Battery Box Build - Sealing Door!

Hi, this is my first time posting (hopefully in the right place!).

I am in the process of designing my first ever ebike build using a Specialised Sirrus Sport 2015 bike with Bafang BBS01B mid drive motor (250W) and custom made 18650-based battery pack.

I want to make the battery casing out of fibreglass due to its structural properties (as well as an opportunity to try out the fibre glassing process for the first time). The container will be closed with a removable panel lid on the side of the pack (see picture attached) and will be secured with bolts around the perimeter of the opening.

--- My question is: Does anybody have any ideas on how to create a seal between the panel lid and the container itself? I thought about taking inspiration from engine block gaskets and 'DIYing' a rudimentary gasket of my own but am not sure what material to use. Other alternatives may be silicon/caulk sealant, neoprene self adhesive tape but I'm not sure what will be most suitable for my application. ---

Any advice would be greatly appreciated.

Thanks

N.

If you make the door panel so it has an inset portion that fits within the "inner frame" of the box, it will help seal as anything going in there must go around that, too, and not just leak into the first edge. Sorry I don't know how to describe this properly. As if you had a door that was two sheets of the same thickness, but inner one is smaller than the outer one for a good fit in the frame. (but actually made as one piece)

If you also inset the door itself (so the part it screws to on the box is not flush with the outer edge) then it can be an even better seal as it has one more step water has to go around to get in.

Add a thin layer of silicone to the entire door's edge / flat surfaces that mate with the box, and let that get a skin (but not solidify).

Coat the box's mating surfaces with something the silicone can't stick to, like wax (there is a product called "mold release" that is a PVA waxy stuff you can spray or brush on).

Then lightly screw the door down to partially compress the skinned silicone gasket against the box's surface, to cause it to conform completely to it. Let it all sit there until it's cured.

Remove the door and clean the mold release off everything.

Now you should get a pretty good seal when you fully tighten the screws down.

You'll want to use nutserts or something to thread the screws into, because the plastic won't take the torque you'll need to compress the seal.

What are you access requirements? (Do you need to open the box frequently or only if something needs to be inspected/repaired)

I used some peel-able/removable caulk for sealing around a window, where I didn’t want a permanent seal and it worked well and was easy to remove when I needed to. You’d need to seal it each time, as opposed to a reusable gasket, but if you’re not accessing it often, it would be a watertight option.


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amberwolf said:

If you make the door panel so it has an inset portion that fits within the "inner frame" of the box, it will help seal as anything going in there must go around that, too, and not just leak into the first edge. Sorry I don't know how to describe this properly. As if you had a door that was two sheets of the same thickness, but inner one is smaller than the outer one for a good fit in the frame. (but actually made as one piece)

etc.

Click to expand...

Thanks for the advice, amberwolf. I think I understand what you mean by an "inset portion"; I've attached another photo from a previous thread of something similar for confirmation.

The suggestion for sealing the lid sounds like a sensible idea and I will already have mold release from making the fibreglass container itself (I think) so that's probably one less material to worry about!

Definitely agree with the use of nutserts or something similar, I will have to look into whether I can insert these into fibreglass.

Do you think this approach would allow semi-regular removal of the container door panel? (Intend to store the charge connector within the box so will have to remove the panel each time for charging. Estimated use around 2 charges a week so maybe 100-200 removals of the panel a year).

E-HP said:

What are you access requirements? (Do you need to open the box frequently or only if something needs to be inspected/repaired)

I used some peel-able/removable caulk for sealing around a window, where I didn’t want a permanent seal and it worked well and was easy to remove when I needed to. You’d need to seal it each time, as opposed to a reusable gasket, but if you’re not accessing it often, it would be a watertight option.

Click to expand...

Also thanks for the help, E-HP! As mentioned to amberwolf, I intend to open the box around twice a week (maybe less) - mainly for charging but also possible maintenance and tweaking design features within the pack.

By the sounds of it, removable caulk may not be the best option for my intended application but perhaps it would be good for sealing holes where wires exit the pack etc(?).

