Introduction: Puncture Resistant and Polycarbonate EBike Battery Case

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this instructable shows how to make a puncture resistant, flame retardant (and even waterproof) enclosure for your eBike battery.

eBikes are becoming more and more popular. Most of the eBike batteries being sold online are protected only by paper and heat shrink tubing. This is insufficient for safe operation as vehicle batteries, especially if you are using Li-ion. Poly-carbonate sheeting, commonly sold by brand names like Lexan and Mercolan, is puncture resistant, shatter resistant, flame retardant and affordable. As such it provides an ideal material with which to construct a protective case for vehicle batteries of all types.

For less than $30 one can easily construct a protective case that can save your $400 battery from a wreck or other physical trauma. The video describes the construction. Even with no 'ahead-of-time' design just winging it, and not really caring how it looks, I ended up with a pretty nice enclosure! With more planning and attention to detail some very nice battery boxes could be made using this method...

Note - This is a work in progress. I'm still finising the waterproofing and mounting and have yet to edit and add most of the photos. Stay tuned for updates!

Step 1: Parts and Tools

Only basic tools are required for this project. Though if you have a sheet metal bender the job becomes easier.

Parts

  • Sheet of poly-carbonate 1/16" thick - measure the battery and buy a sheet that is at least - length + 1 by 2(width + depth) + 1. So for example, my 10AH battery is 11" x 5" x 3".So - 11" + 1" = 12" by 2(3"+5") + 2" = 18". So for my main tube I need a piece at least 12" by 18". (I bought 12" x 24" so I have enough to make some mounts and the endcap too...
  • Sheet of poly-carbonate 1/4" thick - you will need 2 pieces that are each battery width x battery height. So for my battery I needed 2x 3" by 5" pieces. We also need a bit extra if we want to make mounts to attach to the downtube. For most small eBike batteries, 6" x 12" is enough
  • Plastic welding (solvent based) glue - I'm using a product called sci-grip 16 but any solvent based polycarbonate compatible welding agent should work.
  • Silicone RTV sealant. - This goes around the cable exit points to waterproof the battery
  • Small machine screws (optional) - for the lid

Tools

  • Saw - jigsaw or reciprocating saw to cut the PC sheet.
  • Drill - To drill the PC
  • Hand Sander (or sandpaper + time) to smooth and shape the edges of the PC sheet
  • Heat Gun (or a sheet metal brake)
  • Table Vise
  • Wood and Metal Straight Edges to help bend the PC sheet

Step 2: Cut and Bend the 'tube'

First we cut and bend the tube for our battery enclosure. In order to do that we must measure the battery.

The width of the sheet for the tube will be -

(width of battery + height of battery) x 2 + 2"

So for my battery (5" wide x 3" tall)

(5" x 3") x 2 + 2" = 18"


The height of the sheet will be -

length of battery + 1"

So for my battery (11" long) I want a 12" sheet and the size of sheet I need is 12" x 18"

Next we bend this into a tube shape that has an internal diameter of w x h. After doing this we have an extra 1" or so which is folded over at 90 degrees as a tab. This tab is then plastic welded together.

A sheet metal brake press is the best thing for bending poly-carbonate. But since I didn't have one, I improvised and was able to get nice 90 degree bends by clamping the PC to a straight edge then starting the bend cold. Once I had a 'crease' I would use the heat gun to sharpen the bend. (poly-carbonate has a lot of spring-back. Without heat I was only able to get 45 degree bends. Be careful though! Too much heat will cause the poly-carbonate to warp or bubble!)

Once the tube is bent, use the solvent welding compound to weld the open edge and clamp it until it sets (see video).

Step 3: Cut and Weld the Bottom End Cap

In the design, the bottom cap is a rectangular piece of 1/4" PC that is just large enough to fit into the tube like plug or stopper. This should be plastic welded into the bottom of the tube.

Unfortunately, because the piece of PC I bought was short by 1/4" I didn't have enough length in the tube to do a 'stopper style' joint. So a larger piece of 1/4" PC Was glued on to the end of the tube in a simple 'butt-joint'. I do not recommend you do this. If possible make a longer tube and use a 'stopper style' joint for the end cap. It provides 4 times the contact area between pieces and will be much stronger!

Step 4: Make the Top End Cap

This is merely a rectangular piece of 1/4" thick PC that is cut to fit inside the end of the tube *5" x 3" in my case) glued to a slightly larger (3 1/4" x 5 1/4") piece of 1/16" polycarbonate. This makes a cap with a lip to prevent the 1/4" piece from falling into the tube.

Step 5: Fit the Cables an Install the Battery

To do this I installed the battery into the tube then noted where the wires want to exit. Then I thought about where the battery will be mounted so I could decide where I wanted the wires to exit the battery. Then using a drill and some files made slots at the end of the tube to accept those wires. The slots should bee deep enough that with the end cap installed the wires are not pinched.

After everything was shaped and the end cap installed, the battery fit into the box with a slight amount of 'slop'. to keep it from rattling around I took some strips of dense foam 10mm square and used it to make the cells fit snugly in the case. Alternatively you could use vibration isolating pads or sheet to do the same thing. I use a product called sorbothane to mount electronics and cameras in my UAV's and it works extremely well.

Step 6: Assembly and Waterproofing

There are a couple of options to attach the last end cap. You can -

  • plastic weld it like the rest of the box - This yields the strongest box and is easy to make waterproof with silicone RTV sealant. The downside is that if you ever need to gain acces to the battery you will have to cut the case open and probably make a new one. But since they are so cheap to make that's not a big deal... If you do this double and triple check the electrics before gluing. It would be no fun at all to glue the box shut and then look through the clear plastic to see a loose balance connector!
  • Silicone and screws - To seal the battery this way you draw a bead around the lip of the lid, then mate it with the battery case. Once it's set drill small (1/16 or so) pilot holes around the edge and inster small finishing screws. Finish off by sealing the wireing with RTV. This is not quite as strong as welding the lid, but allows for battery access later on without destroying the enclosure and allows us to make a waterproof case.

Step 7: Mounting

I've been riding with the battery in a back bag (on a rack) over the rear tire. While there is plenty of space there, it's not a great place due to it being high up, far back and over a tire with no suspension. So Now that I've got a weather-resistant case, I decided to try to improve battery placement on the bike. this meant moving the battery further forward and down and mounting it on the bicycle frame. To actually mount the case I've added straps onto the box. Each strap consists of a long strip of 1/4" PC with 1/16" spacers underneath. This leaves what are essentially 'belt loops' which hose clamps fit through to secure the case to the down-tube of the bike frame. I also have added larger hose clamps which go all the way around the case.

This has freed up my back rack and bag for general storage (including enough space to carry the charger along if need be...) The clear plastic case turns heads and also allows me to see the charge indicator LEDs on the BMS board during charging. Overall I'm happy with this project.***

*** Note - This is a work in progress. I'll likely build a 'MARK II' version incorporating the lessons I've learned from this prototype. So Stay Tuned for More!!!