Chibikart’s New Unobtainium-Free Sibling

chorl6 Comments

Hi. I’m building another Chibikart.

But this one will be just a little different. Historically speaking, most of my projects have involved a little bit of “unobtainium” – or perhaps to put it more accurately, an Unobtaining Machine. As well documented as they tend to be, they are admittedly difficult to reproduce by anyone outside of a university or similarly well-equipped makerspace setting, the reason being that they tend to involve much manual or CNC machine work; waterjetting, laser cutting, precision boring, etc.. I’ve generally made a attempt to explain the processes involved in addition to investigating ways that things can be built by more people with different resources, but it isn’t usually the focus of a project.

I support the methodology of getting up and building something as an effective way to learn engineering. Part of my research here (if it could be qualified as that yet) is to engage more  students in learning engineering and design concepts using electric vehicles – it was the whole motivation for 2.00EV this past semester, an experiment that I consider to have been successful (and useful as a data point to work from for future iterations of the course). Working with a more life-sized system, in my view, makes you understand the mechanical engineering concepts commonly taught during the 2nd year such as bending/flexing, bearings and constraints, power and work, etc. more than building a small robot would. The numbers are in a range which make more sense to think about – you understand more what 10 pounds-force is like than 0.1 lbf, or why your vehicle can’t have 1/4″ aluminum axles. You can’t bend 1/2″ aluminum vehicle frame plates to suit your misaligned bearings, but 1/32″ sheet metal robot frames just don’t illustrate that well.

And that talking in the customary units system is fun and all, but Charles will make you write everything in SI because things make more sense that way.

While “2.00EV” engaged the students in learning the machining, designing, and analytical skills  that MIT typically expects out of sophomores, I’m wondering if a slightly detuned version would be accessible to more people – younger, older, different university, or just out to learn & build.

Through the activities of the Collegiate Silly Vehicle League, we have both discovered and created a sizeable array of resources and links for collecting knowledge about buildables. Many of the vehicles that we have built (and zip around on) have their own web pages and long, detailed build reports – so many that listing them here is going to be impractical, so feel free to click through the links in the left sidebar. Some of the principles and sources of parts have been amassed together – for instance, the ‘scooter instructable‘ and my very own ‘hub motor‘ instructable. These have all been guides more so than telling people how to build one specific thing in the spirit of Instructables, but both their 5 star ratings and the emails I get related to them speak alot. Some times, knowing the resources is all the push that someone needs to get started.

Despite pushing several billion dollars a year in revenue, very few people know what McMaster-Carr is, or that they deal directly with end users and customers. When they find out for the first time, it’s quite daunting because there’s everything there and how are you going to find the thing you need? Shopping on McMaster (and related enormous catalogs like DigiKey, for example) is a skill which is most definitely not taught here at MIT, at any level, and I’m definitely not okay with that. It’s something you have to pick up by going above and beyond the call of handing in problem sets. That’s why I made it a point for students this year to order things from McMaster that they found by themselves, but I digress.

Very few people anywhere also know that you can in fact have parts custom made for you by places like Big Blue Saw and ShapeWays, just to name two. There is an interesting “uncanny valley” of personal fabrication here – the general nontechnical public, of course, does not know about the existence of it and online rapid prototyping services. Yet the university and academia crowd also tends to not know about it, from my surveys, probably because if you needed an Unobtaining Machine it is usually right there. In the middle of that is the realm of makers and amateur engineers for whom these services are a boon. (The other big reason is that personal fabrication is still quite new, so it’s unfair to equate it to established systems of multi-shop quoting and contracting machining services.) Oddly enough, even as some people complain that infinite waterjet is the hard part of my projects to replicate, it is actually the milling and turning which will stump most people – there is as of yet no equivalent fast service for custom 3d-machined parts.

