So far as I can tell Li Ion batteries have gained about a 1.5x charge per mass improvement in the last 10 years. They may have doubled or trebled over a longer

So far as I can tell Li Ion batteries have gained about a 1.5x charge per mass improvement in the last 10 years. They may have doubled or trebled over a longer time frame though. If there was a big jump due to a particular technology an extra year or two in the time frame, or shift in the time frame might make a big difference.