Build Log Plantraco Bahoma Cell Build -- DIY

[mharms]

PART 1. INTRO

So... you want to make your own Bahoma cells. Here’s how I’ve been making my cells.

Note: This post describes the build of a 10mm spacing Bahoma cell. 5mm spacing cells can be similarly made. I purchased most of the components from Plantraco before they put together their Bahoma Experimenter’s Kit. Except for the TIX solder and flux (which I purchased at a LHS), their kit has the same components I describe, below.

In this thread, I'll be referencing images as "Pic 1", "Pic 2", etc... and the corresponding image filenames will be "01_blah_blah", 02_blah_blah", etc... Hopefully, you'll be able to follow.

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Caution and Disclaimer:
Working with lithium polymer cells is serious business. Even momentary shorting of the LiPo terminals can cause a fire, or send molten metal flying. Do not attempt this build if you are unsure of your soldering or mechanical skills. At all times, be aware of both terminals of the LiPo and insulate at least one of them during handling. Remove magnetic materials from the vicinity of the cell, once the magnets have been attached. Also, be aware that the magnets will probably be attracted to your soldering iron. Undertake this procedure at your own risk. Oh… and have fun.
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(Pic 1) When not directly working with the LiPo cell, I keep a piece of heatshrink tubing pulled over one of the tabs to reduce the chance of shorting the terminals.

[mharms]

Here’s what you’ll need:

Low temp soldering iron (that can be set to 200C)
Low temp solder ( supplied by Plantraco or TIX brand solder)
Flux: TIX flux (Kester SP-44 seems active at low temp, so it may work)
High temp Neodymium magnets (3mmx1.5mm, nickel or gold plated, from Plantraco).
Plastic safety rings (from Plantraco)
Magnet PCB (from Plantraco)
A LiPo cell with solderable nickel tabs
Some toothpicks


I’ll try to answer some questions up front:

Why do you need all these specialized components and equipment?

You need the low temp soldering iron so you don’t demagnetize the magnets.

You need the high temp magnets because ordinary Neo magnets will demagnetize, even at moderately hot temperatures. Nickel or gold plating is needed for good electrical conductivity and solderability.

You need low temp solder (NOT ordinary solder) that melts below 200C and has good conductivity, and meets a bunch of metallurgical criteria that I won’t go into here. Not a lot of choices for the hobbyist, but TIX solder (an alloy containing Indium) works well and is sold in some hobby and jewelry shops. Plantraco may supply TIX or another specialized low temp solder in its Bahoma Kit. Be careful substituting other types of low temp solder (for example: some Bismuth-based solders can be brittle). These are critical solder joints we are talking about here – do your homework if you choose alternative low temp solders.

The little magnet PCB makes assembly much easier. I made my first Bahoma cells by directly soldering to the LiPo tabs, but this can be tricky and dangerous.

The plastic safety guards are a real convenient little assembly and helps prevent accidental shorts across the magnets. You could make something equivalent yourself (I did, at first), but it is much easier to get them from Plantraco.

Flux. I always use it. It helps clean the nickel surfaces (LiPo tabs, magnet plating) that you will be soldering and promotes good flow of the solder. Unlike rosin cored electronics solder, TIX solder has no flux built-in. I’m not sure if any flux is included in the Plantraco Bahoma Kit. At low temperatures (like 200C), some fluxes aren’t active. The TIX flux is specifically made to work at low temps. I also have found that Kester SP-44 flux seems to at least melt and help solder flow at low temps, so this may be an alternative.

What about Conductive epoxy?

It might work, but after a lot of research, I decided it might have issues with temperature cycling and current densities. Also, the really good conductive epoxy usually must be refrigerated, has limited shelf life, is expensive and hard to obtain in small quantities. Low temp solder (TIX) is fairly widely available, and not that expensive. If someone tries conductive epoxy, let us know how it holds up. I’m sticking with solder.

