simple high current balancer - RC Groups
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Sep 01, 2004, 04:18 PM
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Dan Baldwin's Avatar

simple high current balancer


The circuit depicted here is for experimental purposes only. Any person using the circuit, or any part of the circuit depicted here does so at their own risk.

This balancer would be the circuit for 1 cell. One would have to be built for each cell in the pack to be balanced. Although normally balancing current would be very small to keep a battery pack in balance, this balancer is capable of sinking 2 amps (or higher) with an appropriate heat sink. It is intended as an external balancer to be used while charging the battery pack. The LED turns on as low as 5 milliamps balance current. It will not work well with pulsing type chargers such as the Astro Flight 109 charger. Don't be misled by the simplicity of the circuit. It will clamp the voltage to within 2 millivolts from 20 milliamps to 2 amps. The LED will turn on with as little as 5 milliamps balancing current. If the pass transistor should fail, it is protected from putting a short on the cell by using a resettable fuse. A regular glass fuse could also be used. The cost is about $4.56/cell without the circuit board.


Mouser Parts List
qtys shown per balancer

1 652-MFR185 RESETABLE FUSE .58
1 511-TIP137 PNP POWER DARLINGTONS .68
1 531-PT10MH-1K 1K OHM POT .40
1 604-L934SGC T-1 GRN 300 MCD 50 DEG SUP BRITE LED .18
1 511-TL431ACZ ADJUSTABLE SHUNT REGULATOR .28
1 511-2N3906 PNP GENERAL PURPOSE SIGNAL XISTOR .06
1 567-290-1AB T0-220 HEAT SINK .38
1 532-4880M TO-220 mounting kit for TIP137 .77
1 75-516D107M035 100 mfd cap .28
1 581-BF014D0103K .01 mfd cap .09
1 581-BF014D0104K .1 mfd cap .11
1 72-RWM410-1-5 1 ohm 3 watt .32
1 271-10K 10 K 1% RESISTOR .09
1 271-15K 15 K 1% RESISTOR .09
1 660-CF1/4L102J 1K 5% RESISTORS .05
2 660-CF1/4L121J 120 OHM 5% RESISTOR .05
2 660-CF1/4L331J 330 OHM 5% RESISTORS .05

Dan

Disclosure;
although I have built and tested this circuit, I have not used it to balance any battery packs. I (at least at present) do not balance my packs.
Last edited by Dan Baldwin; Sep 17, 2004 at 11:02 AM. Reason: Updated schematic
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Sep 01, 2004, 04:34 PM
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Dan Baldwin---OK it looks good now how about a description about how it works and how to set it up-----please.
TIA--G Pearce
Sep 01, 2004, 05:04 PM
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Dan Baldwin's Avatar
The diagram below shows how to hook up the balancers while charging a 3 cell battery pack, but any number of cells could be charged in series as long as each cell had a balancer hooked up as shown. The charger shouldn't be set any higher than 2 amps with the circuit shown although higher current could be used if a larger fuse was used. This circuit has not been tested higher than 2 amps. To do the initial adjustment, a load ( a resistor as shown, or a 12 volt light bulb) is put in series with balancer, and 12 volts is applied. The LED will be on. An accurate voltmeter (a DVM) is put across the balancer, and the pot is adjusted for a 4.2 volt reading.

Let me know if anything isn't clear enough.

Dan
Last edited by Dan Baldwin; Sep 17, 2004 at 11:18 AM. Reason: diagram changed to show different hookup method
Sep 01, 2004, 05:18 PM
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Dan Baldwin's Avatar
principle of operation;
The TL431 is a precision adjustable shunt regulator. The TIP137 darlington transistor pair is a booster for the TL431, making it capable of handling much higher currents. The TIP137 is rated at 8 amps continuous current. When the TL431 begins to conduct because the voltage has risen to the set voltage, the transistor that controls the LED will turn on before the TIP137, so the LED will turn on with even the slightest current flow.

Dan
Sep 01, 2004, 05:34 PM
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Larry3215's Avatar
Cool!

