Introduction: How to Rebuild a BionX E-Bike Lithium-Ion Battery Pack

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About: Was a maker before makers became a thing. I have always enjoyed solving problems with my hands. I also enjoy writing.

Hello fellow Makers, Hackers and DIYers!

This instructable will show you how you can replace the lithium-ion cells in a worn-out BionX e-bike battery pack to restore your lost range and even greatly exceed it. It will also show you how you can replace your original equipment charger with a high quality balance charger so you can to get the most from your rebuilt battery.

The CANBus, Rear Rack, 37 Volt, 6.4 Amp-hours, 236.8 Watt-hours battery in my Dahon MuP8 e-bike worked reliably for two years, but soon after the warranty expired the range dropped from 10 miles to maybe 2, even when I did all the pedaling, so the time had come to open it up to see what could be done. Inside I found that BionX had packed forty 1,600 mahr US18650V lithium-ion cells into a cavity that could hold sixty and simply filled the empty space with foam blocks. This presented an interesting opportunity as I soon realized that I could replace the original forty cells with new ones having twice the capacity and add twenty more, which would give me three times the original range while adding only a few more pounds of weight. The trick then, was to figure out how to do it without destroying the battery or breaking the bank.

I found tons of videos on the internet showing how to build e-bike battery packs, but few people have rebuilt a BionX pack due to the unusual nature of their construction. I had also wanted to convert my pack to 48 Volts by using a different control board but that turned out to be more expensive than expected, and since the existing Smart Connect ver 5.2 control board still worked well, if I kept it I could retain all the original BionX functions, which as e-bikes go, are quite extensive. That meant the new pack had to stay at 37 Volts, but I could convert it from 10S4P to 10S6P and increase the number of cells to sixty, thus increasing the pack's cell count by 50%. To increase the cell capacity I chose to replace them with Panasonic NCR18650B cells which are the same ones used to power the Tesla Model S electric car. They can deliver a sustained 10 amps and have a 3,400 mahr capacity -- more than twice that of the ones they'd replace -- which would add 100% more capacity giving me a total gain of 150%.

I also wanted to charge the pack with a balance charger rather than use my original BionX charger so I could keep better track of my pack's condition. The BionX 37 Volt charger relies on a low 2 amp charge rate, temperature sensors and some control circuitry to determine the end-of-charge. While this is a perfectly safe method of charging lithium-ion cells under normal conditions, the pack lacks a Battery Management System (BMS) which means that the cells must not lose much of their balanced state in the course of their use. It is still possible for one or more cell groups to become unbalanced as they age which can lead to reduced output and a pack that will no longer charge to its full capacity. It is much better, in my opinion, to use a good quality balance charger as they monitor the voltage of each cell group and automatically balances them to within a few millivolts of each other as the charge progresses. They can also alert you if any cell group under-performs relative to the others and let you make adjustments or intervene if things go awry. But my BionX battery pack didn't come with a balance port, so I would have to add one.

Please note that I present this instructable to you with an Attribution Non-commercial Share Alike license, which allows you to use it for your own use as long as you don't attempt to profit from it commercially. However, in addition I must give you a stern warning and disclaimer: This project is NOT for beginners. Please do NOT attempt it if your soldering skills are only "average" as you can easily short the pack if you're at all sloppy or don't follow proper safety precautions. People have accidentally caused fires and/or injured themselves with lithium-ion cells so before beginning this project I want you to clearly understand the dangers involved. I am in no way responsible for the use or misuse of the information presented here. What you do with it is entirely up to you and at your own risk.

With that out of the way, let's begin!

Step 1: Safety Equipment, Tools and Parts

Despite the dangers, there are ways of protecting yourself and reducing the risk of fire. The first, obviously, is to keep a fire extinguisher on hand at all times. Mine is a First Alert ABC dry chemical type available in any hardware store. Next, during construction make sure you use good eye protection, gloves and ventilation, and remember to take off any rings, watches and jewelry beforehand. Finally, you should always charge your battery in a fire-proof box or bag even after your charging routine has proven itself safe and reliable. I use a large, heavy bag called a "HoverCover Fire Resistant Hoverboard Bag" that I bought for $60 off Amazon. It is designed to fully contain a hoverboard fire. A bag like this is very good, cheap fire insurance and could save you from disaster.

