# Bottom balancer circuit



## jackbauer (Jan 12, 2008)

Possible higher current version. In hindsight may have to substitute a bipolar for the mosfet due to low zener voltage.


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## steven4601 (Nov 11, 2010)

Hi 
Nice tinkering. However using diodes with a thyristor as the cutoff circuit is just prone to error.
Diode forward voltage varies too much as function of die-temperature to be used as a reference. Atleast that is from my experience. 


Requires a maybe few more components but what about an actual 3 pin voltage reference ? LM336 (2.5V 1% adjustable ) or cheaper would suffice. Buffer the voltage reference with a beefy transistor and control the bottom balancer with an Optocoupler to keep the control board isolated from the individual cells.


edit: the most exact and temperature stable zeners are the 6.2 V ones. That has to do with the doping of the material. Lower and higher voltage zeners are temperature transducers .


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## pm_dawn (Sep 14, 2009)

Hi !

Check this.

http://endless-sphere.com/forums/viewtopic.php?f=14&t=5416

There just use the shunting part of the BMS.
The resistor R1-R4 the Q1 and the U2.
Size the resistors to get your shunt to start(and end) at about 2.8v or 2.9 volts.
Size the Q1 a little bigger to carry perhaps 1A-2A of shuninting max.

That would pretty much do it for you, dont you think.....

Regards
/Per


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## bgeery (Oct 17, 2011)

Has anyone got a working and practical and cheap bottom balance circuit? I'm going to need to bottom balance 25 180AH cells, and I'd like an automated way to do it.

A simple circuit to apply the load until the voltage drops to a set voltage. Automatically reapply load as voltage bounces back.

IMO, it should not be powered by the connected battery, but instead by an isolated source. Also, it should have a very accurate voltage reference.

After the circuit takes care of 99% of the work, come in manually adjust any minor differences.

Might be nice if the circuit could work with a large or small load, to speed the process up.


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## DIYguy (Sep 18, 2008)

bgeery said:


> Has anyone got a working and practical and cheap bottom balance circuit? I'm going to need to bottom balance 25 180AH cells, and I'd like an automated way to do it.


Check with Valery at emotorwerks. I believe he has an automated black box bottom balancer that he will rent out. 
Now i am wondering if it was Valery or someone else.. . . it was introduced at EVCCON . . someone will remember better than me. lol


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## bgeery (Oct 17, 2011)

DIYguy said:


> Check with Valery at emotorwerks. I believe he has an automated black box bottom balancer that he will rent out.
> Now i am wondering if it was Valery or someone else.. . . it was introduced at EVCCON . . someone will remember better than me. lol


Thank you for the lead. I'd prefer something I could build on my own, but a rental would not be a bad second option.

I forgot to add, I want to balance the cells individually, and not just tie them all in parallel. Just in case that might make a difference in some way.


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## TomA (Mar 26, 2009)

You could check with Dmitri to see if he is still making his individual BMS modules slightly modified to work as a bottom balancers:

http://www.diyelectriccar.com/forums/showthread.php/lifepo4-bottom-balancer-39025.html


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## bgeery (Oct 17, 2011)

TomA said:


> You could check with Dmitri to see if he is still making his individual BMS modules slightly modified to work as a bottom balancers:
> 
> http://www.diyelectriccar.com/forums/showthread.php/lifepo4-bottom-balancer-39025.html


Already did. He said it's not available anymore.


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## JRoque (Mar 9, 2010)

jackbauer said:


> What am i missing?


 A fuse! 

For manual/temporary operation? If so, I'd take it a bit further and complicate matters a bit by adding PWM control on a FET, along with volt and temp measurements. Not practical, I suppose, if you want to permanently install on each cell.

JR


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## Guest (Nov 10, 2011)

DIYguy said:


> Check with Valery at emotorwerks. I believe he has an automated black box bottom balancer that he will rent out.
> Now i am wondering if it was Valery or someone else.. . . it was introduced at EVCCON . . someone will remember better than me. lol


Actually it is from EVWest and you can talk to Matt or Michael. Or actually email them. They are busy with a build right now.


