It is common for many Braun shavers, that if the indicator light flashes red while the shaver is being used (switched on and running), it means the battery is low and needs to be charged soon.
Below is an example from a manual for a Braun model 7526 shaver. You can search that site for other manuals if this is not the correct one.
This model shaver has 3 indicator lights:
1) Green (Charging)
2) Red (Low battery)
3) Blue (Power cord attached and plugged into the wall)
Download the manual for the model 7526 here: http://www.manualslib.com/manual/20281/Braun-5301.html#manual
"The green charging light shows that the
shaver is being charged. When the rechargeable
battery is fully charged, the charging light
flashes or goes off.
If later on, the charging light comes on
again, this indicates that the shaver is
being recharged to maintain its full
The red low-charge light flashes when the
battery capacity has dropped below 20%,
provided the shaver is switched on. The
remaining capacity will then be sufficient
for 2 to 3 shaves."