There has been a boom in ebike builders making their own battery packs out of the popular 18650-format cells (18mm diameter, 65mm long), and I want to share what I’ve found out about the guts of an 18650, so you will understand more about proper DIY pack-building methods.
Why would somebody make their own pack?
The existing battery pack vendors will only make (and stock) the packs that they think they will sell enough of, to make it worth risking their available cash. Which means, they will only have packs with certain particular sizes, shapes, and with specific cost-effective cells.
But…what if you want a pack with a different cell? Or maybe, you desire a very custom shape? There’s nothing “wrong” with a turn-key battery pack, but…we are under the impression that anyone who is going to go to the trouble and expense of building their own pack, they are likely to also be the type of person who wants high-performance. After all, why go to the expense and trouble of building a low-to-medium performance pack in a conventional shape? They already exist, and they are getting very affordable. (For building a custom pack, the most-often cited DIY cell spot-welder is about $250!)
A third reason (after “custom features”, and using a specific cell), is that if you want to ship a large ebike battery pack internationally, it is very hard right now (and the rules and regulations are likely to get worse over time). If you live in a country where the major ebike battery pack retailers will not ship to you? Then…you can’t get ANY kind of pre-built battery pack. You can still buy all of the components, and build your own, but…buying a completed battery pack is just not available.
Negative cans and shoulder-shorts
I’m putting this fact about an 18650 cell construction first, because…I still run into people today who are surprised to find this out, and a “short” across the shoulder of an 18650 can will cost you trouble and money. It might ruin an entire expensive pack, and also…IT CAN START A FIRE!
I have seen some caps and shells advertised as stainless steel, but…here is a quote from an 18650 parts supplier:
“The case and cap are both made from nickel-plated A3 steel, and the insulating seal is made from nylon”
Even though it is “possible” to draw current from the negative shoulder of an 18650 cell (so that the positive and negative are pulled from the same end of the cell), doing that would also mean you are using the sides of the can as a conductor. The shell is steel, and its conductivity is about 10% of what copper would be, so…it is not unreasonable to call the sides of the 18650 case a “resistor”.
If you are purposefully running current through the sides of the can, it means you are wasting battery watts to heat the shell. Wasted watts and heating-up the cell on purpose is a bad design. Always pull the negative current from the bottom of the 18650, using something that has better conductivity than steel (aluminum or copper, either raw or nickel-plated).
Just under the positive-electrode cap, is the “Positive Temperature Coefficient” device, and it is a thin and compact way to limit the current coming out of an individual cell, when the amount of current is so much that the cell-tip is getting hot.
The PTC is a conductive washer, but…when it starts to get hot? Its resistance dramatically increases, so that hopefully…less current can pass through it. In this way, it is almost like a self-resetting breaker. That brings up the question, how hot does it have to get before it activates?
One seemingly-reliable source states that the current-throttling point begins significant activation at 134C (273F). If that is true for most common cells, I’m not sure what situations exist where the PTC will help…maybe it needs to allow the faulted cell to get “that hot” in order to allow the electrolyte to begin producing gasses, and it is actually the gas pressure build-up which then activates the CID, which provides the safeguard. This same reference states that the PTC returns to full-rated current capabilities when the cell cools back down.
There are several references that indicate a temperature of 60C (140F) is the hottest that any 18650 cell should ever be allowed to get up to (if you want it to last a long time). If you know someone who has gotten their pack hotter than that, and they are proud of the fact that the pack still works, they may not realize that they have thrown away much of their expensive battery packs’ potential life-cycle.
Tesla has an 8-year warranty on their packs, and…they not only designed the pack to never get hot in normal charging and discharging conditions, they also incorporated a pack-cooling system, as did the Chevrolet Volt. The Nissan Leaf was introduced without a liquid-coolant heat-management system for the battery, and they depended on ambient air-cooling, which caused problems that they encountered during the summertime in regions with very hot weather. The Tesla system has a cooling target of 55C (131F), which fits right in with the widely accepted safe max temp of 60C (140F). If they are getting more than eight years out of their battery packs, they are doing something very right.
This stands for “Current Interrupt Device“, and it is a simple and compact device that “pops” when enough pressure has built up inside a cell, and it’s located just below the PTC. There are several variations in the designs. They operate on the same principle, but do it slightly different ways. The only reason any pressure would develop inside a cell is because some of the electrolyte has converted from a thick gel (almost dry) into a gas, from experiencing too much heat.
