I left my RZR for a fortnight and the charge dropped from 12.8v to 12.3v. They self discharge really quickly. 13.0v is "full". You need to keep them charged up inbetween rides. I've not had any issues with my low power light coming on.

That said, the stock battery is a bit shit and others have replaced it with something better. A 2nd battery is a good alternative as this gives you the option of really big amp hours but with the added advantage of being able to quickly remove it and save yourself 20lbs or so when not needed.

But if you're having issues with hardly any lights on then you need to check the charge. Don't forget the RZR has a built in voltmeter. I run 5 PIAA lights on top of all the other lights and the fan working overtime... and not had any trouble at all. Keep it topped up!