Charge HR battery/display trouble

[Merisi]

I got a Charge HR as a gift. It has been long time kept in a cupboard without charging. I put it on charge and everything (connection, etc.) looks in working order but
1. the battery seems not able to take charge anymore. When I disconnect from the charger, it shows random levels, sometime high, sometime low, and the little power it has stored drains quickly (two nights, so there is something wrong with the battery).
2. the display remains dark and hard to read. 
Is there any led behind the screen that can can be broken? I am not going into the process of changing the battery (soldering, etc.) if not sure that it will solve the problem.
Thank you all for your answers.

[LZeeW]

Hello @Merisi and welcome to the Community.  The type of battery in a Fitbit will degrade over time and might not take or hold a charge.  Fitbit released the Charge HR in 2015, so it has a pretty old battery.  The Charge HR is also on Fitbit's Legacy Device list.  It's performance could be problematic, even if you got it back on your account.

[Merisi]

Thank you very much for your answer, LZeeW. Changing the battery is not expensive but requires some soldering skills. Do you think that the low lighting of the screen is connected with the battery problem?

[LZeeW]

@Merisi   If you have the skills and are willing to accept the possibility that it won't work, then it's up to you.  I have this other experience to share.  I tried to put my One on my account a few months ago.  It's also on the Legacy Device list.  My phone couldn't find it to give me a pairing code.  Like I wrote, totally up to you.