HELLO THERE AND WELCOME TO FIXYA
This is a fairly common issue with these radios
The thing about these radios is that the batteries are made to be able to charge for long periods of time HOWEVER the battery will begin to lose its charging capabilities due to long periods of charging then will get overcharged and then they are bad after that so you may have several charges on it then all of the sudden it no longer charges this is a sign of overcharging
what you can do to check is to take the battery out and use a multimeter to test the charge on the battery then test the charger to test it as well to ensure it is charging properly
If the battery is bad you should be able to go to radio shack or any good radio dealer and get a replacement battery or you can order one thru the radios manufacture ok?
Hope this helps please post me back if yo need further assistance or have any more questions ok?
Best regards mike
1) Clean all your battery contacts with a pencil eraser. This includes the contacts on the charger (if a drop-in style), battery and the radio.
2) Check your battery date code. It is a 4 digit number on the white portion of your battery. The first two numbers are the year, followed by the week of the year. (Example: 0946 would be 2009 the 46th week.) Your battery pack should be less than 2 years old to work correctly.
3) Check to see you are using the correct charger. The XTN style charger should have a white label on the cord indicating it is for the XTN radio.
You most likely have batteries which are failing or shorted out. It would be advisable to remove them from the radio and replace them with new batteries. Continued charging of shorted/bad batteries will damage your radio.
Check your charger and battery contacts. Cleaning battery contacts on the radio and charger with a pencil eraser will remove dust, dirt, debris, and hand oil which may cause the battery to charge poorly. Secondly, you might want to check that your charger itself is working properly. If you have a second known good radio you might try charging it on the same charger.
Your batteries may be bad. Sometimes over charging batteries will cause them to wear out.
If radios are brand new then I would take them back to the store for warranty exchange. The other opton is to vist a local radio service center and ask the tech to jump the battery. if the battery falls below a certain voltage it might need a quick jump sart from a power supply with a higher voltage normally 12 volts works well...
The radios will work fine while being charged, but they will charge faster if turned off. Normal charging time is 8-10 hours, but I would recommend an overnight charge - they won't overcharge or be damaged by the longer charge time.