So if you fly your adaptive mountain bike across the world how do you power it when you get there? The battery is too large to fly on an airplane, it’s too specialized of equipment to rent or borrow batteries, and you can’t ship ground to an island.
To allow myself to freely travel with an adaptive bike battery I used my expertise designing electromechanical assemblies to create a modular battery pack using custom high current PCB’s, CNC’ed aluminum enclosures, and 3D printed components.
Background
I was faced with this conundrum for a planned trip to New Zealand. I started researching the rules around flying batteries as a passenger and discovered that airlines abide by the rules set by their industry governing body, the IATA. In the rules I noticed that while a single large battery would be forbidden, that same battery capacity was allowable if broken up into smaller sub 100Wh batteries (kinda makes sense if you think about risks like thermal runaway).
I’m not the only one who noticed this – some engineer nerd friends tipped me off to other products using this strategy to comply with the rules. The Grin LiGo is a modular ebike battery and professional camera equipment is available in airline compliant modules. Dewalt also makes a transport cap for their 20V/60V flexvolt batteries that supposedly separates the internal connection to render the battery into two smaller legal modules. These examples were helpful in demonstrating that the modular approach is accepted by airlines.
Design
Battery
The LiGo, while close to the right idea, doesn’t stack into the right voltage and doesn’t supply enough current for a power hungry Bowhead Reach (15A vs 60A). I also took advantage of the 300Wh allowance for mobility equipment to have one slightly larger module as part of the final battery pack. Off the bat I decided to only target 75% of the stock bowhead battery capacity since I rarely use the full capacity on rides. This meant instead of a 20s4p battery (20 cell groups in series, 4 cells in each group) I dropped one cell of each group and only built a 20s3p battery. I picked Molicel 21700 P45B 4500mAh 45A cells since the math worked out well that 6 cells would be 98Wh and just under the magic 100Wh threshold. Thus I split the battery pack into 7 modules that were 2s3p 98Wh and then one larger 6s3p 294Wh module (the sub 300Wh “mobility device” battery). The resulting battery pack all together is 84V 13.5Ah.
I designed a backplane assembly that would connect all the modules together, enclose the BMS, and have a LCD screen that would show the battery status. A clamping frame secures the modules to the backplane. Upon arriving at my final destination I can assemble the battery pack with just four screws that hold the frame in place.
I spent a sometime thinking about how to connect the modules to the backplane. Since the modules are in series, each connection has to pass the full 60A current of the battery pack. I also wanted the modules to just click into place both mechanically and electrically. Having little stubby cables to connect together at each module just seemed like too big of a failure point. I considered enclosing XT-60 connectors somehow but ultimately decided to go premium and buy something purpose-built. I ended up selecting Molex Microfit 3.0 Blind Mate Interface connectors which is a fancy way of saying they are connectors that are very tolerant of a little misalignment and can just be pressed together. I just ganged together multiple pins to get the current capacity needed. Each module also has a second blind mate connector for the balance sense connections (which are needed because the BMS needs to sense ground, 3.7V and 7.4V of each module to ensure each module is balanced).
Battery Modules
Safety was a top priority designing the modules. Each module is enclosed in a 3D printed shell for electrical insulation and then enclosed in a crush proof aluminum case. I went a little wild with the hexagrid which I normally would have skipped for something more manufacturable but since I was going to machine the parts myself, I didn’t mind the extra CNC operations. The modules have a 80A fuse and a small connector PCB to fixture the blind mount connectors. I used a cheap spot welder to connect nickel plated steel strips to bridge the cells.
One thing I didn’t think about but just barely worked out was the arms with the welding probes on the spot welder are pretty short and I could just barely reach all the cells of the 294Wh module.
Backplane Module
I designed a common “backplane” PCB to interconnect the modules and the BMS. I used an off-the-shelf 20s ANT BMS which would handle all the cell balancing and protection and also comes with the information LCD screen. The BMS has 20+ leads that need to be connected to each cell grouping for voltage monitoring so the backplane PCB also has easy solder points to keep the connections to the BMS organized. My friend Carl pointed out “you can tell a mechie designed it because the wires are all strain relieved on the first rev.” That made me happy to hear.
Another unusual thing to figure out on the backplane PCB was how to make traces that could carry 60A. I ended up paying for a 2oz*in^2 copper thickness and then I removed the solder mask from all the large traces so that I could layer a braid of triple thickness copper solder wick and flood the traces with solder. I probably wouldn’t do this again since it took a while to build up the traces (and would just solder copper bus bars somehow) but there have been no issues with the current capacity.
The PCB is enclosed in a two part CNC’ed aluminum case. The PCB mounts to the top of the case which has openings for the connectors to protrude out. The lower case has the BMS and LCD mounted to it. I applied plenty of kapton tape to insulate any metal surfaces just in case.
The aluminum parts were all machined on my Fadal 3016 CNC. This is one of my first big projects since adding the CNC to my shop and it was exciting to be able to use complex aluminum assemblies freely for the first time. I should have taken more photos but here are a couple of the backplane enclosure in progress.
Real World Testing
Not only did the battery provided plenty of reliable power for the adventures on our trip, it was even relatively easy to travel with! I purchased a fireproof battery bag that looked like a lunchbox to carry the separated cells. I had to explain that I was traveling with batteries for mobility equipment every time we we passed through security but after they verified the modules we below 100Wh, they let us through without issue. The 13.5Ah capacity was plenty for 30 miles (48km) of flat riding, and on any longer rides I just carried the charger and we’d charge at lunch time.
Reflecting on what I’d change in a next generation design, I’d probably try to make the pack take up less space when assembled. The completed pack is about the size of the stock battery, but that is with giving up 25% of the capacity. I’m not sure how much I’d be able to improve though-the cost of having separable rectangular modules is the round Li-on cells can’t be tightly nested and a lot of volume is used to create the interconnections.
