Recycling 18650 cells into Useful Battery Packs

This has been on my todo list ever since I reviewed the Sunnko 709A spot welder machine in Hackspace Magazine issue number 48 and I finally got a bit of time to play this morning. Making battery packs from recycled 18650 cells, the cells commonly found in older end of life laptops and other devices, is nothing new and there are plenty of technical blog posts and youtube guides out there to explain the process. It appeals to me on a few levels, I like the recycling part but also it's handy to make some battery packs up that can be used to power FPVground equipment or even for use with my Pinecil soldering setup in the field. Of course it's also great practice perhaps for making 18650 packs with brand new cells which I may be tempted to do for long range drone or fixed wing projects as you can pack a lot more capacity into a smaller and lighter battery than a LiPo.   

I recovered these three cells from an old Sony laptop battery someone gave me and whilst I recovered 5 cells, I only wanted to build a 3 cell battery. I spent some time charging the cells to find that they were all serviceable and then made sure to charge these three cells so that their voltages matched pretty closely. This is useful when making a battery which you wish to balance charge as then it doesn't take an age in the first instance getting the pack to balance. The cells as recovered retained the little paper disks on the positive end which afford protection from shorts, It is good practice also to add extra insulation between the 18650 cells to protect from accidents and you can see in the above image I arranged the cells with some barley gasket paper between them. Whilst many people use a jig to hold the cells and then the pack covering to reinforce them, I decided a few spots of hot glue make for a robust structure.  

Next up I cut some nickel strips to make the connections and then spot welded them using the Sunnko machine. For the strips that connect the cells I simply made straight cuts but for the positive and negative terminals I wanted to make tabs with smoothed edges to solder to to avoid them snagging or cutting any insulation or wiring. The spot welder can create really nice joints and it's great to be able to avoid soldering directly to the cells as it puts a heap of heat into them and isn't massively safe.

All that remained was to solder up an XT60  battery connector and also a balance connector and associated wiring.  I made up my own balance connector using a JST XH connector and it's relatively straightforward to wire up, soldering to the new spot welded tags in between the adjoined cells, again, with some decent flux and a hot iron it's much easier to avoid putting too much heat into the pack. Finally a good dose of sanity checking and triple checking with the multimeter that everything is connected correctly and it's happily charging correctly. I've just used some Kapton tape to insulate the pack and hold the wiring in place, a more common approach is to find some large heatshrink which is on the list to pick up at some point. As I'm not too worried about the weight or the ascetics I am tempted to just give it a good wrap of PVC tape. 

Tool Tuesday - Wire Brush

This is a tool that perhaps shows my predilection for rusty projects! The wire brush is an excellent tool for loosening and removing rust or loose surfaces. It's a tool that gets a lot of use on my collection of scruffy vehicles but also has been used to remove flaky masonry paint and more. It's not a refined tool and I tend to go for the budget end if possible as they are almost by definition destined to be abused. 

Bonus content! - Hackspace Magazine Lockpicking Articles

I've recently written a 2 part article in Hackspace Magazine about locksport and lockpicking, the first part (issue 48) covers the basics around lockpicking practice and essential stuff such as the golden rules, never pick a lock you don't own and never pick a lock you depend on. The second part (issue 49) looks at DIY lockpicking/locksport tools where we made some picks, tension tools and a practice board. Occasionally when I am writing this type of piece I have to edit out sections to meet the word count requirements of the article, in this instance I culled a section on how to re-pin a lock. I'm posting it here as it's a really useful skill that can mean you need fewer practice locks and can get multiple setups/pinnings per lock. 

Re-pinning a lock is a useful skill that can enable you to create a new challenge out of an existing lock that you have got used to picking. Essentially you are removing the centre core of the lock and removing the pins and replacing them to make the lock a different challenge. A standard 5 or 6 pin tumbler lock has the core of the lock retained with a C clip in a machined groove and this can be removed with a pair of pliers. Once you have removed the clip, be careful to not pull out the lock core until you are ready as all the pins will fly out and potentially get lost. You can’t remove the core until the lock is in the unlocked state. If your lock currently has a key that fits you can insert the key and then unlock the lock, but only turn the key an eight of a turn. With the key turned rotate the whole lock assembly so that the key is now vertical such that if you removed (but don’t do this yet) the core the key pins would remain sat in the now vertical channels in the core. A plug follower is a dowel, or indeed a 3D print in our case, that matches the diameter of the lock core. These are quite commonly half an inch in diameter. The plug follower allows you to remove the lock core without releasing all the pins at once. Whilst removing the lock core hold the plug follower flush with the far end of the core and feed it in as you move the core out. If all has gone well you now have the lock core, with the key inserted, and all the key pins still in the holes and you have the lock body with the springs and driver pins still in each channel hole block by the plug follower. Cover the end of the lock or place it flush onto a table top and then slowly retract the plug follower from the other end. You should hear each driver pin and spring release and once the plug follower is out you can lift the lock body and find all the springs and driver pins safely on your work surface. Re-pinning the lock reverses this process. Note that if you swap the key pins to different locations, making a new challenge from the same lock, your key will no longer fit or operate this lock. You can, of course, pick the lock into an open position and re-pin it back to work with the key.

To re-pin the lock place the plug follower at the back of the lock and with a pair of tweezers carefully place a spring into the hole furthest into the lock, next grab your driver pin with the tweezers and place that pin over the spring and push it until it is slightly seated into the hole. Next push the plug follower into the side of the driver pin, you should be able to trap the driver pin, binding it on the hole edge and then use your tweezers to push the pin fully down into the hole, you can then slide the plug follower over that pin to secure it and move on to repeat the process for the next pin. Finally once all the driver pins are replaced you can place the core, containing the key pins, back into position, lock the lock by rotating the core and then re attach the retaining C clip.