I also believe it’s important that you not immediately drop the textbook on people from the very outset. I sometimes betray a disapproval of ‘academic engineering’ when I talk to people, a fact that I used to be ashamed to admit in a place like the MIT Department of Mechanical Engineering. About 10 years ago (…. whoa…) I started out building things from the opposite end of the spectrum that academia is used to – I didn’t learn mechanical physics, structures, beam bending, electric motor dynamics, etc. first and then go out and build a robot – I started throwing junk together and then wondering why it didn’t work, then went to local robotics clubs and tournaments and asked people where they got parts from. Alot of reading online (and real books! Even in 2002 the Internet wasn’t as everywhere as it was today). While that sounds like alot of “Well it worked for me…”, I understand from personally knowing many people who also became interested in engineering early and then subsequently were driven to excel that relevant experience is of serious value even if you cannot explain fully to a professor why something works. You incrementally… build… both experience and knowledge, which are two important qualifiers of competence people conflate alot.

Building for your own satisfaction and learning is also discredited alot – both academia and the curiously onlooking public contribute to this effect. I’ve been asked countless times why I built something when it’s already on the market, or already been invented or done. I’ve been asked “how is it useful?” both facetiously and seriously. Other students I have known in the past years have told me they don’t really build projects because they don’t think they can invent anything new. It’s a hard mentality to change, especially in academia where cutting-edge research is going on all around you and you feel pressured to take as many advanced classes as possible to get to that level so you can contribute. With the growth of MITERS and awareness of projects on campus in general, I hope this is something that will improve in the future. No matter how much we joke about the MITERS “build fads” – robots, scooters, motor controllers, quadrotors, tesla coils, go-karts… (roughly in that order actually, from my recent memory) the fact is that one person starts out building something and their peers look on, go “hmm, I want one too”, and many people benefit from the learning experience in the end. Even though we’ve made literally 5 of the same thing like 4 times over. So what? There’s now 4 more people who can, for example, more confidently design a power converter or mixed-signal closed-loop controller than who could before.

Is it just me or do I insert a long rant or philosophical essay before every build post now?

So, finally getting to the point. What does “detuning” mean for this project? It will mean fully documenting online the build process of a small electric vehicle using commonly available but modern parts and a reasonable set of garage tools and hardware store purchases, while utilizing available digital/online/personal fabrication resources. It will not feature cutting-edge science nor attempt to teach principles of engineering simultaneously. The goal is a fun buildable project of more difficulty and exploration of the hidden underbelly of engineering than a trip to Home Depot will net someone. Again, some times all you need is the resources and the technique – I’m guessing individual sections of this documentation will be way more useful to more people than the whole project is.

Now, Chibikart is probably the worst possible project to do something like this with. First of, it’s not going to be cheap at all – it’s not a weld-together-a-steel-frame-insert-scrap-lawn-mower-engine-here kind of project. It’s also substantially more complicated than the aforementioned WTSFISLMEH go-kart. I decided to pursue it, though, both because it has novelty factor (while it may be impractical it’s fun as hell to ride around), is made of a pretty adjustable/extensible platform (80/20 extrusion), and because it is harder. It’s abundantly clear to me that not everyone will be able to pick up and do it, but in my opinion, to throw it out there is better than just relegating it to a build category in my sidebar that I will occasionally linkdump to.

Let’s set the ground rules:

  1. “fully documenting online the build process“. I will use Instructables and create a step by step document that shows the vehicle’s construction from start to finish. This is a departure from my usual style, but I will try to keep things as open ended and customizable as possible.
  2. “a small electric vehicle using commonly available but modern parts.” No old car batteries, galvanized frame tubing, and hacked wheelchair DC motors. This will be brushless, lithium ion, aluminum framed, and powerful & lightweight using parts that can be purchased new. Much McMaster, Hobbyking, and Surplus Center (with other minor vendors) will be involved.
  3. “a reasonable set of garage tools and hardware store purchases.” This will be the biggest constraint on technique. I will limit the use of tools to those which could reasonably be found in someone’s garage, or the average high school builder had access to. For me, it was a drill press, hand drill, Dremel tool, a fairly common 29-piece drill set (couldn’t afford the 115 number-letter-fraction set!), and a set of hand tools including some wrenches (not a whole set), pliers, cutters, and limited taps and dies.
  4. Finally, the kicker: “utilizing available digital/online/personal fabrication resources“. The limitations set forth above may make it sound difficult, but this is the part where I get to essentially pick and choose the fabrication exercises. The advent of on-demand waterjetting means that nothing substantial has to really change from Chibikart’s design. Like its conceptual predecessor tinyKart, Chibikart is a pile of 80/20 and waterjet-cut aluminum plates that I really didn’t have to do that much to in order to put together. I’ll publish the flat layout files publicly such that anyone could conceivably Big Blue Saw a copy.