If that has happened?… I would not “reset” the CID and try to use the cell again (cells are cheap, and you don’t need to add a risky cell to an expensive pack). The PTC will not reset by itself, but sometimes?…it can be done manually. I might use such a cell in a flashlight, but not an ebike pack. Wait a minute…actually…I would throw that bitch away. I don’t need my flashlights catching on fire. I have access to plenty of new 18650’s, and I don’t need to spend even one minute of my life wrestling with an insurance adjuster over a house-fire.
The CID is a thin disc of sheetmetal that is in-between the positive cathode cap, and the rest of the interior of the cell. It has a bowl-shaped depression in the center of it that presses down against another flat metal disc to make contact, and by doing so, that will complete the current-path in normal operations.
The Scored Burst Disc
Generic cells do NOT have a “burst disc”. If they get hot enough for the electrolyte to begin turning into gasses, and then expand from the building pressure, the cannister will split…somewhere. If there is a burst disc, it will pop open at that specific location. You should only use name-brand cells from the “big five”, which always have this feature (Panasonic, Samsung, LG, Sony, Sanyo). If they are ever abused in a way where they will burst from internal pressure, the hot electrolyte vapors will always blow towards the burst disc, instead of splitting the sides (sometimes the disc is located on the bottom, sometimes on top…just like my…uh…nevermind).
If the sides split, the heat would be directed towards the cell next to it, and it would make the possibility of a runaway thermal meltdown (and fire) more likely. Here is a paper from NASA on “rupture discs”, if you are interested in this (isn’t everyone?).
Here is a quote from a vendor who sells parts to make 18650’s: “Safety valve will open at 2.8MPa (the valve will open to release interior high pressure if over 2.8MPa, to ensure no explosion from the can)”
MPa = Mega-Pascal, 2.8 MPa = 406-PSI. However, the actual bursting will be the pressure difference between the inside of the cell and the outside. By that I mean, if you are in the mountains where the air pressure is lower, the cell will burst at a lower internal pressure. Air-pressure at sea level is roughly 14.5-PSI / 100 kiloPascals / 100-kPa.
Here is a technical paper on thermal runaway events. The most interesting part for me was to see that the copper foil in the “jelly roll” had melted and formed globules, so the interior temperature had to have reached over 1085C (1985F).
This section may seem out of place, since this section doesn’t specify anything INSIDE an 18650 cell, but…a big reason I’m even writing this article is to preface the acceptable methods for a home battery pack builder to use, when using 18650 cells [in an upcoming article. Insert link here, when article is published].
This pic below is a close-up from a Tesla electric car pack. The electrodes on the 18650 cells has a fuse-wire connecting each one to a thick nickel-plated copper buss. These wires are connected to the cell tip by using an ultrasonic bonding machine (high-speed vibrations), which cause no heat to penetrate even the upper layer of the cell, much less the electrolyte.
Tesla vehicles are designed to draw “low amps” from each cell (to ensure long life, and provide long range), so…a surprisingly thin wire works fine as the connection to carry the current (in order to get high amps at the motor, they use thousands of them). The tiny diameter of the fuse-wire means that if the car is involved in a wreck and then one (or more) of the cells are shorted, any cell that is flowing high amps will get the fuse-wire hot enough that…the fuse melts very quickly.
An internal short of a cell is extremely unlikely, but…whatever the reason for high amps in the cell, heat from high amps will melt the fuse-wire, which will separate that particular cell from the pack. One of the Tesla models has 74 cells in each paralleled group, so…if one cell pops its fuse? that P-group will be just fine running on the remaining 73 cells.
There are quite a few youtube videos about taking some cheap salvaged low-mileage 18650 cells and building them into an off-grid home-electricity-storage system. Fusing is less needed in a stationary system (no crashes into other homes), but…it can be an easy and cheap feature to add to your design, whether it is for a home or a vehicle.
If you read farther below, you will see that I do NOT recommend soldering onto the negative anode of the cell (the flat bottom), but…I actually believe that…with the right tools and techniques? Soldering a fuse-wire onto the positive cathode is very easy and quite safe, with no risk of damaging the cell from overheating it. After a wreck, a damaged cell can be easily un-soldered and replaced.
Prep the surfaces, apply some solder-paste, set the fuse wire where you want it, and press down for a second with a fat-tip 100W soldering iron (thin tips cool off too fast). Or, use a resistance soldering rig, which I will write about soon. If you have a spot-welder? Fuse-wire can be spot-welded onto any 18650 positive terminal (hit it in two places).
