 Ah lithium-ion battery, and GSAT-19 flew an indigenous lithium-ion battery. NASA gives today's commercial lithium-ion cells 150 to 270 watt-hours per kilogram.
A geostationary satellite is high enough to miss Earth's shadow for most of the year. Around the equinoxes, though, the shadow lines up with its orbit once a day. NOAA says its GOES weather satellites see eclipses from late February to mid-April and from late August to mid-October, and must run on batteries for up to 72 minutes a day.
The battery on ISRO's Mars Orbiter Mission was designed for eclipses of only about 1 hour 40 minutes. In early 2017 ISRO saw a coming eclipse of up to 8 hours — long enough to drain the battery beyond its safe limit.
On 17 January 2017 the team fired all eight of its 22 N thrusters for 431 seconds, changing its speed by 97.5 m/s. The new orbit avoided eclipse until September 2017. The burn used about 20 kg of propellant and left 13 kg.
Five years later the story ended in the shadow. ISRO lost contact with the spacecraft after a long eclipse in April 2022.
Follow the ground track across the night side of the map: during that stretch the station is in Earth's shadow and running on batteries. Open the ISS tracker
Eclipse length in a circular low orbit follows from geometry. Treat Earth's shadow as a cylinder of radius R. A satellite at radius r = R + h is in shadow over an arc of 2β of its orbit, where (when the Sun lies in the orbit plane)
sin β = R ÷ r
For the ISS, with R = 6,371 km (NSSDCA) and h ≈ 408 km, r = 6,779 km:
sin β = 6,371 ÷ 6,779 = 0.940, so β ≈ 70°, and the shadow arc ≈ 140° of 360°.
With an orbit of roughly 90 minutes (NASA gives 16 orbits a day), that is about 0.39 × 90 ≈ 35 minutes in shadow, the worst case. When the Sun is out of the orbit plane the eclipse is shorter, or vanishes.
Battery energy needed is E = P × t. A 1,000 W load through a 35-minute eclipse needs about 583 Wh; through the 72-minute geostationary maximum quoted by NOAA it needs 1,200 Wh. NASA gives commercial lithium-ion cells 150–270 Wh/kg, so 1,200 Wh needs at least 4.4–8 kg of cells before any margin.
Cycle count drives battery life. At 16 orbits a day, a low-orbit battery sees about 16 × 365 ≈ 5,840 charge–discharge cycles a year. NASA lists lithium-ion cycle life from 2,000 to 70,000 cycles depending on cell and depth of discharge, which is why low-orbit batteries are run shallow. A geostationary battery, by contrast, sees only two eclipse seasons a year.
Solar panels give no power in Earth's shadow, so a battery charged in sunlight must run the satellite through each eclipse.
Only in two seasons around the equinoxes; for GOES satellites NOAA gives late February to mid-April and late August to mid-October, with up to 72 minutes a day on batteries.
It fired the spacecraft's thrusters to change its orbit so that it would avoid an 8-hour eclipse that its battery, designed for about 1 hour 40 minutes, could not survive.