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

Thursday, May 17th, 2012 @ 6:26 | Beyond Unboxing

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.

 

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    One Response to “Beyond Unboxing: K2 Energy 12v Lithium Iron Phosphate Lead-Acid Replacement Bricks”

    1. Henry Says:

      Any chance the series module limitation is driven by the voltage limit of the FETs? I’d expect they used low voltage parts for the Rds advantages. Although the cumulative nominal/charging voltage discrepancy gets larger with more in series the error amount per cell remains the same…