eBay Hack Charger #7,523

I really should start a project page for random power supplies that I put together with junk from eBay...

A few weeks ago, I re-purposed an Xbox 360 power supply for my battery charger, and commented on how nice it would be to have a flat OEM power supply brick like this one from TDK Lambda instead. Well, I didn't find that on eBay, but I may have found the next best thing. Vicor is another manufacturer of shiny OEM power converter bricks, and I happened to come across a lot of these:

Turns out Azure Dynamics, a local EV technology company, has an eBay store, presumably for stuff they don't need anymore. I'll be sure to check back regularly. The VI-251-09 brick takes 100-300Vdc and puts out 12V at 200W. I also managed to find a matching Vicor input rectifier module, though presumably any bridge rectifier could work. Combined, the rectifier and DC/DC converter are functionally equivalent to the XBox 360 power supply, but a lot smaller. Some assembly is required, though:

The input is 120Vac and the output is trimmed up to 13.2Vdc. In between, I added a two-pack of 200V, 330μF capacitors. This buffers the rectified AC at the input to the DC/DC converter. The disk-looking capacitors between the terminals and the baseplate are recommended in the datasheet for EMI protection, i.e. they are probably not necessary.

And as usual, I found a random heat sink in MITERS that was already the perfect size for fitting the input rectifier and DC/DC converter brick. I added the fans after experimentally determining that they were needed to hit the full 200W output. Through some sort of luck, the 8-32's I happened to have lying around self-tap into the heat sink fins. Since I'm not an HVfrosh, I thought it wise to insulate all the high voltage pins and ground the heat sink in the final configuration.

I also randomly found a box/carrying case that everything fits in perfectly. There's even a little square left for the balancing lead breakout board. I really should stop measuring things and just look around until I find things that are exactly the right size. It works well.

eBay Hack Charger #...idk I lost count.

It's been a while since I thought about battery charging...

...which is probably a good thing.

This might be a good time to mention that you should not take anything I say about lithium battery chargers seriously...and if you do, you are doing so at your own risk. I highly recommend buying a name-brand off-the-shelf charger and using it according to the manufacturer's instructions.

Frankly, though, my habit of going on eBay, buying the most absurd power supply I can find, and making a battery charger out of it is mostly just unfounded EVT urban legend. But I do take pride in the Cell Abuser, pictured above, which is essentially a 4V/250A power supply I bought for $100. And yes, it is attached directly to an A123 26650 cell with copper blocks and a quick-clamp. It's as close to the 8.02 ideal voltage source as you can get. The result was about as unexciting as you can imagine: the cell charged from 0 to 80% SOC in 102 seconds and to 94% SOC in 3 minutes. At the end it was...warm.

That CV-only charger was just to prove a point, though. For actual day-to-day charging of the LiFePO4 packs I use a sophisticated BMS and integrated charging system re-purposed LED power supply. The MeanWell PLC-100 and the HLG-240 lines actually make very capable 100W and 240W (respectively) CC/CV battery chargers, as I've expounded on in the past. I've been using the HLG-240-36A as BWD and Pneu Scooter's dedicated fast-charger for a long time now.

But it doesn't do cell balancing and you have to buy one specific to your pack voltage and it doesn't make cool beeping noises when it's done and and and...

Okay shut up I finally bought one of these.

Yes, I agree, it's sometimes more useful to have a charger that can handle any pack voltage and can also balance the cells at the end of a charge. For Pneu Scooter, I use separate battery balancers every...well pretty much never; it just stays balanced because A123 cells are so good. But a balance charger would come in handy when first assembling and balancing a pack. So I finally caved and bought a 1010B charger while they were in stock.

But dammit, my MeanWell was $135 and this thing was $128 and doesn't even include a way to plug it into the wall. Too bad there aren't high-current 12V power supplies available for $7...

Oh wait, there totally are.

Thanks to Sasha for the tip. This stupid thing is an obsolete Xbox 360 power supply brick. And it literally is the size of a brick, if not a little bigger. Only Microsoft needs an entire support page dedicated to a wall adapter. Apparently, it's obsolete because it's the old 203W model (12V/16.5A), but since I'm looking for high current anyway it's perfect. They're available on eBay for basically the cost of shipping.

I bought a whole box of them for $60 with free shipping.

They seem pretty easy to work with. Inside the DC output cable there are about 8 more wires than there need to be. All the yellow wires seems to be 12V and all the black wires seem to be GND. Though, there is one set of thinner-gauge yellow and black that seems kinda shady. The red and blue wires can be shorted together to turn the output on. (I guess usually the Xbox takes care of this.) A few alligator clips later and...

It's still a little bulky. If I were truly going for ultra-compactness and I had an infinite amount of money to spend, I'd get a shiny 300W TDK Lambda supply or something similar. But for basically free, this is not bad. It's better than lugging around a giant closed-frame power supply. Now, does it work?

