Battery or dc to dc step up converter might be defect

Hello,

i bought a second hand bittle after a lot of back and forth what i should give myself for my birthday :-)

Unfortunately i cannot test it yet as the battery does not change its red led pattern after about eight hours now. I used a standard USB charger 5V, 1.2A and the battery load was about 5V and 0.3A current for the first several hours and dropped to 0.01A current afterwards. When i remove the micro USB cable it turns blue for a very short time (milliseconds) and sticks to red otherwise (short press and 3s press). The Nyboard doe snot turn on when battery is connected.

Now i decided to open the battery and measured volatge. Its about 4.3V directly at the battery contacts but there no voltage at all on +/- contacts on the step up board itself where the cables are soldered for output.

I cannot find the battery from a reseller here in Germany or the step up converter, there are numbers on the board, is it proprietary? Any hints how I can test the bittle with an alternative method?

Thanks in advance

Oi