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Old 06-29-2014, 08:15 AM #1
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Default Will this work as a DIY a 2S lipo charger?

Ignoring for a moment that simple constant-voltage isn't the ideal way to charge lithium ions, I'm trying to wrap my brain around wither something as simple as two nested voltage regulator circuits would be capable of charging a 2S lipo pack.

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Old 06-29-2014, 09:00 AM #2
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Default Re: Will this work as a DIY a 2S lipo charger?

Nope.

Imagine the top cell has a lower voltage than the bottom cell. This means that you'd need more current between the 4.2V point to GND which the second regulator can't supply. The only path would be the bottom cell so it will get overcharged when the top cell is full.

Best way is to use a single 8.4V supply and charge both at the same time then use a balancer circuit to keep both cell voltages under 4.2V.
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Old 06-29-2014, 09:10 AM #3
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Default Re: Will this work as a DIY a 2S lipo charger?

Have you ever come across an IC that does cell balancing for 2S?

If I wasn't concerned with balancing, there are a couple ICs that can do lithium ion charging at 8.2V on the presumption that they're charging 2 cells in series, but I'm looking for a way to DIY a balancing circuit.
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Old 06-29-2014, 09:20 AM #4
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Default Re: Will this work as a DIY a 2S lipo charger?

I haven't seen one yet. Never really researched about it though. There is a balancer circuit in my site that used a voltage reference IC but it takes a bit of parts. Could be possible to make it smaller but with lower dissipation ability.

Or you can get a charger same as shown in here: Replacement 9 V battery - Náhrada baterie 9 V

I got one and it's basically three identical 4054 based single cell chargers with outputs all wired in series and the input to all charger ICs are isolated using a DC-DC converter.
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Old 06-29-2014, 09:54 AM #5
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Default Re: Will this work as a DIY a 2S lipo charger?

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I haven't seen one yet. Never really researched about it though. There is a balancer circuit in my site that used a voltage reference IC but it takes a bit of parts. Could be possible to make it smaller but with lower dissipation ability.

Or you can get a charger same as shown in here: Replacement 9 V battery - Náhrada baterie 9 V

I got one and it's basically three identical 4054 based single cell chargers with outputs all wired in series and the input to all charger ICs are isolated using a DC-DC converter.
The commercial balance charger is too large for my purposes. And wow, I'll give you credit for this! (but it hurts my head to think about reproducing that!)
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Old 06-29-2014, 10:12 AM #6
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Default Re: Will this work as a DIY a 2S lipo charger?

It isn't actually too hard. You won't need the optocouplers for a standalone balancer.

If the cells to be balanced are pretty matched, you can even eliminate the BD140 power transistor. Even the LED indicator current will be enough to balance the cells.

And then, you'd only need to duplicate it twice for two cells
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Old 06-29-2014, 06:47 PM #7
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Default Re: Will this work as a DIY a 2S lipo charger?

What about regulating an 8.2V output, and then using an Op Amp to split the voltage into a +/- 4.2v output, with the virtual ground then serving as the middle lead?

http://www.circuitstoday.com/wp-cont...sing-opamp.JPG
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Old 06-29-2014, 09:53 PM #8
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Default Re: Will this work as a DIY a 2S lipo charger?

What if you put 6 diodes in series? Wouldn't that hold a cell at around 4.2V?
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Old 06-29-2014, 10:03 PM #9
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Default Re: Will this work as a DIY a 2S lipo charger?

There's always the simple but annoying way of using two isolated 4.2V CV sources too. If you're mains powering the thing just use two stepdown transformers instead of one. If you're planning on DC powering the whole thing then isolation becomes a bit trickier.
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Old 06-29-2014, 10:28 PM #10
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Default Re: Will this work as a DIY a 2S lipo charger?

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There's always the simple but annoying way of using two isolated 4.2V CV sources too. If you're mains powering the thing just use two stepdown transformers instead of one. If you're planning on DC powering the whole thing then isolation becomes a bit trickier.
I'm essentially trying to make a very tiny circuit for charging a 2S pack from a 16V DC source. At present, I'm just charging each cell separately, but that takes an extra step (switching from one cell to the next).
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Old 06-29-2014, 10:34 PM #11
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Default Re: Will this work as a DIY a 2S lipo charger?

You know there are very inexpensive off the shelf solutions for such. While I always admire and recommend DIY when possible, sometimes economically it isn't worth the trouble.
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Old 06-29-2014, 10:35 PM #12
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Default Re: Will this work as a DIY a 2S lipo charger?

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You know there are very inexpensive off the shelf solutions for such. While I always admire and recommend DIY when possible, sometimes economically it isn't worth the trouble.
I haven't been able to find any, at least not that fulfill this requirement:

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I'm essentially trying to make a very tiny circuit for charging a 2S pack from a 16V DC source. At present, I'm just charging each cell separately, but that takes an extra step (switching from one cell to the next).
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Old 06-29-2014, 10:38 PM #13
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Default Re: Will this work as a DIY a 2S lipo charger?

Yeah, size can be a problem. Perhaps a reverse engineering and DIY reproduction of a commercial unit would suffice?

DC 2-4S Lipo Balance Charger*
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Old 06-29-2014, 10:58 PM #14
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Default Re: Will this work as a DIY a 2S lipo charger?

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Yeah, size can be a problem. Perhaps a reverse engineering and DIY reproduction of a commercial unit would suffice?

DC 2-4S Lipo Balance Charger*
I don't know that I could get their units down small enough. I'm looking for something the size of a (large) driver. Maybe a 1x1 inch footprint. I recognize that I'll have to make sacrifices, like perhaps needing to rely on CV charging only, not being able to have indicator lights, and perhaps being limited to relatively small charge currents, but those are constraints I can live with. I'm only going to be charging utterly tiny 2S Lipo packs - truly, packs so small that you can cut them in half with scissors! (and so small that doing so is pretty uneventful).

Is there any reason that a +/- 4.2V split rail setup wouldn't work? IE, my idea of regulating 8.2V output via voltage regulator, and then splitting it with an OP Amp to create a virtual ground at the midpoint?
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Old 06-29-2014, 11:05 PM #15
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Default Re: Will this work as a DIY a 2S lipo charger?

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Is there any reason that a +/- 4.2V split rail setup wouldn't work? IE, my idea of regulating 8.2V output via voltage regulator, and then splitting it with an OP Amp to create a virtual ground at the midpoint?
Not that I can think of right now, if you're willing to live with straight CV charging and not balance charging. Though *I think* balancing isn't much of an issue with 2S as typically it is the middle cells which are in danger in an unbalanced regime. Virtual ground +-4.2V is essentially (only with current limit and no voltage sag) the same as connecting a second 2S in parallel through the balance cable afaik.
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Old 06-29-2014, 11:47 PM #16
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Default Re: Will this work as a DIY a 2S lipo charger?

If you don't mind undercharging a bit

D3Z3V9BF-7
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