Wiring batteries in parallel

[SweareDeep]

Working through the details of my upcoming install of 3 x 200 Ah Victron LFP batteries ...

Standard practice is to wire the batteries individually to a common positive connection (with fuses on each connection) and to a common negative connection, with each positive connecting cable and each negative connecting cable the same length.

However, if the common positive and negative connections are at opposite ends of the set of batteries it would be neater (less excess cable) to instead ensure that the sum of the length of the positive connecting cable and the negative connecting cable is the same across the batteries. In other words, the battery at one end could have a shorter positive cable and a longer negative cable, the middle battery equal medium length cables, and the battery at the other end a longer positive cable and a shorter negative cable. Provided longer plus shorter equals two times medium the cabling resistance will be the same for each battery, which is the goal.

Granted, there will be a small potential difference between the positive terminals on the different batteries but that will be balanced by a corresponding potential difference between the negative terminals, though there are some circumstances involving battery monitoring relative to a common negative where this might cause a small (likely inconsequential) discrepancy.

I can't see any shortcomings but I may have missed something.

[BillKny]

Originally Posted by SweareDeep

Working through the details of my upcoming install of 3 x 200 Ah Victron LFP batteries ...

Standard practice is to wire the batteries individually to a common positive connection (with fuses on each connection) and to a common negative connection, with each positive connecting cable and each negative connecting cable the same length.

However, if the common positive and negative connections are at opposite ends of the set of batteries it would be neater (less excess cable) to instead ensure that the sum of the length of the positive connecting cable and the negative connecting cable is the same across the batteries. In other words, the battery at one end could have a shorter positive cable and a longer negative cable, the middle battery equal medium length cables, and the battery at the other end a longer positive cable and a shorter negative cable. Provided longer plus shorter equals two times medium the cabling resistance will be the same for each battery, which is the goal.

Granted, there will be a small potential difference between the positive terminals on the different batteries but that will be balanced by a corresponding potential difference between the negative terminals, though there are some circumstances involving battery monitoring relative to a common negative where this might cause a small (likely inconsequential) discrepancy.

I can't see any shortcomings but I may have missed something.

I have tried to read through what you are suggesting, but got lost in the words. A picture would be very helpful...

[SweareDeep]

This diagram shows Victron's recommended practice. I'm simply saying that instead of each positive wire being the same length and each negative wire being the same length, what matters is that the combined length of the positive wire and the negative wire should be the same across the three batteries.

[GordMay]

The best parallel connection scheme (Pos from one end & Neg from the other - not both from same batt), shown below, charges/discharges the entire bank equally.

[Jamesiv1]

Please forgive a newbie question:

If a set of batteries is not in "parallel", then what is it?

Why do they need to be in parallel?

[BillKny]

Originally Posted by GordMay

The best parallel connection scheme (Pos from one end & Neg from the other - not both from same batt), shown below, charges/discharges the entire bank equally.

Actually, the shown diagram most certainly NOT best. For TWO batteries it is good, but for more than that, the center batteries get lower and lower charge/discharge cycles.

Connecting batteries in parallel to get higher capacity at the same voltage results some issues. We have a couple different ways we can accomplish this. At a first glance, they might appear functionally identical, but it turns out that is not the case.



Connection geometry “A” is in most cases the simplest to assemble. Fewer, shorter cables are needed. Unfortunately, is is the worst arrangement for the batteries. The very small, but non-zero resistances, of each of the cables, connection points and the batteries’ internal resistances matter when charging currents, or discharging currents, are high. Calculating the result is not simple, but it can be fairly easily accomplished with a circuit simulator and under high loads the battery closest to the load can easily work twice as hard as the one furthest “downstream.” It will cycle deeper and work harder, and die earlier.

Geometry “B” is an improvement. For a bank consisting of only two batteries, this is as good as it gets, but as we add more batteries to the bank, the differences between batteries in the middle of the bank and those at the “ends” grow larger and larger.

By now you have probably guessed that Geometry “C” is where we want to be. You would be correct. Each battery is not directly connected to any of its neighbors, but rather to a central connection point. Each cable from a battery terminal to the connection point is as close to the same length and construction as possible. As a result, each battery has exactly the same path to the load and charging sources as all of the others. Each sees exactly the same voltage during charging. Everybody is happy. Should one battery develop a problem, it can be easily removed from the system without extensive rewiring.

Any other arrangement is second-best, at best.

[Discovery 15797]

Originally Posted by Jamesiv1

Please forgive a newbie question:

If a set of batteries is not in "parallel", then what is it?

Why do they need to be in parallel?

Hi Jamesiv1,

Batteries can be wired in "parallel" or in "series."

In parallel wiring positives are interconnected and negatives are interconnected (as shown in the diagrams in this thread). Parallel wiring of batteries serves to increase the overall amperage.

Series wiring connects the positive of one batter to the negative of another battery. The purpose of series wiring doubles the voltage, but overall amperage is unaffected.

For example, I myself use 4 6 volt 220 amp hour batteries. Wiring 2 batteries in series gives me 12 volts. The with the other 2 (also wired in series) I wire the 2 battery 2x banks in parallel to give me 440 amp hours at 12 vdc.

This assumes all batteries are same voltage and amperage ratings.

[SweareDeep]

Originally Posted by GordMay

The best parallel connection scheme (Pos from one end & Neg from the other - not both from same batt), shown below, charges/discharges the entire bank equally.

That's a perfectly good configuration for lead acid batteries but for LFP the recommendation is to fuse each individual battery hence the configuration shown in the Victron schematic.

