The Downspout E-bike Battery Case

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Building up a battery case for a DIY e-bike is a challenge. It must be lightweight but rugged and must be easily detachable. Achieving this on a budget is even more difficult.

Before proceeding with this project I suggest you perform a bit of research regarding the construction of e-bike batteries. It is a bit dangerous to the uninitiated. There are serious voltage and currents involved here so make sure you know what you are doing. There are plenty of educational videos available on youtube regarding this topic.

The following case is made up of 3d printed ABS components included here and some 3"x2" PVC downspout available from your local home center for about $3 (it is sold in a 15" length called a downspout extension")

The 1" x 1/8" aluminum bracket is also procured from the hardware isle of the home center for about $8. You should have enough to make two brackets. This formed bracket will bolt to the water bottle mount on the bike with two countersunk screws. The 3d printed mounts on the battery will dovetail into the bracket and the battery drops down into place, the top portion of the bracket then snaps over the top of the battery to lock it securely into place.

While at the home center also pick up a small can of ABS-PVC transition adhesive and some flat head screws (countersunk taper) that will fit in your bike bottle mount holes. Also look for some very small self tapping machine screws (or very tiny wood screws)

It turns out that a three row, hexagonally packed, 18650 cell group fits perfectly inside the downspout - with some ample room on either side to accommodate the sensing wires for the BMS board. However, the cells must be packed closely and hotmelt glue is used to connect them all together. My bike is 36V and I used 40 cells. However your system may be different. Naturally you may make the length longer/shorter to accommodate a different number of cells. I use a box as shown in the photos to cluster the batteries together in order to spot weld the contacting nickel ribbon together,

Speaking of BMS boards - I have thus far made two of these batteries using two different kinds of BMS boards. In my case these are 10S (36V) BMS boards as shown in the pictures and are available on ebay. They both fit fine inside the case, but the 40 amp white board is a close fit. I use some 1/4" thick construction foam between the batteries and the BMS. See the diagram image.

Since every bike frame is different and you may be using an alternate number of batteries I do not provide overall dimensions of the downspout and the aluminum mount. You have to work this out for your particular bike frame configuration and battery case length.

Here is the general workflow..

1. Decide on the number of cells you want and determine what the overall length of the PVC tube will be. Take into account the space needed for the BMS board. Give yourself some extra space as well for the wires at the top of the pack and the charging connector.
2. Take this measurement and "mock up" on the bike. Make sure you have some clearance between the top of the battery and the top tube of the frame - this allows for the battery to insert into the dovetail and then drop down about 7/8". Basically whatever length your downspout ends up it needs to be able to fit in the allocated space on the bike with at least 7/8" above it and it must not hit the bottom bracket or the front frame tube when it drops into final position.
3. Print two endcaps and build up your battery. These caps get printed in ABS with 3 outer layers, 5-6 top/bottom layers and about 30% infill.
4. I presume you know how to safely build up a lithium ion battery pack. It is a dangerous process. If you don't know what your doing then you better study some of the videos on youtube. You will need a spot welder and the Nickle strips, and you need to know what a BMS is and how to solder up the BMS sense wires.
5. I don't provide holes in the model for the harness or for the charging connector. These are also variable. I happen to use an RCA jack for my charge connection and I just drilled a hole for the silicone insulated wires to exit.
6. I glue the bottom cap in place. I then drill four small holes and use four small machine screws that provide some extra hold.
7. The top cap is only held in by the 4 small screws, in case the battery must be serviced later on.
8. At this point your battery is built and tested and everything is inside the downspout case. Print the two dovetail mounts that slip over the downspout. Print these in ABS at 100% infill. Use concentric infill to get the best strength for these parts.
9. You will probably have to file the dovetail on these parts to get them to slide easily over the aluminum bar. Not too loose mind you, just enough to ensure no binding.
10. Take your battery over to the bike for another mockup. Slide the mounts over the downspout case. Position the battery where it will be in its final "locked in" on the frame and slide the mounts so each are centered over each bottle mount hole. Mark the position of each of the mounts location on the downspout tube with a permanent marker.
11. Slide the mounts just above their final locations and apply ABS-PVC transition cement to the final location of the mounts. Then slide each mount to it's final position over the glued area. Carefully wipe off the excess glue and let them set.
12. Now cut the aluminum bar about 2 inches longer that the overall length of the case. This should allow just under an inch to be bend over. Center the bar along the length of the case and mark the top and bottom places where you will bend the bar. The bar will be bent in a vise.
13. The bottom "L" bend should be left at about 1 inch. The top "L" should be cut down to about 1/4"
14. Now final mockup on the bike. Take the aluminum bracket and the battery over to the bike determine final location of the aluminum bracket and mark the two hole in the bracket for the screws to go into the bottle mount.
15. Drill the holes in the bar and then follow up with a 45 degree counter sink. The screwheads cannot protrude above the bar's surface when the bar is bolted to the bottle mount on the bike.
16. Finally the dovetail cutouts are made on the bar. These are not done at the final rest position of the case in the aluminum bracket, but instead are cut just above the final rest positions. Such that the top "L" bend is pushed back, the dovetails engage the bar, and the battery then drops down about 7/8" to finally rest on the bottom "L" bend. The top "L" bend then snaps back thus locking the battery case onto the bar. I cut the dovetail notches on a bandsaw, but you can do it with multiple cuts with a hacksaw and then file them to finish. Be sure to deburr and slightly round over the edges of the notches to allow easy engagement.
17. When you are all done wipe the case down in alcohol and then spray paint. I used Rustoleum fast dry flat black.
18. To prevent water ingress use a dab of silicone sealant where the wires exit the top cap. A very thin smear of silicone can be applied around the perimeter of the top cap as well. Just don't overdo it as it can make the cap difficult to remove later on.

