Liftmaster 475LM Battery Backup DIY

[cwolanyk]

I'm trying to add a battery backup to my Liftmaster 8500C. I've purchased the batteries and a 41A5726 circuit board. The wiring seems straightforward except for an extra ground. In video clips I've seen, then seems to be a thin wire connected to the middle (-ve) terminal. The connect to the 8500C has two pins. Anyone know where this wire comes out of the harness from. Is it just connect the the -ve pin on the connector?

 

[IsotopeX]

Did you figure this out? I noticed that it might be connected to a ground terminal on the connector to the 8500, like on the 580LM: https://www.ebay.com/itm/305595666255