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## DavidDymaxion (Dec 1, 2008)

It is a DIY solution (but hey, this is DIYelectriccar, right?). The Lee Hart balancer could be easily modified to do bottom balancing (well, that's easy if you understand it and are up to building it in the first place!):

http://www3.telus.net/nook/balancerland/balancer/index.htm


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## brainzel (Jun 15, 2009)

I want to get my cells to a bottom point as well.
My thought was to get a resistor (0.1 Ohm in this case) across the terminals, switched by a relay.
The Relay would be driven by an arduino board which will measure the voltage constantly.
When the voltage reached 2.5V, the circuit would open.


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## TomA (Mar 26, 2009)

Presumably once the circuit opens and the 25A load is removed, the cell voltage will creep back up to whatever the open circuit voltage is at this point.

Would you then "hand cycle" the cells to get them to whatever OCV you want, say 3.85 or the like?

I'm just asking because a device that would reliably drain an individual cell to a consistent OPEN CIRCUIT voltage seems pretty sophisticated indeed, given the time needed for the cells to recover voltage after being loaded, and of course the risk of having the process go wrong at this SoC level is relatively high. 

I don't think Lee Hart's battery balancer circuit can do the waiting and measuring of OCV necessary to actually dial in the cell. It seems that anything that balances cells under charge or discharge loads without waiting for cell voltages to stabilize is highly likely to produce variations, and not produce accurate open circuit values. The simple resistor cutting out at 2.5V is a good mechanism, but the tweaking necessary to bottom balance at a given OCV takes more effort than that, no? 

A little programming of the Arduino to run through ever smaller cycles of discharge, wait, measure, discharge less, wait, measure would do it, but it would probably take time and attention to dial that in on the specific cells in any given pack reliably enough that you would turn your back on it...


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## JRoque (Mar 9, 2010)

Hi. I like it! You can make it fancier by adding a thermistor to check for temperature for full peace of mind.

JR


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## bgeery (Oct 17, 2011)

TomA said:


> A little programming of the Arduino to run through ever smaller cycles of discharge, wait, measure, discharge less, wait, measure would do it, but it would probably take time and attention to dial that in on the specific cells in any given pack reliably enough that you would turn your back on it...


Ideally, you should not have to do a discharge, wait, measure, discharge less. Just do what our chargers do in reverse.

Discharge at full capacity until the desired voltage is reached, then taper current to maintain voltage. Eventually it will taper to 0 amps load. I would think voltage would have been rising to resting voltage as the load tapered closer and closer to 0.000 amps, and would reliably get to to any DoD. Your Arduino or a cheap PIC should be able to do this easy.


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## palmer_md (Jul 22, 2011)

TomA said:


> A little programming of the Arduino to run through ever smaller cycles of discharge, wait, measure, discharge less, wait, measure would do it, but it would probably take time and attention to dial that in on the specific cells in any given pack reliably enough that you would turn your back on it...


Is it really necessary to be that precise? All you are really trying to do is discharge all the cells so they are at the same starting point before charging them up. This simple circuit will get the cells reasonably close to one another for this purpose. If you want to get them closer yet after this, you could just put all the cells in parallel for a while before assembling the pack into series and charging them, but this probably is not necessary.


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## TomA (Mar 26, 2009)

I'm thinking it is absolutely necessary to be precise with the discharged open circuit cell voltages when bottom balancing. They're already going to be "about" the same voltage without the balance, so why bother if you can't put them as closely as possible to the same spot on the discharge curve? 

The proposed device would be handy more often that just on initial pack commissioning, like when adding a cell, or bringing down either a larger cell or a smaller capacity cell after charging it up some. Putting the cells in parallel isn't practical after the pack is assembled, so it would be nice to have the circuit dial the voltage in more accurately than just pulling the cell down to 2.5V under a 25A discharge, and quitting. 