Possible future improvements:
I was thinking about adding a molded in hoop on the bottom cap, so you can run a bike lock cable through it when you lock up the bike.

Update 3/25/23 : I just wanted to add some additional information... On my second battery I used "hexagonal" nickel strips (instead of linear nickel strips. This goes together easier and provides an overall stronger pack. This material along with a variety of discount 18650 batteries are available from batteryhookup.com.

In early 2022 I was hit by a car that blew through a stop sign. The battery described here was knocked loose from the bike, but sustained only scuffed damage. My helmet saved me.

In early 2023 there have been several reports of e-bike battery fires in NYC and elsewhere. Please employ safe practices with regard to e-bike batteries. Do not leave them on a charger for extended periods. While charging and storing make sure the batteries are kept far away from flammable materials.

Do not leave your batteries outside to freeze in the winter! I'm wondering if this may be a source of the fires reported. Given the timing of the reports. In other words, folks let their batteries freeze over the winter and then started charging them in February/March due to the mild weather.

The battery spot-welder I used is available from banggood.com. However there are other options available. I used three, high current, spim08h batteries to run the spot welder.

https://www.banggood.com/Portable-DIY-Mini-Spot-Welder-Machine-with-LCD-Display-Automatic-Touch-Welding-Mode-for-18650-Battery-12V-Car-Battery-Super-Capcitor-p-1694146.html

https://batteryhookup.com/products/2x-spim08hp-3-7v-8ah-cells-with-threaded-insert

A few details about the bike. It is an aluminum track frame from bikeisland.com (probably no longer available). The e-bike kit was from ebikeling,com , a 700c 500w rear wheel kit.

I think sometime in the future I'd like to lace lighter spokes between the hub motor and rim. Handlebars are 27mm "bullhorn". The controller is concealed in the saddle pack.

I have a single 43 tooth chainring, But I purchased a 11-34 7 speed cluster. The intent was a road bike I could happily ride even if the battery died. It has meet all the design goals.

With a 500w motor, this bike easily out performs any of the modern offerings from Trek, Cannondale, etc.. With the wide range cluster (specifically the 34 tooth low gear - see photo) no hill is insurmountable even without electric assist.

If I desire I can ride this bike as an electric moped without pedaling at all. But I seldom do this. If you only utilized the electric assist on a long ride for only the tough hills you could go for hundreds of miles on this bike.

I have ridden this bike without using any electric assist for 30 miles in the rolling hills of Bucks County, PA.

Overall I have been quite pleased with my pack's design. If executed properly it is probably one of the lightest and most compact pack designs available. Enjoy!

Update 10/1/24 I see that BikeIsland.com has the Dawes SST AL frame I used in this build available again. Unfortunately only in 50 52 cm sizes. But it is a nice frame at a good price.

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