Where to Locate one odd cable in a dual parallel string bank

Farlander · May 2017

Thanks in advance to on topic, thoughtful replies!

I have a 48V AGM battery bank comprised of 8, 12V "Energy Power" (Vision brand) EV8D batts. The batts are wired as dual parallel strings of 4. During installation, one of the 4/0 battery cable terminal ends got damaged from an arc. I'm trying to figure out if there is a "best location" to put this particular cable in the bank. It made sense to me that it might be best to install this cable as one of the string interconnecting cables, rather than as a string "intra"connecting cable, to keep the resistance of each strings relatively the same.
Thoughts?

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dennis461 · May 2017

Put a new terminal on the cable.

mcgivor · May 2017

You could use an L lug, cut the ring off, insert the end and torque it down.

Photowhit · May 2017

Yeah, find a battery shop or an auto parts store and get a new lug, ask ahead if they have a crimper that will handle it, if not find a welding parts supply (they likely will have the lug also) and mount a new lug, The old one can be cut off so that they are the same length, cut down the sides away from the compression. You'll do it at some point and the connection might be questionable other wise...

Lumisol · May 2017

Get a crimper and a new lug and fix it.
I got one and made my own custom length cables so I don't have extra loops anywhere.

Farlander · May 2017

Making a new lug / adding a new lug is basically impossible as this system is located in a third world country on an island in the middle of the ocean VERY far away from any type of hardware store or machine shop and I am not planning on going back there. Only option is have the island staff relocate or nothing. Original question still stands.

Horsefly · May 2017

I may be misunderstanding the question. If you want to keep two parallel strings of four batteries, you need both your current wire and the proposed wire. If you can only do one or the other, it doesn't matter which you do. You'll have only one string of four batteries. Still 48V, but half of the batteries won't be contributing.

mcgivor · May 2017

Farlander said:

Making a new lug / adding a new lug is basically impossible as this system is located in a third world country on an island in the middle of the ocean VERY far away from any type of hardware store or machine shop and I am not planning on going back there. Only option is have the island staff relocate or nothing. Original question still stands.

Could the existing terminal end be flattened and a new hole drilled to improve contact, or at very least, clean with a file and add a washer after flattening down the existing terminal, again to improve contact area. Both existing and proposed locations are equal as far as current is concerned so relocation depends on wether there is enough slack to do as mentioned above. Just some thoughts.

What sort of loads are the batteries supporting?

Farlander · May 2017

Hi, the question pertains to balancing the resistance of the two strings to make them as even as possible. Would it be advantageous to move one "odd" cable to the a string interconnecting position rather than having it "inside" of one string. All the cables are good enough to use it's just that I'm stuck with one damaged / odd cable. Batts total 660 Ah, charged by a 6.6 kW PV array with generator backup. Loads are all AC off a 4.4 kw inverter. Average DC amp draw is 7 amps minimum, maximum of 30, with surges up to 80 amps. Refrigerators, water pumps, lights, fans, etc.

Farlander · May 2017

@ Mcgivor,

Yes I drilled a new hole in the damaged cable and cleaned it up. All of the cables are the same length (they were part of an older battery system) except for the main pos and neg cables, they are 8' long. The rest are 24". Here's a pic of the bank, notice we had to turn one battery sideways to make the cables reach. Isn't it possible that by making that cable the interconnecting cable it would affect both strings instead of just one? Maybe I'm over thinking this. Not sure what the effect on resistance is given the small difference in surface area.

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mcgivor · May 2017

Difficult to say wether one position is better than another, assuming the peak load is intermittent, such as a motor start, a volt meter across the affected connection, post to terminal, under maximum load would reveal if there is in fact a resistive loss, useful if the meter has a max-min setting, if a voltage is detected there is a resistive loss. Other than that increasing the contact area is as good as you could get......on a remote island in the middle of nowhere, it might not be as bad as it looks.

jonr · May 2017

Probably not a practical answer for you, but best would be to measure balance between the two strings with a DC clamp ammeter and adjust cables and/or shuffle batteries accordingly. My guess is that internal variations in the batteries are more significant than that cable end.

Keep in mind that temperature has a huge effect on lead acid battery life - every degree matters.

Farlander · May 2017

Thanks everyone - so making the cable an "interconnecting" versus an "intraconneting" is not going to improve string balance? My idea was to attach the damaged cable ON TOP of the main negative or positive cable, with the goal of making the damaged cable basically "shared" by both strings. Is it very common for banks with only 2 strings to actually get out of balance?

Estragon · May 2017

Wouldn't putting it under the main cable be slightly better (more contact area with lugs than the nut)?

With sealed batteries you'll want to measure current balance periodically as you can't tell from SG. I suspect any parallel bank will eventually have some degree of imbalance just from manufacturing variance, temp differences, etc. Even cells in the same battery get out of balance over time.

Horsefly · May 2017

Now that I understand what you are asking.... Having the odd cable in either of the two positions will have the exact same effect. In both cases, the resistance in the upper string will be different than that of the lower string, by the same amount in both cases. If the wire you are talking about is smaller than the others, the resistance in the upper string will be higher, so the lower string will bear more of the load, so there will be some imbalance introduced over time.

mvas · May 2017

What if you moved the damaged connector to the "Common Negative" Terminal?
Common Negative = the Lower Left Negative Terminal in your schematic.
Stack the connectors as follows, from top down ...

[ Nut ]
[ Temperature Sensor? ]
[ Main Neg Power Cable ]
[ Damaged Connector ]
[ Battery ]

Does this re-wiring cause the damaged connector to be "partially shared" by both strings?
Would this arrangement be "less worse" than what you have?

There are absolutely no deliveries to the island, ever?
What will happen when a battery goes bad?

Where to Locate one odd cable in a dual parallel string bank

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