Ideal Charging Strategy for Cordless Drill Batteries
Question:
The Dewalt cordless drill manual says to charge after every use to maximize the power tool battery’s life. This would essentially keep it fully charged, for me that is, because I tend to use it very briefly. I understand that the most important thing is to avoid fully discharging the Dewalt drill battery, but is keeping it topped up really the best charging strategy? I ask because I read somewhere that lithium batteries should ideally be discharged to 50% before charging, because the total run time is better than, say, recharging every time it’s 90% charged. This also implies that keeping it topped up at all times is not the best way.
Is my understanding on this topic accurate? Or does the ideal strategy differ depending on the particular lithium power tool battery? Supposing that I’m wrong, how exactly should I charge it? Keep it topped up like the manual says?
Answer:
It depends on the use. Li-Ions do not really self-discharge, they lose battery capacity. Permanently. High and low State of Charge, and elevated temperatures, both speed this. Really, you’re best off keeping the cordless power tools battery something like 20-80% charged in good temperatures, but that leaves you 60% of the total HITACHI BCL1815 Power Tool Battery capacity ‘available’ for use.
You should try and keep Li-ions in a low-ish average state of charge. Charge the batt. just before using and try and target ~ 40% capacity for storage until next use. Li-ions will self discharge, but quite slowly, and who knows what the parasitic drain the device might have. If you leave it too close to empty, depending upon how long it sits, the self-discharge and/or parasitic drain my deplete it too low.
Here’s an interesting military study on different charging habits… the lower the average voltage, the better for the battery.
I’ve been using a 12v bosch litheon system since it came out. I generally use it and toss it in the bag unless I plan on doing more work the next day, in which case I’ll top off the charge. Back when I was doing commercial electrical work I mostly used 18v nicad tools, and the 12v lithium stuff only got brought out for renovation work above ceilings and a few other odd jobs. Those were kind of sporadic and the 12v lithium batteries spent months at a time fully charged, nearly discharged and being recharged every day.
They’ve been going strong for 5 years and nowdays mostly get used around the house on a weekly basis. The runtime does seem a bit shorter than new but not enough to be a bother. A single 1.3ah DEWALT DW9095 Power Tool Battery in my first gen pocket driver had enough charge to drive 40 or so durock cement board screws when I did a shower stall last year with a bit to spare. When bosch releases the 2ah 12v batteries hopefully this year I’ll be expecting the same or better from them at the 5 year mark.
Storing at lower voltage is better, but it’s been my experience that they handle being stored at full charge pretty gracefully. There is something to be said about being able to toss the tool in your bag, grab it a few months later and have it ready to go without needing a recharge.
