Could i use a pair of 12v 20ah agm batteries instead.
Obviously make a new battery box and upgrade wiring
for a bit more range and safer option really.[/quote]
Not sure those would fit and give you enough ground clearance. Even if they did, you'd have to make the box very carefully, so as not to impair the chair's ability to fold.
I've had mine for nearly 3 years now, and it continues to perform flawlessly with the original battery pack (I use the maximum 3 batteries). My range is easily 20 miles on the few occasions I've pushed it that far. In everyday use (around the home/neighborhood and around the office) I only need to recharge it about once every couple of weeks.
The only safety issue I've had is when I took a curb cutout too quickly and got dumped on my back last February. The rear casters were useless and just folded flat under me. I removed the casters, learned to be more careful and haven't had a recurrence since.