Calculation of parallel string battery currents

[Solarod]

To keep it simple, let's assume that the load is in the middle. Reality is a bit more complicated as my bus bar has two rows of studs, so batteries are connected on one set of studs and all load/charge devices are on the second row. The Blue Sea Bus Bar is one solid piece of copper. Even that's not how I have it wired at this time, but it's close enough.

My calculations in this thread have shown that just where the load connection is made to the busbars can have an effect on balance, but if the busbars are very thick the effect will be quite small. I suppose that there might be a particular connection of load to your busbars, with which you might achieve the "perfect" balance, but it looks to me like your busbars are very thick, and it probably won't be worth going for a minuscule improvement.

An advantage of the "perfect" balance connections I've shown in this thread is that the balance is not upset by higher link resistance, so that less thick busbars could be used and still get "perfect" balance. But you already have very thick busbars so there is no advantage to you to change.

 

[Solarod]

Can you split the 5 parallel into one with 3 in parallel and one with 2 in parallel, get the optimized connections for those two and then connect the output from the two bus bars to an additional bus bar and try to optimize the connection from that bus bar?

Are all the batteries the same, or do you have a group of 3 of one kind and another group of 2 of a different kind?

 

[Solarod]

All the same in this scenario

I'm going to deal with this in another thread. There are some additional factors I haven't been addressing in this thread.

 

[Solarod]

Finally, here are some results for a parallel string of 6 batteries. I probably won't go beyond this number of batteries.

I'm using a value of 1 milliohm for the battery IR, and .025 milliohm for the link resistances which I think is more representative of what one gets with heavy busbars rather than cable links. The load current is 100 amps. I'll reference this image:

First the calculated theoretical battery currents in amps for a standard diagonal connection like the red/black connections above:

Example 29
18.0
16.4
15.6
15.6
16.4
18.0


Let's try moving the load connection toward the middle of the string. If the black load connection is moved to the point on the busbar adjacent to the negative terminal of battery 2, and the red load connection is moved to the point on the busbar adjacent to positive terminal of battery 5, we get these theoretical currents in amps:

Example 30
16.4
17.2
16.4
16.4
17.2
16.4

This seems to be fairly good, and it's an easy connection to make. I would say that it's definitely worth while to move the load connection here rather than at the full diagonal location.

In some of the earlier hookups it was possible to get a perfect balance by connecting the load to the right place on the busbars between the last and next to last batteries in the string. I tried to do that for this 6 battery string by varying the connection point. It isn't possible to get perfect balance, but it is possible to get improvement. I had to consider how to measure the improvement, and what seems reasonable is to get the least variation in the battery currents. I had my calculations include the standard deviation of the battery currents, and tried all the connections between the end batteries and the next-to-the-end batteries in 5% increments. For this 6 battery string, the connection which is 67% of the way from the last battery to the next battery inward gives the minimum variance in the battery currents. This would be a connection like the yellow/blue connection shown in the image above, at the 67% point. Here is the result of the 67% connection theoretical battery currents in amps:

Example 31
16.9
16.9
16.1
16.1
16.9
16.9

There is a definite improvement.

 

[RCinFLA]

Having identical wiring and terminal connection resistance will only take you so far. Variations in matching of individual battery's over-potential voltage slump vs battery current will also cause current distribution variance between parallel batteries.

An older cell or abused cell with greater internal impedance will be at a disadvantage against a newer cell with lower internal impedance. Just manufacturing tolerances causes cell matching variance.

Temperature variations between batteries, like being near an outside wall versus inside positioned batteries will cause matched cells to be unmatched in discharge overpotential voltage slump with current to be different.

Better than 10% variance in battery current should be considered good.

 

[Solarod]

Having identical wiring and terminal connection resistance will only take you so far. Variations in matching of individual battery's over-potential voltage slump vs battery current will also cause current distribution variance between parallel batteries.

An older cell or abused cell with greater internal impedance will be at a disadvantage against a newer cell with lower internal impedance. Just manufacturing tolerances causes cell matching variance.

Temperature variations between batteries, like being near an outside wall versus inside positioned batteries will cause matched cells to be unmatched in discharge overpotential voltage slump with current to be different.

Better than 10% variance in battery current should be considered good.

The very first paragraph in my post #1 says:

"Readers should understand that the results I'm going to post are theoretical values obtained with mathematical circuit analysis. In the real world things will be different since battery IR is quite variable due to temperature, age, SOC, etc., whereas in the mathematical analysis the parameters are exact and don't vary. But the mathematical results are a useful starting point."