NiCat23 said:

I intend to open the box around twice a week (maybe less) - mainly for charging but also possible maintenance and tweaking design features within the pack.

Click to expand...

RTV silicone may work if you need to remove the cover that frequently (similar to what Amberwolf described):

"One note we should mention is that RTV silicon is not just for semi-permanent seals, it can also be used in applications where repeated removal is necessary. When a semi-permanent seal is made, you apply the silicone, mate the parts and bolt them down. For removable seal, you apply the silicone and allow it to fully cure before installing the part. This creates flexible seal that does not bond to the other side of the mating surface. This can be used for valve covers and other parts that require regular removal. The caveat here is that you have to use just the right about of build-up and ensure that the silicone is in the right place to yield a proper seal."

http://knowhow.napaonline.com/jb-weld-rtv-silicone-guide/

Are you planing to mold this box, or are you going to build up the box over a foam form? I use the foam block method, and generally make it a solid form first, then use dremel to cut the door out, and attach a lip to the inside of the box after. That makes it much easier to get the panel flush, and get the seams to line up.

Instead of screws, you could use Neodymium magnets. I use them on my bike, and they hold very well. it makes it much easier to open and close the hatches quickly, and without tools

How sealed do you want to make this? Airtight, or just keep the rain off the sparky bits?

If you're looking for airtight, you might want to rethink that for a couple reasons. One is the heat cycle. they get hot in use and while charging, up to 80C (180F) in normal use. That will cause a significant expansion of the air sealed in the box.
Two, you'll get much longer life out of them if you can keep the temps down. letting some air get across them will help.
Three, batteries need to be able to vent for safety reasons. Lithium batteries will vent when they have an internal issue. the gasses vented are explosive. the last thing you want is a sealed box full of explosive gas, high energy combustible material, and an ignition source. That would be a bomb. Instead, the gasses need a way to disperse

NiCat23 said:

Thanks for the advice, amberwolf. I think I understand what you mean by an "inset portion"; I've attached another photo from a previous thread of something similar for confirmation.

Click to expand...

Sort of like that, but a double step, so the door itself also has a matching inner layer. If I can use all my fingers again later, I'll try to sketch what I mean in paint (steam scalded the back of the middle three fingers on my trackball hand earlier; got 'em wrapped together with aloevera plant whcih always helps).

Definitely agree with the use of nutserts or something similar, I will have to look into whether I can insert these into fibreglass.

Click to expand...

You would mold them into the unit. Leave scrap screws threaded into them coated in mold release to prevent material contaminating the threads.

Do you think this approach would allow semi-regular removal of the container door panel? (Intend to store the charge connector within the box so will have to remove the panel each time for charging. Estimated use around 2 charges a week so maybe 100-200 removals of the panel a year).

Click to expand...

I'm sure you could do that, but I don't recommend it.

First, you're going to get awfully tired of doing that every time you charge.

Next, it's not too difficult ot get (or make) a waterproof charge connector for just positive and negative mains.

If you need balancing connectors, its' been done with DB-25's and similar, mounted to the bottom of the housing, and using rubber caps over them to keep splashes out. There are also the Canon (Cannon) connectors, which are commonly the round types you see on assorted aircraft and military and test equipment, and those come in waterproof versions, with screw on waterproof caps with chain retainers, too. They come in just about any number of pins you might want...but they are expensive if you have to get them new. If you have access to scrapyards you cna pull them off junked equipment....


Alternately, you could mount a waterproof charger (like the Meanwell HLG series, or the Cycle Satiator) on the bike (or whatever it is), and leave it wired into the battery via a diode or switch (to prevent the "on" LED of the HLG from very slowly draining the pack).

You could build the charger into the box, but you'd have to also build a heatsink into it to transfer the heat out of the charger and case, and having the charger operating in there is going to heat the batteries every time you charge, too.

Hello, I met the same problem a few years ago.

1. If frequent disassembly is considered, custom sealing rings are recommended. Cut according to the size of the drawing. IP67 at this location can be easily achieved by using two seals. But other holes still need to be sealed well.