the interesting part

I should be clear that this Chibikart is not like the current Chibikart. In fact, I took Chibikart’s Inventor assembly file and pretty much ripped everything out of it, and the first things to go were the hub motors. That’s right – no more hub motors, since as much as I would love to corner the market on small hub motors, I don’t have the time or patience yet.

We start with a redesign of the back end, now featuring indirect drive. The horror.

The motor is a Turnigy Sk3 5065. It’s pretty weird for an outrunner, since it’s long and skinny, but it’s compact. I chose this largely because it was left over from 2.00EV – I had purchased a “sample plate” of motors at the start of term for students to fondle. It should be more than powerful enough for this purpose.

The Jasontrollers are staying. Even though I’ve seen that they are not very good at starting low inductance and resistance loads like R/C outrunners, I’m interested to see how well they perform at this scale. My experience is with Melon-sized motors and 8″ tires, and the very small 4″ wheels (those are staying, because lol) will give a comparatively larger force at the ground. The reduction is also pretty high – 2.5:1 as designed, resulting in a max speed of 20mph as limited by the Jasontroller.

Note the “live axle” suspended in bearings – this is actually an outdated design that I scrapped in the interest of making people build as few live spinny things as possible. I couldn’t figure out a good way to couple sprocket to wheel except through risky bore-shimming and drilling of steel sprockets in precise alignment with the wheel bore.

The updated method takes a page from Jamison and uses a sprocket cut from 1/8″ aluminum. I hope to avoid needing as much “drill chamfering” since the material is closer to the native width of the #25 chains. The wheel is now a 1.25″ wide, 4″ diameter Colson wheel, of robot fame, because of their softness and pliability in the face of cutting tools. I picked the ball bearing version, and the bore of the sprocket hub is designed to be two soda can wall thicknesses larger such that a shim can be inserted for drilling the bolt holes, then removed for smooth rotation. This is risky, and I have to build one soon to make sure it can work.

Not being able to do this whole “precise right angle hole” thing means that I need to think of creative ways to retain a right angle dead axle. I used methods commonly seen in the DIY 3d printer universe here – trapping the hex head of a bolt against an internal pocket. Once the front wheels are cranked down with a nut, then the bolt head will have to strip the entire aluminum thickness around it to fail.

Because I only have control of material in Z resolution of 1/8″, I decided to make ‘frame thickness spacers’ such that I have a full 1.25″ between the uprights. Enough to put in a few bronze washers to space the wheel block away from the bearings so there can never be a case of my aluminum parts wiping off on eachother like I experienced with Chibikart’s first steering arrangement.

Up front, I’ve changed the big mill handle to a waterjet-cut yoke, since it’s actually quite hard to interface with a 3/4″ hex shaft. The lovely flange-bolt shaft collars make a return, and they will be the adjustable hubs which hold the steering components in place.

Working out the brakes took the whole weekend. Mostly because I wanted to avoid an excessively complex solution like Chibikart’s eggy cam thing. I was also kind of stuck on the mechanism to use, until I started staring at bike parts at Cambridge Bicycle across the street from MITERS. The result after some more thinking is this half-of-a-single-pivot-brake dealie that uses a stock road bike brake pad and stock brake cable mount. In this case, the cable sleeve actually moves too – usually I’m used to seeing fixed cable sleeves. The modeled spring thing is commonly found with scooter/bike brake part kits (I plucked a random one off McMaster to make it complete the look).

