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Old Oct 08, 2003, 02:37 AM
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Idea
LiPo Balancer - Schematics & PCB

This is a switching shunt regulator that will show if there is an imbalance in your pack. It will also automatically rebalance the cells at the end of each charging cycle.

Each device must be adjusted so that the red light goes on at 4.15 Volts. One device must be paralleled to each cell group. If you have a 10s4p pack, you need 10 devices. For a 2s1p you need 2.

The max. current available for re-balancing is set by R1A..R1D. These are SMD metal film resistors rated for 0.25W each. 100 Ohms for each give a max. balancing current of 165mA, which is plenty for normal packs, because a pack protected by these devices will not develop a major imbalance in the first place.

Up to 10A are possible with a suitable PCB layout, but then you need fairly big resistors for R1A..R1D, as each one would dissipate up to 10 Watts. Also, for higher currents you will want to connect Pins 1..3 and Pins 4-8 of Q1.

If there is a huge imbalance, the re-balancing current may not be sufficient to rebalance the pack during one charge, and one or more red lights will go on permanently. This is an alarm condition because it means that some cells are too high in voltage and that the charging must be limited to less than 165mA or stopped immediately. The overcharged cells will automatically be discharged to 4.15V and the balancing process will continue during the next charge. After some charges, the pack will be fully balanced.

To completely balance a pack with unknown cell status in one go, slow charge at 100-150 mA. You may use a current limited power supply or even a NiCd charger set to 100mA formatting mode. As some cells become fully charged, their balancers will activate and their red lights will flash. All cells are fully charged and balanced,
when all red lights are flashing.

The circuit draws less than 10 Microamps form each cell while idle.

Please note:

This circuit is intended for keeping a pack balanced when charging it with a suitable LiPo charger. With the listed parts, it can absorb up to 165mA charge current, which is sufficient for the intended purpose. However, if you overcharge your pack by setting the wrong number of cells or by using an unsuitable charger, you will still damage your cells and risk a fire. To be safe, watch your pack while charging. If one or more LEDs stay on permanently, these cells are being overcharged, and the charging process must be stopped immediately, or current must be reduced to less than 165mA to avoid damage and possibly fire.
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Last edited by Suzanne; Oct 08, 2003 at 03:02 AM.
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Old Oct 08, 2003, 02:42 AM
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This is a single sided board for SMD components. It is 1.0 by 0.75 inches and weighs 1 gram. It is intended to be taped to each cell.
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Old Oct 08, 2003, 08:13 AM
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Suzanne,

This is an excellent solution to the balancing problem. The true elegance comes from being able to peak each cell/parrallel pack with a single existing charger, plus having some visual indication of cell balance/charge completion.

My only concern would be variance in the reference divider due to temperature of the cells. This probably wouldn't hurt too much since all the resistors would vary at the same rate.

People using this circuit would find the mAh the charger says it delivers to teh pack is greater than the actual figure since some will have been burnt by the balancing resistors on a cell that peaked early. However it will not alter the value by much in a nearly balanced pack and I'm not really sure how accurate these measures are to start with.

I don't think that the 165mA max balance current is a problem if new cells are fitted with this device when assembled into packs - all the cells I have had from li-poly factories have been exactly 3.80v. With a properly configured 4.2v per cell charger the fully charged balance current will only be 2mA.

In my view the only reason for uprating the current balancing of the unit would be to deal with over voltage conditions from the charger (e.g. faulty charger or wrongly set cell count). 10watt resistors would weigh lots more.

Do you have any ideas on how to protect the cells against short circuit. The momentary high current destroys the cells internally, I wonder if some current ramp device might work.

Cheers

Joe
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Old Oct 08, 2003, 12:10 PM
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Quote:
My only concern would be variance in the reference divider due to temperature of the cells. This probably wouldn't hurt too much since all the resistors would vary at the same rate.
I use 1% metal film resistors.

Quote:
I don't think that the 165mA max balance current is a problem if new cells are fitted with this device when assembled into packs
That's the idea, yes.

Quote:
10watt resistors would weigh lots more.
In this case I would put the devices outside the pack, i.e., connect only when charging.

Quote:
Do you have any ideas on how to protect the cells against short circuit.
Basically you need a shunt to measure the current and a few FETs in parallel to pass/break the current. Plus some amplifier / voltage reference / glue logic. Not really difficult. But people don't want it because it reduces voltage under load and introduces an additional point of failure into the system.
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Old Oct 08, 2003, 03:33 PM
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Could you build it with larger components just for use when charging?

phil
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Old Oct 09, 2003, 12:05 PM
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Phil,

yes, this shouldn't be a problem. The MAX 921 is available in a DIP-8 package, and logic level FETs area vailable in the TO-220 package. If size and weight is not an issue, you may as well replace R1 with a 3.9 Ohm / 5W type to increase the balancing current to more than 1A.

