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Will A Waterproof Bag Cause Overheating?

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    Will A Waterproof Bag Cause Overheating?

    I'm building my first ebike and want to mount the battery on the underside of my down tube (by front wheel) on a step-through frame Electra Townie. There isn't enough clearance for a Wolf pack plus the tube is curved which makes it difficult to mount a 14" cradle for a Wolf or Shark style hard case battery. I don't want to mount it on the rear rack and would like to avoid using two mini-packs in parallel. I'm considering using an Anything Bag from Salsa strapped to their cage. Since it is pretty much sealed up (it's a dry bag with a roll-down top) will this cause an overheating problem? The battery is a Luna 52v 14ah. I'm open to other ideas.
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    Last edited by Lakejumper; 08-20-2018, 03:23 PM.

    A lot of people use a soft triangle bag, so soft material must be okay if it is secured in place. However, a triangle bag has multiple mounting points approximately 60 degrees apart from one another.

    If you do use that bag, you will have to have it cinched down tightly. Even then, it seems to me that it could sway back and forth. Maybe four cinch straps would hold it tightly enough. You could periodically check the tightness of your cinch straps and tighten if necessary.

    Many people in your position would fabricate a metal box and secure it to the frame with Rivet Nuts I weld if you want to design a box. The box would need to allow access to the bolts that thread into the Rivet Nuts.

    Maybe you could find a metal box with your desired dimensions and mount it upside down under your down tube. In this case, you would need to secure the lid of the box because the box would be mounted upside down. I'm sure you could find a waterproof box that has a top that bolts on. And then use a waterproof grommet for your battery connection.

    You could also use a combination of these ideas and fab a metal frame that mounts to the down tube with Riv Nuts and then fab a bag made of the DryBag material that goes around that skeletal frame. In this case, you would have to take care to prevent water from entering the battery bag at the mounting points (thanks, gravity!).

    Would you consider putting your battery in a bag or box on a rack over the rear tire? That is easy peasy. You would have room for a larger battery if you ever wanted to extend your range.

    DIY fun, right?
    commuter ebikes
    Super Moderator
    Last edited by commuter ebikes; 08-20-2018, 06:12 PM.


      If thats an Anything bag, you have room for I believe five straps if you use the HD cage. Use all five and you'll be fine. ASSUMING that cage is sturdy enough. Further, do not use the Salsa straps. Use a velcro cinch strap with a buckle. If you use a buckle you can tighten the strap down a lot better than if you were trying to use the more serious Velcro Onewrap, which would be my second choice.

      Attach the straps around the frame. So go thru the HD cage and then around.

      I have used five 2" buckled velcro cinch straps to secure a 25R, 14S8P 20ah brick battery. It was so secure, when the bike was hit by a car (T-bone impact at speed) the battery stayed put and survived with no damage (surrounding it in Therma Rest closed cell foam helped).

      Worth noting: Panasonic GAs get hot under load. For a low amp draw like a hub they are ok but if you are going to be pumping out 30 amps on a BBSHD then you are right to consider heat as an issue. If this was a 25R pack I wouldn't have the same concern. But GAs... Might be best to not use a bag at all and do what you can to waterproof the connections.

      Therma Rest original (non thermal) closed cell foam mattresses make for cheap and highly effective padding around that battery... but again heat may prevent you from going there.


        You can avoid heat by simply going with a less thermal reactive cell. The GA cells are not good for this but the 25R and 30Q cells are.


          I was in your position of not having enough space in the frame triangle to mount a standard battery pack and case. I tried everywhere and everything. I investigated the underside of the downtube solution but found that on my bike I ran the risk of the front tyre coming in contact with the battery when the forks where under compression.

          I decided that the only viable solution was to find a battery pack that would fit in my frames's triangle and fabricate a case. However I couldn't find anyone that sold a battery pack of the correct size and specification. I considered building a pack myself but thought better of it due to the cost of a battery welding tool.

          SOLUTION: I specified the battery pack that I needed and a company on aliexpress built it for me, sending photos throughout the build process to check it was correct. I initially tried to build a battery case out of a single piece of sheet aluminium but it didn't turn out good enough. I then bought 3 aluminium electrical enclosures and fabricated them together using suitable expoxy resin for fixing aluminum with strips of aluminum to brace the joins. After spraying with a car primer and finish coat the look was pretty good - well to me anyhow :-). I've used sealed wire grommets, and heat resistant silicon sealant on all joining surfaces to create a gasket for water sealing. I have used a remote on/off switch from Lunacycles so that I can isolate the power when cleaning or charging as I can't quickly clip out the battery pack whilst doing either.


            Sorry for the late response. Thank you for all of the ideas and info on the battery and enclosures. I failed to mention that I haven't purchased a battery yet. I will wait until I install the motor to see how much room I have on the downtube with the front shock compressed.
            Last edited by Lakejumper; 08-26-2018, 02:26 PM.


            It depends on what kind of thermal reactive cell you're using. I know for sure that the 30Q cells should work since they're less thermal reactive. However, I've also read on several forums that the 25R should work. The GAs usually get hot under load since they're more thermal reactive. Anyways, let us know what battery you're gonna get and whether it worked out well! By the way, I was thinking of getting a backpack for cycling, and I found a few websites online, but I'm not sure from which one I should get it. I liked this one, but I'd love to get to know what other brands you can recommend! I'm looking for one that shouldn't cost me more than 200 dollars!
            Last edited by cleopants; 05-18-2021, 04:43 AM.