2. Considering the particularity of fiberglass and preventing wear and tear caused by many disassemblies, we should also consider the deformation caused by high temperature, the corrosion caused by rainwater and other acidic sewage. I recommend chloroprene rubber. The price is also relatively low.


Other comments. For reference only.
1. I see you have reserved a power output hole. Does your battery pack support all in one (in/out)? If supported, the battery can be recharged without opening the battery box. If not supported, a power input hole can also be added to the housing. Reduce the opening frequency of the battery box.

2. Ultra-sonic welder is recommended for all plastic battery boxes if they do not need to be disassembled.

3. Heat dissipation may be a hidden danger. Temperature test can be carried out. BMS or Temperature sensor can judge whether the temperature is too high.

4. In the case of not being completely sealed, I recommend adding a leak hole at the bottom of the horizontal position. In order to prevent the leakage hole from entering water, “one-way valve” can be used.

Thank you all for the advice and support, I really appreciate all of it - and sorry I could not reply sooner, it has been a busy weekend!

---

E-HP, that RTV silicon looks like a good bet. Having reconsidered the practicality of removing the lid 100-200 times (as mentioned by amberwolf), I think that I'll try to limit the number of removals to when urgent maintenance is necessary.

---

Drunkskunk said:

Are you planing to mold this box, or are you going to build up the box over a foam form? I use the foam block method, and generally make it a solid form first, then use dremel to cut the door out, and attach a lip to the inside of the box after. That makes it much easier to get the panel flush, and get the seams to line up.

Instead of screws, you could use Neodymium magnets. I use them on my bike, and they hold very well. it makes it much easier to open and close the hatches quickly, and without tools

etc.

Click to expand...

Drunkskunk, I intend to use the foam form method. May I ask how you add the lip after the door is cut out? I was thinking about creating a foam form for the box with the lip feature already present, then making the lid separately using a mold.

Regarding the use of Neodymium magnets, how do you install them onto the container and lid? And what about size, quantity of magnets, spacing and installation of magnets etc? Magnets over bolts is definitely something I would like to consider!

I just want to keep the rain off the sparky bits. I'm not a hardcore cyclist so won't be going out in torrential rain, but would like to have peace of mind when going through occasional, moderate rain showers and puddles.

If I'm looking to keep rain off sparky bits, would you recommend doing away with the seal entirely?

What I would say is that I have two 2P10S batteries in parallel, each battery is set to kick out a maximum of 7.5A each for a total of 15A (limited by the controller). That means that each parallel string is providing 3.75A of current (making some assumptions). Considering the internal resistance of these cells, is this current really enough to reach 80 degreeC battery temps? However, I am aiming for tightly packed batteries with not much space for airflow inside the container so having a no seal approach may help with airflow.
I think point three on your list could possibly be resolved using a pressure release valve. I worked on an electric car project at university where one valve was used on every battery pack we used, I wouldn't be surprised if this was standard in the automotive industry.

---

amberwolf, hope your hand gets better soon! I think I get what you mean now (I've mocked up another example on CAD).

If I go with bolts over magnets, I will be following your advice on the nutserts.

I won't need balance connectors but will need something like the Cannon connectors, I will have a look around! I am quite interested in using panel-mount connectors to reduce the number of wires I have outside the box too.

---

QiaoNan, I intend to seal the fibreglass and then coat it (not sure what with yet) to protect it from the elements. I live in the UK countryside which doesn't get too hot (maybe 2 weeks of hot weather a year if we're lucky!).

I want panel mounted charging and discharging connectors so that I have to remove the panel door less frequently.

I am not sure what you mean by the use of an ultra-sonic welder, could you possibly re-explain what you meant?

I will definitely be conducting temperature tests with temperature sensors or through the BMS (if it supports such a feature).

I'll add one-way leakage valves on my list of things I need to look into! Thanks for the help! .

You can mold the lip into the foam, but it's kind of a pain in the ass, and nearly impossible to make the lid and the box match. It's something you can do when building a mold for a part, but not something that you would do on the Lost foam method like you're going to use.

