# wire choice for cell monitoring/balancing ?



## jockepocke (Nov 24, 2009)

8 A is not very much current... compare with, for example a power drill: They can have a power of lets say 1000 W (my crappy one has 500 W). Running that at 110 VAC (as in many american homes) would require 9 A. Then take a look at the size of the cord... pretty small, and it does not get warm! Maybe double it to be sure!  

About the insulation, I have used a lot of 250 VAC indoors-insulated (the kind I have scavenged from broken electronics, often with two or three insulated strands in one outer insulation) cable in my (cheap)conversion, and as long as it does not wear at any point, it should not be an issue. 

But if you want the numbers: copper has the resistance of (0,0172 Ohm*mm^2)/m. (Yes I use the metric system.) So using 1mm^2 cable (quite thin that is) will give you 0,0172 Ohm resistance/m. At 8 A that is 1,1 W heat loss/m.


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## rwaudio (May 22, 2008)

dtbaker said:


> I need to make a decision on what kind of wiring to use for monitoring/balancing my new TS cells. Because I want to have the OPTION of charging/discharging individual cell(s) as needed at a reasonable amperage, I think I may need larger gauge than the typical 4-strand 24ga telephone wire that a lot of people seem to be using for BMS monitoring.
> 
> The inexpensive power supply I got to do occasional charge of cells to balance is sitting in the garage right now pumping about 8 amps at 3.7volts into my new pack which I have wired in parallel with 16ga (indoor) stereo wire for the initial top-balance. The 16ga is not even warm, so I know that is probably overkill, and not sure the indoor insulation would hold up anyway so I am not too keen on using indoor stereo wire for the install.
> 
> ...


There is some great info here:
http://www.powerstream.com/Wire_Size.htm

There is even a little calculator at the bottom to help you tweak your application based on the awg and length of your cables. I personally wouldn't go smaller than 18awg for that and even larger if possible. Technically virtually any wire can handle the 3.7v that you will be using however you should find something with stranded wire with an appropriate jacket that will stand up against abrasion etc. Just make sure you fuse every wire as close to the cell as possible. And remember that even though each cell only carries 3.7v if you short two together near the ends of the pack it could be your full pack voltage or close to it.


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## azdeltawye (Dec 30, 2008)

dtbaker said:


> I need to make a decision on what kind of wiring to use for monitoring/balancing my new TS cells. Because I want to have the OPTION of charging/discharging individual cell(s) as needed at a reasonable amperage, I think I may need larger gauge than the typical 4-strand 24ga telephone wire that a lot of people seem to be using for BMS monitoring.
> 
> The inexpensive power supply I got to do occasional charge of cells to balance is sitting in the garage right now pumping about 8 amps at 3.7volts into my new pack which I have wired in parallel with 16ga (indoor) stereo wire for the initial top-balance. The 16ga is not even warm, so I know that is probably overkill, and not sure the indoor insulation would hold up anyway so I am not too keen on using indoor stereo wire for the install.
> 
> ...


 
I used 16AWG 'RVI' wire for my shunt-type BMS modules. $0.14/ft from Del City:

http://www.delcity.net/cartviewitem?item=2916105&search=y


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## EVfun (Mar 14, 2010)

You are planning to install a fuse in each wire at the battery terminal, right? Anything less is just begging for a fire.


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## dtbaker (Jan 5, 2008)

rwaudio said:


> There is some great info here:
> http://www.powerstream.com/Wire_Size.htm


thanks for that.... its looking like 18ga is pretty safe. Going to add up to a thick bundle (18 pairs) from the rear rack to the engine bay. 18 pairs x 10 feet long!



rwaudio said:


> Just make sure you fuse every wire as close to the cell as possible. And remember that even though each cell only carries 3.7v if you short two together near the ends of the pack it could be your full pack voltage or close to it.


hhmm, I hadn't really thought of wires from different pairs shorting. I guess it would be POSSIBLE if they wore through the insulation somehow and happened to be + and -. hhhmmm, seems like a really big stretch, but possible.


