I am trying to replace the battery in a handheld recorder (m-audio microtrack). I am following this guide. The guide instructs to use a 3.7V battery, and this value is the same as written on the original battery which I pulled out of the device. I am trying to turn the device on with a Li-Ion battery that I took from a mobile phone - the battery is also rated 3.7V, is fully charged and functional - the mobile phone works OK and indicates a fully charged battery. However, the device is not turned on (polarity is correct).
I measured the output voltage of both batteries, and the original reads 4.15V with open terminals, and about 4V with load (the device connected and turned on). On the other hand, the mobile phone battery reads 3.8V.
Can this voltage difference be the reason the device is not turned on with the replacement battery? Is there another possible reason? It is weird especially because the info written on the original battery also says 3.7V.