Seems to handle a 5A charge (into a 1.8Ah LiPo) just fine. I let it run until the alligator clips started to smell funny. 

And finally, to show how it compares to the 240W MeanWell:

The 1010B/Xbox Frankenstein charger has the clear advantage of being able to handle any pack size and it can do balance charging. But the MeanWell LED sign supply still wins on size, price, robustness, and power. Verdict: I'll stick with the MeanWell for dedicated vehicle charging, but the 1010B rig will be useful for small robot packs, which vary more in size and charge current rating.

Super Mean Well: The Scooter Fast Charger

Disclaimer: Do not try this unless you are very comfortable with the concepts and dangers of battery charging. I am pursuing this as an experiment, not advocating it for general use. Attempt at your own risk.

A while ago, I had the idea of repurposing large LED power supplies as battery chargers. The premise is that both use the same output characteristic. That is, they will apply a constant current or a constant voltage, whichever is lower. For batteries, the constant current is applied first and supplies most of the charge. Then, when the applied voltage reaches a certain threshold, the voltage remains constant and the current ramps down. If you don't understand how this works, you probably don't want to work with batteries anyway.

Mean Well makes inexpensive 100W LED power supplies (presumably for commercial lighting) under the PLC-100 line. They are roughly the size and weight of laptop power supplies and are available in a range of voltage outputs, up to 48V. They have a constant-current and constant-voltage trim pot that allow adjustment within a limited range around the nominal output. They retail for about $65 and are available from several electronics distributors including Mouser.

I have been using the PLC-100-36 to charge the batteries on the BWD scooter for some time now. The scooter battery pack is lithium iron-phosphate. (Note that these are generally safer than other lithium-ion chemistries, which is why I am more comfortable using a non-standard charger.) It's a 33V, 4.4Ah pack (equivalent to two 36V DeWalt drill batteries). I set the constant current to 2.2A, so it's a C/2 charge, meaning it would take approximately two hours to fully charge the pack.

There is obviously no balancing circuit (though I did try to make one). It's important that the cells be well-balanced before charging and that the voltages be monitored in case one starts to peak early. Cell voltage monitors are widely available, for example this one and this one that also balances. They will not automatically cut off the charger, though, so constant observation is required. (Generally a good idea for battery charging anyway.) For general use, an integrated charger/balancer such as this takes care of all the monitoring and cutoffs. The downside is that for most of these RC chargers, you still need a bulky front-end DC power supply.

Though it is compact, inexpensive, and plugs directly into the wall, the 100W Mean Well supply falls short of the RC chargers in output power. So I was very glad when I came across the new HLG-240 line. This is a 240W version of the Mean Well LED power supply that retails for about $130. It comes in a compact (but dense) aluminum enclosure with wires already attached. Here's the HLG-240-36:

And for scale, here it is next to the scooter:

It's not as light as the 100W version, but it's still something you could see carrying in a backpack. And just like the 100W version, it plugs straight into the wall. This particular model has a voltage trim range of 33.5-38.5V and a current trim range of 3.3-6.7A, a good match for the scooter battery pack. Other models offer other ranges. (Be careful: Only the model with the "A" suffix are adjustable!) And here it is doing its thing:

With a maximum output current of 6.7A, this supply can now charge the scooter pack in about 45 minutes (~1.5C). Whether or not this is practical or necessary, I don't know. I've never had a case where I absolutely needed to charge it in less than an hour. But it's interesting to know that the capability exists for about the same cost and size as a laptop power supply. (Okay, this is a little bigger than a laptop supply.) I still don't recommend it for casual use, since it doesn't implement balancing or voltage monitoring. But if somebody made an small adapter that did these things, I could see it being a good, inexpensive solution to small EV charging.

What's Next: Charger / Balancer

Not ideal.

Charging LiXX batteries is an annoying, time-consuming process and can easily result in a dead battery pack, or in the case of Lithium Polymer batteries, massive destruction. There are two main problems:

1. They need a constant-current, constant-voltage (CC/CV) supply. You can't just hook them up to an AC adaptor or a variable-voltage power supply. The internal resistance is so low that even a small difference in voltage between the supply and the pack will cause a large current to flow.
2. Cells need to be protected from overcharging on an individual basis. Unlike lead-acid, they will not self-balance and are not tolerant of temporary overcharing. The voltage of every cell should be kept below a certain value at all times.

There are plenty of solutions to Problem 1. Most lead-acid battery chargers have a CC/CV strategy of some sort. Unfortunately, they only make them in increments of 12V (14V cutoff). So for my target application of a 33V pack with a 36V cutoff, I'm out of luck.

The more expensive option is an adjustable charger such as the Astroflight 109. This is basically a lab-style power supply in a small package. It will automatically monitor and adjust voltage and current, taking whichever hits a limit first as its operating point. So for a fully dischaged, 30V pack, it may only need to put out 31-32V to supply 5A. But as the pack voltage rises, it will adjust its own voltage to maintain 5A until the upper limit of 36V is reached. Fancy. But it also has problems. The biggest of which is that it runs on a 12V supply with no provisions for plugging into the wall. So for $120, you still need an AC adaptor. And it still doesn't do balancing.

