If you do a lot of outdoor work or play in foul weather, you've probably noticed that keeping your batteries dry can be a bit of a hassle - they are metal so they attract condensation. And you've probably also noticed the considerable importance of keeping them dry. Not only will a wet battery rust rapidly, the water can create a conductive path around the top of the battery which rapidly discharges it - leading to a nasty surprise if you are counting on your gps or flashlight! I've also discovered that even if i keep my spares in a ziploc bag, some of my "waterproof" electronics (like my gps) are actually waterproof on the inside, but not the battery compartment - so while the gps electronics are happy and dry, the batteries powering it are sitting out in the damp cold still!
so, for a while now i've been waterproofing my batteries. its incredibly easy! so easy in fact that i really wonder why some of the manufacturers don't do it already.
This project is brought to you by MonkeyLectric and the Monkey Light bike light
(which now includes a 100% waterproof battery holder so this tip isn't needed)
Step 1: What you need
All you need is a waterproof coating material. There are a wide variety of products which work well: at your home improvement store there are lots of options such as urethane waterproof coatings, rubberized paints, etc. from an electronics supply story you can find silicone and urethane conformal coatings which also work very well. these coatings soak in well, stick to metal, and dry to a rubbery finish. if you are in pinch, you can do quite well just using standard oil-based paint, or nailpolish. these soak in and stick well, but are a bit hard when they dry so i suspect they may chip off over time (although i have not tried them enough to say for sure).
Step 2: do it!
all you need to do is put a dab of your waterproofer around the dimple-end of the battery!
ideally you want a waterproofer that starts as a thin liquid so that it soaks in well - there is a fiber washer around the metal dimple, which should soak up your waterproofer, and it will also soak into all the corners under the washer.
that's it! .... unless your battery is already well used? the 2nd photo below is a battery that had already suffered quite a bit of wear and tear - the plastic wrap was partly ripped, and the fiber washer was damaged so i removed it. in this case its still very easy - just put a few drops into the dimple area, and use something thin like a paper clip to spread it all around into the gaps.
if you accidentally get some on the dimple, you can wipe it off before it dries, or scrape off afterwards.
Step 3: How it works
of course you are wondering - how does it help to put the waterproofer only around the dimple? what about the rest of the battery?
The entire can of the battery is made from stainless steel - totally rust-proof. so why does it rust? its because right around the dimple area is where the positive and negative of the battery meet at their closest point - and when water gets into this area, it allows electrolytic corrosion to occur - the electricity from the battery flows through the water, and this causes corrosion of our otherwise impervious stainless steel (as well as rapidly discharging the battery!)
So - all we need to do to make the battery impervious is to apply our waterproof coating around the dimple area, covering the area where the positive and negative are near each other.
Check out the photos below, where i've peeled away the plastic wrapper at the top of a battery to show this in detail. the bottom (negative) end of the battery actually extends under the label and up the sides of the battery all the way to the top - where it is separated from the positive (dimple) end only by a thin rubber seal. its very easy for water to get in the groove next to the seal and create a conductive path from positive to negative. the battery also is made with a fiber washer around the dimple - this protects this area from getting damaged (without it it would be too easy to short out the battery by dropping a screw next to the dimple) - but the standard fiber washer that is used soaks up water quite well, providing lots of nice water to corrode the battery.
in the 2nd photo i show the safety-pressure valve. if you accidentally over-charge your battery this valve will open to prevent the battery from exploding. if you are using a fairly thin or flexible waterproofing material the valve should not have a problem still working, but you probably don't want to use epoxy or any really thick or strong material as it will block the pressure valve from working in the rare case that it needs to.
Step 4: What about other types of batteries?
You can waterproof other shapes of batteries too, the main thing is to take care to keep the contacts clean while you seal all the gaps. you might need to put some tape over the contacts (cut to shape with a knife), then apply the waterproofer, then peel off the tape after it is dry.