And the outrunner can is the brake drum. Isn’t this a wonderful idea? I decided to not do rubber (pad)-on-rubber (colson) because the engagement would have been less predictable, it would have eaten the colson pretty fast, and the motor can was just right there. This is not possible unless the motor has a “can bearing” – the brake pad is basically pressing right on top of it, so there’s no forces which can cause the can to become misaligned.

Oh, and during this process I found out what all of those random little brake parts are called. For future reference, the cross-drilled screw doohickey is a “Brake Cable Anchor Bolt” and this tubey thing is a “6mm Barrel Adjuster”. I went through so many iterations of “brake cable adjusting tubey thing with the nut on it” or similar search terms before finding the correct looking object.

On the other end is a custom brake pedal. The Jasontrollers’ don’t have regenerative braking anyway, and so having the left Hall sensor pedal on Chibikart was kind of silly. It’s also a bit fragile, and the pedal plate actually bends significantly since it’s made from some seriously bullshit steel. Having it as a mechanical brake pedal was therefore suboptimal, so I got rid of it in favor of a stronger aluminum one.

This design will use two of those cross-drilled-anchor-bolt-thingies and pull on both brake cables simultaneously. It does not have a ‘wiffletree’ or force splitting hinge, so some adjustment of the tension on either side (using the tubey-things-with-nuts) will be needed. For rear wheel braking, though, it’s not as bad because unbalanced brakes aren’t as likely to jerk the vehicle around as much as having unbalanced front brakes, which might cause it to pull suddenly in one direction.

One minor change to the steering – I’m reconfiguring the location of the collars and bushings a little because of the flange-bolt collars being annoying.

They use clearance holes for a #10-32 screw which are actually tapped holes for 1/4″-28. Well, nothing else on this design so far uses #10 screws, and it would have required a 3rd tap size (#10-32 or #10-24) since the collar’s holes are clearance (necessitating threads elsewhere). To avoid the need to do this, I elected to expand the holes in the linkages to be 1/4″-28 clearance so the tapped holes in the shaft collar can be used. McMaster conveniently sells 1/4″-28 screws for cheap in packs of 10.

The protruding screw heads necessitated a bit of rearrangement. I actually like this better, because currently the lower shaft collar is the lowest point on Chibikart and it is the first to hit anything.

So here’s where it’s at so far. It still looks vaguely like Chibikart, is still the same size (34 x 26 at the outer ends of the wheels), and the estimated weight is 28 pounds without hardware (so maybe 30-32 with hardware, wiring, duct tape, etc.)

Through a bit of late night Wikipedia jiggling, I’ve decided to christen this the Democratic People’s Republic of Chibikart. Because like North Korea, I swear this is for everyone to build and learn from but the Instructable will just mean it’s me telling everyone what to do anyway.

(Occupy Chibikart was briefly considered as a name before I realized that DC motors and lead-acid batteries are more 99%-ish than brushless and lithium)

More details to come. Parts are due to arrive in massive amounts this week, and I want it rolling by next week.

Beyond Unboxing: K2 Energy 12v Lithium Iron Phosphate Lead-Acid Replacement Bricks

chorl1 Comment

Whoa, it’s back! Another episode of Beyond Unboxing, the series of random curiosity-driven posts by me which has so far shown light (literally speaking) on the inner workings of a few shady motor controllers. This time, I explore what is going on inside a commercially available generic lithium ion based “lead-acid replacement” – a battery made of a bunch of lithium iron phosphate cells hiding inside the shell of a standard-size lead-acid battery, with (or….without?) a battery management system to make them act like a plain lead acid battery.

