I never guess it's the battery. I pay the 6 BUCKS and swap it out.
I find that fretting about it takes far too much time.
I bought a refurbished ipod Nano 1st gen from the Apple store a year ago. Worked great until a few weeks ago when I suddenly noticed the battery drains in about 2 days whether I use it a lot or little, even if the Hold switch is activated. It can drain so low that there is not even enough power in the battery for it to be "seen" by my PowerBook until it charges for a few minutes (the Please Wait Very Low Battery message on the nano). Can there be a short in the nano? I think this happened too quickly to just be bad battery. Any way to test it out on my end? Does this happen often with Nano 1st gens?