Try experimenting around with "hal-device bme|grep erc". It uses about 6 % of the CPU time of "lshal|grep erc". Put 'time' in front of the command to get timed result.

Also it might help to get it into a sleep state whenever the screen is off. For example you can see the desktop clock showing old time for a moment before it is updated to current time. And given the battery will last at least some 3 hours of very intensive usage, the time it takes for battery to drop a single percentage is about 2 minutes at lowest. Thus, the polling interval could be even lower if there is no way to make the program sleep when screen is off. Possibly allow user to configure it to his liking.

Also would be nice if the widget could show the battery percentage in place of the normal battery display next to the clock. Like "systeminfowidget" shows CPU and MEM usage there. Possibly make it user configurable where it is shown.

As for the 'click' for more information. The widget could add "Battery information & config" button into the menu that opens when you click the clock. Like "systeminfowidget" does few features.

If it isn't possible to get the widget to show in 'add widget' area, it can be made as automatically active when installed as many other widgets are. Like "systeminfowidget" works.