Tiger Trouble

If it is the original battery it will likely be time expired and simply not hold a charge.
If you have a multimeter start it up and check the voltage between the battery posts. At tickover it will hover around high 12 to 13 volts and jump up to 14 ish on a blipped throttle. That will tell you if the problem is the battery or charging system.
Otherwise charge it disconnected and see if still loses charge (don’t reconnect!) over 48 hrs.

 

Triumph standard fit batteries are pretty rubbish so if it's still on the original battery it's doing well. Stop complaining.

 

Yup check battery first and even if the readings are good stay suspicious. If replacing it look at one with a greater cca
Also, somewhere on here is a simple run through to check reg and rec

 

Keep us updated