Tuesday, April 26, 2011

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.

2 comments:

  1. Interesting app note which does sensorless commutation in a seemingly low-tech, single control loop way:
    http://www.silabs.com/Support%20Documents/TechnicalDocs/AN208.pdf

    Also, you should make a long, technical post about "full DC Bus utilization", and what it actually means.

    Also, any ideas on how to build a motor dynamometer to find the peak efficiency point?

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  2. That is an interesting app note. It's still block commutation, not sinusoidal control, but I like the way they sample back EMF. I also like the explanation of startup.

    By full DC bus utilization, do you mean things like third harmonic injection? As usual I will refer to James Mevey's awesome thesis of awesomeness, page 175-177:
    http://krex.k-state.edu/dspace/bitstream/2097/1507/1/JamesMevey2009.pdf

    And I guess finding the peak efficiency is a matter of measuring both power input (voltage, current) and power output (torque, speed) variables simultaneously and sweeping through the range of operating points you are considering. Easier said than done, I know. Those would be the relevant measurements the dyno would have to make, though.

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