Thank you for explaining in greater detail than I did how much variation in all this occurs in the real world. I don't want readers of this thread to think that a theoretically balanced connection such as Victron's "Post" or "Halfway" connection, or the connection I show in Post #4 somehow isn't subject to the effect of real world variations of battery parameters on balance.

Because the modern LFP batteries have such low IR, achieving good balance requires paying attention to things like good clamping resistance. A mathematical analysis is a quick way to explore sensitivity to small changes in circuit resistances.

Starting with hookup configurations that are inherently balanced is a good thing, such as the one I disclosed in post #4, which is trivially easy to do with busbars. My discovery of this connection was quite fortuitous, a result of analyzing several connections, and realizing that there was a way to go halfway between the Example 4 and Example 8 connections.

Consider the unbalanced currents in Example 7 even with very low link resistance. It's astounding to me that a simple change in the load connection point to the busbars could lead to theoretically perfect balance. Even though in the real world all the factors you mention (and I mentioned) will prevent a perfect balance, that starting point cannot be bettered for the 4 battery parallel string.

 

[RCinFLA]

For the individual cells, like 280 AH EVA's, a lot of folks have difficulty achieving good low resistance connections with the aluminum terminals. Aluminum oxide build up is a problem and should be cleaned off before making bus bar clamp downs. Keep your fingers off cleaned contact surfaces. A good terminal connection resistance is less than about 0.07 milliohms but it is easy to get over 0.2 milliohms if not prepared well.

Make sure you have copper core bus bars, not brass. It would be terrible to have an unknown mixture of both types randomly applied between multiple parallel strings. Best to sample bus bars by grinding off end plating to expose base metal. Copper is brownish, brass has a lighter more yellow-brown color. Very easy to tell the difference by resistance reading with a YR1035+ battery impedance meter, no grinding off plating required. Worth the $35 to have one. Copper bus bars are about 0.05 milliohms where brass is like 0.18 milliohm, for typical 2 mm thick, 20 mm wide, 72 mm hole spacing bus bars. YR1035+ are not super accurate below about 0.3 milliohms but you can check calibration with a good quality current shunt. 500A/50mV shunt is 0.10 milliohms, 500A/75mV shunt is 0.15 milliohms. My YR1035+, as received, reads my Victron 500A/50mV shunt as 0.11 milliohms. 10% off at 0.1 milliohm is not bad.

Self-contained 12v LFP battery doesn't have as much trouble with their non-aluminum terminal connections but questionable quality internal BMS series resistance and having BMS in each series connected 12v battery adds its own variability to net battery impedance for that approach.

 

[Henderson]

I've seen comments on the forum suggesting that, given a string with load connected in the diagonal manner, there might be a benefit to making the load connections more toward the middle of the string rather than at the very corners. For the 4 battery parallel string that would be done like this:

Setting the initial parameters back to: battery IR of 5 milliohms, link resistance of 1 milliohm, load current of 100 amps, the calculation gives this result for the theoretical battery currents in amps:

Example 8
20.8333
29.1667
29.1667
20.8333

Compare this to Example 4. The values of the currents are exactly the same, but distributed among the batteries in kind of an anti-symmetric way. When I saw this, I thought "I wonder if there is a way to make a connection halfway between the two methods?". Consider the link cable between the negative terminals of battery 1 and battery 2. At a point halfway between the ends of the cable strip away an inch of insulation and expose the copper conductor. Connect the negative load cable at that point. I know this is impractical but bear with me. Now in a similar manner expose the conductor at a point halfway between the ends of the link cable connecting the positive terminals of battery 3 and battery 4; connect the positive load cable at that point. It should be as illustrated in this image:

Now with 4 identical batteries having indentical IR of 5 milliohms, link cable resistance of 1 milliohm and load current of 100 amps the calculation shows that the battery currents in amps are:

Example 9
25.0
25.0
25.0
25.0

We have (theoretically) perfect balance! Is this a previously unknown connection giving perfect balance, like the "Halfway" connection shown in the Victron document? Does anyone know if this been published anywhere?

A very practical way of making this connection is possible when busbars are being used. Just drill a couple of holes at points on the busbars at the pertinent points halfway between the appropriate battery terminals and make the load connection there. This image shows what I mean:

The usual diagonal connection would be as shown with the red and black wires. The new connection would be as shown with the yellow and blue wires.

