How closely matched do parallel battery cable lengths really need to be?

[crossy]

We always say that when connecting batteries in parallel that the cable length to each battery should be "the same". This is, of course, very sensible advice.

But, just how much difference in length (as a percentage of overall) is actually acceptable?

Would it be wise to actually have spare cable on the run to the closer packs?

I'm asking because I'm re-vamping my battery arrangements somewhat whilst adding additional capacity, but, due to geography (and some laziness) a couple of packs will be about 0.5m further away from the bus bars than others.

Current run length is about 5m so we're adding about 10%.

So, make all the cables actually the same length and coil the spare (in a way that means it won't overheat) or ...

Thoughts?

 

[rhino]

I'm actually curious about this as well and if there is any hard evidence that you need to worry that much about this when using LiFePO4 batteries. With lead acid I can easily see this would be an issue but with LiFePO4 I'm not convinced. Even if one ends up working slightly more than others does that really matter when it should have a lifetime of 1000's of cycles?

 

[Ellcon123]

My cables are the length required to go from the battery to the busbar. No idea if they are the same length but parallel batteries keep them selves sorted out. So many points of possible resistance that I feel cable length is the least of my problems.

 

[MurphyGuy]

We always say that when connecting batteries in parallel that the cable length to each battery should be "the same". This is, of course, very sensible advice.

But, just how much difference in length (as a percentage of overall) is actually acceptable?

Would it be wise to actually have spare cable on the run to the closer packs?

I'm asking because I'm re-vamping my battery arrangements somewhat whilst adding additional capacity, but, due to geography (and some laziness) a couple of packs will be about 0.5m further away from the bus bars than others.

Current run length is about 5m so we're adding about 10%.

So, make all the cables actually the same length and coil the spare (in a way that means it won't overheat) or ...

Thoughts?

The reason for the suggestion that they be the same length is due to resistance in the cable. But what they are referring to is one battery bank run being 3 feet long and the other battery being 40 feet away.

Two parallel battery banks are always going to balance themselves to the same voltage. Whichever bank has the higher voltage is going to charge up the other bank.. So its not an issue when the batteries are just sitting there doing nothing.

However, when charging the battery banks (or discharging), the one with the longer cable run isn't going to charge as fast as the shorter one because of the extra resistance in the cable.. This would cause the shorter (closer) bank to always be at a slightly higher voltage when charging, and a slightly lower voltage when discharging.

But they're talking meters of difference, not inches.. Also, the larger the cable you use to push the same amount of current, the less the effect will be.

So for example, if your current draw is 100 amps and you use a wire rated for 100 amps, the imbalance effect would be greater than if you used wire rated for 150 amps.

A 10% difference on a 5 meter run shouldn't cause any issues unless you're using the minimum size wire you can get away with and you plan to be running that wire near its rating all the time. Even then, a 1/2 meter at that distance probably wouldn't cause a problem.

 

[timselectric]

Between batteries, isn't as much of a problem as Between stacked inverters/AIO's.
Paralleled batteries will always self balance by voltage.
The goal is to keep them close in SOC, as much as possible. But, never expect them to be perfect. No matter how hard you try.

 

[Vigo]

Im paralleling lifepo4 with horribly unequal wiring. Its working fine so far. As mentioned its mostly a problem if you are running them up or down at high C rates. If you are using them at a low c rate most of the time it should be fine.