The 1P battery: capacity, depth of discharge and cycle life

5500 eclipses per year#

In a $525\text{km}$ orbit ($95.1\text{min}$ period), a spacecraft enters Earth's shadow $15.14$ times every day—about $5530$ charge–discharge cycles per year[1]. Consumer lithium-ion pouch and cylindrical 18650 cells are rated for $300\text{-}800$ deep cycles ($80\text{-}100\%$ DoD). Surviving two to three years in LEO requires restricting the eclipse depth of discharge to a small fraction of nominal capacity[2]:

$$\text{DoD}=\frac{P_{\text{ecl}}{t}_{\text{ecl}}}{E_{\text{nom}}}$$

For an eclipse load $P_{\text{ecl}}=0.45\text{W}$ lasting $t_{\text{ecl}}=35.7\text{min}$ ($0.595\text{h}$), the energy drawn per eclipse is $0.268\text{Wh}$. Against a single $3.7\text{V}$, $2500\text{mAh}$ ($9.25\text{Wh}$) 18650 cell or a $600\text{mAh}$ ($2.22\text{Wh}$) LiPo pouch cell, this corresponds to $\text{DoD}=2.9\%$ or $12.1\%$[3].

DoD per eclipse	Charge cut-off (V)	Usable energy / 9.25 Wh cell (Wh)	Cycles to 80% SoH	Years to 80% SoH	3-year retention
10%	4.10	0.93	24,429	4.4	84.4%
15%	4.10	1.39	12,873	2.3	76.4%
20%	4.10	1.85	8,171	1.5	68.3%
20%	4.20	1.85	4,085	0.7	50.2%
30%	4.10	2.78	4,306	0.8	51.9%
45%	4.20	4.16	1,134	0.2	40.0%

Charge-voltage derating and the 0 °C plating threshold#

Two operational limits govern cell longevity in orbit[1]:

1. End-of-charge voltage ceiling: Lowering the maximum charge termination voltage from $4.20\text{V}$ to $4.10\text{V}$ sacrifices about $10\%$ of initial capacity but nearly doubles the number of cycles before solid-electrolyte interphase (SEI) growth reduces state of health to $80\%$.
2. Sub-zero charging inhibit: Charging a graphite-anode Li-ion cell below $0^{\circ }\text{C}$ causes metallic lithium plating on the anode, permanent capacity loss and internal short-circuit risk. Because a $1\text{P}$ has a thermal time constant near $56\text{min}$, the battery exits eclipse near its coldest point—precisely when solar arrays hit peak illumination. An EPS hardware comparator or thermistor-gated charge switch must block charging or divert solar power to a $0.3\text{-}0.5\text{W}$ Kapton heater until cell temperature exceeds $+2^{\circ }\text{C}$[2].