Tool Tuesday - Bahco Hacksaw

The dedicated reader might have noticed that Tool Tuesday has been quieter of late. I decided that I would move from every Tuesday to a more relaxed every now and again approach, hopefully equating to around one Tool Tuesday a month. The idea being then I also do more blogging on personal projects.... well never say never!

Anyway today's Tool Tuesday is this excellent Bahco hacksaw. Whilst a great step up from a no brand cheap hacksaw, this Bahco sits firmly in the middle ground and is available for around £12-16. It's a very nice frame and handle, comfortable in hand and the blade tension bolt and handle are excellent. The black moulding on the far end works well for a guiding hand and some pressure application and as such it returns very accurate cuts. What really makes this hacksaw shine though in my humble opinion are the Bahco Sandflex Bi metal blades. I've yet to break one and they seem to last a very long time. Mine is due a new blade but I can't remember the last time I opened my spare blade pack to replace one which tells me all I need to know. 

Tool Tuesday - Record Ball Peen Hammer

I mostly associate Record with excellent engineering vices but it's fair to say they have had their branding on a lot of tools over the years. I've had this Record branded ball peen hammer since my mid teens (late 1980's) and it's an excellent bit of kit. Sadly the model number is obscured/worn but reading around a lot of this era Record hammers are actually rebadged "Marples" hammers that are very well regarded. The castings on the head are nicely made and the hickory handle is still the original, despite a fair amount of use and abuse. It's at the lighter end of ball peens but is perfect for small engineering work, stamping and centre punching etc. If you ever see one second hand I'd say grab it for a quality little tool. 

Tool Tuesday - Telescopic Bore Gauges

 

Today's Tool Tuesday is a particular favourite set of tools as they often are my first port of call when beginning to design or build a rocket! I often need accurate measurements of internal tube diameters and the Telescopic Bore Gauges are an excellent approach. 

Each gauge covers a range of internal diameters and features 2 sprung probe sections that can be pushed back into the centre body of the gauge somewhat. In the handle mechanism there is a spring pushed bar which when you tighten a knurled nut at the back of the handle will lock the 2 probes in their current position. To get an accurate result you push the probes into the desired internal bore and hold the gauge at a slight angle and lock the probes to finger tight. You then pull the probes so that they span flat across the bore to bring it to it's final measured size.  You then remove the bore gauge and use an accurate device to measure across the jaws of the probes, a well calibrated micrometer is best but often a vernier caliper across the probes is sufficiently accurate for most needs.  

Building a Doritos STAX rocket!

Photo Credit Peter Barrett

I'd never built an "odd rock"! Odd rocks are a category of rocketry reserved for those who create rockets from odd materials or make odd objects fly. I've seen loads of examples from flying cars to an office fan but it's never been an area I've delved into. At the milder end of odd rocks there has been a long history of people making rockets using confectionery packaging and in particular Pringles tubes. Pringles tubes are a very worthy rocket building material but they have been done a lot and I wanted to do something different. The Dorito's STAX tubes are a pleasing rounded triangular shape and I thought it might be a worthy candidate for a build. 

This build was very quick and dirty and everything was done in between work and other commitments and definitely took a back seat priority wise to other rocket projects. I started by drawing around the tube to get a rough sketch of the tube profile and took a picture of it on my phone. I dragged the image into inkscape and then hand traced to get a vector of the tube shape. The tubes aren't quite an equilateral triangle which adds a bit of faff! I laser cut some plywood centring rings into which I cut a hole to receive the motor mount tube. This was finished off with a PETG 3d printed motor retainer. 

The fins are 3mm balsa laser cut to a shape that kind of looked a bit dorito-esque! I originally planned to put the fins through the wall but ended up just gluing them on. I reinforced the oversize fins with a layer of tissue paper and glue. Due to the shape of the tube this rocket is impossible to simulate using a package such as openrocket. As such I decided to go very large with the fins to try and definitely pull the centre of pressure rearward in the airframe. The nosecone offered some challenges and I didn't really have time to create an accurate fitting cone. As a first attempt I decided to model a shoulder-less nosecone which would overlap the top of the tube somewhat rather than trying to accurately model to the tube. Having my rough nosecone model in FreeCAD to 3d print I then created a stack of lasercut plywood triangles to create a shoulder piece that could be sanded to a decent fit inside the tube. The recovery bridle was some thin kevlar and I flew this using a proven hexagonal annular parachute I'd made some time ago. Finished with a bit of satin black spraypaint and a quick vinyl cut graphic, DSTAX was finished! 

Photo credit Peter Barrett

Due to this not being sim'able, once together I decided to add around 18 grams of nose weight (some small flat lead panels epoxied into the nosecone bulkhead) and this seemed to make the airframe stable when performing the classis "swing test". The swing test is where you attach a string at the centre of gravity (with the airframe loaded with a motor etc) and then swing it in a circle, it should correct itself slightly into the wind and then remain stable, an old school approach that works well to give you some indication of the stability.  The other consideration was the maximum lift off weight limit of the target motor, an estes D12-5, which is 283 grams. All up weight of the rocket came in under the maximum at 224g. 

As you can see in the video the first light was successful albeit with a slightly late deploy of the recovery system. I think it's a little over stable by the way it turned into the breeze but it's certainly within acceptable limits. If I built another I would consider reducing the ridiculously large fin area somewhat!