I left the original BionX charging circuitry and 4-pin XLR connector intact so I could use the original charger if my balance charger idea didn't work out, but I'm happy to report that the new charger was so successful I no longer use the old one. I can still, however, use it as a portable spare and might take it with me on some long trips.

For regular charging, however, I highly recommend using a good quality balance charger as it gives you complete control over all ten cell groups, allows you to monitor their charge and discharge voltages, display their internal resistances, change the voltage and current settings and easily track the overall condition of your pack. My charger is an iCharger 4010 Duo that I bought from Progressive RC for $350. Not cheap, but it charges most anything at up to 75 amps and was one of the few I'd found that could balance charge a 10S pack without my having to divide the pack into two parts. It does not come with a DC power supply, however, so be prepared to buy or provide that separately if you purchase one.

To do this upgrade you'll need 60 new Panasonic NCR18650B cells (or equivalent), which are available from a number of vendors online. Be mindful of the postal regulations if you live overseas as hazmat shipping methods may be required. You could use recycled cells from old laptop batteries but you won't get the capacity you can with these new cells, and matching their internal resistances over time is problematic at best. You also can't escape the fact that the older the cells, the more quickly they will deteriorate, and since you don't know their age to begin with, you may find yourself with a poorly performing pack sooner rather than later. My advice is to spend the money now on high quality new cells so you won't have to spend it again later.

You've probably guessed by now that this is not a cheap project. With new batteries, charger, assorted parts, tools and international shipping I probably spent more than $1,000 -- more than even what a new pack would have cost me from BionX. However, my new battery will (hopefully) last twice as long as my old one did and go three times further per charge giving me a much lower cost per mile, and the knowledge I gained by successfully taking one apart and rebuilding it is, to me, priceless. But mistakes can be expensive so be certain you know exactly what you are doing before tackling this project. Also know that this project will very likely void your warranty. I therefore suggest you not attempt it until after your warranty expires so as to not lose your support from BionX, who are generally good about warranty replacements. My warranty had already expired and overseas shipping regulations made my buying a replacement battery difficult and expensive so rebuilding my pack was really the only option I had.

You'll need these parts:

  • 60 Panasonic NCR18650B cells or equivalent - Do not get cells with built-in protection circuits as those will not fit the enclosure.
  • 1 10S balance charger - e.g. iCharger 3010B or iCharger 4010 Duo.
  • 1 Fire-proof bag or box for charging
  • 1 ABC fire extinguisher
  • 10 feet of 12 AWG super-flex stranded wire, 5ft red, 5ft black
  • 10 feet of 14 AWG super-flex stranded wire, 5ft red, 5ft black
  • 10 feet of 12 conductor, 22 AWG, multi-colored stranded wire
  • 25 feet of 1/4" flat pre-tinned copper grounding braid
  • 5 ft of 190 mm heat shrink tubing
  • 1 box of heat shrink tubing, assorted sizes from 1.5 mm to 13 mm
  • 2 sets, aviation style 12-pin connectors, male and female, with 5 amp pins and end cap (search eBay: "Aviation plug Disc flange assemblies DF-20 12-Pin XLR Radio 20mm Panel waterproof")
  • 3 sets, XT60 connectors, male and female
  • 3 sets Deans Ultra Plugs, male and female
  • 4 sets, 6-pin JST-XH connectors, male and female
  • 2 11-pin JST-XH connectors, female with pins
  • 1 sheet of stick-on fish paper - Insulates battery assemblies from each other.
  • 1 sheet of stick-on fish paper discs, 18 mm diam with 8 mm holes - Insulates the positive battery ends.
  • 2 sets of small brushless motor connectors, male and female
  • 1 spool of 60/40 rosin core solder, 1mm diam. - 63/37 rosin core solder is better, if available.
  • 1 bottle of good quality liquid brush-on flux
  • 1 bottle of black CA glue
  • 1 bottle of CA accelerator
  • 1 two-part Arctic Alumina thermal adhesive
  • 1 13mm roll of Kapton tape
  • 1 roll of 1 inch blue masking tape
  • 2" x 2" x 1/8" birch plywood, aircraft grade
  • Assorted warp-free scrap-wood, 1/8" to 1/2" thick (see pictures)

