I have inherited a set of Dewalt 18v cordless tools -- 2 batteries (pretty sure they're lithium DC9096, 18v XRP), charger, two drills, flashlight, impact driver, cutoff saw. I've avoided cordless ever since a Craftsman drill I had several years ago died from inactivity. I just don't use cordless often enough.
What, if anything can I do to keep these batteries usable? I wasn't sure of the charge when I got them, so I charged each of the batteries and they are now on a shelf, off the charger. The Craftsman was NiCad.
What to do to preserve battery life is revealed by what is done to preserve it in commercial applications, Protera buses for example. The battery packs are climate controlled because charging or discharging too hot or too cold degrades the capacity. The charge on the battery is controlled in a mid range of charge.
There is sophisticated voltage and current control during charging. Battery packs last 10+ years with daily use.
You might not have much choice on discharge temperature, but you can charge indoors where you live.
Over charging or over "draining" is very bad. To prevent these situations all Li ion batteries have battery management electronics to prevent over charging or over discharging. You will note that Li tools don't fade, they stop! The battery management electronics shut them off before the batter is damaged.
We have to assume the charger is smartly designed for how the battery charges is critical.
Store charged, ideally indoors. Don't store for long periods(many months) without "topping off" the charge.
A quick search online of that battery number seems to indicate to me that they are NiCad batteries, not lithium. You can usually tell by the number of terminals on the battery. If there are 2-3, they're probably NiCad or NiMH, but if there are 5-6 terminals, then they're lithium, as each battery has internal circuitry to prevent overcharging and over-discharging, either of which cause very bad, hot things to happen.
Another way to tell with some manufacturers (and I believe DeWalt is this way, too), is that the NiCad batteries have a post that is inserted into the tool, much like a pistol magazine, whereas the lithium batteries have two rails on top that engage two similar rails on the bottom of the tool handle, and slide towards the front or rear of the tool when removed or attached.
FYI - I have both Bosch and Ridgid 12V cordless tools that have Li batteries that are inserted batteries. Many of the current 18 or 20V tools do in fact fit your description of rails vs post, but not necessarily the 12V line.
Discharge of NiCd is different from Li ion or NiH. It is best to discharge them fully before recharging. A quality charger will have a deep discharge mode that should be activated once in a while. You will know what you have by how they power a tool. If they loose power toward the end but keep running they are not Li ion. Li ion goes from full power to shut off instantly.
A li ion battery can not be charged to full capacity or it will, as they say in the trade, spontaneously deconstruct. Details vary but assume the battery management electronics knows what is best. Charge until the charger cuts off, which will be well short of 100% maximum, but might indicate 100% operation capacity.
Is the 40% value quoted 40 % of theoretical maximum or 40% of working range. If charged to 40% of working range, presuming the charger provided a reliable measure of this charge, the battery would not stay at 40% do to a slow self discharge. After some time the battery would have too little stored capacity to be useful when needed. As a practical matter charge till the charger cuts off and hope the battery management system does what is best. Charging and use temperatures are the main variables that affect batter life (also rate but one has no control of this variable).
Details of Li ion batteries vary and improvements are being implemented every year. Follow manufacturers recommendations.
The XRP batteries are NiCAD, not LiIon. I have had a BUNCH of the 18V and I'm down to maybe four as they die. I have a couple of Dewalt LiIon batteries too, less than a year and one already won't hold a full charge. These came in a kit to let them be used almost anywhere the 18v XRP batteries are used.
The XRP on the left, the LiIons and adapter on the right. The LiIons are MUCH lighter in weight.
Read on the Battery University web site about why batteries with Nickle-based chemistry (NiCad, NiMh, etc) can go bad just sitting on the shelf.
When I have some that start to hold only a partial charge, I do a deep discharge. My buddy the battery expert built a circuit with a big resistive load to discharge at a controlled rate but I usually run them down in my cordless circular saw. When building my shop and cutting notches in posts for beams I started keeping track of how many cuts I could get with a charge before the battery went dead. (I ignore advice to avoid running them all the way down - I ran them until they were dead dead dead.)
Notches for the first of doubled or tripled 2x beams. I made a bunch of horizontal cuts then cleaned out the notch with a hatchet.
I had some batteries that would barely make a couple of cuts. After some cycles of charge/deep discharge I recorded the number of cuts going from low values on some batteries back to what I got from new batteries. A couple of these are still among my best remaining DeWalt NiCad batteries.
Edit: I wrote this part earlier but lost it somehow. I'm giving up on DeWalt batteries even though I have maybe 8-10 DeWalt cordless tools. When the last battery dies I'll sell all the tools or convert them to use batteries from another brand. I have a Hitachi impact driver I've used extensively over the last 8 years or more and the batteries are just like new. Yesterday I bought a Milwaukee cordless grease gun after talking to some mechanics at a construction equipment maintenance shop - they have used them exclusively for a long time. I've always been happy with anything Milwaukee.
Thanks to all for the advice. They are actually Nicads. I looked on line last night and saw something that made me think they were lithium. I guess the key is going to be to try to remember to do to a discharge / charge cycle every so often to try to keep them going. I don't think the charger that came with them will do a discharge. Previous owner has tossed all the documentation, so I'll have to try to track that down on the interwebs.
