When first switched on with no battery fitted the green light is on as expected, immediately a battery is fitted the red light flashes, which I understand means there is something preventing the battery from receiving charge eg connections dirty/damaged. The connections on the charger and both batteries are clean and not damage. Fault is the same with each battery.

Don Stanley, March 2017

You either have two bad batteries or one bad charger.

Amazon sell the charger for £86 and a 36v battery 2.6AH for £123

My guess is a defective charger - there's an article about repairing the 3640 charger here...

https://galooph.com/blog/2015/11/13/repairing-a-bosch-al-3640-cv-charger/

Good Luck..., March 2017