Assembling a 4s LFP battery

Con­fi­dence
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Domain expe­ri­ence
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Audi­ence
Peo­ple wor­ried about the elec­tri­cal grid
Sum­mary
I put my ideas to the test by build­ing a small 4s1p bat­tery.

With a func­tion­ing spot welder in hand, I set out to build a bat­tery with Lithium Werks ANR26650M1B cells. These are LFP cells nom­i­nally at 3.3 V with a capac­ity of 2500 A·h.

Four green ANR26650M1B cells, three on their side showing the cathodes and one upright showing its anode. Unlike most cylindrical cells, the cathode is the can.

Sev­eral of the ANR26650M1B cells I ordered as they appeared when deliv­ered. Note the unusual ori­en­ta­tion of the cath­ode and anode.

Design

Since this bat­tery is going into a 1:6 scale RC trac­tor, I have some oper­a­tional con­straints. First, I’m going to try to fool a Hob­by­wing QuicRun 1080 G2 ESC into believ­ing it’s run­ning with power from a LiPo. This ESC sup­ports 2s and 3s LiPo bat­ter­ies, which means it should oper­ate between 6.4 V and 12.6 V. A 3s LFP con­fig­u­ra­tion will give us a usable range of 7.5 V to 11 V; for 4s, 10 V to 14.8 V.

In gen­eral, a 4s LFP bat­tery is going to be more use­ful than a 3s one, sim­ply because 12 V nom­i­nal cir­cuits are more com­mon. I sus­pect I can get away with a 4s pack here as the elec­tron­ics in the ESC should be rel­a­tively sim­ple. I hope the worst case is that I might fry the BEC com­po­nents, which would force me to use an exter­nal BEC instead.

The other con­straint is size. This bat­tery has to fit under a hood that is only 77 mm wide on a plat­form that lim­its its height to about 45 mm, so there isn’t a ton of room for wires or even the BMS. Ini­tially, I wanted to have two cells on top of two cells (so the axial ori­en­ta­tion would look like a square), but since I had more length than height to work with, I opted to make it inline:

+ - + - BMS

Safety

Since we’re using a BMS, our first step is to syn­chro­nize the cell volt­ages. A BMS won’t work at all if the volt­ages of indi­vid­ual cells are too far apart: it assumes some of the cells have been dam­aged. Since cells ship at a low SoC, I gen­er­ally just pick a volt­age above their cur­rent charge level and let them all charge up to that point. Then I dou­ble check every­thing with a volt­meter.

I won’t do a full charge until they’re in the bat­tery as I’d rather spot weld mostly dis­charged cells.

Two mate­ri­als are usu­ally used to limit the risk of short cir­cuits: adhe­sive-backed fish­pa­per and Kap­ton tape. We use the thicker fish­pa­per for sep­a­rat­ing elec­trodes and Kap­ton tape to hold the fish­pa­per (which seems to have a rather weak adhe­sive) and other elec­tri­cally iso­lated com­po­nents in place.

For most cylin­dri­cal cells, the entire hous­ing is the anode, and the cath­ode pro­trudes on one side, sep­a­rated by a thin layer of plas­tic insu­la­tion. (For the ANR26650M1B, the hous­ing is the cath­ode, but it is oth­er­wise struc­turally sim­i­lar.) To pro­tect the plas­tic insu­la­tion, we use a fish­pa­per gas­ket matched to the cell diam­e­ter.

Like­wise, we want to pro­tect each par­al­lel group from each other. To do that, we wrap the hous­ings of the entire group in fish­pa­per after assem­bling them, leav­ing only the elec­trodes exposed. Once the groups are assem­bled in series, we can addi­tion­ally wrap them, leav­ing only a small area exposed for attach­ing the bal­ance wires. With the 1p bat­tery I’m build­ing, I’m going to put a layer of fish­pa­per over each cell.

The four ANR26650M1B cells shown earlier wrapped in fishpaper around their bodies and over the anode.

Fish­pa­per applied to the cell bod­ies, and fish­pa­per gas­kets on the areas sep­a­rat­ing the anodes from the cath­odes.

We’ll also use fish­pa­per any­where wires cross each other, around the area the BMS over­lays the cells, and when­ever nickel strips or wire are near cell hous­ings.

