I bought an old tennis tutor model 2 that had no charger. I replaced the batteries, replaced the control panel and found a power adapter that had the right connection and was 12v/3ah. The 1st time I plugged it in with the new batteries the control panel light showed yellow and did not change. I then used the machine for about an hour and came back and plugged in the adapter and the control panel light was red. It never changed from red. So I pulled the top off again and put a voltmeter to the batteries and the batteries were charged - but the light still shows red. I assume my power adapter is causing the control panel light not to register correctly. How many hours should I charge the batteries with the 3ah power adapter, it's the same amp's as the quick charger. I think the quick charger says to charge for 3 hours ? What can cause the control panel light not to show green when the batteries are charged ?