Hello,
I have just come back from troubleshooting which really intrigued me.

When you plug in the charger with completely empty batteries, it turns off after 30 secs and lights up the green LED.

I have pluged in the battery connector from another Wave. The charger works great. Now I'm shure it was batteries.

The batteries are 1.5 years old max and slightly bulged. Manufacturer is Vtrack or something....

I checked each battery voltage - great! 6,37 V each
I checked each battery with load tester - great! 5 V each.
Now I'm confused?!...

I started replacing batteries from the other Wave.
When I changed two batteries and plugged the charger it started to charge continuously.

My tought is that it has to do something with the resistance of the batteries.

Could this trick the "smart" charger?
How can I check this batteries?

Regards!