To aid in heat dis­si­pa­tion and reduce the risk of a sin­gle mal­func­tion prop­a­gat­ing to adja­cent cells, it’s usu­ally a good idea to phys­i­cally sep­a­rate cells from each other unless size con­straints pre­vent it. You can buy gen­eral-pur­pose plas­tic spac­ers, but I decided to 3D print a model spe­cific to this bat­tery. It has raised areas between each series con­nec­tion. These serve both as addi­tional short-cir­cuit pro­tec­tions and as build guides. The over­all width of my spac­ers is just over 30 mm, which means I can lay my nickel strips directly on top of the cells with­out wor­ry­ing about wrap­ping them around the cell hous­ings.

The first revision of my cell holders for 26650 cells, shown side-by-side. Each cell holder has raised ridges to guide the placement of the nickel strips.

The first revi­sion of the cell hold­ers. I want good sep­a­ra­tion between cells (cf. the com­mon prac­tice of hot glu­ing cells together) and a tem­plate for plac­ing the nickel strips.

The wrapped cells placed into the cell holders to test their fit.

A rough fit test of the cells into the cell hold­ers. The fish­pa­per was thicker than I designed for, so it is pretty snug.

The cell holders, again side-by-side, with precut nickel strips shown in their correct alignments.

I cut the nickel strip to size using the cell hold­ers as a ref­er­ence. The two strips that over­hang the edges of the cell hold­ers will be the pos­i­tive and neg­a­tive ter­mi­nals for the bat­tery; I plan to sol­der 10 AWG wires to them and wrap them around.

Finally, many peo­ple like to round off the (poten­tially quite sharp) edges of the nickel strips before weld­ing them to pre­vent them from poten­tially dig­ging into the fish­pa­per and even­tu­ally short­ing the cell hous­ing. This isn’t nec­es­sary with my cell spacer design.

Preparing the nickel strips and lead wires

I had a really, really hard time fig­ur­ing out how to attach the bat­tery leads, which are 10 AWG wires, to the nickel strips. Even though I der­ated my over­all design to 25 A, I have a 60 A BMS: bet­ter safe than sorry for every­thing in front of it, I guess. Any­way, every­one just says “sol­der them”—well, eas­ier said than done!

I’m using a Hakko FX-888D, which has plenty of power (you’ll need it), a 3.2 mm chisel tip, high qual­ity eutec­tic sol­der, and RMA flux paste. I wouldn’t skimp on any of the mate­ri­als, per­son­ally. It’s hard to get enough heat into the chonky nickel strip.

I started by tin­ning the wire and the area of a “pad” I wanted to cre­ate on the nickel strip. The sol­der did not stick well to the nickel, even when cleaned with IPA. I had to scratch up the sur­face using some medium-grit sand­pa­per and apply a lib­eral amount of flux to the area. (I also used a lot of flux on the wire itself.)

To hold every­thing in place, I put the strip in my vise and aligned the wire using a help­ing hand. Even so, I found it very dif­fi­cult to get the sol­der to flow prop­erly. I prob­a­bly tried 5–10 dif­fer­ent approaches (vary­ing tem­per­a­ture, amount of flux, etc.) in this vein before I went back to the draw­ing board (that is, search­ing the inter­net for clues).

The most use­ful tip I found was to fold and crimp the nickel over the wire before sol­der­ing. In prac­tice, this made it dif­fi­cult to tell when the sol­der had filled the gaps inside the fold, and I also had trou­ble ori­ent­ing the iron to get con­sis­tent heat­ing, so I ended up with a slightly mod­i­fied pat­tern: mak­ing a semi­cir­cu­lar cup around the wire and fill­ing it in, sim­i­lar to how you’d attach a wire to any other sol­dered con­nec­tor. I lightly pressed the nickel into the wire with pli­ers. Then I was able to wedge the iron into the cup on one side and feed sol­der from the other.

My final rev­e­la­tion was to sig­nif­i­cantly increase the tem­per­a­ture for this step. I ini­tially tinned the wire and metal strip at around 375 °C, but I bumped my iron up to 415 °C to make the con­nec­tion. (And it still took an awk­wardly long time.)

A close-up of the negative lead wire in the pocket I created for it.

With the wire cupped into the nickel strip, it’s pos­si­ble to flow sol­der nicely over it and still visu­ally inspect the result. You can see some rene­gade flux residue, too. It took for­ever to clean up all of it.

