I have a 99 arctic cat 300 and it runs good when the battery is fully charged. runs like crap when the battery runs low

 

How old is the battery? if its 3 years old ,get rid of it and get a new one. Its not worth screwing with. The best is to buy an actual motorcycle battery. if you buy the cheap walmart battery for a riding lawnmower,chances are it wont last as long. lawnmower batteries are not designed to take the beating like atv batteries do on trails. Unless you ride on a farm real slow,then it wouldn't matter probably.

You can also load test your battery to see if it is still holding the correct voltage. Auto zone does it for free.

On the other hand, Are you saying it will start up fine then as the day goes on it starts to run like crap? if this is the case then its a possability your rectifier/regulator/charging system(whatever you wanna call it) is not working correctly. You may want to check it first. They are around $80.00 a few years ago.