Storing powertool Li-Ion batteries

We know it's good to store Li-Ion cells around 50% for storage. I just got some decent powertools but don't use them often. How can I get these to 50%...or even 75%...if they're close to 100%?

They're 20v Dewalt packs. I'd rather not just run the drill forever and guess. Has anyone ever hooked them up to a charger to discharge?

Sorry I posted this in the wrong section. Can we move this to the Batteries Included area?

Do you have a tool light? Especially one of the older incandescent models? If you know the current draw, you can calculate how long to leave it running to get to the approximate state of charge you want, and set an alarm for yourself.

Resting voltage also is a semi-reliable way to get rough state of charge. 16-19 Volts seems to me like a pretty good level for a 5-cell battery.

Personally, I don't give mine much thought unless I think they might be near empty, in which case I check the voltage. If it's low, I'll throw in on the charger for 30 minutes.

By the way, the only decent piece of research on this I've found so far is here:
http://jes.ecsdl.org/content/163/9/A1872.full.pdf+html

Temperature seems to be the biggest factor. A hot garage is not the best place for longevity. Cell capacity actually appears better the lower you go in state of charge, with the caveat that if you go too low, the battery may self-discharge to a dangerous level, hence why 40% seems to be the most commonly recommended level. Cell capacity takes a notable drop if stored above 60% SoC. At room temperature, I'd say even that drop isn't too bad, but it gets significantly worse at elevated temps.

Internal resistance seems to start creeping significantly upward around 70-80% SoC, and again, this effect is significantly worse at higher temperatures.

if "Temperature seems to be the biggest factor." then it does not make sense to toll them thermally by discharging them to 50% and recharging them to 100 for every use.
The 40-50% number maeks eense for long term storage like from the maker, to the store, to the consumer.

Your Li-ion batetries will die around the 10 year mark no matter how well you babysit them. Just use them....

Just use them now and them. Top the up as well waiting until they've run dead isn't good anyway and interrupts your work. It varies with makes while some of them will be ok at -4C the winter cold can be harmful. I bring mine in for the winter until the weather is back above freezing temps.

The manufacture should have a section on battery care on their website.

Dewalt makes a 20v work-light for your pack.

Most of these 20v packs have 5 cells inside. So, charge fully, and run light for an hour. Measure voltage with a voltmeter.

If you wanted to store at 3.8v per cell, that would be a resting voltage of 19 volts. You'll run through a few hours of discharge until you find the level you want to achieve, and after that the procedure is to just charge to full, and run light for how many hours you figured out.

A work-light is a great accessory for just this kind of thing if you don't want to go bananas with hobby-charger/analyzers.

Thanks everyone. Unfortunately I have bigger problems now. One of my packs is defective right out of the box and only runs the drill for a few seconds right off the charger. It will only charge up to 15.2v max, so it's definitely a dud. I sent it to their service center for a fix/replacement. Hopefully I don't get charged anything being a few weeks old with no real use.