# sizing wires for cell monitoring/balancing



## dtbaker (Jan 5, 2008)

I want to add a loom of paired wire to be able to monitor/balance all (TS or SE Li) cells from a central location, like in the motor bay... and wanted to get some feedback on advisable gauge, and if anyone had a source for pre-grouped pairs or ribbons to ease installation for groups of multiple cells.

It seems that any active shunting system that is attempting to bypass a cell would have to be sized to carry a couple amps, and if there were manual balancing with either a load, or a power source to a target voltage to balance a specific cell, it would be good to carry a couple amps so it doesn't go too slowly.

What is the suggested (typical) load for active monitor/shunt/charge? It only has to carry maybe 3.7 volts if being used for a single cell top balance, and only a small load if attempting to drain a little for bottom balancing.... right?

so.... is 18ga enough, or 16ga?

Do people think its easier to use pairs, ribbons, or just roll your own from individual strands?


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## TigerNut (Dec 18, 2009)

dtbaker said:


> I want to add a loom of paired wire to be able to monitor/balance all (TS or SE Li) cells from a central location, like in the motor bay... and wanted to get some feedback on advisable gauge, and if anyone had a source for pre-grouped pairs or ribbons to ease installation for groups of multiple cells.
> 
> It seems that any active shunting system that is attempting to bypass a cell would have to be sized to carry a couple amps, and if there were manual balancing with either a load, or a power source to a target voltage to balance a specific cell, it would be good to carry a couple amps so it doesn't go too slowly.
> 
> ...


18 gauge is probably "enough", especially if your balancer does current based shunting, with cell terminal voltage monitoring using a separate set of leads. The other option is to PWM the balancing shunt and then read the terminal voltage while the shunt current is set to zero. 16 gauge would allow you to pull probably double the current for a given amount of resistive loss in the wire, but the terminations may contribute more resistance than the wire loss in either case.

For 18 gauge you could use home lamp cord - it's paired and some varieties have one lead identified so you can keep the polarization straight. Only thing is that the insulation may not be rated over your anticipated temperature range, which can make it fragile at cold temperatures. 16 gauge is commonly used for (home) interior speaker wiring, so you can get colored, paired or 4-conductor cables in spool form at your local home improvement center.

If you use automotive wire or another single-lead source, you can easily make up a multiconductor cable by clamping one end of your conductors in a bench vise, chucking the other end of all the conductors in your hand drill, and then spinning the drill (while keeping it all taut) until the wires are all twisted together. Braiding is nice but it takes a long time.


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## dtbaker (Jan 5, 2008)

TigerNut said:


> 16 gauge would allow you to pull probably double the current for a given amount of resistive loss in the wire, but the terminations may contribute more resistance than the wire loss in either case.


I am finding mini-spools of 16-ga 4wire bonded ribbon for trailer light wiring for $14... not too bad, but this is going to add up running to the rear battery box at 8' away or so.... and I'm going to have to run a new conduit under car I think as it will add up to a hefty bundle for 20 cells....


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## TigerNut (Dec 18, 2009)

Our local FSAE team wired their car with Tefzel wiring - one of the students had some connections with a local airplane repair place.
http://www.aircraftspruce.com/catalog/elpages/unshieldlwire.php
The insulation is much thinner than automotive stuff, so the bulk of the wiring is reduced by a considerable amount. However, you would need to bear in mind that if you're pulling some significant power through a bundle of closely located wire, then all the wire will heat up, and at the center of your bundle the temps might get too high.

The pricing of this wire may actually not be too bad compared to your source of trailer wiring... 100 feet for $28, and you can get different colors.


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## etischer (Jun 16, 2008)

the insulation on trailer wiring always seems to get dry and crack. I would use some high quality jacketed multi-conductor cable. 

