Yes, but to use this parameter, one needs to predict usage to properly squeeze more calendar life from this chemistry.
Consider as example we are using a Dremel 8200 cordless, which is fairly well studied on the net:
http://www.coptool.com/blog/2010/06/dremel_8200_cordless_rotary_to.htmlWhen you pull it out of storage, it will have ~ < = 50% power. Will your current need only require such runtime? If you need more, you must wait 90 min to charge the battery up to 100%, which still limits runtime to ~ 30min under load. Now if one did not use down to 50% of existing power, or exhaust the 50% left in the battery in the last task, a user would ideally have to run down the battery or charge it up, to ~ 50% before storage.
So, effectively, you've replaced the convenience of cordless with the the inconvenience of managing the battery charge state and anticipating your runtime needs, to extend the calendar life.
Storage at 50% charge is most beneficial for inventory considerations, where batteries are kept awaiting sale, rather than active use. Not only will it be kept at < 50% of full charge, but likely they'd benefit from refrigeration too.
I would guess most users charge batteries to full before storage, so when called to duty, a tool will deliver its maximum performance and runtime. Its the same way I see most users treat laptops and cellphones, so 4 years is likely to be nominal rather than worse case. All one need do is reflect on your own cellphone or laptop batteries and ask, how long does it last before replacement?
Li-Ion chemistry has an ~ 4 year useful life, whether you use it or not. ..
With normal cordless drills I believe Li-ion batteries give more power ..
Li-ion does have a shelf life, but if stored at about 50% charge it can be longer than 4 years. The 4 years down to 80% would be a worst case scenario.