Alternator is weak and needs replacing. Drive belt on alternator can also be slipping due to being loose. Check the alternator output at the battery with the engine running. It should read from 13.5-16 volts DC. Any less and it needs replacing.
Sounds like the battery has a dead cell
dead battery
battery light indication means a dead battery or a dead alternater/regulater or a fault in the charging/ignition system
A little more detail please, is this after the battery has been charged and goes dead while the vehicle is being used or is it after it has been sitting for a few days??
It sounds like your alternator is no longer recharging your battery. You probably need a new alternator.
bad ground
Best guess would be that you have a short to ground somewhere that is draining your battery.
More than likely the battery is dead or has a loose or corroded connection.
Bad battery or drain in the electrical system
lots of causes: -battery might be dead... try to jump start it... if it starts... change the battery(most common problem) -no more gasoline -dead fuel pump -clogged fuel line -dead starter etc...
If you keep the key on your ATV on for two days, the battery will probably go dead and need a charge or jump start.
Connect the - cable from the negative battery terminal on the "live" battery to a clean bare metal part on the "dead" car. Make sure it wil not be touched when "dead" car starts