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
Step 2: Do it!
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
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.