There’s been relatively little site activity lately, mostly because I’ve been diddling with minor and other in-progress projects, which will be detailed soon. I have a lower threshold of build report typing with respect to effort, so unless I’ve done something worth writing about or just stacked up enough work, I don’t update the site.

Testing Fankart has shown me that the battery was the limiting reagent. I’ve been using a 10S (37 volt) lithium ion battery from an old electric bicycle kit that we disassembled long ago at the Media Lab. It was clearly designed for long range, but low power operation, since it had all of 18 gauge leads. This was a battery with 6 18650 lithium ion cells in parallel as a single “cell unit”, so I know it can certainly dump more than enough amps to overwhelm the small wiring. In testing, the battery voltage routinely dropped to 30 volts from a full charge of 42, while flowing 60 to 70 amps in bursts.

Lame. I’ve been in a pinch for batteries lately – this same lithium pack, in fact, is swapped between Fankart and whatever else might need 10S at 9Ah, which includes two of my “minor supporting cast” EVs, the minibike and the heap-scooter (so called because it is a heap of shit). I also had another 10S, 9AH lithium polymer, cube-shaped pack, but that has been put out of commission after one of the cells went completely bad.

So what am I to do? Well, for the past year or so, our EV clique has been sitting on top (and besides, and between) a sizeable quantity of lithium iron-phosphate cells (the DeWalt drill batteries that everyone keeps hacking apart) which were originally a donation to the legitimate MIT Electric Vehicle Team (not to an il-legit experimenter like me, of course). I detailed some of this last year back when LOLriokart first gained unwarranted Internet fame. I had a box full from the “sampler plate” the Media Lab and EVT were originally given. It had been parked on top of my EV parts box since then.

What a box of the aforementioned batteries might look like.

I’ve nicked some of them for Cold Arbor and Überclocker, but the thought of making a large pack out of them was still one relatively easy to suppress, since they were known to not be the highest grade of cells. I’d have to make sure they were matched in characteristics (or just not plain dead like 5 were to start with!) before puttting together a nontrivial pack, else I’d end up with LOLrioKart’s never-ending battery tragedy again, except this time with more alkali metals.

But I finally caved. A resource which most DIY engineers would kill for just sitting in a box is totally not me. The usage experience of peers and The Internet At Large has shown that LiNP batteries aren’t nearly as fragile and temperamental as lithium polymer packs. They don’t like to be babied and trickled charged and balanced like lipolies – or at least, careful battery management system design is less critical for casual use. Plus, my awesome charger can charge and balance up to 10 LiNP cells at once anyway. I knew I bought that thing for a reason.

So I took the dive.

This is \M/ËTÄLPAKKK, so-called because it’s TOTALLY \M/ËTÄL.

I had only one design goal with this battery pack – that it must equal or exceed the watt-hours stored in the electric bike pack. 12S LiNP cells is roughly 10S lithium polymer. Each cell is 2.3Ah, so 4 in parallel would be approximately 9.2Ah. But my charger can’t care and feed for a straight 12S pack, and it would also be unwieldy because of the size. Thus, I decided to split it into two 6S4P modules.

Well – two design goals. These modules should also fit in the minibike. You probably see where this is going.

Above is the pack in mid-construction. Each vertical column is 4 cells in parallel acting as a metacell. Cell selection and arrangement was a quasi-scientific process involving the following:

• Group the cells by open circuit voltage, in clusters of 8. I made 4 “voltage groups” comprising cells which measure 3.30-3.33 volts, 3.34-3.36 volts, 3.37-3.40 volts, and over 3.4 volts. There was only 1 “over 3.4v” cell, and I had to put it aside as an oddball.
• Put the 8-cell clusters into a custom cell-holding rig which had balance and charge leads, and wait for the charger to do its magic. The cell-holder was originally designed with 8 cells in mind, which is why I used groups of 8.
• After the cycle was over, divide the cells into subgroups of 4. The voltage differential between the cells in each subgroup were no more than .01 volts. Legit cells are nice.
• Cram each subgroup together using Automotive Goop and leave to set.
• Goop 6 subgroups together

I used my favorite interconnects – grounding braid – to solder the cells together. Soldering these cells is a bit of a shady practice, but by using an 80 watt, huge chisel tipped iron, I was able to keep the “dwell time” per cell under 3 seconds, then move on immediately. It’s never good to park a soldering iron on a  battery, and on lithium batteries in particular, excess heat could melt the polymer separator which keeps your cell from shorting itself internally.