Components, Supplies, and Tools:

(Pic 2) TIX solder, TIX flux, and a raw LiPo cell (160mAh from Max Amps). This particular LiPo cell is nice because the external portions of both tabs are solderable nickel. Many cells have aluminum tabs coming out on the positive side, with an added nickel tab spot welded at the end. You must only solder to the nickel part.

I found a glazed ceramic tile made a good work surface for many of the assembly steps.

(Pic 3) High temp Neo magnets, plastic safety guard, and thin, double-sided PCB that interfaces the LiPo tabs and magnets.

(Pic 4) You need to keep the soldering temp at 200C or even a bit below. To avoid contamination with standard electronic solder, I dedicated a new soldering tip for use with TIX only. A somewhat broad “chisel point” style tip is good for this work.

[mharms]

Here We Go…
We’ll start with “tinning” the surfaces to be soldered with low temp solder.

Pic 5) Clean the both side of the tabs. I used alcohol applied with a Q-tip and then use a fiberglass brush that is very slightly abrasive. Extra fine sandpaper can be used, but be gentle. You don’t want to sand through the (nickel) plating or the solder may not stick to the underlying metal (ask me how I know). It helps to support the tabs while you work on them (obviously, use something non-conductive, like a wooden popsicle stick).

(Pic 6) Apply flux to the solder site.

(Pic 7, 8, 9) The amount of solder used here is small, so for convenience and to control the amount used, I cut small pellets of TIX solder and set them at the site to be soldered. Then I just hit the site with the iron until I see the solder has flowed (turns shiny). Hold everything in place for a few seconds after removing the iron until you see the solder solidify (turns dull).

(Pic 10) Cut a single section of PCB from the strip that Plantraco supplies. Sharp scissors will work.

Pic 11, 12) As with the LiPo tabs, tin both sides of the PCB with TIX solder. I apply a tiny droplet of flux beforehand. The toothpick just holds the tiny PCB in position.

However, before tinning the magnets, lets build an alignment jig.

[mharms]

Part 4 to come later tonight (have to do some shopping before the stores close).

Mark

[mharms]

(Pic 13, 14) I made a little alignment jig to hold the magnets at the correct spacing and to position the PCB over the magnets during the soldering of the PCB to the magnets. It is also used when soldering the LiPo tabs to the PCB.

The jig is made of balsa and barrier strip paper (a resin-impregnated cardboard, about 1mm thick, used commercially with electric terminal strips). I punched 2 holes at exactly 10mm spacing in the barrier strip paper. The hole diameters are nominally 2.8mm, so they are a snug fit for the 3mm magnets. The balsa pieces simply follow the outline of the PCB and position it so the PCB pads (the round ones) are directly over the magnets. The balsa only conforms to 3 sides of the PCB, since the LiPo tabs will come in from the 4th side when the tabs get soldered to the PCB.

The barrier strip paper seems to handle the heat of soldering just fine. Instead of barrier strip paper, you might be able to use a similar dense cardboard or a high temp plastic.

(Pic 15) For punching the holes in the barrier strip paper, I used a punch I found at the craft store (in the leather crafts isle, I think). It was under $10.

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(Pic 16) You can use your Plantraco HFX900 TX charger to determine your magnet polarities.

(Pic 17) I used a fine point permanent marker to mark, on the side of each magnet, which end attracted to the positive charger magnet. This end of the magnet is the correct magnetic polarity for presenting the positive cell voltage to your RX and charger. The other end (away from the mark) is the right polarity for presenting the negative cell voltage to your RX and charger.

(Pic 18) Here is a pic showing which magnet side goes where.

(Pic 18a, 18b, 18c) It may also be convenient to place the magnets on top of the TX charging magnets and just set the alignment jig over the magnets and press the magnets into the holes in the jig. If you slide the jig off the charging magnets in a sideways motion, the magnets will stay in the holes and maintain the proper orientation for the next steps.

[mharms]

With the magnets in the alignment jig (and polarities correct), the side of the magnets that will be soldered to the PCB should be cleaned with alcohol and a tiny droplet of TIX flux applied. I don’t recommend sanding the magnet surface, as you don’t want to remove the thin plating.