You could easily build this version on a prototyping board.

OTOH, I have just received confirmation from the board manufacturer that the PC-boards should be available in a few days. They will be made from lightweight 0.8mm laminate, tinned, CNC cut, etc. Very professional.

If there is enough interest, I can ship some to the US for $3 (each) plus postage.
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Old Oct 09, 2003, 01:56 PM
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Thanks Suzanne, as i plan only to use it when charging with my home made CC/CV chargers(electron_head's design)
the larger design is of no matter. Also what's a prototyping board?(sorry if this is a stupid question but i'm relatively
new to electronics)

phil
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Old Oct 09, 2003, 05:12 PM
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Prototyping board

This is what it looks like:
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Old Oct 09, 2003, 05:56 PM
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Suzanne,
What type and value would C1 be?. 47 Microfarad?. What voltage? Electrolitick?.

Thank you for the information,
Robert.
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Old Oct 10, 2003, 12:45 AM
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LiPo Balancer Questions

Suzanne: Why is the balancing voltage set at 4.15 rather than 4.2 volts? Also, could you clarify what you mean by connecting pins 1..3 and 4 - 8 of Q1 for higher currents? I seem to recall the new Astro LiPo charger pulses the LiPo cells as they approach a fully charged state. If I in fact remember that correctly, will your balancing circuit work correctly with the Astro charger? In a 5S4P pack, the charger is looking for 21 volts before it will stop charging (5 times 4.2). If your balancing circuit allows each cell (or parallel group of cells) to reach only 4.15 volts instead of 4.2, will the LiPo charger ever sense a fully charged pack and cut off the charging current, or, in the case of the 5S4P pack, will the charger see a maximum of only 20.75 volts and never completely terminate the charge?

Michael
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Old Oct 10, 2003, 03:36 AM
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Quote:
Originally posted by RMFISH
[B]Suzanne,
What type and value would C1 be?
47nF (0.047uF) should be a suitable starting point. It is not critical. You may want to adjust it for the desired blinking frequency.
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Old Oct 10, 2003, 03:48 AM
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Re: LiPo Balancer Questions

Quote:
Why is the balancing voltage set at 4.15 rather than 4.2 volts?
Simply a safety measure. Not everyone has a super precise voltmeter. 4.2 is ok if you don't need the additional safety margin.

Quote:
Also, could you clarify what you mean by connecting pins 1..3 and 4 - 8 of Q1 for higher currents?
Pin 1..3 are all connected to source, Pin 4..8 are drain. For higher currents, you should connect all pins. For 165mA, one pin is sufficient. I have since changed my PCB layout so that the pins are connected.

Quote:
I seem to recall the new Astro LiPo charger pulses the LiPo cells as they approach a fully charged state. If I in fact remember that correctly, will your balancing circuit work correctly with the Astro charger? In a 5S4P pack, the charger is looking for 21 volts before it will stop charging (5 times 4.2). If your balancing circuit allows each cell (or parallel group of cells) to reach only 4.15 volts instead of 4.2, will the LiPo charger ever sense a fully charged pack and cut off the charging current, or, in the case of the 5S4P pack, will the charger see a maximum of only 20.75 volts and never completely terminate the charge?
This depends on many factors and it doesn't really mattter. The cells are safe either way. The normal behaviour would be that the charger does not terminate and that all lights are flashing, indicating a fully charged and balanced pack.

If you don't want this, just set the threshold to 4.2V or slightly above. In this case the charger will terminate as usual.
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Old Oct 16, 2003, 10:21 AM
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I just got confirmation from the board manufacturer that the first batch of circuit boards is in the mail.

Should be here Friday.

If anyone's interested, please let me know.
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Old Oct 16, 2003, 10:56 AM
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Re: Re: LiPo Balancer Questions

Quote:
Originally posted by Suzanne
Simply a safety measure. Not everyone has a super precise voltmeter. 4.2 is ok if you don't need the additional safety margin.
as far as i've read each cell voltage can be as much as 4.25V(4.2V +/- 0.05V) and still be within the safety margin.
so a setting of 4.2V for the balancing voltage should be ok

phil
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Old Oct 17, 2003, 06:29 AM
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Phil,

Yes. 4.2V should still be a safe limit. After I have assembled the first batch, I will experiment with various settings and report here.

Unfortunately, my distributor just informed me that the lead time for the MAX921ESA is 6 weeks...

Well, patience....
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