If you want to use fuse-wire on your design, maybe consider flattening the tip of the fuse-wire to improve the contact area onto the positive 18650 electrode nipple. In order to get a consistent thickness on the fuse-wire tip, maybe put some steel sheet-metal on either side of the fuse-wire tip, and then when you whack it with a hammer, the wire tip-thickness will be very consistent. The sheet-metal that determines the thickness of the whacked-wire tip should be roughly about “half-to-one-third” the diameter of the round cross-section of solid wire.
Individual cell-fusing doesn’t have anything to do with the internals of the 18650 cells, but…they are a safety feature that is easy and cheap for anyone to add to an 18650-cell pack. If you look back over the info above on how the positive end of a cell is internally constructed, you can see that the positive electrode end can take some heat without even coming close to being damaged.
A “Protected” cell circuit
Ebike battery packs are made from UN-protected cells, because the BMS and controller decide how many amps you will be drawing from them. I’m adding this section to this article because…the pictures in some web-catalogs do not show why protected cells are different. If you are buying cells to build an ebike pack, make sure you do not order the ones with these protection circuits.
Protected cells are also a little longer than an 18650 that has no protection circuit. This current-limiter is attached to the negative electrode, and then the current passes through a very thin conductive ribbon up the side of the can to the positive electrode (ribbon shown in the pic above). That ribbon is typically then attached to the underside of a false cap that “snaps on” over the factory positive electrode cap.
Flat top vs button top
Some flashlights (or other commercial devices) have a hollow cylindrical protrusion inside it where the cells’ positive cathode tip presses against the housing contact that the 18650 is inserted into. This prevents the cells’ negative electrode from making any contact, if the cell is inserted backwards by a drunk customer (who you lookin’ at? I’m not talkin’ about me…*breaks down and sobs uncontrollably “Why did you leave me?…WHY?” ).
That protective socket-shape means that a raw 18650 cell cathode will also not make any contact, even if it is inserted in the proper orientation. For those devices, you must order a “button top” cell. It has an additional “snap-on” cap that is narrower and protrudes farther out. This also makes the button-top 18650 a little longer, and you should not order these when building an ebike pack.
The Jelly Roll
If you want to cut open an 18650 cell, then first… you must fully discharge it for safety. One way is to hook it up to any incandescent filament bulb, such as a 12V automobile tail-light bulb. A 12V LED will not work, because they require a fairly exact voltage to work, and you will be draining the volts down to zero. Also, a very large bulb (like an old headlight) might allow so much current that the cell overheats, so…a small 12V incandescent filament bulb is safer.
Of course, that also means that when using a small incandescent bulb, the cell will take longer to discharge. When you no longer see any dim light coming from the filament, I have even read about putting the cell in a bucket of water with a spoonful of table salt (overnight), to ensure that it is absolutely 100% drained.
To cut open the metal can of an 18650, I recommend a Dremel with a thin abrasive disc, instead of a tubing cutter. Those tubing-cutters cause an indentation on the edge of the cut, making a removal of the jelly-roll difficult. Once you get the jelly-roll out and begin to unroll it, you will notice there is a copper foil as a base for the anode chemicals, and aluminum foil as a base for the cathode chemicals.
There is a thin carbon rod that runs up the center to connect the edge of the roll to the positive cathode (#13 in the patent drawing below).
Inside the bottom of the can, the Anode
Once you cut the can open, you can see that the only thing between the jelly-roll and the bottom of the metal can is a thin plastic insulation washer.
One of the most important things I want to get across in this article is that when you are assembling a bunch of cells into a pack, it is the negative anode (the sides and bottom) that are the most sensitive to heat.
How is the whole enchilada stacked up?
Here’s a couple of images that I thought would help to get my points across. They are from the official patent of a Samsung 18650 cell. The top of the cell has all those pieces stacked up above the electrolyte in the jelly-roll, but the bottom? It only has that one thin plastic washer…good old #11.
If you’ve read this far, here is another paper on 18650 cell construction and safety from Dr Wesselmark, who holds a PhD in Applied Electrochemistry from the Royal Institute of Technology, Stockholm…co-written by Tom O’Hara, who has over twenty years R&D experience with Energizer battery.
If Dr Wesselmark or Tom reads this? I owe you a beer, just like Niels Bohr.
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Thanks for reading, and send any additional info, suggestions, or death threats to: Prisoner #41, Kansas state correctional facility for the mentally unstable.
Written by Ron/spinningmagnets, July 2017