Sorta stuck on kid's bikes at the moment. Apologies. As I mentioned in my review of the Start, I was going to swap the stock lead acid batteries for LiPo's. Did it, and spent a whopping 30 minutes doing the conversion.
I got the idea from the Oset owners over on TrialsCentral so I owe them all the trial and error. I ordered two 5000mAh batteries for about 20% more range than the stock ones. My Start had 8Ah SLA batteries even though the Kuberg website says that the Start comes with 9Ah. Oh well no matter. Anyway the Kuberg already uses XT-60 connectors so all I had to do was solder XT-60's to the batteries toss them into some LiPo charge bags and drop em in.
LiPo's are a bit more maintenance than SLA's and require special chargers so don't undertake this if lack of maintenance is your favorite aspect about an e-bike. They don't last as many years, require special chargers, and they can catch fire if used improperly - mostly just during charging but it is possible to start a fire if the battery is shorted or damaged. I spent just under $200 including chargers and a power supply to run them and ordered everything from Hobby King.
Tool kit that came with the Start
2x XT-60 female leads
XT-60 Parallel connector
2x Zippy 5000mAh 6S LiPo batteries
2x LiPo safe small charging bags
2x battery monitors (optional, but a good idea to prevent excessive discharging of the batteries)
2x XK balancing lead extensions for monitors
Misc solder and heat shrink supplies
Remove the seat and plastics and the upper bolt to the rear shock. Prop the bike up on a stand and let the swingarm drop to the floor. Disconnect the battery leads, release the battery strap, and slide the batteries out the back where the shock used to be. Pop the new batteries in their bags and slide them in. Use the battery strap to hold them in place. Plug the batteries into the parallel connector. Plug the speed controller into the parallel connector. Connect the battery monitors to the balancing leads (if you bought monitors). Bolt everything back up.
For now I just shoved the monitors in the back next to the bags. I'll mount them more permanently if needed but they're rather handy there - just a bit ugly with all the wires from the balancing leads exposed.
With a slightly higher voltage the bike is a tiny bit quicker, although with my kid still on speed setting 2 its maybe 1/3 mph so hardly noticeable. With the larger 5000mAh batteries I gained 20% more range, but didn't save as much weight as I could have - only 7.7 lbs including wiring, bags, and monitors. But still, that's a nearly 18% reduction in weight over stock. That would be the same as shaving 45.5 lbs off my KTM for $200 so that's extremely significant! Aside from that the bike performs the same as before so nothing really to report.
I bought 2 chargers and a power supply big enough to run both so that I could charge both packs simultaneously. Charge time is about an hour although I could have reduced this to 30 minutes if I'd opted for a larger power supply.