Something like Belden 8466 (18 awg x 12c) or 8467 (18 awg x 7c)


For long runs I would go with 16 or 14 awg. 18 awg should handle 7 amps, but the voltage drop across the wire might cause your BMS to go crazy if it starts to shunt. You can look up the ohms/foot for various gauge wire, and calculate the voltage drops you will have when shunting. This would be the best indication of what your gauge should be. If you are loosing half your voltage across the wire, you will be shunting half the voltage (if using a simple resistive load bms)


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## Zanthic (Jul 2, 2008)

just one thing to consider, and I know this makes a difference on what style of BMS you choose, but, should you use a BMS that has the balancing circuit mounted at the cell than you only need a very small gauge wire back to the BMS for reading the voltage and a second one for turning on the balancing mosfet. The advantage to this is that should there be a wiring error or a short sometime in the future, having small wires would mean they would act as a fuse and burn up quickly with less chance of a fire. If you short a 18 gauge wire it will handle enough current for long enough that there is a much higher chance of fire. Maybe that's being overly cautious, I don't know, but you can decide!


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## Roy Von Rogers (Mar 21, 2009)

Zanthic said:


> just one thing to consider, and I know this makes a difference on what style of BMS you choose, but, should you use a BMS that has the balancing circuit mounted at the cell than you only need a very small gauge wire back to the BMS for reading the voltage and a second one for turning on the balancing mosfet. The advantage to this is that should there be a wiring error or a short sometime in the future, having small wires would mean they would act as a fuse and burn up quickly with less chance of a fire. If you short a 18 gauge wire it will handle enough current for long enough that there is a much higher chance of fire. Maybe that's being overly cautious, I don't know, but you can decide!


Exactly my contention, one should never use a shunt in a remote location for reasons that should be obvious. I'm not a believer of those type of balancers to begin with, and I dont care to discuss why.

First of all your wires are part of the resistance and better be heavy enough to handle it, not to mention you better have a fuse at the battery connection in case of a short, your dealing with high voltages.

Now if one wanted to just monitor cell voltages, cat5 cable would be good to use and easy to get, not to mention it has twisted pairs.
And you will still need small pigtail fuses at the battery even if you just use it for voltage indication.

Roy


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## dimitri (May 16, 2008)

Since all cells are connected in series already, you don't need to run 2 wires per cell back to central location. You only need N+1 wires where N is number of cells. You run negative wire from most negative cell and then only positive wire from all cells. I have several customers using MiniBMS in centralized fashion and it works very well with 18Ga wire.


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## dtbaker (Jan 5, 2008)

Roy Von Rogers said:


> Now if one wanted to just monitor cell voltages, cat5 cable would be good to use and easy to get, not to mention it has twisted pairs.


I want to have heavy enough gauge so that I can check and potentially balance cells manually from a central location. I am NOT starting with a BMS; I want to monitor manually, see how much balancing is required, try top-balancing with either charge/discharge to individual cells if needed... so I think I need pairs to each cell, and heavy enough to take a couple amps either direction if I want to balance a specific cell.


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## etischer (Jun 16, 2008)

You would use 2 wires if you wanted to measure the battery voltage while shunting. If measuring voltage using the same wires the shunting current flows though, there will be some voltage drop. 



dimitri said:


> Since all cells are connected in series already, you don't need to run 2 wires per cell back to central location. You only need N+1 wires where N is number of cells. You run negative wire from most negative cell and then only positive wire from all cells. I have several customers using MiniBMS in centralized fashion and it works very well with 18Ga wire.


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## dtbaker (Jan 5, 2008)

etischer said:


> You would use 2 wires if you wanted to measure the battery voltage while shunting. If measuring voltage using the same wires the shunting current flows though, there will be some voltage drop.


right, I figured I needed pairs to each cell to allow charge/discharge of individual cells... what I would like to learn more about is the best time to monitor, and best techniqe to attempt 'manual balancing'.

I am assuming that at the end of a charge cycle, all the cells *should* be very close to the average 3.65 per cell given the charge curve I intend to use. I am thinking that if i periodically catch the system at or near the end of the charge cycle i would be able to spot cell(s) that are +/- more than.....0.03? .05? compared to the rest, and either add a little juice with an individual charger, or drain a little with a small load.

Does anyone have any more specific input for 'manual balancing'?


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## Dolphyn (Nov 17, 2009)

I've found that very thin wire can be used for monitoring. I used over 10 feet of 28-gauge ribbon wire, and the results seem to be accurate. I used a readily-available 50-pin connector to connect the ribbon wire to a pair of CellLog 8S devices.

On the other hand, at one point I tried connecting some 2-amp chargers part way along the ribbon wire, which was a disaster; poor charging and skewed measurements. 

I think the best bet is to use completely separate wire for monitoring, vs any external charging or shunting.


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