Larry
Sep 01, 2004, 05:51 PM
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Dan Baldwin--Thanks for the additional info.
I guess there is no hope for cells being charged in parallel??
G Pearce
Sep 01, 2004, 06:02 PM
DNA
DNA
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Lipo cell balancers such as this circuit eliminate the need for charging cells in parallel, so they can be left in their series connections in the pack without having to disconnect the cells and rewire them in parallel. You will need cell taps on the pack.

However, if you want to charge cells in parallel, you could do that too and just use one of the 4.2v cell balancers for the cells you have connected in parallel as long as you don't go over the 2 amp charge rating.
Sep 01, 2004, 06:13 PM
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Dan Baldwin's Avatar
Another method of balancing cells is to use a separate charger and power supply for each cell. That method was being discussed in another thread (https://www.rcgroups.com/forums/show...=248268&page=4), and it's what prompted me to post this information. I will be posting another circuit that would work for charging 1 lipoly cell from a 5 volt power supply within a few days if I get the time to breadboard it to make sure it works as advertised.

Dan
Sep 01, 2004, 08:05 PM
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Just a thought.
If the packs are always charged with the balancer connected then it is unlikely that the balancer will need to conduct much current.
You could limit the current through the pass transistor with a resistor instead of the polyfuse.
For the cases where the pack is severely out of balance something like the PQ PCM Guard wired up with the balancers would disconnect charging power if any cell exceeded 4.35V. You may then need to manualy restart charging in this case.
Sep 01, 2004, 08:51 PM
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Dan Baldwin's Avatar
Your absoutely right. If I were building this for myself, I would probably do as you suggest and put a current limiting resistor in series with the pass transistor, but I have noticed that some of the guys on the board are concerned that the "wimpy" balancers won't balance their big packs. If the balancer were limited to 1 amp, and your pack was severely out of balance, you might have to charge the pack once at 1 amp to get it balanced, but you could charge it normally after that.

Dan
Sep 01, 2004, 09:22 PM
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I think the method I was suggesting in conjunction with an overvoltage guard like the PQ PCM Guard will balance a large pack.
With my Supernova 3000 you may have to manualy restart charging.
If you used a charger which did not require a manual start such as one of the simple DIY CC/CV chargers you will end up with a charged and balanced pack eventually without manual intervention. The PQ PCM Guard will cycle the charge on and off to allow the balancers to do there job.
Sep 02, 2004, 12:10 AM
DNA
DNA
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If you're using a CC/CV charger to charge the pack, as the voltage nears 4.2v per cell, the current near the end of the charge will be decreasing and will be much less than the 2 amps that was set initially.
Sep 02, 2004, 12:49 AM
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Larry3215's Avatar
The only real down side, from my point of view, in useing balancers on packs is that you need to know which tap goes to which cell and in which order. This is not hard to determine, but you will have to be carefull to plug the correct balancer lead into the correct tap every time.

Thats where isolated, 1 cell chargers would be more plugNplay and safer for newbies and the electricaly chalanged. It would not matter which tap you pluged which charger into.

Larry
Sep 02, 2004, 10:16 AM
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Dan Baldwin's Avatar
Larry
You're right that you have to be careful to get the balancers hooked to the right lead every time, but if you use a small connector like the Deans Micro 4r, you only have to worry about it once, when you wire the connector. Even if you use separate chargers for each cell, you still have to get the leads right. I think that newbies and the electrically challenged would have trouble tearing their packs apart to attach connectors regardless of the balancing technique used.

DNA
The only time a balancer would see the full charger current is in the unlikely event that one cell reaches 4.2 volts while others in the pack are still far enough below that voltage that the charger is still putting out full current. I thing you're right. The capacity of this balancer is overkill.

dkselw
The point I was trying to make is that if you have balancers that can handle near full charge current, you don't need the PQ PCM Guard at all. It would certainly be a good idea to use the PQ PCM Guard as an additional safety.

Dan
Sep 02, 2004, 06:58 PM
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Using the tap connector on the polyquest batteries you can't get the order wrong either.
I just brought up the idea of using the PQ PCM Guard partly for extra safety and also because if you limit the current through the balancers you can make the circuitry smaller and you don't have to worry about getting rid of as much heat.
I guess its all a trade off.
Last edited by dkselw; Sep 02, 2004 at 06:59 PM. Reason: Typo


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