To my knowledge, the battery in the TF300, and the one in the multi function docking station is a non removable, save for sending it to an authorized service center where they do that type of thing for a living. Though its been my experience..if and when the battery is depleted due to normal usage, it will probably be time to replace the tablet and docking station.
Now then, if your unit is defective..that is entirely a different matter. If the battery goes before the manufacturers warranty expires, then ASUS should be responsible for having the battery changed or a new unit issued to you. If it is after the manufacturers warranty, and you have an extended warranty..same thing applies, if you do not have an extended warranty, then my friend..you are on your own. You'll have to shell out of your wallet for a new battery/batteries.
I would think under normal wear and tear the batteries should last a good long time, as stated..under normal circumstances..you'll be looking to replace both by the time the batteries wear out. Figure 2-4 years, just imagine where technology will be in 4 years, where tablet technology will be next year, the year after, it boggles the mind at times. Of course, with all thats been said.. I may be wrong and your batter could conk out toorrow..I hope not, but stranger things have happened. When it comes to electronics, nothing is forsure.