Why would you want such a thing? Reduction of weight, longer lifecycle and shelf life (1500-2000+ charge cycles compared to 500 or less), no lead or acid, and higher power density and efficiency under high discharge loads, among other things. First, a little bit of philosophizing and rationale, though:

I’m looking at them as a possible prepackaged lithium ion battery solution for hobbyist & amateur electric vehicle construction – it’s a solution that is less white-hair inducing to suggest to people than “Oh, you buy this shady hobby lithium ion battery with no hard shell or output protection and use this rather complex multi-purpose charger to charge it, occasionally making sure to use the ‘balance’ function so your battery doesn’t explode”. While that solution is perfectly workable for someone with some technical ability or experience in the field, a plug-and-play solution can reach a wider audience.  I can reasonably assume that the people I see day to day in the engineering departments can handle such instructions as “use this battery charger on this setting and don’t plug it in backwards”, for instance, but from experience with the average curious audience member at the Maker Faires who wants to build a vehicle, it is not something I would suggest immediately without gauging their technical experience more.

Batteries are one of the last finicky items on the list of cheap & repurposed EV parts that we have compiled here over the years – I consider the problem of motors and controllers to be well-solved, especially with things which are coming down the pipe locally, but the answer to “What battery do I use?” has always been sort of difficult. The first response is the one stated above – hobby equipment and soft, plushy lithium pouch cells. Workable? Absolutely. Unforgiving if you are an idiot or just don’t pay attention for a second? Definitely. The second class of answers usually centers on chopping power tool batteries, e.g. DeWalt 18v XRP or 36v lithium ion batteries, which are nice because they already come with the charger for the hapless drill or saw you are about to dissect.

For example, the DeWalt DC9360 36v pack, popular with electric bicycle hackers, is only 2.3Ah and $150 (about $2/Wh), not including charger, but you either have to find an interface for the proprietary DeWalt drill connection or modify the battery/solder discharge leads yourself. They are by far the closest to plug-and-play generic battery solutions I’ve seen yet, though. In terms of modern Li battery solutions for small vehicles, there’s a clear trend between cheapest but highest user experience required (hobby and R/C batteries, chargers), and most expensive but plug and play.  There’s always the classic fallback of nickel cadmium or nickel metal hydride batteries – easy to charge, easy to use. But as it turns out, a good NiMH pack is actually as expensive, if not more expensive, than a lithium battery of the same watt hours these days. It would seem that the magic Chinese manufacturing cloud has largely moved towards producing cheap lithium.

Now, the other big question is, do I really want everyone to be scurrying around on Chibikarts? Probably not, but one step in pushing the construction of electric vehicles as a…… vehicle…. for engineering design education (I’m going to use that pun so much it’s somehow going to end up in my thesis, just watch…) is making sure that more people can do it, and there exist many different starting points and “upgrade paths” as you go. It’s similar to what happened to the robot fighting/combat robots community that I’ve been part of since the beginning of it all for me over 10 years ago: Starting out as a niche sport for people with tons of money and expertise with access to expensive tools, and gradually having commercial solutions and clever hacks for common problems emerge and letting more builders participate.

Anyways, rant over. Onto the relevant subject matter.

A product exactly like this, the A123Systems ALM12V7 module, is what powered this year’s 2.007 EV section to victory:

They are pretty awesome, I’ll admit. Externally accessible ATO fuse, automatic charge cutoff, automatic discharge cutoff too. These were pretty much foolproof, and fainted like Pokemon when the fuse blew or they were overdischarged. Really, that was the word we settled on describing it – when the battery shut off its outputs due to undervoltage or fuse tripping, a brief application of charging voltage would wake them right back up.

But what I found disappointing about them is that nobody else can get them. We were lucky to get them as a sponsorship / donation, but just typing “ALM 12v7” into Google shows alot of press releases, datasheets, and articles, but no product. This is consistent with A123’s (and most American companies’) position of not dealing with the public directly, something which kind of irks me a little – no matter how much I love the fact that they are an MIT-affiliated battery company that regularly throws prototypes and QC line rejects at us for our own consumption, it’s not reproducible in a setting which is not us. Kind of hard to test your hypotheses then, huh?

They are not the only producers of these lead acid replacements, though. In fact, here’s a whole page of them.  And another.  And that’s just two of my favorite shady Chinese back-of-the-truck battery dealers, out of many! It’s my firm opinion that A123 has already lost the game here – when the 12V7 finally hits the market, they will be seen as just another player in the game, not the player or the leader.

