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Discussion Starter · #1 ·
Considering a bottom balance on my pack and came up with the circuit attached. What am i missing? Obviously the number and type of diodes have to be chosen for the desired cuttoff point.
 

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Discussion Starter · #2 ·
Possible higher current version. In hindsight may have to substitute a bipolar for the mosfet due to low zener voltage.
 

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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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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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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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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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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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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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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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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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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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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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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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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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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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