Touch and go, touch and go…

A 6 cell pack needs 7 wires on its balance lead. Luckily, MITERS had a ton of little 7 pin headers that fit perfectly into the charger’s balance port. Add a dab of rainbow wire and you have… I don’t know, gender-diverse batteries?

After adding the balance lead, it was time for closure:

I am fond of the 2 liter soda bottle school of battery pack DIY. In this case, I actually had to buy (and subsquently empty out in a productive if unhealthy fashion) three-liter soda bottles.

The bottles are generally blow-molded polyethylene, so they shrink significantly under the influence of a heat gun. After it cools, the bottle material effectively becomes a hard shell for your pack!

As usual, I added foam rubber padding to the top and bottom before shrinking the bottle section around the cells.

And suddenly, TWO \M/ËTÄLPAXXX. In all, this took about 2 afternoons and evenings of effort. Check the dual 10 gauge leads and 8mm bullet connectors for when I need to sink some more amps.

After I finished each pack, they were put on the charger/balancer and left to ripen for a while.

For common sense reasons, I used only female side bullet connectors on the modules.  But if I were to connect them in series, I’d need to join two of those together.

So, I made this bullet-bridge using 2 male side connectors soldered literally back to back.

What’s next?

put it in fankart

I changed out the prop adaptor bolt to one which was drilled straighter. For the longest time now, the hub has a mild but disturbingly visible wobble because the original prop bolt. The second bolt I made was much straighter because of better machining practice. Past that, the electricals remained the same, and the fan was re-taped to the underside of the cart body.

Best run yet! The \M/ËTÄLPAXXX held at 41 volts under 75 amps of discharge, which beats out the old lithium ion pack by far.

Knowing the no-load voltage and the under-load operating point means that I can sort of calculate the system resistance. The batteries dropped from 44.6 volts to 41 volts (3.6V drop) while flowing 75 amps, so the resistance in the system is 48 milliohms. Not bad.

and then the whole thing

Boom.

I was just barely out of the Plane of Interdiction before the duct shattered into a few large pieces of PVC shrapnel.

The failure was almost instantaneous and occured at about 75% throttle. I never got a RPM reading on the propeller blades, so all I can say is that they sounded “much faster” this time around.

The impact knocked the two props in line from their original 60 degree offset!

And the aftermath from the top.

Theories about the failure range from deformation of the props themselves at high speed such that they hit the duct (which seems to me like that would result in a prop failure too) to the duct hitting a vibration resonance and deforming cyclically enough to slam into the propellors (my favorite) to the duct being deformed by the change in air pressure alone (seems possible, but out of my realm of comfortable guessing).

Or it could be the fact that the whole thing was suspended with tape.

Lots of circular scrape marks around the perimeter and a weird red powder residue… neither prop nor duct is red in color, so what the hell is going on here?

the end of fankart?

Fankart might return later, built with more engineering and aforethought. But for now, the damage is terminal enough for me to finally put it away and start working on more productive and meaningful things again.  Like finding another home for my 12S4P giant ass-battery… I wonder where it could possibly go.

In the mean time, you should friend Fankart on failbook.

Fankart has turned out to be a concentrated form of the kind of projects projects I’m used to building. It was originally built as an engineering joke, it’s only been existence for a week, it’s utterly useless, and it’s already more internet-famous than it deserves. I mean, even LOLrioKart took a full year before people finally noticed how utterly useless it was. Then the Internet fame is at least somewhat warranted.

And RazEr has never been Internet-famous because it’s actually somewhat useful..

I don’t know what I keep working Fankart. Probably because the time delays between when I ordered parts and upgrades to when they actually arrived have all expired within the past week, so I had an essentially continuous stream of resources to keep working on the HFFan. In my opinion, the HFFan in its latest iteration (to be detailed, of course) has almost reached the limit of what I feel like building without actually engineering something. The whole point of the HFFan, originally, was to see what I could pitch together given only McMaster, Hobbyking, and laziness. To that end, it has accomplished more than I had planned.

