I'm used to using a F1 which gives you quite a good idea of battery life remaining by the number of flashes when you first turn it on.

Last year I was using a SOS beacon for a bit which only gives a red/green light indication.

I once got the beacon out to use (practise) at about lunch time and noticed the light was red when it had been green in the morning. The beacon seemed to have a shortened range and i had trouble pin pointing with it.

I experimented and found that after turning the unit off and putting it in a warm place for a while and certainly overnight the light would be green at first.
This could lead you into going out with the impression that your batteries were OK when the beacons performance might be compromised when you need it.