Hi a 420 has two batteries
I got mine from Ebay
Fitting is easy
No written instruction any where on the net but its easy peasy.
Firstly turn of the power to the bus bar at the socket. Then turn off the chair under the arm and around the back.
You need to remove the cover from around the back. The batteries are under the cover that is facing down the stairs. I would suggest that when you get the cover off you take a picture of the wiring as its individual to each Stannah.
Then the batteries can be disconnected.its an easy thing as they just are a spay bit so grasp battery and connection and pull apart horizontally. Hope that helps . Batteries are available on ebay but will need to be charged, dont move the chair for 24 hrs so it can charge. Dont forget to turn on all three power switches after the batteries are fitted.
Ps chair will only charge at the top or the bottom of your bus rail.so make sure its there before you start the whole process.
Dawn
October 2014