my trx 300 4x4 battery keeps going flat, but charges up ok, could it be the charging system, how do i check this. thanx in advance.

 

you can put a dc voltmeter on the battery, start the machine up and it should read over 13.5 vdc, when you rev it up a bit. This test depends on a battery that's at least good enough to spin the starter. Rule of thumb is if the battery goes dead while you're riding, a potential charging problem. If it goes dead when not in use, bad battery.