Batteries & energy storage

Cell formats in a 50 mm envelope

A $1\text{P}$ PocketQube has an internal board footprint of $42\times 42\text{mm}$[1]. A standard cylindrical 18650 cell ($18\text{mm}$ diameter by $65\text{mm}$ length, $\approx 45\text{-}48\text{g}$, $9\text{-}12\text{Wh}$) cannot lie flat inside a $1\text{P}$ cube; it fits only along the vertical axis of a $1.5\text{P}$, $2\text{P}$ or $3\text{P}$ spacecraft or diagonally with a custom structural bracket[2]. Most $1\text{P}$ designs therefore use flat lithium-polymer (LiPo) pouch cells ($2.0\text{-}5.5\text{Wh}$) or shorter cylindrical cells (14500, 16340, 18350)[3].

Cycle life, vacuum containment and thermal limits

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%

Three failure modes require dedicated hardware protection on small satellites[3]:

1. Vacuum pouch delamination and swelling: Unlike hard-can 18650 cells, laminated aluminium-polymer pouch cells expand under $0\text{atm}$ external pressure and gas generation. Flight pouch cells must be clamped between rigid FR-4 or aluminium plates with a controlled preload pad.
2. Sub-zero lithium plating: Charging below $0^{\circ }\text{C}$ plates metallic lithium onto the graphite anode. A thermistor-controlled heater ($0.3\text{-}0.5\text{W}$) and hardware charge inhibit are mandatory when eclipse minimum temperatures approach $0^{\circ }\text{C}$[2].
3. Magnetic dipole of steel 18650 cans: Standard 18650 shells are nickel-plated steel and can acquire a remanent dipole after spot welding or vibration, perturbing passive magnetic attitude control[4].