And these tools:

  • 50 Watt soldering iron with a new 5/16" or 4 mm flat bladed tip - for soldering braids onto cells.
  • 25 Watt soldering iron with small conical tip - for soldering small connectors.
  • Brass-wool soldering iron tip cleaner
  • Good quality hot-melt glue gun with dual temperature glue
  • Electric drill with drill bits 1/16" to 1/2" diam
  • DC voltmeter
  • Dremel Moto-tool with drum sander and various plastic cutting tips
  • Dremel Flex Shaft
  • Heat gun or modified hair blower
  • Thin razor saw, 9 mm wide - e.g. 4-in-1 Zona-tool saw set or equivalent
  • Xacto knife set with multiple blades
  • 2 Goot heatsink clamps - for electronics work - very useful!
  • JST-XH crimp tool - useful but not essential - careful soldering works as well.
  • Panavise or equivalent
  • Helping Hand with magnifier
  • Safety goggles
  • Workman's gloves - dish-washing gloves work well.
  • 1 sheet Scotch-Brite non-metallic pot cleaner
  • Various needle-nose pliers, diagonal cutters, screw, Torx and nut drivers, small files, etc.

Step 2: Battery Dissassembly

As you disassemble your battery you may find another surprise waiting for you: point-to-point wiring. To save space and lower costs BionX apparently chose to forego connectors and solder their packs together using very stiff coarsely stranded wire. Soldering point-to-point by hand with live voltages present, however, can be dangerous, so I installed Deans Ultra Plugs and XT60 connectors to improve safety and make it easier for me to do my own repairs later. I can now pull the battery or electronics board out with just a pull of a few connectors instead of having to cut them out with diagonal cutters. I also found the coarsely stranded power wires difficult to bend in the tight space available to them so I replaced most of them with super-flex wires of the same or heavier gauge to make reassembly easier.

Before cutting the wires be sure to pull the 30 Amp auto-style fuse off the electronics board and take pictures of everything so as to have a record of how it all connects together. I like to make "Pencil CAD" drawings as well as that forces me to trace each wire end-to-end and learn more of the layout. As space is very tight in there you'll have to be extra careful with the connectors and wire lengths when the time comes to stuff it all back inside.

My pack came with two temperature sensors -- one a thermal switch glued to the top of the pack and the other a Type K thermistor glued to a cell inside the pack. The thermal switch opens during charging when the pack's temperature goes beyond a certain level and acts as a fail-safe cutoff in case the pack overheats. The thermistor monitors the pack's temperature during discharge and can signal the microprocessor to reduce power to the motor during, say, a climb up a long hill to prevent damage to the pack. I recommend you retain both these devices and glue them onto your new pack using Arctic Alumina thermal adhesive. You should also reconnect them to the circuitry using suitable connectors rather than splicing them back together. For that I used some old, small brushless motor connectors that I found in my junk box and covered them with heatshrink for protection against shorts.

Step 3: Balance Port

And now we come to the new balance port. This part took a lot of thought but came out very well. What was needed was a small but robust 12-pin connector set that could take 22 gauge wires and frequent insertions. After much searching I found a 20 mm aviation-style 12-pin plug and socket set on eBay. The silver-plated pins of this pair can pass up to 5 amps each and both connectors will just fit in the space opposite the battery's plunger lock. The wires came from a length of 12-conductor stranded 22 gauge cable I also found on eBay. Each wire has a different color which can help you avoid confusion when wiring up the different cell groups. When soldering the pins, make sure you tin both wire and pin before joining them together, then cover the joint with 1.5 mm heatshrink. Work from the center out, that is, pin 12 on down, as you won't be able to get to the inner pins if you go from pin 1 up. You don't need pin 12 but I put a wire on that anyway just in case it was needed later. If you organize the colors to follow the standard resistor color code, i.e., black for ground, brown for cell 1, red for cell 2, etc., you'll find it easy to put them in the connectors in the right order. Also, stripping off the cable's outer insulation will make the individual wires easier to work with. When done, you can always twist them back together by chucking them into an electric drill.

