I do not know. I prefer using tablet for web browsing. And if I find something long and interesting I print it to pdf and read it in M92 later on. Generaly, I do not use M92 for web browsing. So, I use tablet or PC to find information (it is faster, you may have large screen or two or more and colour) and M92 to process the information found in step 1 (less eystrain, annotation, scribling).
Well, if you think you need spare battery, buy one, charge it to 50% and store it in the fridge. Maybe that, if battery is empty, it may be stored without big decay at room temperature. I have no idea.