The configuration you show can also be used for paralleling cells when building an LFP battery from cells, but paralleling cells to build a battery is different from paralleling batteries.

[Jamesiv1]

Originally Posted by Discovery 15797

Hi Jamesiv1,

Batteries can be wired in "parallel" or in "series."

In parallel wiring positives are interconnected and negatives are interconnected (as shown in the diagrams in this thread). Parallel wiring of batteries serves to increase the overall amperage.

Series wiring connects the positive of one batter to the negative of another battery. The purpose of series wiring doubles the voltage, but overall amperage is unaffected.

For example, I myself use 4 6 volt 220 amp hour batteries. Wiring 2 batteries in series gives me 12 volts. The with the other 2 (also wired in series) I wire the 2 battery 2x banks in parallel to give me 440 amp hours at 12 vdc.

This assumes all batteries are same voltage and amperage ratings.

Thanks Discovery. I appreciate the explanation.

[Dsanduril]

The problem with "C", as shown, is that the middle battery will see a greater charge/discharge cycle because its cables are shorter and thus less resistance than the end batteries. Essentially the same as the "B" case, although with a lesser magnitude of difference. Good enough? Probably....

Victron's advice it to keep all battery cables the same length, in order to get the same losses along each cable. I get the OP's question, using that reasoning, if the length of the positive + negative cable for each battery is the same, does that provide the same effect? The overall voltage drop would be the same at each battery, but the voltage drop would occur at different places. In one battery it would be mostly in the positive cable, in another about equal in both cables, and in the final battery it would be mostly in the negative cable. Does that make a difference when compared with the "all cables the same length" installation? Sorry, OP, I can reason it out lots of ways so don't have an answer.

[PirateGuy]

Originally Posted by Dsanduril

The problem with "C", as shown, is that the middle battery will see a greater charge/discharge cycle because its cables are shorter and thus less resistance than the end batteries. Essentially the same as the "B" case, although with a lesser magnitude of difference. Good enough? Probably....

Victron's advice it to keep all battery cables the same length, in order to get the same losses along each cable. I get the OP's question, using that reasoning, if the length of the positive + negative cable for each battery is the same, does that provide the same effect? The overall voltage drop would be the same at each battery, but the voltage drop would occur at different places. In one battery it would be mostly in the positive cable, in another about equal in both cables, and in the final battery it would be mostly in the negative cable. Does that make a difference when compared with the "all cables the same length" installation? Sorry, OP, I can reason it out lots of ways so don't have an answer.

While Sketch “C” is drawn with apparently shorter cables for the middle battery, BillKny’s post clearly states that all of the battery cables need to be the same length - ie the sketch isn’t to scale, and only shows the nodal geometry. It would be relatively easy (with suitably flexible cable) to loop battery cables of inner batteries wired in manner of Sketch “C” so all the battery cables are actually of the same length.
I currently use the wiring of Sketch “B” for my small (about 150 A*hr) LFP bank, but might at least consider changing it at some point.

[Dsanduril]

Yes, but the "same length" was the OP's original question. What if the negative of one is shorter, but the positive is longer in compensation. If length(negative + positive) is the same for all three (or more) batteries does this achieve the same purpose as having all positive cables and all negative cables the same length?

[PirateGuy]

Originally Posted by Dsanduril

Yes, but the "same length" was the OP's original question. What if the negative of one is shorter, but the positive is longer in compensation. If length(negative + positive) is the same for all three (or more) batteries does this achieve the same purpose as having all positive cables and all negative cables the same length?

I’m not sure - my one and only required electrical engineering course was about 45 years ago.
Being it’s just simple DC electricity, one might easily draw a circuit diagram, and using Ohm’s Law (and substituting R resistors to model the cables) figure out whether or not compensation between the negative and positive cable lengths makes any difference.
My gut reaction is to say as long as the resistances in total (positive AND negative) for the battery attachment cables are the same, the voltage drop across each battery will be the same.

As people have commented, this only matters for battery chemistries where internal battery resistance is very low, as in LFP. Lead acid batteries, AFAIK, have such high internal resistances that differences in attachment cable lengths make little if any difference.

[BillKny]

Originally Posted by PirateGuy

As people have commented, this only matters for battery chemistries where internal battery resistance is very low, as in LFP. Lead acid batteries, AFAIK, have such high internal resistances that differences in attachment cable lengths make little if any difference.

Just because people SAY it doesn't make it true.

Here is the data: https://coastalclimatecontrol.com/in...nnections.html

As you can see if you actually look at what Coast Climate presents, you'll see it DOES matter how you connect the batteries. It is hard to do a proper analysis with Ohms Law, because you have so many moving pieces, but it you use a circuit simulator to solve all the simultaneous equations, you'll see it does matter.

Now, it does matter only at high currents, but at least sometimes that is how we discharge our batteries, and hopefully charge them!

Of course you can go with gut feel... but I prefer data.

[Uncle Bob]

Originally Posted by BillKny

Just because people SAY it doesn't make it true.

Here is the data: https://coastalclimatecontrol.com/in...nnections.html

As you can see if you actually look at what Coast Climate presents, you'll see it DOES matter how you connect the batteries. It is hard to do a proper analysis with Ohms Law, because you have so many moving pieces, but it you use a circuit simulator to solve all the simultaneous equations, you'll see it does matter.

Now, it does matter only at high currents, but at least sometimes that is how we discharge our batteries, and hopefully charge them!

Of course you can go with gut feel... but I prefer data.

Sketch C is pretty much how I have my three pairs of six volt lead acid wet cell batteries wired up. seven years now and still going strong.