European Space Agency

Objectives

The bypass removal after qualification should allow to:

  • Improve the battery reliability as each bypass unit may fail to operate beside a weak module or operate prematurely beside a healthy module or short the module (in case of motion interrupt) when connected in a battery, - Reduce by at least 30 % of the total price of the battery,
  • Reduce significant weight/volume at battery level,
  • Shorten the battery manufacturing lead time:
    • Simplified battery harness
    • Bypass status: 6 months for US by-pass starting from knowledge of satellite application in order to submit export license request to US Department of Defense up to delivery and about the same for European source,
  • Avoid US export licenses if US by-pass is used,
  • Improve battery resistance in nominal mode.

This test program will show that:

  • Concept is valid for all identified cell failure modes,
  • Concept is reliable (robust and stable self-short resistance versus conditions (cycling, time, pulses, etc) and safe versus all types of modules and conditions.

Challenges

  • No 'safety issue' during the 50 reversals and similar predictable self-short behaviour show that concept is valuable.
  • Unbalanced currents in cells in parallel lead to adapted battery design and limitations.
  • Self-short applicability depends of spacecraft/battery management constraints possibly interfering with recommended procedure for best conditions (low & stable resistance to limit thermal dissipation)
  • Conducted tests were not covering 'worst case' which should have given direct applicability for wide use ahead of model release and predictions. Case by case analysis required consequently.
  • Short characterization performed on modules lead to need of self-short knowledge at cell level.

Benefits

  • Self-short is automatic and reliable:
    • No more bypass for 3P modules except otherwise agreed depending of possibility to be within qualification limits or electrical/thermal/mechanical acceptable limits predictable by analysis (depending of model refinement and correlation degree)
      • Lighter/smaller (along width)/cheaper batteries
    • Bypass TC/management replaced by global battery management at satellite level
  • Self-short occurrence is predictable:
    • Self-short can be managed and optimized versus validated formulas (typical events = f(DOD, N cells)).

Features

No extra component than cells and their connecting.

Concept:
Self-short occurs 'automatically' during Li-ion Saft cell reversal


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CVES140 cell = 40Ah capacity when discharged at C/1,5 from 4,1V to 2,7V; 20°C

Discharge to Self-short Sequences:


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Parameters influences on Self-short:

  • Current rate: Rcc = f(I/C)


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  • Cycling and process interruption versus completed steady state discharge: Resistance along ?tendency + 3? merges to "1shot self-short average tendency" due to stabilization cycling.
    Self-short stopped on [126,146]%DOD leads to a resistance above "tendency + 3".
  • Parallel connecting: correction of Rcc by 3/N ratio from Rcc = f(I/C) of 3P configuration gives a good magnitude for N cells in parallel.
  • Current rate, Temperature and Stabilization cycling:
    Rcci, Rcc =f(Imodule, T, after short/after cycling)


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Parallel connecting: unbalanced individual current up to 100%.

Rcc is on stabilization slope after 10 cycles (C/1,5 and 20°C) and merges to the target of Rcc= f(I/C= 1/1,5)


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  • Cycling aging prior self-short: None
  • High rate charge current right after self-short: self-short resistance increased. Cu dissolution and dendrite growth may be affected at the most critical moment.
  • Calendar storage/open-circuit mode: Rcc stabilizes at a value above other cells when cumulated to other conditions (short stopped on non stabilized high Rcc + C/1,5 high charge rate).
  • Leakage: 1mm hole leads to progressive predictable behaviour.


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In case of no thermal control in vacuum, the leaky cell in parallel to a healthy one overdischarges, heats, generates gasses, dries and increases its resistance to open leading to current running in the reversed good cell which pops without safety issue.

In case of thermal control, for discharge at C/1,5 to 120% DOD: Umin/95°C and correct self-short (6,4mOhm) obtained.

  • Pulse (3C/250ms): none.
  • Other: no special interest to match cells versus internal resistance or capacity within a module.

Plan

The plan has been established in accordance with ESA quality standards.

  • Validation of concept/formulas/model (typical events = f(DOD, N cells)),
  • Evaluation of self-short characteristics,
  • Identification of findings, missing data and further investigation.

Current status

Bypass removal qualification within this Artès 3 program was successfully completed in May 2005.

  • 1 program shall fly soon without bypass considering possibility to accept 'constraints' by typical satellite/battery management.
  • Saft has been granted of new ESA programs (Artès 8, GSTPv3) for identified opened points and further analysis:
    • Get single cell 'Ri/Rcc, OCV = f(DOD)' to assess assumptions (localization of dead short for OCV= 0V and maximum Rcc value) and 'Rcc1 shot = f(I/C)' to prepare a thermal model.
    • Conduct tests covering 'worst case' versus electrical profiles (low rate reversal stopped at Rcc max + high rate eclipse starting from ½ season longest duration; direct high rate reversal; high rate being linked to 'full unbalanced module status'), temperature, pressure (vacuum) and cell configuration (aged prior short for high internal cell resistance and possibly cycled at low rate after short for frozen high short resistance). Fine thermal mapping will allow to track maximum temperature/self-short location and pressure measurement will allow to determine mechanical margins of safety and classify cell versus MIL STD 1522A regulation. Pass criteria will not be limited to self-shorted cells (see above) but also to healthy neighbours (temperature limitation).

Status date

Wednesday, September 3, 2008 - 12:37