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## DJBecker (Nov 3, 2010)

rwaudio said:


> Technically virtually any wire can handle the 3.7v that you will be using however you should find something with stranded wire with an appropriate jacket that will stand up against abrasion etc .


You implied it, but I'll state it explicitly: the wire insulation isn't handling 3.7V, it's handling up to the full pack voltage plus inductive spikes. You need insulation that can handle automotive conditions (wide temperature range, exposure to oil, resistance to vibration/abrasion) plus high voltages.

Most battery management systems use thin wire because they balance cells by bypassing some of the current. The more heat that is dissipated in the connecting wire, the less has to be cooled from the control module.

The TI BMS chip uses a charge pump instead of an explicit resistive bypass, but even there a good mental model is "spreading the excess power/heat elsewhere" rather than "efficiently moving excess charge to the least full cell".


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## dtbaker (Jan 5, 2008)

DJBecker said:


> You implied it, but I'll state it explicitly: the wire insulation isn't handling 3.7V, it's handling up to the full pack voltage plus inductive spikes. You need insulation that can handle automotive conditions (wide temperature range, exposure to oil, resistance to vibration/abrasion) plus high voltages.


I m running pairs to each cell... so the wire will only be seeing less than 4v, and only up to 8 amps when and if I ever manually load/charge with power supply to balance. 

Only way the wire would ever see more is if TWO strands from different cells happen to wear thru insulation and they happen to be + and -, and they happen to be from cells that are several cells apart. Not a good situation, but seems very remote.

I do agree slight over-size is sounding better than undersize, to reduce voltage drop in measurement if nothing else! It is also leading me to think that it may be best NOT to run the length of the car to the engine bay, but have a separate monitor block for the front and rear cells with short runs having consistent length wire.

Other good point is that automotive insulation looks way better than maybe cheaper audio speaker wire, interior lamp cord or things of that nature.

.... so, I think I am settling on 16ga auto wire and doing two separate blocks for front and rear, but still resisting the idea of fusing every damn cell...


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## jockepocke (Nov 24, 2009)

Please do use fuses!


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## Sunking (Aug 10, 2009)

My two cents worth. 

Before you go thinking how small you can go, stop and think about IR voltage drop. With Low Voltage applications, you do not have much room to work with as you want to limit VD to 2 % or less. 2% of 3.7 volts = .07 volts. 18 AWG stranded copper wire has .00751 ohms/ft. So at 8 amps, 2% VD, with 18 AWG wire only gives you a loop distance of 1.2 feet if my math is correct. 

Second consideration is of course the insulation. In a vehicle I would have 5 major concerns. Temperature, Abrasion, Vibration, Oil Resistance, and Flexibility. Rather than go through all the insulation types available, I would opt for type MTW or SA.


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## EVfun (Mar 14, 2010)

dtbaker said:


> .... so, I think I am settling on 16ga auto wire and doing two separate blocks for front and rear, but still resisting the idea of fusing every damn cell...


I think the infamous Jack might have experience with not fusing every piece of HV spaghetti coming out of his pack than in turn contributed to his rabid dislike of battery management systems.


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## dtbaker (Jan 5, 2008)

Sunking said:


> Before you go thinking how small you can go, stop and think about IR voltage drop.


got it... so decided on 16ga of auto wire and making my run to a terminal block as short as possible. One block just outside the rear cell box, and one outside the front box. Also trying to make all wires the same length for consistent voltage drop when checking balance.


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## dtbaker (Jan 5, 2008)

EVfun said:


> I think the infamous Jack might have experience with not fusing every piece of HV spaghetti coming out of his pack than in turn contributed to his rabid dislike of battery management systems.


 aaahhhhh, but these are (or should be) LV pieces of spaghetti.


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## EVfun (Mar 14, 2010)

dtbaker said:


> aaahhhhh, but these are (or should be) LV pieces of spaghetti.