As with all Chinese products, I viewed the generic SLA replacements with a healthy skepticism. I’ve already observed how the A123 modules behave under “out-of-spec” charging and discharging conditions, but I also understand some of the engineering and testing that went into them. The Chinese modules? Not so much. Thus, even though I knew of their existence for a long time, I hesitated recommending them to people. For all I know, they could be just cells in a box.

I was lucky enough to come upon some “class droppings” recently, in the form of these guys:

As far as I know, a 2.009 team purchased 2 of these units for a project last fall, but they were unused. They were found sitting in the Course II teaching laboratory and conveniently hijacked to run the 2.007 tables this year for a little while. Afterwards, they were moved back to the graduate student nest. My interest in them fell to a low point after finding out they were pretty well sealed – the A123 brick prototypes we got last year still had removable shells for testing purposes – and I just figured these were the same thing inside, or somehow knocked off of A123.

Renewed interest in their internals came during a battery search for a new variant of Chibikart (to warrant its own discussion later). So, last night, I decided to crack one open just to see what was going on.

The module above is sold by K2 Energy, which I can’t quite tell from their website if they’re a front for a shady Chinese operation or not, but who cares?

update!

A representative from K2 Battery actually wrote to me in response to the above:

“K2 is a Nevada corporation founded by myself and 5 other partners in 2006.  Most of the technical group, myself included, came to K2 from other battery companies, where we had been developing and manufacturing phosphate-based cathode materials.” -K2 Battery dude

Well there you have it!

end update!

The funny thing about it is, if I search ‘K2 lead acid replacement” or “K2B12V7EB” , the only things that show up are places selling them! It’s kind of a negative A123 problem – I can’t find anyone who’s used them in depth, dissected them, dissed them in general, or anything. They exist. In product form. That’s definitely a sign that something needs poking internally.

It’s interesting to note that these come in two forms – there’s an “EB” version which, according to the short shopping site blurbs, has an internal BMS, and the “E” version which does not. Presumably the latter is literally the box of cells of legend, but here I had an EB version, so it was a chance to see just how BMS-y it was.

I had Shane randomly select one (of two…) to be sacrificed to the scooter gods. Next was figuring out how to crack them open – I didn’t want to do that literally, since I did want to use them as batteries. I settled upon a more dramatic but significantly cleaner solution:

If there is one image which I should not post on the Internet, it is this one. Please, everybody, do not mill your batteries.

I positioned a thin slitting saw right on the green-black plastic seam which is cemented together very thoroughly. I made sure to do very light and shallow passes, poking with a screwdriver each time to see if I broke through the welds – if the top moved, then it was a sign to stop increasing the cut depth and move on to another side.

Whatever they make the casings out of, it smells positively disgusting. Also, the case is thick. Like well over 2mm thick!

After all four sides, the lid is popped off… and we have…

…

……

Well that was anti-climactic.

It’s cells. In a box.

With what appears to be a generic “PCM” board glued to the underside of the lid. These boards are sold throughout the Inexpensive Chinese Battery Markets (YES, ANOTHER USE FOR “ICBM”!) to append to your own battery packs. In my adventures in EV design at the MIT Media Lab, I’ve had the joy? of using them for a few different custom packs.

They function primarily as output limiters – the 3 bars of metal soldered to the board are current sense shunts – and will turn off the output FETs if the voltage becomes too low or the discharge current becomes excessive. These are nominal 25 amps, peak 40 amps (according to manufacturer datasheet). They also usually perform low-current charge balancing if any of the cells become out of line by a certain amount – purely voltage-differential triggered. As far as I can tell, they also do not limit input current – presumably because the shunt is only set up as a single-sided measurement so the simple logic cannot handle negative voltages. As black-box devices to append to your battery for some semblance of failsafe behavior, they are more than adequate.