The connectors on the end that attaches to the battery had to be smaller but needn't be so robust so I chose two inexpensive male 6-pin JST-XH connectors, one for ground and cells 1-5, and one for cells 6-10. The pins on these, normally meant for circuit boards, must be soldered to the wires very carefully as they are very thin and can easily melt the plastic. The battery gets the females of those connectors and the different wire colors will tell you which connector goes to which mate. A separate 2 ft. cable is needed for the charger. It will take the female aviation plug on one end and an 11-pin female JST-XH connector on the other, following the same color order.

To install the balance port socket, I built a mount out of a piece of 1/8" thick birch plywood and used a Dremel drum sander to sand and bevel it to exactly the right shape to fit inside the empty space opposite the plunger lock. If you do the same, mount the socket to the plywood without the rubber gasket that came with it using M2 flat head bolts (with nuts), then use a strong glue, such as black CA, on the beveled edges making as much wood-to-plastic contact as possible. The bond needs to be strong as the 12-pin aviation plug requires some force to pull from its socket. Glue it in at an angle to accommodate the long plug and make sure the socket won't contact the rack assembly when you slide the battery in place. I had just enough room to fit on the dust cap after trimming away the excess rubber (see pictures). Remember that the cable must also pass through a 1/2" hole into the battery compartment. To cut the hole I used a 90 deg attachment on my Dremel but the result was less than pretty. A Dremel Flex Shaft might have been a better tool for this but I didn't have one at the time.

Step 4: Optional Charge Connector

For the charger power inlet, I chose to glue in a female XT60 connector near the plunger lock, mostly because I wanted it protected from the rain and I couldn't find an XLR connector that fitted the original BionX side connector. Although the XT60 fits in the small space and works well, it did add more wires to the electronics cavity making it that much more crowded. If you can find a male XLR connector to fit your BionX connector then I suggest you go with that, or cut off the one from your old BionX charger and use it to make a new cable for your new charger. This will save you some space in the cavity. If you do use the XT60 connector fit it in very carefully as there is just enough room for it to fit without striking the rear rack as you slide the battery in place.

Step 5: Battery Layout and Preparation

The original battery was assembled in a layered 7-6-7 cell scheme using custom-cut nickel sheets that were CNC spot-welded to overlapping 8-cell clusters. I didn't have a spot-welder, nor did I feel like building or buying one, so I used 1/4" pre-tinned grounding braid and solder to build my pack. This worked quite well and was easier to build than soldering nickel sheets, or even strips onto the cells, but one must be very careful to solder the right cells together.

After much thought I decided on a flat, cell-to-cell layout connecting the cells in a single line (see the drawings) rather than modifying the original 8-cell overlapping cluster design for a 10S6P pack. The latter would have required 12 cells overlapping each other and that would have made constructing a neat pack quite difficult. Using braids, which are more flexible than nickel strips, and connecting them cell-to-cell also allowed more precise soldering and the least transfer of heat into the cells. The braids did require a bit of custom shaping of their own but that was easily accomplished by cutting notches and pre-soldering angles into them to accommodate the changes of direction (see Step 7).

At this point it is a good idea to make the sheet insulators out of stick-on fish-paper or other stick-on battery insulator material. Peel off one of the insulators from the old battery and use it as a stencil to make six exact copies. Don't try to use the old insulators in the new battery as they will likely not stick on well. Also, make or acquire 60 life-saver shaped 18 mm diam fish-paper insulators for the positive end terminals -- you don't need them for the negative ends. These will help prevent the braid from bridging the short distance between the positive pole and the negative can lip. Mine did not come with the holes so I had to invent an 8 mm hole punch and punch them out by hand.

Also, while they are still loose, rub both ends of every cell in a twisting motion under your thumb with a clean sheet of non-metallic Scotch-Brite (supermarket variety is fine) and blow away any dust that forms. Do not use sandpaper as the grit can lodge under the can's lip and cause shorts if it is at all conductive.

Step 6: Battery Assembly

I recommend you make full-sized drawings of the battery ends (both ends, all three assemblies) to be able to visualize the battery and braid placements. On a long piece of paper (such as a cut open envelop) again use the old insulator as a stencil and draw six images in two rows of three (see pictures). Use an 18 mm diam washer with a 10 mm hole to draw in the positive and negative poles, then draw in the braids. Double check the drawing at least a dozen times - (seriously!) to be sure you have the batteries oriented properly. You don't want any designed in shorts, trust me.

