Batteries Flashcards
how does the battery labelling convention work?
It is labelled according to its cell requirements:
what kind of missions are primary batteries used for?
Short missions as they are not rechargeable
What is the Wh efficiency of a secondary battery?
ratio of energy provided by discharge over energy accumulated during charge E_D / E_c
What Wh efficiencies do modern Li-ion batteries have? What about NiCd and NiH2?
95 %
80 %
For a battery what is the k-factor?
ratio of charge accumulated during charging to the charge released during discharging
What is the nameplate capacity, C?
It is the maximum amount of available charge stored by a battery, Q_D,max, also known as the Ah capacity
What are the specific energy and energy density of a battery?
Energy per unit mass (Wh / kg) and energy per unit volume (Wh / l)
How is the stored energy of a battery represented?
As the Wh capacity
What do charged batteries do when left unused?
They self-discharge. More prevalent in NiCd and NiH2 than Li-ion
what are the 1st 5 requirements for a secondary battery in space?
- peak power delivery should be good
- rapid re-charge at high rates (care not to overcharge)
- large number of charge cycles
- High recharge efficiency or k-factor
- good hermetic seal for vacuum operation
How is depth of discharge defined?
Measure of the charge removed from the battery as a percentage of the nameplate capacity
How is depth of discharge related to battery lifetime?
Larger DoD means shorter lifetime
what are the 2nd 5 requirements for a secondary battery in space?
- ability to survive launch vibrations and shock
- stable electrical characteristics
- availability of electrically well-matched cells
- ability to withstand overcharge (NiCd and NiH2 can Li_ion cannot)
- maximum specific energy or energy density (Li-ion best)
Describe the redundancy approach for cells to protect against them failing both open and short. In particular describe the large and small cells approach
small cells approach:
cell strings in parallel without interconnections
extra string for protection against failure to any cell
large cells approach:
cells in parallel interconnected
extra group of cells in series to protect against failure of any cell
How is reconditioning of a NiCd or NiH2 battery carried out?
A deep discharge followed by a full recharge. Not appropriate for all batteries
What can be said about the overcharge and over-discharge capability of a Li-Ion cell
they have none, overcharging them can lead to a fire above or below V_end-of-charge and V_end-of-discharge
What is the maximum safe rate for charging of a NiCd and NiH2 cell?
C/10 though C/20 is safer
What happens if you attempt to charge at C/100
The rate of charge is less than the rate of discharge i.e. it is ineffective
What are the two different charging methods for NiCd and NiH2 cells?
Constant current(C/10, C/100) then Constant voltage (taper charged)
For Li-Ion cells what are the two charging methods?
Constant current at C/4 to C/2 until terminal voltage approaches Vendofcharge then Constant voltage at Vendofcharge
How many eclipses per year are expected in GEO and LEO?
GEO: 90 eclipses at 72 minutes for each eclipse
LEO: 5500 at 30 minutes each
smaller depth of discharge used in leo to allow for more charge cycles and thus longer life, hence NiCd and Li-Ion most appropriate
What battery technologies are most suited to GEO and LEO and why?
smaller depth of discharge used in leo (~20%) to allow for more charge cycles and thus longer life, hence NiCd and Li-Ion most appropriate
larger depth of discharge used in leo (~20%) to allow for more charge cycles and thus longer life, hence NiCd and Li-Ion most appropriate
