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Discussion Starter · #1 ·
I just found this constant current / constant voltage step down (buck) charger module that can supply up to 3 amps and 30 VDC from a DC source, and it has an LED indicating charging and full charge (OK). This should work for small lithium battery packs or single cells, or even one or two 12V lead acid batteries. Just the three adjustment trimpots are worth $5! I might get a few of these to use for my battery powered drill retrofit with the LiFePO4 18650 cells I got.

http://www.mpja.com/08-26-14.asp?r=10315&s=4

The schematic is:
http://www.mpja.com/download/31562ps.pdf

You should be able to use this with a shunting BMS for multiple cells in series. It would just shut down the power source or disconnect the pack when the BMS shows imbalance or if the circuit fails to shut off upon full charge.
 

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Discussion Starter · #3 ·

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Discussion Starter · #6 ·
I have ordered two of these. They should work well enough for my pack of 4 or 6 LiFePO4 18650 cells rated at 1250 mAh.

The TO-263 package for the LM2596 exhibits 20 to 50 Deg C / watt depending on the area of copper on the PCB. So for 100 C above ambient it can dissipate 2 to 5 watts. The efficiency varies from about 73% at 3 VDC output to 92% at 20 VDC. So for six cells at 3.5 VDC (21V) and 3 amps that is 63 watts, which would be about 6 watts of losses. It may be possible to add a small heatsink, but for my purposes I could use just 1.5 amps and charge the pack in one hour.

This is certainly not a viable solution for charging larger EV battery packs, but it may work well enough for a small vehicle like a bike or tractor, especially one which is used occasionally. This operates at 150 kHz which means the inductor and capacitor can be very small. The same basic design might be extended to make a much higher power unit.
 

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The same company has laptop style power supplies that provide isolation, so you could use several of them for each set of 4-6 cells in series.

http://www.mpja.com/19-Volt-Adapter-Power-Supply-26A-Bi-Pin/productinfo/19077 PS/ (19V 2.6A $3.95)

http://www.mpja.com/24-Volt-Adapter-Power-Supply-265A/productinfo/18509 PS/ (24V 2.65A $5.95)

http://www.mpja.com/16-Volt-Adapter-Power-Supply-374A-Sunny/productinfo/19385 PS/ (16V 3.75A $2.95)
No.... That would short all the negatives of each cell in the series string. These aren't isolated, so the input and output grounds are common. If you put one on each cell of a 6 cell string and connect the inputs together from a laptop supply you have just shorted the minus on each cell together.

You need isolated dc-dc for each cell so they don't short to the next cell. Isolating the incoming power does nothing for you.
 

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Discussion Starter · #8 ·
You do have a good point about some laptop power supplies which have a three wire line cord with ground.

The outputs of all (legal and functional) laptop power supplies MUST be isolated. Otherwise I'd get a nasty shock if I touched the exposed shell of the charger power plug as I inserted it in my computer. And after I did, the USB and other exposed I/O would also be connected to the line.

You may be thinking of converters with 3 prong plugs and a ground, which IS the case for the second power supply I listed above. In that case, you are right, and I do need to see if any of the supplies I have are three-wire and thus grounded.

The ones I purchased have two-prong plugs and are OK for my purposes. But several of my laptop computer power supplies do have three prongs, and the ground pin is connected to the negative of the output. It probably would not hurt anything to disconnect the ground, as there are no exposed metal enclosures. It is something to consider, however, as there will be some leakage current from the AC line through the capacitance of the transformer. And that may be a consideration if you connect a USB or serial or Ethernet cable between a computer and a peripheral such as a printer.
 

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I got a lot of ten dc-dc converters from ebay for $11..
http://www.ebay.com/itm/191149186067?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
Even shipped from california, got them in 2 days.

That is about $1 each, they will do 2.5amps without a heatsink, I added a heatsink to them, and they will do 3.5amps.

