Just wanted to share a reliable, reputable, and reasonbly priced source of replacement (or supplementary) 12v 20ah SLA batteries.
http://www.electricvehiclesusa.com/product_p/ba-ty-12-20.htm
$37.99 each, which is a much better deal than the $59.00 price tag at X-treme.
I've replaced all of my stock batteries as well as my worn out 5th battery with these.
X-treme batteries come with free shipping. When you add shipping costs it is about the same price. Who knows if these batts are made for deep cycle applications or if they will last any amount of time.
X-treme price: $236 shipped for a set of 4
EVUSA price: $156 + $40 shipping
You save $40. I didn't want to go with X-treme when I was replacing my original set because I was having problems with corroded terminals on the stock batteries.
12v 40ah batteries are used for larger Uninterruptable Power Supplies.