To make a lip and a gasket spacer for a lost foam box, you start by making them as separate pieces. Start with a flat, texture free surface. I like to use a sheet of glass, like the smooth side of a glass cutting board. Cover it with wax paper, and then lay out some fiber glass about the size of the side of the box. You might want to make this several layers thick, as it will need to be strong.
Now make a second one the same size, but the thickness of the gasket you plan to use. There are ways to make this precise, but if you plan to use a tube of RTV for the gasket, then make a rough guess, it will be fine. 1 to 2 layers maybe.

So you now have 2 fiberglass sheets. 1 will be the lip, the other the gasket spacer.

As I said before, you cut the hatch out of the finished box. a Dremel blade and a straight edge work well. Once you have that done, use the hatch to trace a hole on the gasket spacer. Now cut that tracing out so that the hatch will fall through, but not by much. cutting on the line of the tracing should be just enough oversized. you want the hatch to be pressing on the gasket, not an edge of this layer. now trim around the outside of panel so it's between 1/2 and 1 in wide. The wider the better, but it needs to fit inside the box.

Next, trace out the hatch on the lip panel. but this time, measure in from the edge of the tracing inward, so that you are cutting a smaller hole. this hole needs to be wide enough that you can pass your battery through it. Trim up the outside edge the same way you did the gasket spacer, and you should have 2 panels that look like the pic below







Now it's time to install. First, put them in and make sure they will fit. trim and sand as need, then take out the lip. You're going to glue in the gasket spacer first. try not to use any clamps or weights for this part, as they can distort the box and then the hatch won't fit clean any more. Once it's cured, slide in the lip panel and bond it in place as well.






Once it's cured, slide in the lip panel and bond it in place as well. And they should look like the picture below.

Bonus part. (No pics) if you're using RTV, you can get the thickness right by taking the center part you didn't use from the gasket spacer and cutting 1/4 inch a strip off it. cut that stip into 4 to 8 pieces about twice as long as the lip is wide, and sticking them on the lip so an end hangs off the lip. These are your depth spacers. A dab of RTV will be enough, you'll take them off soon. Next, run a bead of RTV around the lip taller than these spacer. now lay a sheet of Wax paper over the entire hole, or use some mold release on the hatch, and press the hatch into place. let it cure, and then remove the hatch and the those depth spacers. You'll need a razor to avoid pulling off the gasket. Next, Fill the holes they left behind, and replace the wax paper and put the hatch back on. When it cures, you'll have a fully formed gasket that sits at the right depth to keep the hatch flush with the side of the box. if it doesn't, you can paint on epoxy to the back side of the hatch until it does.




For magnets, I use pushpin Refrigerator magnets. One tap with the hammer, and the plastic shatters, leaving me with a tiny powerful magnet.
To add the magnets, set the hatch in place and make sure the gap is even all the way around. Then drill your holes all the way through the hatch and the lip




Once you have the holes, glue your magnets to the hatch first, and let the fully cure. next, use a long cure time epoxy, and glue the magnets into the lip . put a sheet of wax paper over them, and put the lid back on. be sure to add some weight so the seal is kept tight. Give this a day to cure so it doesn't pull the magnets out. then remove the hatch and give the lip magnets another coat of epoxy from the back side, just to keep them secure.

And that's all there is to it.

Foam tape? For Draught exclusion.

I just use pvc tape between the panels and it look good as my box is all black.

Wow, drunkskunk that is an excellent set of instructions and example in how to make the box properly using the lost-foam method! Thank you for all the diagrams and explanations, it was incredibly easy to follow, even for me!

As you have to make the box bigger than the actual battery pack to fit it through the hatch and lip, you'll end up with quite a lot of empty space inside I'd imagine. This may be good for temperature reasons but I was wondering about the batteries 'rattling around' inside. Do you use some packing material on the inside of the pack to counteract this or is this a non-issue?

Thanks again,

N.

The inside of the box will be rough after removing the foam, and a real pain tto sans smooth, so some thin foam inserts would be good, especially on the floor of the box.

a battery that can rattle and bounce around is a bad thing, so you may also want to add some tie down straps. either Velcro or just a string of zip ties.. it won't take much to hold it in place.