I also tinned the loca­tions on the nickel strip where I plan to attach the BMS bal­ance wires. These wires are very small and shouldn’t take much time to sol­der, but I fig­ured I’d get every­thing ready as long as I had my iron out.


Most of the time, a BMS is inte­grated into either a bat­tery itself (“pro­tected”) or part of the con­trol cir­cuitry for the prod­uct that uses the bat­tery. I bought a DALY BMS to attach to the bat­tery, but I don’t really want it wrapped into the bat­tery itself or sol­dered directly to the bat­tery ter­mi­nals. In my mind, the fail­ure modes and lifes­pan for the cells and the BMS seem dis­tinct, so they should be sep­a­rate parts.

To con­nect the BMS to the neg­a­tive bat­tery lead, I’m using Amass XT150s, which are sin­gle-pole con­nec­tors that can han­dle up to 100 A con­tin­u­ously over 8 AWG wire. The DALY BMS also has ter­mi­nal hous­ings for its bal­ance and ther­mis­tor wires, so the whole unit should be swap­pable with only minor incon­ve­nience.

To con­nect to the load, I also opted to use XT150s, mostly for their sin­gle-pole design, which allows me to con­tinue to keep the BMS and cells phys­i­cally sep­a­rated. Also, with an XT150, I should be able to split the load side out into four 14 AWG wires (equiv­a­lent to about 8 AWG, using the total coss-sec­tional area of the wires) to con­nect to the ESC, charger, or any­thing else I want to power.

Power sources are always attached to the female con­nec­tor for safety rea­sons (it’s harder to acci­den­tally short a female con­nec­tor, which is usu­ally fully obscured by its hous­ing), so I went ahead and sol­dered them onto my pos­i­tive lead wire, neg­a­tive lead wire, and the load side of the BMS. I also sol­dered a male con­nec­tor to the bat­tery side of the BMS.

From left to right, the BMS with XT150 connectors on its lead wires and the nickel strips placed on the cell holders. The nickel strips have their lead wires attached and they have tinned areas for the balance wires.

The BMS is ready to go at this point. The nickel strips are also ready to be attached to the cells once I work out how to weld them cor­rectly.

XT150 con­nec­tors can be a bit dif­fi­cult to manip­u­late. Here’s what I learned:

  • The female con­nec­tor goes in the smaller hous­ing.
  • It’s eas­i­est to push the hous­ing over the con­nec­tor while it’s still hot, within a minute or so after sol­der­ing.
  • If the hous­ing gets stuck and won’t budge, I actu­ally had suc­cess spray­ing into it with a bit of IPA to tem­porar­ily lubri­cate it, as unin­tu­itive as that sounds.
  • The hous­ing needs to click over the con­nec­tor. Until it clicks, it can still be pulled back with sig­nif­i­cant enough force.

Cell spacer improvements

With my cell spacer design, the 30 mm-wide nickel strips com­pletely obscure the cell con­tacts, so it’s annoy­ing (or dan­ger­ous, depend­ing on your per­spec­tive) to align the spot welder elec­trodes in the right place. I added small notches to help with visual ori­en­ta­tion. I also could have used a Sharpie, but I was going to reprint the spacer any­way, so I fig­ured I’d make it pre­cise.

A more sig­nif­i­cant change is adding some rout­ing guides for the BMS bal­ance wires. It’s impor­tant to pre­vent the bal­ance wires from cross­ing over each other or the lead wires because fric­tion dur­ing move­ment could wear the insu­la­tion, even­tu­ally lead­ing to a short cir­cuit. I think this is actu­ally eas­ier to man­age with larger packs because you sim­ply have more sur­face area at your dis­posal; either way, this par­tic­u­lar approach unfor­tu­nately doesn’t scale to a dif­fer­ent pack design, but I do like it for an inline con­fig­u­ra­tion.

I also decided to add a bracket for hold­ing the BMS. This neces­si­tated a few holes for heat set inserts in the spac­ers.

Finally, late in the process I dis­cov­ered the way the ANR26650M1B cells vent is by split­ting the can on the pos­i­tive elec­trode. Accord­ing to their design doc­u­ments, this requires 1 mm of space avail­able at the edge of the cell, which I hadn’t accounted for.

