Battery of Braun 5514 shaver

I have a cordless hair&beard "styling kit" from Braun, model 5514, that doesn't hold the charge anymore. I use my soldering iron more often than this shaver so I have no idea how long the stand-by period is. Still, loosing the charge in one week is not acceptable.

There is a good video on YT showing how to open it. You need a thin and a thicker plastic card/tool to pop open the top cover and a small straight screwdriver. 

There are two batteries wired in series, in my case they are marked as "SUPPO HS-AAA0.75" NIMH 1.2V. The guy in the video shows the same model.

Now, if you search for that string you can buy a replacement at 1/3rd of the price for a new tool. But those batteries are just AAA NiMH rechargeable cells with solder tabs, with a capacity of at least 750 mAh (so if you buy them of 1000 mAh that's fine, it will take longer to reach full charge).

Then it's a matter of cutting off old batteries and soldering in new ones.

Someone might mumble that it should not be so hard to replace a battery. This tool is designed to be used in wet environments so it is waterproof. This means that it's already gold it is not sealed. Also the motor in the shaver creates vibrations and it's better to have batteries (and everything else) held together as strong as possible.