IEEE Electrification - December 2020 - 14

Battery Current

Random Positive Current Based
on the Power Surplus From the PV
Random Negative Current Based
on the Power Deficit From the PV

Time

Battery Voltage
vEOC
vNOM
vEOD

Current-Mode
Charging

Voltage-Mode
Charging

Discharging
Time

Figure 8. A typical Li-ion battery's charging and discharging characteristics.

violation of these voltage levels would cause deformations in the electrochemistry of the battery, which significantly negatively affects its lifetime. These voltage limits
are displayed in Figure 8, together with the battery's
nominal voltage.
The energy injected into the battery is not always fully
recovered due to the electrochemical process involved as
well as due to its internal impedance. Accordingly, the battery is also characterized by its energy efficiency (h BT-CH/DCH),
which varies from one technology to another. Finally, the
energy that the battery stores with respect to its weight,
known as the gravimetric energy density (Wh kg -1), as well
as its storable energy with respect to its size, known as the
volumetric energy density (Wh l -1), are other key factors
that are usually taken into account when selecting the
right battery for the mission.

3.3-V
LDO

Passive Heater
(Kapton)

Active Heater

Heater
Switch

Overcurrent
Protection

MCU
Housekeeping
Heater Control

Temperature
Sensors

GND
Break

Figure 9. A simplified thermal management schematic of batteries for
satellites. MCU: microcontroller unit; LDO: low dropout; GND: ground.

I E E E E l e c t r i f i cati o n M agaz ine / DECEMBER 2020

It is important to note that the DOD of the battery varies in the whole range, i.e., from 0 to 100%, when the battery voltage is in the regions with lower steepness that
correspond to the nominal voltage (see Figure 8). As can
also be seen from this figure, the battery charges under a
randomly changing positive current, where the level of
this latter is dependent on the harvested PV power and
consumed one. Once the battery voltage reaches v EOC, the
battery charge control shifts to voltage mode to not
exceed this limit, while keeping the battery charge at the
maximum. In practice, the battery would still consume
some current (see Figure 8) even when it is fully charged
due to the fact that it loses some of the stored energy. During the discharging phase of the battery, it shifts again to
current mode, where the current value is determined
based on the difference between the harvested PV power
and the load demand.
Onboard the satellite, the most constrained element
by the temperature range is the battery, even if it is not
being used (neither charging nor discharging) (Kang and
Oh 2016). Moreover, despite using the battery in the
allowed range, its performance degrades as the temperature goes toward one of the extreme limits (very hot or
very cold). Under low temperatures the battery internal
resistance increases, which worsens the power losses. If
the temperature goes even lower, the battery's internal
resistance may increase to a point where its discharging
becomes challenging. Accordingly, batteries for satellites
are equipped with both passive and active heaters. For
the former, Kapton film is wrapped around the battery to
maintain its temperature at average levels and as steadily as possible. For the active heater, resistor-based ones
are usually used, as depicted in Figure 9. As can be seen
in the figure, this heater is fed from the battery itself,

IEEE Electrification - December 2020

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