(Pic 19) When tinning the magnets, you can set them down directly on a piece of flat steel to help dissipate heat during soldering. CAUTION: DO NOT SET THE MAGNETS DIRECTLY ON THE STEEL WHEN IT COMES TO SOLDERING THE LIPO TABS!!!

(Pic 20) Since the magnets may be attracted strongly to your soldering iron, a toothpick can be used to hold them down during soldering. Apply a small pellet of TIX solder and hit it with the 200C soldering iron. Briefly rub the tip around as the solder melts, so you get solder spread fully across the top face of the magnet. Be quick with this step. It should take less than 2 seconds to do this.

(Pic 20a) The tinned magnets.

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(Pic 21) At this point, I move the alignment jig and tinned magnets to another flat piece of steel COVERED WITH AN INSULATING TAPE. THE MAGNETS DO NOT MAKE ELECTRICAL CONTACT TO THE STEEL. MAKE SURE THAT THE TAPE CAN HANDLE 200C SOLDERING TEMPERATURES WITHOUT MELTING. KAPTON OR SILICONE TAPE WILL WORK.

NOTE: You could actually leave the tinned magnets on the previous bare steel plate for this step (soldering the PCB to the magnets). This would help cool the magnets while soldering the PCB to them, but I have not found this makes any difference in the final strength of the magnets. As long as the soldering iron is set to 190-200C, you should be fine. HOWEVER: BEFORE YOU BRING THE LIPO TABS ANYWHERE NEAR THE PCB ASSEMBLY, MOVE THE PCB ASSEMBLY ONTO AN INSULATED STEEL PLATE, AS SHOWN, BELOW (PIC 21).

(Pic 21) Tinned magnets moved onto an insulated steel plate. The attraction to the steel plate helps hold the magnets in place during soldering.

(Pic 22) The round pads on the PCB are for the magnets, the larger square pads are for the LiPo tabs. The balsa pieces in the jig position the PCB properly over the magnets.

(Pic 23) A toothpick with a flattened end can be used to hold down the PCB during soldering. There should be enough solder on the surfaces after tinning so that you don’t need to add any additional solder at this point. A tiny droplet of flux on the tinned magnets will help the solder flow. Apply the 200C soldering iron to the topside square pads until the solder liquefies across the full pad. This should take less than two seconds. After removing the soldering iron, keep the toothpick holding down the PCB until you see the solder solidify (TIX solder turns from shiny to a dull sheen).

(Sorry, not enough hands to photograph the actual soldering)

(Pic 24) Here are the magnets soldered to the PCB. Inspect the solder joints. You should see a fillet of solder where the magnet meets the PCB pad. If not, you can place the assembly back in the jig and reflow it. If necessary, you can add a pellet of solder on the topside and this will allow additional solder flow through the PCB via holes down to the magnet.

Now is a good time to check the magnet polarities, so you know which magnet should be the positive battery terminal and which should be negative. If you didn’t mark the magnets previously (or if the alcohol cleaning removed the marks), use your TX charger magnets to determine the polarity of your PCB assembly. The magnet on the PCB that is attracted to the positive charger magnet will become the positive side of the completed battery.

[mharms]

(Pic 25) Here is a basic diagram for how the PCB (with magnets attached) will be bent into the final position. Take note of which side of the tabs the PCB will be soldered. I call the space where the PCB and spacer reside the “pocket”.

If your cell is the type with a nickel piece spot welded to the aluminum tab, you may need to fold that tab an extra time to make the nickel piece face the right way for soldering. You must only solder to the nickel part. Also, do not make any tight radius bends in the aluminum portion of the tab or bend it back and forth repeatedly or you may break it.

(Pic 26) Confirm that the magnetic polarity is correct, then insert the PCB assembly back into the jig as shown. YOU MUST HAVE A HEAT RESISTANT INSULATOR UNDER THE MAGNETS IF YOU ARE WORKING ON A STEEL PLATE. OTHERWISE, YOU WILL SHORT OUT THE LIPO WHEN YOU SOLDER IT TO THE PCB.

