I bought a cheap Onewheel plus for testing the different range extenders.
Since the Plus is out of warranty long ago, it's a good platform to hack as my XR is still under warranty and not much need to extend the range anyways.
-The extenders are home built based on commercial types.
-Parallel extender where the Onewheel battery is accessed .
-Xlr type where charging through the charging port while riding. Seen by the Plus as regeneration.
Thoughts and experience:
--- Parallel extender using a 16 -18650 cells is tricky as both batteries should be the same state of charge and not at 100%.
My mistake was the batteries were at 100% , the board went into "overcharge" warning and went into non balance mode.
The board still wouldn't balance after disconnecting the auxiliary battery, but after leaving the board with the lights on for 10 minutes, it recovered and worked fine.
---Xlr system worked great as it will add to the battery percentage slowly. Uphills will pull the percentage down, but evens out.
My own analysis:
-Parallel system , less parts, more work to install.
-Xlr system, more parts and cost ( MTP-7210 solar controller, Hoverboard battery and charger). Very easy to install.
Note the Xlr system won't charge the board if it isn't turn on.
To charge the board without it being turned on, a different Xlr connector is used where the center pin is powered up.