I've got 4 of my own dc-dc's that I run in parallel to charge single cells at 12amps, but I can't even bother to sold up the parts I ALREADY have when I can buy them already made for $1 !!
 

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You do have a good point about some laptop power supplies which have a three wire line cord with ground.

The outputs of all (legal and functional) laptop power supplies MUST be isolated. Otherwise I'd get a nasty shock if I touched the exposed shell of the charger power plug as I inserted it in my computer. And after I did, the USB and other exposed I/O would also be connected to the line.

You may be thinking of converters with 3 prong plugs and a ground, which IS the case for the second power supply I listed above. In that case, you are right, and I do need to see if any of the supplies I have are three-wire and thus grounded.

The ones I purchased have two-prong plugs and are OK for my purposes. But several of my laptop computer power supplies do have three prongs, and the ground pin is connected to the negative of the output. It probably would not hurt anything to disconnect the ground, as there are no exposed metal enclosures. It is something to consider, however, as there will be some leakage current from the AC line through the capacitance of the transformer. And that may be a consideration if you connect a USB or serial or Ethernet cable between a computer and a peripheral such as a printer.
Draw out your circuit. If you parallel the input power to the buck converters (ie you bus the inputs) and connect each output to a different battery.... You will cause a short between negatives between each cell if they're not isolated dc-dc converters.

Isolation on the laptop supply that goes into the buck converters has nothing to do with what I'm talking about.

Those buck converters are not isolated from input to output. So they share a common ground with the input. If you have one on cell 1 and another on cell 2, and power each buck converter from the same source, you just created a short from cell 1 negative and cell 2 negative, which would be a short across cell 1 plus and minus.

You need to use isolated dc-dc on each cell if you want to charge each individual cell with a common bus voltage on the input to the dc-dc on each cell.

I've done it a few times with vicor and you can't do it unless the converter on each cell is isolated.
 

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Yep, you need an isolated power supply for each DC-DC if the cells are connected in series. The good news is they don't need to be expensive regulated supplies as the DC-DC does that work, so just a dumb transformer wallwart should work.

Draw out your circuit. If you parallel the input power to the buck converters (ie you bus the inputs) and connect each output to a different battery.... You will cause a short between negatives between each cell if they're not isolated dc-dc converters.

Isolation on the laptop supply that goes into the buck converters has nothing to do with what I'm talking about.

Those buck converters are not isolated from input to output. So they share a common ground with the input. If you have one on cell 1 and another on cell 2, and power each buck converter from the same source, you just created a short from cell 1 negative and cell 2 negative, which would be a short across cell 1 plus and minus.

You need to use isolated dc-dc on each cell if you want to charge each individual cell with a common bus voltage on the input to the dc-dc on each cell.

I've done it a few times with vicor and you can't do it unless the converter on each cell is isolated.
 

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Discussion Starter · #12 ·
OK, I see where the misunderstanding is. My idea is to use the buck converters as a voltage-limited constant current supply for each set of up to 6 or maybe 8 lithium cells in series (6x3.5=21V, 8x3.5=28V), and use separate BMS modules for each cell. So the DC-DC does not need to be isolated, and you can obtain the maximum power of 2-3 amps at 30V or 60-90 watts. The laptop supplies I referenced have at most about 24V out so that will limit the maximum charging voltage to about 21V and 6 cells. But I also have a couple of PSUs that provide up to 36V.

My needs are quite different from those of full size EV owners, so this idea has only marginal use for providing a slow charge for occasional use. It may be more useful for an electric bike or small tractor as I am working on, and I will have probably no more than a 48V pack which might be charged with two of the PSUs in series. Mostly, I plan to use these for my electric power tools which will have only 12-18-24 volt packs of 1-2 Ah capacity, so these should be perfect.

Also, the buck converter I purchased is specially designed for battery charging, and is not the same as the ones you get for $1 each. But it uses the same "Simple Regulator" IC and you can't buy them for $1, much less also get the inductor and ten turn pot and a PCB. How these vendors make any money is a mystery to me.
 
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