I agree that bgeery's device is overkill, though. Isn't a constant current/constant voltage discharge device that behaves like a reverse charger going to be pretty pricey compared to a simple resistor? 

I though this was supposed to be a trifle of a thing that sat in a drawer until pulled out once in a while to bring down an individual cell without having to watch it for an hour or three.

An arduino cycling a $30 resistor is cheap. The CC/CV discharger, I dunno...


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## bgeery (Oct 17, 2011)

TomA said:


> I agree that bgeery's device is overkill, though. Isn't a constant current/constant voltage discharge device that behaves like a reverse charger going to be pretty pricey compared to a simple resistor?
> ....
> An arduino cycling a $30 resistor is cheap. The CC/CV discharger, I dunno...


Our ideas are essentially the exact same. A CC/CV discharger is simply an Arduino (or even cheaper, a PIC) cycling a $30 resistor, as your own idea suggested. My suggestion simply does it faster and potentially more accurately via PWM vs "dumb" on-off cycling. The electronics are exactly the same for either method.

Brent


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## TomA (Mar 26, 2009)

That's cool, but isn't the PWM a constant load?

The reason I want to dumbly cycle the load is to give the cell time to recover voltage so that what's being measured is the open circuit voltage _at rest_, not the voltage under load.

It would seem a combination of the two ideas we expressed would be ideal...


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## bgeery (Oct 17, 2011)

TomA said:


> That's cool, but isn't the PWM a constant load?
> 
> The reason I want to dumbly cycle the load is to give the cell time to recover voltage so that what's being measured is the open circuit voltage _at rest_, not the voltage under load.
> 
> It would seem a combination of the two ideas we expressed would be ideal...


In theory, the voltage should rise to the resting voltage even with a PWM load on it. Towards the end, we might be talking about a load of 0.0001 amps perhaps. That's not going to effect the voltage stabilizing at whatever the resting voltage we choose.

All this assums that the circuit itself is powered by an independent source of power so we don't have an additional parasitic draw on the battery, and only the load itself is connected to the battery via a switching transistor.


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## brainzel (Jun 15, 2009)

The resting voltage doesn't matter in both cases (upper and lower knee).
The dicharge voltage and charge voltage is fixed by the datasheet.
In case of the CALB 130Ah:
3.6V @ 39A (0.3C) High
2.5V @ 39A (0.3C) Low
So cut off at lower point and load to the upper one and you got a fully charged battery (and one cycle).

If you don't want to stress them too much, or charge them every day, take a little lower current and move the cut off points to 3.5V or whatever you think might be better.

Much more simpler as take a single resistor on and off at this points should be a challenge 

This circuit is only for myself detecting the amphours for each battery and get them all to the same level.
I drove them one year now and I want to do this procedure once a year to get the data of possible aging or capacity losses.
So it is not made for dayly usage


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## aCoupe (Sep 11, 2013)

did you ever get this requested bottom balancer complete and working?


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## brainzel (Jun 15, 2009)

This is my bottom balance circuit.
Above the resistors, there runs a fan to cool them.


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## aCoupe (Sep 11, 2013)

Very nice work. I am just beginning and it helps to see what's up. What size resistor?


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## brainzel (Jun 15, 2009)

JLD404 DC AH Counter
Tyco Kilovac EV200 Relay
7 x 0,2 Ω 100W resistors (parallel)
aluminum heatsink (under the resistors)
12V MeanWell power supply
250 A / 75mV Shunt
2 x 12V fan

The result for a TS300AHA cells is a starting current of 100 amps, quickly dropping (heat and voltage drop) after a minute to 80 amps, down to about 70 amps.

For every resistor parallel you get 10 to 7 amps more current draw, so with this circuit I can vestigial adjust the current for different cells.

If you have lower cells, you could use a Junsi 3010B or CellPro charger/discharger, so you could discharge at a fixed current level.

Michael


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