Batteries

Question 1

how does the battery labelling convention work?

Answer 1

It is labelled according to its cell requirements:

Question 2

what kind of missions are primary batteries used for?

Answer 2

Short missions as they are not rechargeable

Question 3

What is the Wh efficiency of a secondary battery?

Answer 3

ratio of energy provided by discharge over energy accumulated during charge E_D / E_c

Question 4

What Wh efficiencies do modern Li-ion batteries have? What about NiCd and NiH2?

Answer 4

95 %
80 %

Question 5

For a battery what is the k-factor?

Answer 5

ratio of charge accumulated during charging to the charge released during discharging

Question 6

What is the nameplate capacity, C?

Answer 6

It is the maximum amount of available charge stored by a battery, Q_D,max, also known as the Ah capacity

Question 7

What are the specific energy and energy density of a battery?

Answer 7

Energy per unit mass (Wh / kg) and energy per unit volume (Wh / l)

Question 8

How is the stored energy of a battery represented?

Answer 8

As the Wh capacity

Question 9

What do charged batteries do when left unused?

Answer 9

They self-discharge. More prevalent in NiCd and NiH2 than Li-ion

Question 10

what are the 1st 5 requirements for a secondary battery in space?

Answer 10

1. peak power delivery should be good
2. rapid re-charge at high rates (care not to overcharge)
3. large number of charge cycles
4. High recharge efficiency or k-factor
5. good hermetic seal for vacuum operation

Question 11

How is depth of discharge defined?

Answer 11

Measure of the charge removed from the battery as a percentage of the nameplate capacity

Question 12

How is depth of discharge related to battery lifetime?

Answer 12

Larger DoD means shorter lifetime

Question 13

what are the 2nd 5 requirements for a secondary battery in space?

Answer 13

6. ability to survive launch vibrations and shock
7. stable electrical characteristics
8. availability of electrically well-matched cells
9. ability to withstand overcharge (NiCd and NiH2 can Li_ion cannot)
10. maximum specific energy or energy density (Li-ion best)

Question 14

Describe the redundancy approach for cells to protect against them failing both open and short. In particular describe the large and small cells approach

Answer 14

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

Question 15

How is reconditioning of a NiCd or NiH2 battery carried out?

Answer 15

A deep discharge followed by a full recharge. Not appropriate for all batteries

Question 16

What can be said about the overcharge and over-discharge capability of a Li-Ion cell

Answer 16

they have none, overcharging them can lead to a fire above or below V_end-of-charge and V_end-of-discharge

Question 17

What is the maximum safe rate for charging of a NiCd and NiH2 cell?

Answer 17

C/10 though C/20 is safer

Question 18

What happens if you attempt to charge at C/100

Answer 18

The rate of charge is less than the rate of discharge i.e. it is ineffective

Question 19

What are the two different charging methods for NiCd and NiH2 cells?

Answer 19

Constant current(C/10, C/100) then Constant voltage (taper charged)

Question 20

For Li-Ion cells what are the two charging methods?

Answer 20

Constant current at C/4 to C/2 until terminal voltage approaches Vendofcharge then Constant voltage at Vendofcharge

Question 21

How many eclipses per year are expected in GEO and LEO?

Answer 21

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

Question 22

What battery technologies are most suited to GEO and LEO and why?

Answer 22

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