I made another change to the chair so I can fast charge both chair batteries at 100 amps or so at the same time directly from my van yesterday...
I disconnected the two centre battery terminals from the chairs loom... Now it wont drive without an anderson plug and a short loop of cable across the centre two charge socket pins... It means I can charge both battheries in parallel directly from the van. Just by plugging into the dash and starting the engine. And I can use any quality 12v battery charger on both batteries at the same time which balances them perfectly at the same time... Or ... I am going to have to do a page! And a list of charging methods...