These boards, though, seem to have the FETs back-to-back – the Source leads of the row of devices goes to both the negative output terminal and the shunts. So it very well could be more sophisticated – I didn’t take the opportunity to disassemble the board, since it was plastic-welded in place and I accidentally stripped two of the screws trying to get them out.

So I guess it’s not just cells in a box…but that still doesn’t say anything about the “E” version. Now I kind of want to get one, but also don’t feel like dumping $100 on cells in a box.

(By the way, this is my favorite PCM board by far – the 100 amp model. It really shows the Chinese design paradigm of “CTRL-C CTRL-V” well.)

Well there it is. I do like this arrangement of cells, though – it’s almost like there’s enough space in the case, with the cells laying down and the BMS board in the otherwise wasted space of the lid, to add another row of paralleled cells for more capacity…

….oh.

i see what you did there.

There is a plastic insert that is conveniently one-cell in height. I wonder why?

There is a “10Ah” model which is oddly enough the exact same dimensions despite it having 50% more cells. Could it be that it’s just rows of 3 paralleled cells instead of 2, and the insert is not used?

Nah. Couldn’t be.

A better view of the board, for anyone who wants to do some armchair trace-guessing. In keeping with the tradition of posting FET datasheets, here’s the AOD4184, which is pretty reasonable I suppose since 6 devices are used in parallel to share the output current.

My assessment of these modules:

I’m going to guess that anything which looks superficially similar will use a similar setup inside (The Law of Chinese Packaging Inertia), just with different PCM boards. The rudimentary charge-balance and output protection these devices offer is reasonable if the battery is similar in voltage to the lead-acid systems they are designed to replace. Most common non-automotive SLA chargers do a rough constant-current constant-voltage charge profile anyway – the former is called  “fast charge” and the later “float charge”. With 4 cells being 12.8v nominal and 14.4v charging, that’s pretty damn close to a 12v SLA setup. 24v systems are similar, with the nominal voltage being 25.6 and peak voltage being 28.8v – most 24v SLAs charge at 28 volts.

Some of them come with a “2 maximum modules in series” specification and it’s clear why that’s the case. A “36v” system made of these would actually be 38.4v nominal and 43.6v charging – the deviation from SLA nominal voltages gets more severe with each added cell. There is also a potential for inter-module imbalance. As some 2.007 students have already found out, the modules themselves can take on different levels of charge and during heavy use, one will fall under its minimum voltage and shut off. It is exactly like the inter-cell balance problem, just more meta. The way around this would be to make sure the packs are at the same level of charge (e.g. full) before connecting them in a string. Real SLAs and Ni cells will all suffer from this problem too, so it’s not necessarily any worse. With the output protection circuitry, I think it is manageable, but over charging is still a concern. I haven’t tested to see if these generic modules shut off on overcharging yet.

In terms of Wh / $, it’s expensive but not terrible – on par with the DeWalt DC9360, but more general purpose. The K2 “EB” module sells for $140 and at 82 Wh that works out to be about $1.70 per Wh. The “10Ah” version is in fact a better deal overall at $1.40/Wh. For a 24v 7Ah battery system with decent plug-in-wall charger (6+ amps), it would run about $350 and charge in an hour. If you don’t mind waiting all day, then generic 24v scooter chargers are like $20. As a reference, two of these 8.4Ah Hobbyking LiFe packs would run $160 (That’s only $0.75 per Wh… damn you Hobbking), and a charger which can handle 8 cells starts at $45 or so. But you will need a 12v power supply which can handle the wattage (example – $40). So the hobby solution is certainly still the cheapest for those who can accept its lesser level of integration and can properly secure the battery from damage.

Perhaps most importantly, the modules do have a (very) tough shell and rectangular brick form factor. It’s way better than watching someone zip tie a soft-pouch pack to a bicycle frame. I did a little bit of further shopping on BatterySpace, and found that I could buy the parts for my own 8-cell, 12.8v 6.6Ah pack with a 30 amp PCM board for about $100. However, then I would spend several hours putting it together and then still not have a cute box in the end to put it in. So certainly I could buy the parts for much less, but the balance of the $140 price is made up in convenience.