Spot welding woes

I was mak­ing the changes to the cell spacer design, but it hap­pened to be an unusu­ally nice March evening in Port­land, so I took my kWeld out­side and started play­ing with one of the cells instead. My goal was to dial in the energy required to spot weld the pack, which I expected to be around 30 J for the neg­a­tive elec­trode and 40 J for the pos­i­tive elec­trode.

Frus­trat­ingly, the pos­i­tive elec­trode sim­ply would not make a reli­able weld nugget. Why? Why?! Well, the ANR26650M1B can is actu­ally made of alu­minum with a small 0.2 mm-thick pure nickel disc lam­i­nated to the end. It dis­si­pates heat extremely effi­ciently, which meant I needed to push more cur­rent than a sin­gle kCap unit could pro­vide. (I tried up to 150 J, but the time scaled with the cur­rent, so the instan­ta­neous heat trans­fer remained the same.)

Two cells, upright, showing their cathodes. On the left, the cell I repeatedly tried to weld with seemingly hundreds of failed weld nuggets. On the right, a clean cell.

Jux­ta­po­si­tion of the cell I used to try to fig­ure out what energy was required for a proper weld and a pris­tine cell. I don’t know how many attempts I made. Feel free to count and make fun of me for it.

I mulled over some options:

  • Import another kCap mod­ule from Canada and run them in par­al­lel. This would get the kWeld up to about 1700 A, which would be pretty neat and I’m sure it would work, but it’s also a rather expen­sive option, con­sid­er­ing I still didn’t have even a sim­ple func­tional bat­tery at this point.
  • Dis­as­sem­ble the kWeld, buy a lead-acid starter bat­tery (or LiPo), and try to use it that way. Also expen­sive and annoy­ing to boot.
  • Try 0.15 mm-thick nickel strips instead. No guar­an­tee I’d fare any bet­ter (although I sus­pect I would). I’d have to increase the width to 40 mm to carry the same cur­rent.
  • Try another cell model that doesn’t have the same can design. Assum­ing I can locate one with a more tra­di­tional nickel-plated or stain­less steel can, I should be able to suc­cess­fully weld to it.

I haven’t found a good source for 40 mm-wide nickel strip (should I be buy­ing sheets and cut­ting them myself?), so I started look­ing for alter­na­tive cells. The clos­est one may be the Vap­cell IFR26650. My under­stand­ing is Vap­cell rewraps large Chi­nese man­u­fac­tur­ers’ cells to sell to con­sumers, and there are no real datasheets or design guide­lines to go off of for this cell, so we’re more or less tak­ing a gam­ble they will be drop-in replace­ments for the ANR26650M1B. But they seem to per­form ade­quately, they have a rep­u­ta­tion for main­tain­ing their capac­ity for a lot of cycles, and they’re widely avail­able domes­ti­cally, so I’m going to give them a try.

Once I have a work­ing bat­tery and a bet­ter under­stand­ing of the entire process, I’ll prob­a­bly buy a sec­ond kCap to fin­ish mak­ing packs with my ANR26650M1B cells.

Spot welding Vapcell IFR26650 cells

The Vap­cell IFR26650 cells I received were mar­gin­ally larger than their Lithium Werks cousins, and while they fit in the spacer I designed, I felt they were now way too snug, so I decided to increase the diam­e­ter by about 0.5 mm. These cells seemed to have a more con­ven­tional mech­a­nism for vent­ing through their cath­odes so I wasn’t as con­cerned about that prob­lem; I just wanted a good fit that didn’t exert any force on the cell hous­ings.

Four Vapcell IFR26650 cells, three upright showing their cathodes and one on its side showing its anode.

Vap­cell IFR26650s with fish­pa­per gas­kets in place.

I was able to suc­cess­fully spot weld 0.2 mm-thick nickel to the cath­odes of these cells at 60 J and the anodes at 70 J. Feel­ing out the qual­ity of the welds is a bit of an art. To cre­ate a good weld, you seem to need to apply a lit­tle bit of pres­sure, but not a lot, to the joint. The elec­trodes of the kWeld should be close to each other and posi­tioned at some­where between a 30 ° and 45 ° angle apart. Aside from pulling on the nickel strip, the most reli­able proxy for qual­ity I’ve found is the resis­tance value given by the kWeld in its post-weld read­out. For this cell, it seems like a value around 0.8 mΩ0.9 mΩ is ideal. Higher val­ues indi­cate a poorer weld, and any­thing over about 1.2 mΩ prob­a­bly isn’t usable.