(Pic 27) Now lay the LiPo down with the tabs positioned on top of the PCB pads. You probably will need to trim the length of the LiPo tabs so that , when folded over (as seen in the previous diagram), the PCB assembly fits in the cell “pocket” space. WHEN TRIMMING THE LIPO TABS, CUT ONE AT A TIME, WITH THE OTHER TAB COVERED WITH HEATSHRINK OR OTHER INSULATOR. DO NOT SHORT THE LIPO TERMINALS!

The LiPo tabs may not exit the cell equidistant from the cell centerline, so you may need to offset the tabs to one side on the PCB pads to insure that the PCB is centered in the cell “pocket” space once the assembly is folded over.

(Pic 28) Apply flux, a tiny pellet of TIX solder on the topside of the tabs, and solder the LiPo tabs to the square PCB pads. A toothpick should be used to hold the PCB still while soldering. Apply the soldering iron until you see the solder flow (turns shiny) across the full tab and pad surface, then remove the iron and hold the assembly still until the solder solidifies (turns dull). The whole soldering process takes about 2 seconds per side.

I didn’t use my “heatsink tool” when soldering the LiPo tabs to the PCB, but it might not be a bad idea to use it to keep the LiPo from getting too hot. The only problem is that I would need another hand to also hold the PCB stationary. Maybe another little jig could be made. However, if you are quick with the soldering, the LiPo seems to handle the heat OK (my experience) without additional heat sinking at this step.

(Pic 29) LiPo tabs soldered to PCB pads.

(Pic 30) Removed from jig, tab side.

(Pic 31) Here’s the magnet side.

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(Pic 32) Make a spacer out of 1/32in balsa. I wrap it with tape so that, if it gets wet, it won’t discharge the cell. A lightweight piece of plastic could also be used. Whatever you use, it just needs to NOT be an electrical conductor. Depending on the cell thickness, you may not need a spacer. You just need the battery magnet contact surfaces raised up enough to allow the RX to clear the battery body when its attached.

(Pic 33) Insert the spacer into the cell “pocket”.

(Pic 34) Push spacer against bend line while bending so that bend will have a wide radius.

(Pic 35) After folding.

(Pic 36) After folding, side view.

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(Pic 37) There are many ways to “tape up” a cell. Here is the first piece I apply.

(Pic 38) Use that handy punch to make clearance holes for the magnets.

(Pic 39) Stick tape over the magnets and fold it over on the backside.

(Pic 40) Set the plastic safety guard over the magnets (magnets centered within rings) and use a thin strip of tape to anchor it. You could also apply this strip before the previous tape piece.

(Pic 41) The final wrap is a piece of tape around the mid section (its hard to see -- I used clear packing tape here). You are done!

(Pic 42) Here is the other side.

[mharms]

Yes, this process adds a little weight, but it adds a lot of convenience.

(Pic 43) Raw cell weight.

(Pic 44) Final Bahoma cell weight.

There you go. All done!

Questions and comments welcome.

Mark

[micro_builder]

hey Mark,

thats one heck of a tutorial, very well written. the weight gain is pretty minimal, adding wires and micro connectors like normal probably adds close to the same amount, and you dont have to go through the ordeal of trying to plug the cell into the RX while its inside the plane, just click and go.

thanks

nick

[Zlatko]

Mark,

Thank you very much for this, my hat off to you .

Thank you very much for the idea about TIX solder, I have played with Bismuth and it was a pain .....

Searching on the Net for TIX solder I also came across this
http://www.ares-server.com/Ares/Ares...oduct&ID=80765
a miniature spot welder ( resistance soldering ) .

I have chased something like this for close to 2 years , so a double thank you !!!

Cheers
Zlatko

[Allan Wright]

This post deserves a sticky. And to be published in Fly RC.