I got a set of 16×10 propellers from Hobbyking along with my latest impulse purchase. The accidental perspective makes it harder to discern that they’re larger than the 13×8 props in the foreground – but rest assured, they’re bigger.

The idea is to cut these back down to ~13 inches in diameter such that the portion left retains most of the original’s steep pitch.

Oh, about that impulse buy… Remember when I was talking about the Hobbyking 80/100 “HKrunner” before? They are so rarely ever in stock that to actually see a positive number on their stock count is like sighting a carbide-tipped parabolic flute unicorn. But last week, it finally happened – by the time I saw it, there were six left.

Along with motors #3 and 4 for the HFFans, I snagged two HKrunners. The shipment at this point was probably a few kilos short of when it had to go ocean mail, and shipping fees came out to $100 alone.

My bank probably just shit itself and wiped using my checking account. But, now I have two HKrunners.

What ever will I do with them?

HKRunners aside, I began on boring out and trimming the propellers.

Problem: Even the largest lathe I had access to at the time couldn’t…er… swing the props, since they were too large in diameter. And so, I had to cut the tips off using a bandsaw first. The line indicates roughly where “13 inch diameter” is.

Then came the drill-to-12mm process on the heavy \m/etal machine.

The fine trimming process was the same, except this time I used a straight-fluted cutter on the highest speed to avoid the up-and-down flapping of the prop blades that had occurred last time.

The straight-fluted cutter in question was actually a reamer.

… and the new props drop right into place!

The tips were trimmed to less than .02″ clearance this time. Legitimately, I mean – last time, I forgot that endmills have diameters, and came up 0.050″ too short!

Here is the HFFan with the new propeller setup and a new lower mounting position! It turns out that the whole thing fits snugly between the uprights on the cart frame, blocked from forward movement by one of the basket spars.

The same fiber tape I had used to retain the duct in arrangements past sees a return here, pulled as tightly as possible. The whole setup is reasonably stiff, despite not looking like it should be.

Hey, at least now the basket is empty and ready to accept groceries!

The rear view.

Note the scrape marks in the duct – it turns out that even though trimming the props to sub-millimeter clearances was done with the best of intentions, the high-tension tape mounting still causes the thin PVC to deform some.

I decided to not play the tension adjustment game and just let the prop break in its own duct by running it at full bore until the skull-grinding noises stopped. Now that‘s engineering.

The heavier pitch, longer chord, and tighter duct are all reflected in the increased power draw. Clearly, the 18 gauge battery leads are the bottleneck in this system – the voltage drooped to 31 volts under the highest load. Should it have held relatively steady, I would probably have seen the 3 kilowatt mark.

steering the fankart

This is failrudder.

After I got sick of retying the front wheel knots (or having them retied), I started throwing together other possible solutions for steering. One of them was this dead-fish-esque rudder, made by zip tying a cut piece of copper-coad fiberglass board to a servo zip tied to the frame.

No, it did not work. At all.

Next, here is differential failbrake. Real airplanes generally use differential braking force to turn using their propellers or jet engines (for single engined planes), so I figured why not try it on fankart?

Well, it would have  probably worked if the brakes weren’t raw servo horns, the servos weren’t mounted with zip ties and tape, and the wheels didn’t have so much slop they could tilt 10 degrees off axis and still rotate. Those are some seriously worn-ass wheel bores.

All things considered, I just decided to retie the damned servo knot to test Fankart 3.

So here it is – the collection of test video from the past two days or so!

…

Yeah, that didn’t end well. Something about “night time and shadows obscuring the unforgiving curbside of life”.

There was no major (or expensive) damage to anything. The HFFan just flew off, and surprisingly, all the electronics stayed in the basket.  Maybe I’ll throw it back together and actually try testing during the day some time… New concept, I know.

What’s next for Fankart?  I’m not really sure, but one of things I still want to do is get an actual thrust number on the HFFan. If it’s two digits, I’ll consider making more. If not, it’ll become the next high-five machine. I’m pretty stoked by the fact that the latest HFFan could accelerate the whole thing up a roughly 6 or 7 degree ramp faster than the previous could accelerate on flat ground.