A top-down view of the anode of the cell I used to determine required weld energies. There are not nearly as many attempts.

The anode of the test cell I used to deter­mine weld ener­gies. In the upper half as pic­tured, you can see where chunks of the nickel strip have become fused to the cell, leav­ing a hole in the strip when it was pried off. This indi­cates a suc­cess­ful weld.

Making the series connections

After I printed my updated cell holder design, I made sure every­thing looked good before weld­ing the “pro­duc­tion” cells.

The cells shown upright, loosely placed into the holder. The BMS lead wires are routed through the guide holes. A small part of the BMS is visible behind the cells.

Test­ing sev­eral of the cell spacer improve­ments. The indents are visual guides for me to use to align the welder’s elec­trodes on the cen­ters of the cells. The diag­o­nal holes allow me to route the BMS bal­ance wires from the inte­rior of the pack to the metal strips with­out depend­ing on tape to hold them in place.

A side view of the same test configuration, showing the balance wires connected to the BMS. Two of the balance wires go above the cells and three go below.

To pre­vent the bal­ance wires from over­lap­ping, two go on one side of the cells, and two go on the other. I couldn’t fig­ure out a good way to sep­a­rate the neg­a­tive bal­ance lead wire phys­i­cally, so we’ll use a bit of tape for it. A sled that screws into the cell spac­ers holds the BMS in place and gives the cell spac­ers struc­tural integrity so they don’t put undue force on the ends of the cells.

A screenshot taken by FreeCAD showing a representation of the enhanced cell holder.

The design for the cell holder, show­ing the spacer ends and the BMS bracket. You can down­load it for free if you’d like to try it your­self.

Weld­ing the cells was fairly unevent­ful. I went in order of the series (that is, I flipped the assem­bly over after I attached each nickel strip to a cell) and dou­ble-checked volt­ages and inter­nal resis­tance as I went. These cells had an inter­nal resis­tance around 4 mΩ5 mΩ each and I charged them up to about 3.35 V.

I had a men­tal check­list for each con­nec­tion I made (which I have repro­duced here any­way, so I should have just writ­ten down in the first place, but I digress):

  1. Set the kWeld to the proper energy, either 60 J or 70 J depend­ing on whether we’re weld­ing to the cath­ode or anode, respec­tively.
  2. Posi­tion the elec­trodes using the guides in the cell holder. Make sure the angle is good. I put a lit­tle torque on them because I felt it made it eas­ier to apply con­sis­tent pres­sure as I clearly favor my right hand oth­er­wise.
  3. Weld the cells. Leave the foot pedal engaged after each weld to make sure the resis­tance was less than about 1.2 mΩ. Aim for at least two and ide­ally three very high qual­ity welds per con­nec­tion.
  4. Check the volt­age of the entire bat­tery so far and make sure it adds up cor­rectly.
  5. Check the inter­nal resis­tance of the entire bat­tery using a microohm­me­ter and make sure it isn’t get­ting too out of whack.
  6. Flip the bat­tery over and repeat.

The battery after welding all the cells. The side of the battery with its terminals is shown.

The bat­tery after I fin­ished weld­ing. As you can see by my lack of con­fi­dence in the weld qual­ity, I did the neg­a­tive ter­mi­nal first.

Wrapping everything up

Before we start lay­er­ing tape over every­thing that shouldn’t be exposed, we need to make sure the ther­mis­tor is in a prac­ti­cally use­ful place for the BMS to be able to detect over­heat­ing. I decided to put it roughly in the enclosed area between two cells, although I also con­sid­ered tap­ing it directly to one of the ter­mi­nals. (The way I chose felt a lit­tle cleaner, but I think in the­ory it might take slightly longer to trip the BMS if some­thing goes wrong.)

The thermistor and its wire placed inside the battery pack and held in place with Kapton tape, which wraps around the cell bodies.

I used Kap­ton tape to posi­tion the ther­mis­tor for the BMS in the mid­dle of the pack.

The other side of the battery, showing fishpaper placed over the Kapton tape.

A layer of fish­pa­per over the Kap­ton tape gives addi­tional struc­ture to the pack.

