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Uneven Parallel wire length

1051 Views 8 Replies 4 Participants Last post by  major
Hello all,

Planning on putting 20 leaf cells (in series using the stock connections) in a battery box in the back of my car, and another 20 cells in the front engine bay. I was planning on paralleling the two packs with bus bars in the main wiring box of the car, but I recently watched Jehu's video where he had a wire in a small parallel bundle completely burn as it was the shortest, and took the majority of the current (least resistance). I'm just wondering if this would affect my pack, as it would draw more from the front pack and negate the bottom balancing of the pack.

Cheers!
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I'm just wondering if this would affect my pack, as it would draw more from the front pack and negate the bottom balancing of the pack.
You could connect to opposite corners of the combined pack; take the positive lead to the motor controller from the front pack, and the negative lead from the back pack, or the other way around. Two cells or sub-batteries can be made to share current perfectly, in theory. But that means an extra cable that could be a nuisance.

The other thing is that nearly every battery chemistry other than LiFePO₄ will share SOC pretty well. If the front gets lower in State Of Charge (SOC) than the back, then its voltage will be lower than the pack at the back, so current will flow from the back to the front to even out the SOC. This happens continuously while you drive or while stopped (edit: with the contactors on, e.g. stopped at lights), though it could take some time to even up the SOCs since the balancing currents tend to be low. That effect should prevent balancing problems, but I think it should only be relied on to "mop up" minor imbalances resulting from differing cable lengths and differing resistance of connections.

I think it's worth doing the corner connection thing, just so that the front pack doesn't do most of the heavy discharge work (assuming front wheel drive), with the back pack lazily charging the front pack and taking only a modest share of the motor load. The front pack would wear out faster than the back in that situation.

In theory, the paralleling connections from front to back will only take half the load current, so they could be thinner than the connections to the motor controller. I would not make them half the cross section however; perhaps 1 or 2 AWG higher than the cables to the motor controller. That mitigates a little the space and money cost for the extra full-area cable.
 

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Jehu's video where he had a wire in a small parallel bundle completely burn as it was the shortest, and took the majority of the current (least resistance). ...
There's some flawed logic because copper resistance rises enough with temperature that current division between unequal lengths in parallel would equalize enough to prevent only one of the four from overheating to that degree. There most certainly was some other factor causing that failure.

Also I fail to see why someone would have made a multistrand cable from different lengths. It didn't appear they actually took a close look or dissected the crimps. Maybe all 4 strands were equal and all but the burnt one had pulled loose out of the crimped lug leaving only the "short" one to carry all current.

But copper parallels very well due to its positive resistivity coefficient. Lenght tolerance isn't normally a factor for wires and bars.

My opinions.

major
 

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Discussion Starter · #4 ·
I think it's worth doing the corner connection thing, just so that the front pack doesn't do most of the heavy discharge work (assuming front wheel drive), with the back pack lazily charging the front pack and taking only a modest share of the motor load. The front pack would wear out faster than the back in that situation.
Sorry Coulomb, I don't quite understand what you mean. If I were to take the positive connection the front pack and the negative connection or the rear pack to the motor controller, I would have a series connection which would give me 366V rather than the 168 max that my controller can handle. I do however understand the idea behind the natural balancing over time at rest, but while driving I was unsure about the balance in capacity of the front/back over time as I am not planning on running a BMS.

But copper parallels very well due to its positive resistivity coefficient. Lenght tolerance isn't normally a factor for wires and bars.

My opinions.
Thanks major! I was never taught about increased resistance with heat, so that never crossed my mind. As for why Jehu would possible use a shorter cable on one side, not too sure: possibly going around a very tight bend?

I only ask as there is going to be about 9 feet of cable connecting the rear batteries to the front bus bar, so I figured at that length the resistance would play a bit more of an effect then a couple of inches difference.
 

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As long as your cable is properly sized and quality, I don't see a problem. If the load (or charge) share is unequal due to resistance of the 9 ft cable, then it will self equalize as the load (or charge) current diminishes because the voltage drop (I*R) in the cable goes to zero and voltage on both sides of the cable will be the same. If you're going to lose sleep over it just use a gauge size or two larger.

Regards,

major
 

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My Solectria E10 originally had 3 parallel 144V 40 AH strings, one in the front battery box, two in the back battery box:

All the negative cables when to the current shunt on the firewall.
All the positive cables when to the back battery box.


When I switched to gel cells I had 2 parallel 144V 75 AH strings:

2 negative cables when to the current shunt on the firewall.
2 positive cables when to the back battery box.
1 of the cables was isolated and used to run from the front battery box to the back battery box.

All the cables are 2AWG, never had any problems with imbalance between the packs. :)
 

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Sorry Coulomb, I don't quite understand what you mean.
Like the attached. Half the current comes from the nearest battery, the other half comes from the further away battery via the thinner cables.

I only ask as there is going to be about 9 feet of cable connecting the rear batteries to the front bus bar, so I figured at that length the resistance would play a bit more of an effect then a couple of inches difference.
The inches of difference is negligible. But in my humble opinion, connecting both controller cables to the front pack is asking for trouble.
 

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I don't know if it'd cause trouble but your method leaves a large area in the battery side circuit to controller which will put inductance where you don't want it. Tesseract used to say to twist the power cables together on the input side of his controllers IIRC.

major
 
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