Battery Life
If you suspect the battery, check the generator. I can never understand why the battery has to have a cap check so regularly while the generator seems to be allowed to go on until it fails.
If your battery is well up, then essential instruments should continue to work for half an hour. That doesn't mean that the batt would then have enough go in it to deal with an engine start.
The GPS 100 does have integral batteries in it - at least, mine does. A useful standby!