# Has anyone used this Battery Balance circuit?



## Wattmotion (Sep 10, 2008)

I am putting together an AGM pack and need a simple battery balance circuit. I don't want to use light bulbs and zeners but something slightly more advanced that I can dial in an exact voltage to dump. I found this circuit on another thread but wondered if anyone has built this for an EV and has comments about it. 

Here's the link: http://axiom.anu.edu.au/~luke/xr4000batbal.html

If someone out there has a similar circuit that I can put across each battery and set the voltage, I'd love to hear from them.


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## Jeff (Sep 25, 2008)

I don't know about that variation, but I have used the clamping circuit below in electric bikes, as well as a large EV running 3600 A123 M1 cells.

The ebike packs are 6p, and the EV pack is 30 parallel. The transistor is the sink, not an external resistor. The circuit is used to detect when 3.6v is achieved (knee) and charging current is reduced linearly until the clamp releases, or bottoms out at 25w, whereas the charger enters a leveling cycle until all clamps report 3.6v.

Yo can trim the threshold for whatever voltage you want. The threshold for this clamp is designed to be very tight +-20mv from off to full clamp current. The circuit includes a 10a fuse for protection.

schematic:
http://www.evbones.com/vclamp.pdf

ebike pack using this circuit:
http://www.evbones.com/tfa123pack.jpg


Jeff


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## Wattmotion (Sep 10, 2008)

Thanks Jeff,
I might test this circuit out and see how it works on an AGM. I assume all resistors are probably 1/8 watt with exception of R1. Just wondering what does the optos interface into? Is there a source for the board or complete unit? 

Steve


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## Jeff (Sep 25, 2008)

Wattmotion said:


> Thanks Jeff,
> I might test this circuit out and see how it works on an AGM. I assume all resistors are probably 1/8 watt with exception of R1. Just wondering what does the optos interface into? Is there a source for the board or complete unit?
> 
> Steve


Steve, the circuit will work fine for AGM. You will want to set the clamp at approx 14.75v, or whatever the mfgr specs for a finish voltage.

The clamp includes a LV detect that triggers at 2.55v so you can ignore that portion of the circuit. This was specific to my cutoff voltage for the A123 M1, and does not apply to PbA.

The other opto is a flag to the host processor to indicate the clamp is active, and used to control charge current. The clamps are not designed to sink a lot of power, and would be damaged if not able to flag the host to reduce power.
The opto isolators provide the galvanic isolation needed in the 400V system.

This clamp is a secondary component of the vehicle BMS, where voltage and temperature of each battery cluster (120 in total) is reported back to the host via a daisy chained fiber link.

I have some spare blank pcb's of the tiny ebike clamps that do not include the LV cutoff. 

I'd sell the blank (unstuffed) boards for $2ea if you're interested. I think I have close to fifty blanks.
Here's a closeup image of what the board looks like assembled. You will need to provide a heat sink for the transistor that is scaled to the current you want it to sink. A 4x8 inch piece of thin sheet aluminum bonded to the side of the battery would sink 2A, and function as a battery warmer too.
http://www.evbones.com/cellclamp.jpg

The schematic shows all of the component values, although the values assigned to R2,R3,R7 would be quadrupled, and R1 would be 1k for a 12v PbA application. 

An LED at D1 indicates the clamp is active. All components are 0603, so unless you have the steady hands of a surgeon, and the soldering Kung Fu of a Guangzhou sweatshop, you might want to consider designing your own using larger through hole parts.

Jeff


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## Wattmotion (Sep 10, 2008)

Thanks for the offer of boards but I think I would be further ahead designing one for my application. I have something designed around the 431 zener that I think I can use for 6 or 12 volt AGMs and tie into my monitoring system. Thanks again.. 
Steve


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