Next, build a 130 mm x 66 mm box out of warp-free scrap-wood and extend the base a foot or so in one direction so it can be clamped to your tabletop (see pictures). With your full-sized cell layout drawing in front of you, place strips of Kapton tape on the sides of the cells that will touch each other and put the first layer of seven cells in the box per the drawing. Spread hot-melt glue on the tape in the valleys avoiding the ends and making as small a bead as possible, then invert the cells and glue the other side in like manner. Kapton tape resists temperatures to 500 deg F. and can easily take the heat of the hot-melt glue. If you ever have to disassemble the pack, you can just peel off the tape without ruining the cells' heatshrink coverings. I didn't learn this trick until after I'd laid down my first layer of cells but each succeeding layer did get the tape treatment.

The middle layer has six cells and the top layer, like the bottom, has seven. When all three layers are assembled, stack them in the right order and tape them together with Kapton tape. Put a few tacks of glue on the end cells to keep the layers from shifting. Assemble the remaining clusters in the same manner, following your pencil drawing exactly and noting the different cell placements in each.

Step 7: Braid Preparation

Before you start soldering, a word about solder technique: A new 4 mm chisel-point tip in a 50-watt iron, frequently applied flux, and counting to three every time you touch a cell with a hot iron is essential to avoid damaging them with excessive heat. Make it a rule to remove the iron after three seconds regardless of whether you've made a good bond or not. You can try again after the cell cools, but avoid doing that more than twice. It takes a bit of practice, but if your surfaces are clean, your iron is clean, you've applied liquid flux to every surface top and bottom and pre-tinned each surface then you should have little trouble soldering the cells together.

Next, stand an assembled cell cluster inside your box to hold it in place and put the life-saver shaped insulators on each positive end, then brush on some liquid flux on all ends, positive and negative. With a new 4-mm chisel-point tip in a 50-watt soldering iron, put a blob of solder in the center of each end taking care to not touch the cell with the iron for more than three seconds. Good quality 60/40 or 63/37 lead solder should flow on quickly and almost cover the positive electrode and about 1/4" of the center of the negative one. Do not use lead-free solder as it requires too much heat.

Prepare the braids according to your drawing. Make the shorter ones first and save the longer ones that connect the clusters together for later. Always make the braids in one piece to avoid soldering them on top of one another which will require too much heat and make the cells too long to fit the enclosure. Cut notches 3/4 the way through at the turns with sharp diagonal cutters, form the bend, and solder the bend together using as little solder as possible (the braid will need to absorb more solder later to bond with the cell ends). Tap the soldered bends with a small hammer to flatten them to the same thickness as the rest of the braid. Each braid should exactly fit its corresponding cell tops with no off-center bends and no excess length. The two exceptions are the 6-cell positive and negative end braids which will get one inch or so of extra length to allow you to solder on the red and black power wires. Brush flux on the entire solder side of the braid and mark it with a Sharpie pen at the central contact points for each cell. Finally, put a small blob of solder on all the marked spots, again following the three second rule to avoid flowing on too much solder. Just a small blob to match the cells is all you need.

Before test fitting the braids, be sure to put lots of blue masking tape on the cell ends that you are not connecting together and only leave exposed those that you are. This will help minimize accidental shorts. Just to be safe, I also tested every exposed cell against its neighbor with a voltmeter before laying down the braid to be certain there was 0 volts between them. If you ever find you're about to solder two adjacent cells that show a voltage between them, you WILL get sparks when you lay down the braid! You should get something between 0 and maybe 20 millivolts depending on how well equalized the cells' charge states were before you began your construction. My cells were new and even though I delayed construction for 3 months, they still showed less than 10 mv between them when I finally built my pack.

