Has anyone tried replacing the battery on a Nike+ Sportwatch?

The battery life on workout mode is suppose to be around 8 hours but I went for a 2.5 hour run today and got the low battery warning. Admittedly I didn't charge it since my 1 hour run the previous day but still, that's only 3.5 hours.

I need it to last at least 5 hours and preferably the 8 hours quoted

Searching the net I see it's fairly easy to take apart:


and the battery looks like a 3.7v 170mah

I can pick one up for around $9 on ebay but will have to make sure the dimensions will fit.

I'm tempted to see if I can find a higher capacity battery to fit though