The most common reason for DIY ebike battery failure is damage due to improper mounting. Frame packs connect directly to the frame of your bike and generally do not require extensive shock or waterproofing. If the battery is in a triangle bag, in a rack bag, in a backpack or bouncing around anywhere else loose on your bike then it should be water & shock proofed.
A typical lithium ebike battery cluster (other than frame packs) often comes shipped in a bundle surrounded by oversized heat shrink wrap. The ends of the battery pack are often covered with small squares of plastic, or it has been heat-shrinked twice, once width-wise and once length-wise. This setup works well as a way to transport your batteries and BMS around, but it is still quite easy for water to work its way into the end of the packs. There is also little to no shock proofing so if you throw a loose triangle pack into a triangle bag without padding often around the perimeter where the batteries contact the frame it will create serious problems over time. This article goes over some of the ways I’ve found to protect your valuable investment using only Ziplock freezer bags, packing tape, Thermarest sleeping pads and duct tape.
When I first started packing batteries in my backpack I was concerned about damaging them if I wiped out on my ebike. For normal people, this would not be an issue, but I seemed to take a pretty bad spill almost weekly on my mountain bike and had gotten into the habit of flipping around and landing on my backpack which usually had just a spare tube and some water in it. I didn’t want to flip over the handlebars and land on top of my $500 battery, and if I did do it, I wanted to make sure my battery would survive a 200lb weight (that’s me) dropped on top of it from a height of about 5 feet.
With my battery in the backpack, another problem I have is that I leave the top of the pack open and snow will fall down onto the top of the battery and melt. Shrink wrapped batteries are water resistant but are not water proof. Because I have to leave the backpack open at the top for cooling, I decided to start waterproofing the batteries as wells as shock proofing them. At first, I tried materials like saran wrap and plastic sheeting but what I ended up using which works better than either one is heavy duty brand name ziplock freezer bags.
Take your battery and put one bag over the end without the wires then put a small cut in the corner of the 2nd ziplock bag big enough to pass the connectors through. Put the second bag on the other side overlapping the first ziplock bag with the wires passed through the tiny cut in the bottom of the bag. Get packing tape and tape the two ziplock bags together. Fold the ends of the bags like you’re wrapping a present and put packing tape around the ends of the battery. It’s important that all the edges of your battery have a strip of packing tape, as it is pretty tough and will help to keep the battery from wearing holes in the freezer bag. Where the cables go into the bag wrap a piece of packing tape tightly around the cords to keep water out. If you want to make it super waterproof then add a little silicone before your wrap the tape around the wires.
Once you are done wrapping the battery in ziplock bags and packing tape it’s time to work on the shock proofing. I’ve tried a lot of materials, but the stuff I’ve found that works better than anything else is cheapo Thermarest pads. They are a heavy duty closed cell foam that you can get on ebay or Amazon for about $15 and one pad will do about 3 batteries. For a small shark pack you’re looking at about 100g, the heaviest shock proof case I’ve ever made was double ply sleeping pad for a very large 20lb battery and weighed about 260g. If you use thermarest pads make sure that the thermal coated (silver) side is on the outside of the pack. It will reflect too much heat back to the battery if it’s on the inside.
The first step is to cut the foam pad to size. You want the ridges to run the length of the battery and wrap the battery around the pad so that it is about 80% the entire way around. Having a large slit in the case will save you a little padding and make it much easier to the battery to release built up heat.
Once the pad is wrapped around the battery cut two slits on the bottom so you can fold one flap up and two flaps in, one from either side. You will want this extra triple layer of padding on the bottom, as this is where most of the impact will come from when you are setting your backpack down. If you are building a protective frame for a triangle battery then you will not need this much padding in one place.
Then I start wrapping duct tape around the bottom to tie it together. Once that is done I will wrap duct tape around the center and near the top of the battery, making sure the foam is loose enough that you can easily remove the battery from the case, but not so loose that it will slip out on its own. Where the duct tape bridges the gap there will be sticky tape towards the battery. Take some small pieces of duct tape and wrap them around the tape so that it can’t stick on anything. I usually cut a few air holes in the side of the padding to help the battery cool as well.
For Triangle packs stuck in triangle bags you do not need to wrap the Thermarest all the way around the entire battery. A thin strip wrapped around the outside edge of the battery that is about 2.5 inches and taped in place should be enough. Since that is the only place the triangle pack will come into contact with anything hard, that is really the best way to do it. If you put padding on the sides of the triangle there is a good chance that it will end up being too big to fit in your bag.
Whether your battery is in your backpack or in a triangle bag or a rear rack pack, waterproofing & shockproofing it costs almost nothing and may add years of life to your pack. Think of it like buying dirt cheap health insurance for your ridiculously expensive lithium packs.