Step 8: Soldering the Cells Together

When you're ready to solder the cells together, reapply flux to the cell tops and the bottom of the braid at the places you tinned, position the braid in place and choose an end cell for the first joint. Since they're pre-soldered you do not need to apply more solder. With one hand apply the iron to the braid as flatly as possible to transfer heat as quickly as possible, count to three, then remove the iron and, in one smooth motion with your other hand, quickly press a small screwdriver onto the joint to hold it in place while the solder hardens. Again, this whole process should not take more than three seconds. The cell should be cool to the touch a few seconds later. If it takes longer than three seconds to make a bond then your iron is likely less than 50 watts and not up to the task. Smaller irons force you to contact the braids for longer periods actually transferring too much heat into the cells, larger irons are clumsy and just as apt to transfer too much heat, so I don't recommend them either. Mine is a Hakko Presto 980 dual power iron that goes from 20 watts to 50 watts nearly instantly at the press of a button. Finish the line of cells from one end to the other, then remove any escaped solder balls with needle-nose pliers to prevent them from causing trouble later. Cover this line of cells with fresh masking tape and uncover the line you wish to solder next.

The four braids that connect the clusters together will be longer and need some extra care in their construction. All four will fold 180 degrees and require some good quality heatshrink to keep them from shorting against each other or the sides of the cans. The short-spanned ones will need no more than 3/16" to go between the clusters -- any longer and the folded braid won't fit inside the enclosure. Also, too much solder in these spans will cause them to not bend easily. Placing a Goot heatsink clamp in the middle of the spans while soldering them in place might help you avoid that problem, but insulate the clamp's jaws with masking tape or glued on bits of cardboard so they don't short against the cans. Don't tape over the jaws' teeth as you need metal to metal contact for good heat flow. These spans will have to be insulated with black tape as you can't get heatshrink tubing on them once they are soldered in place. The long-spanned braids must traverse the length of two cells end-to-end so will need 5 3/8" inches between clusters to cover the distance after they're folded. Put two heatshrink tubes on before you form the braid so you can move them around as you make your bends. Two 3" pieces, slightly overlapping in the middle, will give you some length adjustment later without having to cut them.

Step 9: Balance Wires; Thermistor

Next come the balance wires. Cut 2 ft. lengths of all 12 colors and solder them onto the braids following your drawing and the resistor color code. I put their most convenient braid locations on my drawing and numbered the spots 1 to 10 to match the series cell groups. The black power cable braid also gets a wire for the balance ground reference. When the pack is folded, the wires for ground and cells 1-5 should run along one side of the pack and the wires for cells 6-10 should run along the other side. Test them afterwards with your voltmeter to be sure each colored wire goes to the correct series cell group.

For the thermistor, carefully trim off most of the old glue and re-glue it to one of the middle layer cells in the black power cable side recess using Arctic Alumina thermal epoxy. Solder one thermistor wire to the black power cable braid and use the 12th wire (pink) to extend the other to the electronics board area. I added a small brushless motor connector here to allow for an easy disconnect later.

Step 10: Folding the Cell Clusters Together

It takes a bit of finesse to fold the three clusters together. You don't want the braids to touch each other, even briefly, during this process so tape some temporary cardboard insulators on the ends to prevent shorts. You also don't want the braids to twist so be very careful when flipping the clusters around while you tape up the balance wires in the side recesses. The wires must not add any width or length to the pack.

With all wires loosely in place and the pack still in a Z- shape remove the cardboard and stick on the fish-paper sheet insulators you made earlier. Fold the pack into its final shape making sure that the clusters fold evenly and tightly together, then pull and tape the balance wires into their final position. A slight variation in the width of the pack of even 1/16", such as a slight skew or a balance wire resting on the outer edge of a cell, could keep the enclosure halves from closing properly.

The power cables will also take some careful attention. Cut 11" lengths of black and red 12 gauge super-flex wires and solder them to a female XT60 connector. I again used a heatsink clamp here to keep the solder from wicking up the cable and stiffening the wire. Don't forget to put on heatshrink tubing for both the connector pins and the braid ends, then solder them to their respective braids. The black negative cable will run along a side recess next to the middle layer of cells. The red positive cable just has to exit the front of the battery, but you will likely have to loop it around 180 deg in its recess to avoid damaging the braid to which it is attached. Loop and tape any excess 22 ga.wire in the side recesses to keep the lengths as short as possible as there is very little space in the front cavity for excess wiring.

