During Mars Global Surveyor system test, it was discovered that horizontally mounted nickel-hydrogen battery cells are subject to electrolyte “pooling” in a 1 g. environment. This can damage the cells; high charge rates (and temperatures) increase the likelihood of damage. Some measures to prevent battery popping (rapid oxygen-hydrogen recombination under fast charge conditions) are recommended for nickel-hydrogen batteries during any ground phase.
Description of Driving Event:
| Mars Global Surveyor (MGS) carries two nickel-hydrogen (NiH2) battery assemblies: one mounted to the +Y and one to the -Y propulsion module. Each assembly consists of eight 20Ahr NiH2 common pressure vessels (CPVs) in a honeycomb composite enclosure mounted on an aluminum baseplate. During MGS system test, shortly after installation of the batteries on the spacecraft, an intermittent "popping" or "snapping" sound issued from both batteries. This noise was most likely caused by oxygen-hydrogen (H2/O2) recombination in the battery under the fast charge rate conditions normally present throughout the charge cycle. |
H2/O2 recombination is a natural chemical process which occurs mostly during the late charge and early discharge stages of NiH2 battery operation. Audible popping occurs during H2/O2 recombination when a large enough oxygen bubble is allowed to form on an electrode plate inside the cell stack before it can recombine with hydrogen. When this bubble finally does recombine, a popping sound results. Such popping may induce significant damage to cell plates and may reduce battery cycle life.
This phenomenon was exacerbated by the MGS practice of mounting and testing the battery on its side (i.e., horizontally) in a 1 g. environment. Instead of remaining fully contained within the porous electrodes, a small amount of the liquid electrolyte was drawn by the force of gravity to the bottom of the horizontal CPV. The liquid pressure from this "pooling" of electrolyte promoted the formation of oxygen bubbles large enough to recombine violently. Such pooling cannot occur in a zero g. environment.
Use of the flight batteries was judged to represent a low risk to MGS mission success. X-ray inspection showed that the popping had caused some deformation of the cell plates. However, charge retention testing showed no degradation in battery performance, and the item features a heritage design and robust capacity.
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Figure 1. NiH2 CPV Positioned Vertically
(only the upper of two stack sets is shown)
Additional Keyword(s): Battery Charging
- Problem/Failure Report No. B0M9XU
- Walker, S., "Battery Noise Anomaly Close-Out Engineering Analysis," September 26, 1996
A nickel-hydrogen battery design where the cells are mounted on their sides (i.e., horizontally), instead of in the vertical position shown in Figure 1, increases the potential for electrolyte "pooling" in a 1 g. environment and for possible damage to cells. High charge rates (and temperatures) increase the likelihood of battery popping.
For nickel-hydrogen batteries, take precautions against battery popping during any ground phase:
- Reduce the charge rate as the batteries approach full charge.
- To minimize the likelihood of pooling, ensure that battery cells contain no excess electrolyte.
- Mount batteries to minimize electrolyte pooling in a 1 g. environment.
Evidence of Recurrence Control Effectiveness:
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