Sorry for any delay - I had network issues.
I missed your question about the battery location, which is INSIDE the device. That means that the device will have to be taken apart (carefully) to replace the battery.
And lifespan really depends on how much the device is used (daily, weekly, etc.), and the environment it is used in (changes in temperature mostly).
ALL batteries have a limited number of charge and recharge "cycles" - and the batteries in these units are no different.
So if it was used every day (mostly) for a period of 2.5 years, then the battery itself could be the issue.
I found a replacement battery listed online, and have a link to it below.
NOTE: I have no affiliation with this seller at all, and you may be able to find the battery at other places online OR locally - such as at a "battery store". And SOME battery store places may also offer to replace the battery for you - either at no cost or a low fee.
I also have a link to a video that shows how the battery can be replaced on a SIMILAR device (same case style). You may want to take a look to see what is involved, if you want to try this yourself.
Please let me know what you find or if you have any more questions.
Thanks, ***** *****