Even more useful are the following properties. For this new connection, it doesn't matter what the battery IR is!! The calculation of the battery currents remains (theoretically) perfectly balanced for any value of IR as long as all 4 batteries have the same IR. This is very handy because as the batteries are discharged, their IR will change but this change will not upset the (theoretical) perfect balance. Also, the resistance of the links has no effect on the (theoretically) perfect balance! This means that the busbar need not be very thick copper; it can be thinner and higher resistance than would normally be needed for good balance. The only restriction is that its resistance shouldn't be so high that it gets too hot.

A mathematician would say that the (theoretically) perfect balance is invariant with respect to battery IR and link resistance. These properties are crucially dependent on the batteries having identical IR.

I expect that some reading about this may be skeptical. I was skeptical myself at first, but I've checked it several times. I invite verification by members of the community. Perhaps someone will do a simulation with Spice. The ultimate proof would be a hardware proof. If someone already has a string of 4 identical batteries in parallel with busbars, they could drill the two new holes and connect the load there, subsequently checking the balance.

This analysis is awesome. Thank you for this. If I were to bring the battery situation in fully to the DIY side of things like what I am building, wouldn't it be better then to build one large battery? So in a case where your picture shows 4 separate batteries (I'm assuming 48V), for the purpose of avoiding balancing issues, would it be better then to build a single 4P16S battery? I know redundancy wouldn't be present with a single large battery and they are BMS-es available that can handle large amounts of current.

 

[AndyRonLI]

My interpretation of all this:
The goal is to have the exact same line resistance from each terminal to the common point where the load current is drawn.
Is that correct?
Andy

 

[chrisski]

My interpretation of all this:
The goal is to have the exact same line resistance from each terminal to the common point where the load current is drawn.
Is that correct?
Andy

Yes, to the load itself; the point where all wires from the terminals come together. In my case I join at a busbar.

 

[Solarod]

My interpretation of all this:
The goal is to have the exact same line resistance from each terminal to the common point where the load current is drawn.
Is that correct?
Andy

For the "Posts" configuration, yes, the goal is to have the same cable resistance from each battery to the load points; also you need the batteries to be nominally identical.

But for the diagonal connection of 4 batteries in parallel, as in this image, the cable resistance from each battery to the load connection point is the same, but the configuration is not perfectly balanced. The simple rule of thumb leads to a wrong result:

In order to get a theoretically perfect balance this connection must be used. In this case the cable resistance from batteries 1 and 4 is different than the cable resistance from batteries 2 and 3 to the load connection point. One's intuition is fooled here; only a full circuit analysis of the hookup reveals the truth. See post #4 of this thread for more about this.

 

[SniperX]

I literally marvel at how smart people are and the fact that they are willing to share that with others. Thank you. I followed the whole thread. I may not be a math genius, but I can see the results and know that what you have suggested (figure number 30 for 6 parallel batteries) is the best configuration to use. Thank you!

 

[copec]

This is great! I studied EE for a short time before finishing in pure Math (although I've only ever worked in IT), and I took the intuitive approach and figured the path is equal for each battery with the diagonal connection, but yet again, intuition fails!

 

[Danke]

I was going to use wires in diagonal style, but this seems interesting. Will it make much difference?

I will be paralleling 5 batteries per bank. Where does on find such bus bars? And because the bars will be exposed, will Polyolefin Heat Shrink work on the exposed portions?

 

[Solarod]

I was going to use wires in diagonal style, but this seems interesting. Will it make much difference?

I will be paralleling 5 batteries per bank. Where does on find such bus bars? And because the bars will be exposed, will Polyolefin Heat Shrink work on the exposed portions?

This post: https://diysolarforum.com/threads/c...ing-battery-currents.37937/page-3#post-488325
discusses what can be done to improve a 5 battery parallel string.

I can't answer your questions about where to get bus bars, but there are many posts on these topics. Do a search of the forum, and look here: https://diysolarforum.com/resources/

 

[Danke]

Found this with a search.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

 

[HRTKD]

Found this with a search.

McMaster-Carr

McMaster-Carr is the complete source for your plant with over 595,000 products. 98% of products ordered ship from stock and deliver same or next day.

www.mcmaster.com

This is where I buy my copper flat bar.

110 Copper Flat Bar | Midwest Steel & Aluminum

www.midweststeelsupply.com

 

[chrisski]

I buy my 110 copper bar where available.

I’d rather buy it in person, but the local supplier, metal supermarket, was out the three times I wanted, so I went online. I would not want to use a busbar I made myself with a copper pipe and hammer. Others have. Perhaps if I had machining tools like a press, but I don’t.

