For three years, a consortium led by Airbus, tested batteries under abusive conditions. Why did ESA contracted them to run this activity? The Agency wanted to understand how to keep batteries safe after the end of a spacecraft’s mission.
What led ESA to carry out such a study?
The ‘Requirements on Space Debris Mitigation for ESA projects‘ require the passivation of a satellite at its End-of-Life. This rule applies both to electrical and propulsion systems aboard a satellite.
But why does the policy include such a rule?
In the past, we have observed spacecraft explosions due to some battery failures, leading to space debris generation. Today, we do not use the type of battery responsible for these explosions.’ Yet, although we have never noticed in-flight explosions caused by the failure of current Li-Ion batteries, there is still a potential risk, highlighted by our activity,’, explains Francois Bausier, ESA Technical Officer for the project, working with fellow ESA battery specialist Maria Nestoridi.
The goal of the activity was to test Li-Ion battery cells and modules (meaning several cells connected together) under extreme conditions, of the kind expected to be encountered following spacecraft disposal, in order to assess their safety.
The batteries have been tested under abuse conditions
As we were expecting violent reactions a safe place had to be identified to run the tests. So all tests were carried out in bunkers located in France. The ESA team had the chance to visit one of these locations, situated in the Pays de la Loire region. They had a fascinating tour of these CEA facilities; the bunkers are highly secure since they are also used to test explosives.
Most of the tests have been conducted in an inert atmosphere to mimic as much as possible the space environment. This was achieved by reducing the amount of oxygen as much as possible.
Cells and Battery were tested under various ‘abusive’ conditions:
- External short circuit
- Internal short circuit
- High temperature
- Micrometeoroid (or space debris) impact
For this test, the cell was shot by an aluminium bullet of 8mm diameter in order to observe the reaction triggered by this impact. The bullet’s energy was representative of that of a micrometeoroid.
Here, the internal short circuit was caused by a nail injected into the cell.
During the external short-circuit test, the positive and negative terminals of the battery cell were connected together. In space, the cause of an external short circuit can be:
- a faulty connection between the positive and negative terminals
- an insulation failure
- structural failures.
Overcharge was one of the most reactive tests as can be seen on this picture during a module level test. The results turned out to be quite dependent on the charge current and on the battery configuration (cell or module).
Finally, high temperature tests were conducted to identify at which temperature that thermal runaway would be initiated. Sometimes this exothermic reaction occurs so rapidly that cell protections don’t have time to react on time and the battery can blow up.
The result of all these conditions can be catastrophic, with the potential to generate space debris if occurring in orbit.
The tests in numbers:
- 3 years of research
- 200 to 300 tests completed
- over 200 battery cells and modules used for the tests. Part of the batteries were brand new, and the other part aged and irradiated. ‘Aged’ means that the batteries were charged and discharged many times.
- a consortium of one prime contractor, Airbus, and three sub-primes, CEA, SAFT and ABSL.
Conclusions and recommendations
‘During these tests it has been noticed that there is a risk of explosion at end-of-life if the battery is not passivated,’ explains Francois Bausier. ‘Moreover, abuse conditions could lead to catastrophic reactions even for cells with internal protections.’
Thus, the study led to the creation of recommendations to keep the battery safe at end of mission. These recommendations should be of particular benefit to low Earth orbiting class of satellites, which have to follow space debris mitigation requirements.
So the main recommendations arising from the project:
- discharge the batteries as much as possible.
- isolate the batteries from solar array to avoid recharging, or even overcharging. The use of a switch helps isolating the battery from the solar array.
- keep the battery temperature below the relevant safety threshold, as defined by the battery manufacturer.