To con­nect the bal­ance wires, I first routed them through their respec­tive guide holes. I was happy the other end was already assem­bled in a hous­ing because I didn’t have to worry about shorts. I made them fairly taut because I’m going to back them out a bit once I strip and sol­der them. With the tinned pads on the nickel strips, it took less than a sec­ond to sol­der each wire, so I have no con­cerns about excess heat affect­ing the cells.

The pack showing some of the balance wires routed near their corresponding cell connections.

The pack after soldering on the balance wires, with a new piece of fishpaper applied to keep the negative terminal's balance wire from coming into contact with any of the others.

The pack after sol­der­ing on the bal­ance wires. As planned, I added another piece of fish­pa­per over some of the wires to pre­vent them from poten­tially rub­bing against each other. You can also see how nicely the guide holes work once the wires are short­ened: only tiny parts are vis­i­ble near the nickel strips, and they’re imme­di­ately drawn away and into a safer inte­rior area through the cell holder.

At this point, I no longer had a coher­ent plan, and it shows, because things got ugly. Not dan­ger­ous, just aes­thet­i­cally dis­pleas­ing. I started indis­crim­i­nately wrap­ping every­thing that looked slightly metal­lic in lay­ers of fish­pa­per and Kap­ton tape.

In ret­ro­spect, I could have built out the cell holder a bit more to pro­vide addi­tional struc­ture for the fish­pa­per, or even to replace it entirely to get more of a lap­top bat­tery look. I’ll prob­a­bly take the whole thing apart at some point to do just that, but for now, I’m excited to have some­thing that works!

The complete battery, wrapped in a startling amount of Kapton tape, which is all wrinkly and weird.

To be hon­est, I kind of assumed I would be good at tap­ing things. How hard could it be? Is using tape cor­rectly an actual skill? I guess the answers are “sur­pris­ingly” and “yes,” respec­tively.

A voltmeter reading 13.3 V through the BMS, meaning the battery is fully functional.

I con­nected the bat­tery side of the BMS to the neg­a­tive ter­mi­nal of the pack, and—voilà—we read 13.3 V through it. Going through the BMS also val­i­dates the cells are within bal­anc­ing spec­i­fi­ca­tion (close enough in indi­vid­ual volt­age).

Parts list

PartModelQuantityPriceLink
CellVap­cell IFR266504
  • USD 6.39
ea.Liion Whole­sale
Tool
Microohm­me­ter (DLRO)
RC3563
  • USD 23.13
AliEx­press
BMSDALY H Series Stan­dard BMS1
  • USD 14.11
AliEx­press
Rated for cur­rent
60 A
Rated for series
4
Rated for volt­age
3.2 V
Cell holder 3D model CC-BY-NC-SA-4.01
  • ~USD 1.00
Mate­r­ial
PETG
Fish­pa­per~5 m
  • USD 3.61
AliEx­press
Size (W)
65 mm
Fish­pa­per gas­ket~25
  • USD 0.95
AliEx­press
Size (⌀)
26 mm
Nickel strip~2 m
  • USD 15.05
AliEx­press
Dimen­sions (W × T)
30 mm × 0.2 mm
Kap­ton tape~33 m
  • USD 6.71
AliEx­press
Size (W)
50 mm
Wire~1 m
Size (Ga)
10 AWG
Insu­la­tion color
Black
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Wire~1 m
Size (Ga)
10 AWG
Insu­la­tion color
Red
Insu­la­tion mate­r­ial
PVC
Wire mate­r­ial
Cop­per
Wire type
Stranded
Bul­let con­nec­torAmass XT150~10
  • USD 11.99
Ama­zon
Size (Ga)
10 AWG
Rated for cur­rent
60 A
Heat set insert2
  • USD 0.09
ea.Ama­zon
Dimen­sions (⌀ × L)
M3 × 5.7 mm
Screw2
  • USD 0.16
ea.Bolt Depot
Dimen­sions (⌀ × L)
M3 × 30 mm
Screw head style
Pan
Screw type
Machine
Down­load the parts list in TOML for­mat.

Conclusion

What’s that famous adage? “Mea­sure twice, cut once.” Well, it doesn’t help when you delib­er­ately mod­ify the plan after you for­got what the con­straints were. Adding the bracket and sled for the BMS made my bat­tery too tall and it didn’t fit inside the trac­tor. So, for now, some vel­cro straps are doing the hard work of keep­ing every­thing roughly in place while I design a new hood.

The battery shown wired up inside the RC tractor.