With nothing to loose and no mood to wait two or three weeks for a replacement USB board I decided to go the firmware way.
And after some flashing it now works as it should.
Here is what I did to get rid of the flashing battery symbol and to get better performance:
Wipe all, user data, dalvik, cache, battery stats - I installed Philz Touch recovery for that.
To my surprise Kies refuses to update the phone, similar to mine I get the not supported message even when trying it over the model and serial number.
Since I am not alone with that problem I assume Samsung is deliberatly blocking the process.
Anyway...
Phone was running on 4.0.3 and I did not want anything with Knox so the first flash I tried was the original 4.0.3 AU firmware with the Vodaphone branding.
For the above reasons Odin was used from now on for the firmware needs.
Surprise, surprise! After the first start the battery symbol was no longer flashing!
Let is run out of juice and recharged, still no flashing.
But after playing around with it and performing all the updates for the Vodaphone and Samsung programs on it the flshing was back
Flashed the firmware again and did not update anything but manually installed a few programs games to drain the battery faster.
No blinking battery symbol but once all was updated it was back.
This made me think it could be related to the branded software.
As it was also locked to Vodaphone I decided to install a custom kernel to get full root access and to remove both lock and branding.
The firmware used for this was the current CyanogenMod based on Android 4.4.
To be able to install it I had to change the recovery to the current Clockwork Mod recovery to get full support for 4.4.
Have been testing the phone now for over a day with all sorts of programs from WiFi hacking over GPS to games.
It is just amasing!
Not only is the battery fine now but there is no blinking and the lifetime is about 2 hours longer while playing games had no time to check the standby times though...
Overall performance with the CM11 is in a different league to the original.
Response is basically instantly and programs like Tomtom or big games load almost twice as fast now.
It is even faster than my S4 running on the stock firmware and that is impressive for such an old phone.
With no branding and useless software cluttering the phone the menus are clean and simple.
But best of all I don't have to waste hours for updates to programs I don't use anyway.
Showed it to my friend and asked if he is happy with it now and once he got used to the new look I got a slab of beer for my work and everyone is happy.
The conclusion for me is that not always is the USB board at fault when it comes to the flashing battery icon and poor battery life.
Since the full data wipe only helped until the stock firmware updated all programs I have to assume that there are cases where electronics are not the cause the problem.
Apart from the missing hardware features an old S2 is able to play along with the modern models if you install a proper firmware on it.
But if your USB connector has connection problems so you have to wiggle it to get contact for charging or the USB tranfer constantly fails it is best to replace the 5$ board.
One other thought that pooped into mind together with this repair was the time the phone is without juice.
Several tutorials tell you to fully charge the phone and then to remove the battery for a few minutes.
This is claimed to "reset" the internal battery meter so after the next full charge the phone is aware of the power loss in the old battery.
Considering that the procedure of replacing the board tke some time I suggest to try the battery reset first and use the time to clean the connector with a soft brush and some air.
I hope someone will find this helpful and reports back if it was a success or still a failure for the phone.
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