There are a number of reasons for not including a rechargeable battery inside your Timesulin unit.
1) Safety. After a year of clicking the plastic cap on and off your insulin pen every day (and for short-acting insulins it'll be at least three times a day), the fit won't be as snug anymore, risking the cap coming off your pen without you intending for that to happen. It's easier for us to ensure that the cap gets replaced once a year and that the fit stays tight. You'll see for yourself what happens to a regular ballpoint pen you've had lying around for a while - the cap just comes off too easily for us to consider it 'safe' after a year.
 2) Simplicity. We wanted to ensure that nothing could go wrong / needed to be programmed / could open up accidentally for our customers. If we had to add a replaceable battery it would have required steps in usage that would have prevented us from presenting the simplest product possible.
 3) Bulk. We tried our very best to keep Timesulin as close to the original pen cap's size and weight as possible. To include a replaceable battery container would have added quite a bit of bulk and weight, which we wanted to avoid.

BTW, we have over-engineered Timesulin a bit - in reality you may get more than a year out of the battery but we don't want to make any promises here because it has to do with temperature, humidity, and other factors which makes it hard to predict for everyone.