I often wonder why people don't call the manufacturer first. Is it that
difficult?

I would contact iRobot and ask them what the recommended procedure is.

On the assumption that iRobot is not stupid, and knows that the batteries
will eventually have to be replaced, it seems reasonable that the wires from
the batteries connect to a push-on terminal on the main board. If this is
so, you'll have to get the iRobot batteries.