the lite has a 9ah battery while the other 2 edge models have either 18 or 20ah look for a battery distributor in your area and ask for a 12 volt sla 20ah battery. if it meaures around 6" x 6" x 3" and costs between $30 and $40 then that will probably work.
Some have tie wrapped them in. I made a clamp strap out of thin aluminum.
It does add 8 or 10 pounds to the weight of your machine, so you will feel the difference, if picking it up is an issue.
Eventually, I will leave the battery out of the machine and just run some wires out of the base, so that when I am finished, I can just leave the machine in my vehicle and take the battery in to be charged
Another problem is the charger....I am waiting on a 5 amp charger, so that I can reduce charging time to maybe 3 or 4 hours rather than 10-12 ($50 online)
Only problem with an external battery MIGHT be that the machine could move around more easily.
2x APDGT, Ektelon H, SP Lite on steroids