from https://members.gliding.co.uk/wp-
content/uploads/sites/3/2017/09/BGA-Newsletter-September-17.pdf

Airworthiness
FES Battery Fires. On 10th August, a FES-equipped sailplane
experienced a severe fire in the forward FES battery after landing
normally at Southdown Gliding Club. The pilot was uninjured and the
fire was extinguished, although the battery compartment and
surrounding structure were damaged by the fire. This event is the
subject of an AAIB Field Investigation and no findings of cause have
yet been established. The AAIB is aware of a second FES battery fire
that occurred on a different FES-equipped sailplane in May 2017, and
is working with the sailplane manufacturer and LZ Design, the
manufacturer of the FES system, to determine the cause of both
battery fires. Whilst the facts have yet to be determined, it is possible
that there could be internal fault within battery cells that could cause
an electrical short between cells and thereby a spontaneous
combustion which battery temperature monitoring will not warn of.
Part of the AAIB work programme involves CT-scanning battery packs
to identify any foreign objects. The potential lack of fire warning is
also being carefully considered.
If you have any questions about operating and maintaining a FES
installation and associated battery, please contact the glider agent in
the first instance.