Under ordinary circumstances this frustrating lack of good options would be the cue for me to build something from scratch. One idea I had was to use a Dell notebook adaptor as the front-end, since I carry one (or two) of those around on a regular basis anyway. But that still means I would have the build the CC/CV stage AND the balancer. And I don't want to make a big project out of this...I just need something convenient. So I started looking for hybrid off-the-shelf/custom solution and stubled upon this:

It means well.

The Mean Well PLC-100 is a 100W power supply module "suitable for LED lighting and moving sign applications." Clearly, exactly what you need for charging batteries... But actually LED drivers tend to be current-controlled devices, and in fact this power supply does have a constant-current output from 75% to 100% of its voltage rating. So the 36V version can provide a stable, adjustable current of about 2-2.65A from 27-36V. Perfect for a 33V battery pack, which should rarely drop below 27V anyway. 100W will charge the pack in under two hours. The cut-off voltage is also adjustable to between 85-100% of the rating. So the 36V version can be adjusted from 30.6-36V. For different-sized packs, there are versions for 12, 15, 20, 24, 27, 36, and 48V, all with a similar 85-100% trim range.

That takes care of the front-end supply. Now all that's left is the monitoring and balancing part. It would be excellent if this part didn't require a microcontroller or any programming. It would be even more excellent if each cell had its own, independent, floating circuit that handled balancing. Turns out this isn't actually that hard:

The monomer, if you will. (Click for a clearer picture.)

This circuit runs entirely off the voltage of a single battery cell. The core is a voltage comparator, the LTC1440, which has a built-in 1.182V reference and optional hysteresis setting (not used in the circuit above). The comparator check the voltage of the cell, through a 1% resistor divider. When the cell voltage reaches 3.6, it will turn on its output. The output drives an indicator LED and a transistor which bleeds away power. If the transistor can bleed away more current than the charger can supply, it can even be used to balance and the end of a charge. In this case, that would mean a transistor that can sink 3.6V*2A=7.2W. That's a pretty beefy heat sink, but not unheard of.

The boards will look something like this:

Don't ask why it only has five cells.

You can chain them together indefinitely, or use only a fraction of a board. No programming. No extra power supplies. Very simple.

When combined with the Meanwell front-end, the whole thing will be small enough to fit in a backpack and comparable in price to a Li+ charger/balancer/AC adapter combo. I'm doing it more for the convenience of not having to look around for power supplies, but I never can pass up the opportunity to use things for other-than-their-intended purposes.

The epic journey of the spinning disks of doom.

Well, maybe not epic, but at least humorous. The spinning disks of doom were five "steel" plates I ordered from Big Blue Saw because they were cheaper than material + machining time at MIT even though we have six abrasive waterjets. Go figure. I say "steel" because the first set I got was aluminum. I had actually figured out this mistake before they arrived because the shipping weight was off by exactly the ratio of densities. Anyway, I eventually got the steel plates and installed them for motor testing:

DO NOT TRY THIS AT HOME...OR ANYWHERE.

This was an incredibly bad idea. 55lbs of steel dangling off the edge of a motor shaft with no other bearings and no enclosure is a disaster waiting to happen. It happened to be very well balanced due to the fact that the waterjet cuts the ID and OD at the same time, and it ran very quietly, but NO NO NO DO NOT DO IT. It stores as much energy as 500 lbs moving at 40mph. This makeshift inertial dynamometer provided some invaluable testing data for the kart regen system, and then I decided it must be destroyed. It was the most dangerous thing I've ever built, and I decided this while standing next to a 110F ultracapacitor on an electric go-kart...

So where do flywheels go to die? When I got involved with the MIT Electric Vehicle Team outreach project, they were looking to make a tabletop demonstration of regenerative braking. Sounds familiar. Turns out they had enough time to do it the right way...bearings, hubs, enclosure, etc. It was also much smaller...8" instead of 14" diameter disks. So, I gave them the plates and they produced a mini-flywheel out of the insides:

Much more sane.

So that takes care of the donut holes, but then what do you do with a bunch of 1/4" steel donuts? Pretty useless, right? Not if you believe in conservation of usefulness. After looking around for a microwave transformer or something to smooth out some extremely high battery charging current, also for an EVT project, I remembered these donuts. Toroidal inductor core!

Also surprisingly less dangerous than the previous usage.

Yes, I know this is not a great inductor, as a lot of field is wasted in the excessively-large steel ring. But according to some maths it is >200uH, which is good enough for the job. The job, btw, is insane-charging a motorcycle Li-Ion battery pack. (4-6C charge rates.) This will smooth out the 100A charge to about a 5A or 10A ripple current. Hopefully...

In any case, the spinning disks of doom have found new homes.