The last of the battery wiring is to put the two 6-pin female JST-XH connectors on the balance wires. As with the aviation connector cable, the wires for ground and cells 1-5 go in one JST-XH connector and the wires for cells 6-10 go in the other. I soldered the female pins onto the wires using a "helping hand" but if you have a crimper then by all means use that. Be sure to avoid "insulation creep" at the connector pins and be careful inserting the pins into the connector bodies as you don't want them to touch each other. You should really put them in the connectors first, then solder the wires onto the battery, but measuring the wire lengths exactly while the pack is still open can be tricky. Another option is to pre-wire your connectors and solder the battery points with separate lengths of wire, then splice and heatshrink them together somewhere in the side recesses after the pack is fully folded. Commercially made pre-wired connectors will also work but they use thinner wires than my 22 gauge wires and don't follow my color scheme so I chose not to use them.

Step 11: Final Pack Assembly

Once the pack is folded and the wires tidied up you can tape the three cell clusters together using Kapton tape. The pack should then hold its shape well enough to be handled. Check that the clusters are not askew with each other and that the interconnecting braids protrude as little as possible from the sides. Now you can cut a piece of the large 190 mm shrink wrap to fit the battery with four inches of overhang on each end, slip in the battery and shrink it down. Flatten the end flaps with gloved fingers. Do not use foam padding as there is no room for it.

The final step is to glue the old thermal switch back in its rightful place on top of the pack. Cut it off the old shrink wrap (don't try to peel it completely off as you might damage the switch) and glue it on the new shrink wrap directly over a cell using Arctic Alumina thermal glue. I also replaced its wires with more flexible ones and added a Deans Ultra Connector to allow easy removal later. One switch wire goes to the XLR charge connector and one goes to the electronics board, it makes no difference which. After a final check with your voltmeter to be sure that all is well with the pack and balance wires, it's time to sit back, have a drink and admire your handiwork!

Step 12: Fitting the Battery in Its Enclosure

It's a good idea to at this time replace some of the really stiff BionX power wires with more flexible ones to make the final assembly easier, but only do that if you're skilled at it. The board is conformal coated for moisture protection and durability and that can make pulling wires off it without damaging the board or components difficult. Use of a desoldering station, if you have one, or at least a good solder-sucker and various pliers and hold-down tools is mandatory.

You can now test fit the finished battery in its case. If you used braids you'll probably find that the battery is too long to fit within the enclosure's molded-in stops, which were designed for spot-welded batteries. However, I was able to make my battery fit by cutting away the stops in the rear only, including two that were just straight bars of plastic about an 1/8" wide and an inch or so long running parallel to and near the rear screw channel (see pictures). The screw channel, only a 1/4" away, will serve nicely as your new stop. Choosing the right tools to cut the excess plastic out, though, is important. For me, the best tool was a 9 mm wide backless razor saw from a Zona-tool 4-in-1 saw set. It cut the thin bars out nice and flush with the sides. The Z-shaped stops were dealt with using a combination of Xacto knife blades and Dremel plastic cutting bits.

If you find your enclosure halves don't quite close all the way, it may be because something on your battery is protruding too much. I found that one of the short folded braids protruded just a 1/6" at the fold point contacting the side wall of the enclosure. Fortunately, at that location there is another bar of plastic about 1/2" wide by 1/16" deep running the full length of the enclosure's side. Some careful work with a Dremel cut out a nice divot in that bar and that gave me just enough room for the folded braid to fit.

With the battery and case successfully test fitted you can now install the electronics board in its cavity. As I had guessed at the lengths, I found my power and braid cables too long so had to shorten them. With live voltages present, though, that was is a bit dicey to say the least! I carefully rebuilt my connectors, but in retrospect I suggest you splice your wires somewhere mid-way in their lengths and cover them with heatshrink to avoid the possibility of creating shorts.

