I would like to know if you have some experience with following problem. When I insert my fully charged original battery in Canon EOS800D the camera does not start at all. Even the indicator for closed battery door does not blink. I brought the camera in the service. They have inserted another battery in the camera and it worked normally. Than they have charged my battery for about 1 min in another battery charger, inserted it back to the camera and the camera starts working normally. It worked like that for several hours - about 2-3hours, because they advised me to fully discharged the battery. The camera turned off automatically and when I have tried to turned it on again - could not. Usually at least give some info that the battery is empty. I`ve decided that the battery was finally fully discharged and left it in the charger for charging. After about 2 hours and green indication for fully charged battery on the charger, I haveinserted the battery in the camera and it did not start again. No power at all. Please, advice if you have some experience and solution. Buying of new battery is not a discussion, but please advice, Could it be some other problem in the electronics? Thanks in advance.
If the camera works with another battery (which is what they demonstrated at the store) then get a new battery. Batteries are consumables, replace them when they fail or even before.
There are many aftermarket brands that are somewhat less expensive than Canon but are just as good and work well with Canon cameras and chargers.
The fact that the camera isn't reacting at all when you try to turn it on suggests that it doesn't get any power at all from the battery.
The concacts in the camera and the contacts on the battery might be oxidized. Check if you can see any discoloration. Using an eraser you can remove any oxide from the contacts.
The charger seems to indicate that the battery can be charged, but it might be wrong. If you have a multimeter you can measure the voltage of the battery. If the battery is working, you should get a voltage of around 7.2 volts.
Based on the information provided in the question, as well as this comment made by the OP to another answer:
I have tried few not original batteries, which worked perfectly before this problem - same , no power.
It sounds to me like the charger is faulty and is not charging your batteries. Any batteries charged using a different charger seem to work to power your camera, including the same batteries that don't seem to work when charged by your charger. The difference seems to always be what charger was used to charge the batteries.