WHAT! All wires attached to your traction pack are HV pieces of spaghetti. The pack is in series. Every wire coming out has a potential to every other wire coming out. If any pair of them have a high voltage potential the wiring is HV. You can't look at each cell in a vacuum because they are not separate from the other cells. Anyway, I think it would be pretty easy to start a fire with just 3 volts - when there is a hundred amp hours behind it to provide heating time.


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## rwaudio (May 22, 2008)

dtbaker said:


> aaahhhhh, but these are (or should be) LV pieces of spaghetti.


Yes but if you pick the wrong two they are HV pieces of spaghetti. I would fuse them if they are over a few inches long and/or leave the battery box.


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## dtbaker (Jan 5, 2008)

ok, ok, I get the concerns with possible shorts.  still not keen on installing 38 fuses though... sigh


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## rwaudio (May 22, 2008)

dtbaker said:


> ok, ok, I get the concerns with possible shorts.  still not keen on installing 38 fuses though... sigh


Atleast you don't have 90 series cells like I will, and all 90 will have fuses on the charger boards for voltage monitoring wires.

Something like this would probably be easy and cheap for your application:
http://cgi.ebay.com/5X-Auto-Car-Boa...902?pt=LH_DefaultDomain_0&hash=item20b2a58306
or
http://cgi.ebay.com/10A-Inline-Stan...cles_Terminals_Cabling_ET&hash=item43a075331c


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## Overlander23 (Jun 15, 2009)

A couple of ideas. You don't absolutely need to run 18 pairs.... actually you only need 19 leads for 18 cells since one cell's positive is the other's negative (except for the ends of series string). 

You also don't *need* to charge at your full 8 amps. If the goal is monitoring and occasional balancing, you might be able to get away with a much lower amperage depending on how out of balance a cell is (which shouldn't be much if properly set up to begin with, which you're doing).

It still tricky, though...


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## dtbaker (Jan 5, 2008)

Overlander23 said:


> A couple of ideas. You don't absolutely need to run 18 pairs.... actually you only need 19 leads for 18 cells since one cell's positive is the other's negative (except for the ends of series string).


right, and good point... saves a lot of wire.  but gotta be careful organizing my terminal block to keep things sequential so I don't apply balance charge with reverse polarity. I was fixated on making sure everything was visually simple with each cell having a black and a red lead, but yes can see that is really not required.



Overlander23 said:


> You also don't *need* to charge at your full 8 amps.


true, just looking to do the balancing reasonably quickly when and if required....


BTW, I forgot to ask if you have added any leads like this to be able to check/balance your 'not easily accessible racks, or are you just planning to do the occasional major inspection and drop you racks to check cells?


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## Overlander23 (Jun 15, 2009)

dtbaker said:


> BTW, I forgot to ask if you have added any leads like this to be able to check/balance your 'not easily accessible racks, or are you just planning to do the occasional major inspection and drop you racks to check cells?


The latter... Although it's still up in the air. For now, the latter. As with all projects, I eventually get bored if I'm not actively working on something. At some point a BMS may just get the best of me.


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## DJBecker (Nov 3, 2010)

rwaudio said:


> ...all 90 will have fuses on the charger boards for voltage monitoring wires.


I hope you aren't putting the fuses on the battery management board. That provides no protection against a wire abrading on the way to the BMS board. It only protects against a fault in the BMS.


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## rwaudio (May 22, 2008)

DJBecker said:


> I hope you aren't putting the fuses on the battery management board. That provides no protection against a wire abrading on the way to the BMS board. It only protects against a fault in the BMS.


Sorry, I should explain further. The Charging boards are going to be integrated into the battery boxes and the wires from cell to board will be 1"-6" so the fuses will be within about 2"-7" of each cell, so any wires leaving the charging board for monitoring etc will be fused. I'm still revising things so I might use optocouplers & multiplexers instead so while also being fused the potential between any two wires from any cells from any of the battery boxes can't exceed about 3-5v and a few mA.


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