Step 13: First Charge and Test Results

I chose 10 amps for my first charge, which I calculated to be slightly less than 0.5 C, but the thermal switch opened about 30 minutes into the charge. I then lowered the charge rate to 8.2 amps, which is about 0.4 C and it worked fine for several more charges with no aborts. That is, until I decided to do a complete cycle test to determine the pack's full capacity. The discharge at 2.0 amps went uneventfully but the following charge, done at 8.2 amps, caused the thermal switch to open again about 3/4 the way through when the pack overheated. Seems the longer charge time from a fully discharged state (3.0 V/cell) didn't take well with my fancy-dancy hoverboard fire protector bag which probably acted like an efficient thermal blanket and didn't allow any ventilation to dissipate the heat. I'm therefore very glad I had reinstalled the switch because it protected my new battery from damage. Lesson thus learned, I now charge the pack at 6.0 amps while leaving the bag's top flap open and have had no heat buildups nor aborts since.

It takes a bit more than two hours at 6.0 amps to charge the pack from the nominal 3.7 volts/cell and maybe five hours from a fully depleted 3.0 volts/cell. However, I don't let the pack get down that low and don't plan to do cycle tests more often than once a year. I also don't charge it beyond 4.1 volts/cell even though the new cells are rated at 4.2 volts, which gives me about 98% capacity. My internet sources tell me that the battery will last through twice as many charge cycles and that the few sacrificed minutes that 0.1 volt represents are well worth it!

BTW, if you leave your pack in storage for long periods it is advisable to lower the charge rate to 2.0 amps and charge the pack every six months or whenever the pack "chirps". At that point the cells are approaching 3.7 volts/cell which is the point when the control board recommends you head home and recharge the pack. The idea is to give you some reserve but avoid as much as possible over discharging the pack on the way home. The same monitoring occurs in storage but the lower charge rate will help equalize the cells without shocking them.

So how well does the new pack perform? The bike easily goes 20 miles now on a single charge up and down the hills of my city which are often quite steep, all the while staying above 36 Volts, and I'm sure I can get it to go 30 miles if I let the voltage go lower and I do more of the pedaling. If I lived in a flat city and only used the BionX torque-assist mode I could probably stretch it to 40 miles. A single charge lasts me at least a week now, including daily commutes to and from work and various shopping trips, whereas before I averaged only one or two days. This is easily three times the old battery's capacity on its best day, and the power is strong with no sags or heat buildup no matter how hard I ride the bike. As I seldom let the charge get lower than 36 volts, I can pretty much avoid the chirps coming from the pack telling me its time to go home and put away my toys.

The cycle test indicated a full 18.4 Ah (680.8 Wh) of capacity and would probably have reached 19.0 Ah (703 Wh) if I had let the charge go to a full 42 volts. That compares with my old battery's 6.4 Ah (236.8 Wh) rating, and I don't think it was ever that good even when new. As for weight, the pack is now an even 9.0 lbs, up from 6.5 lbs for a 2.5 lb gain -- not a bad weight penalty for tripling the range of the old pack! I am quite pleased with this performance!

Step 14: Bonus! How to Wire a 7 Cell Battery Capacity Checker for 10 Cells

So now you have a new 10S6P battery pack and you're zipping along on your e-bike enjoying the fresh air, but from time to time you'd like to see how well the ten cell groups are holding up. You notice that the battery indicator in your BionX control panel shows "empty" at around 10 miles even though you know you're still above 38 volts at that range -- plenty of charge left for another 10 miles at least, maybe 20 if you go easy on the throttle.

To read how much tank you really have left you could use a digital Battery Capacity Checker like those available on eBay and Amazon. They're inexpensive and work surprisingly well, but are built for a maximum of only 7 cells. To make one work on 10 cells you need an adapter cord that connects to your battery's balance port on one end and splits into two 6-pin JST-XH connectors on the other end. You then wire the connectors for two separate cell groups; one for ground and cells 1 to 5, and one for cells 5 to 10. That's not a misprint. The important point to remember is that the positive wire for cell 5, on connector #1, must split and serve the additional function as the ground wire for cell 6 on connector #2. The battery checker doesn't care where the ground is as long as there is one and that the cell voltages go up from there and in the proper order. As both connectors are not connected simultaneously there's no risk of shorts between them. Just remember to always plug your connectors into the checker pins to the right of those marked "6" and "7" (you could also use 8-pin connectors and simply not wire up pins 6 and 7). Though the battery must be removed from its rack every time you check it, the checker's low cost, light weight, and easily read display make up for it.

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Circuits Contest 2016

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