I started at Amazon, but too pricey and went with different vendors off eBay, based off reviews and how quick it could be delivered.

For my use, None of this is super time critical, so I don’t mind waiting a couple of weeks.

 

[HRTKD]

A few months back when I built my system to support the Multiplus 12/3000, I priced copper from the supplier I provided in the link above and also the local Metal Supermarket. The local guy wanted almost three times as much with just about the same lead time since it wasn't in stock. For that much of a difference in price, I can be very patient.

That said, the cost of all these supplies is crazy right now.

 

[Danke]

I buy my 110 copper bar where available.

I’d rather buy it in person, but the local supplier, metal supermarket, was out the three times I wanted, so I went online. I would not want to use a busbar I made myself with a copper pipe and hammer. Others have. Perhaps if I had machining tools like a press, but I don’t.

I started at Amazon, but too pricey and went with different vendors off eBay, based off reviews and how quick it could be delivered.

For my use, None of this is super time critical, so I don’t mind waiting a couple of weeks.

110 bars preferred? What about 101 bars?

And any concern connecting copper bar to tinned battery terminals?

 

[HRTKD]

110 bars preferred? What about 101 bars?

And any concern connecting copper bar to tinned battery terminals?

For our DIY implementations, I doubt that you could measure the difference between copper 110 and 101. Here's a comparison between the two.

What is 110 Copper - Copper Alloy 110 Properties, Copper 101 vs 110, What’s the Difference | CNCLATHING

In this article, CNClathing.com will introduce what is 110 copper with its properties and the difference between copper 101 and 110.

www.cnclathing.com

110 is easier to find and may be easier to work with (drill, cut, etc). 101 has a lower Rockwell Hardness rating which may be important if you need to bend the flat bar. I needed to bend a short length of my copper bar to account for the height difference between two components. I used thinner bar for that, 1/8" I think. No way would 3/8" take a bend like I did. I used 110 copper.

I connected bare copper to tinned connections, using a thin application of NO-OX-ID between the contact surfaces. No problems so far. A tinned surface is a lot less likely to corrode than if you were connecting bare copper to bare aluminum.

 

[chrisski]

And any concern connecting copper bar to tinned battery terminals?

I tinned my copper busbars with nickel.

 

[12kw_2021]

I tinned my copper busbars with nickel.

A few months back when I built my system to support the Multiplus 12/3000, I priced copper from the supplier I provided in the link above and also the local Metal Supermarket. The local guy wanted almost three times as much with just about the same lead time since it wasn't in stock. For that much of a difference in price, I can be very patient.

That said, the cost of all these supplies is crazy right now.

Hello

Just curious if anyone can comment on the following much simpler install. I have two 16 cell 280Ah EVE home built batteries to connect to a Sol-ark inverter. Current plan is to use 9 foot wire on the + and 5 foot wire on the - connection. (Wires for one all ready cut. Goal is to supply power and receive charge equally from the Sol-ark. Is this a problem? And do I need to just make all the 2/0 wires the same length. Thanks for a reply , see crude cartoon drawing attached.

 

[HRTKD]

Hello

Just curious if anyone can comment on the following much simpler install. I have two 16 cell 280Ah EVE home built batteries to connect to a Sol-ark inverter. Current plan is to use 9 foot wire on the + and 5 foot wire on the - connection. (Wires for one all ready cut. Goal is to supply power and receive charge equally from the Sol-ark. Is this a problem? And do I need to just make all the 2/0 wires the same length. Thanks for a reply , see crude cartoon drawing attached.

That will work. However, you should have a disconnect between the batteries and the inverter and you must have a fuse between the batteries and the inverter. The fuse should be as close to the battery as possible. Ideally you want a fuse for each battery, but many of us compromise and combine the two batteries at the fuse, using a single fuse. Class T fuse is what is recommended. They're the best and they aren't inexpensive.

 

[12kw_2021]

That will work. However, you should have a disconnect between the batteries and the inverter and you must have a fuse between the batteries and the inverter. The fuse should be as close to the battery as possible. Ideally you want a fuse for each battery, but many of us compromise and combine the two batteries at the fuse, using a single fuse. Class T fuse is what is recommended. They're the best and they aren't inexpensive.

Hello
Thank you for the confirmation on the cable lengths.. I have purchased 2 circuit breakers that were linked here, and will install them inside a suitable box on the plus side only.


Will Prowse has this linked on this site, and it says the listed 200A circuit breaker is acceptable, so I purchased two of those.

48V Offgrid Solar Power System

If you wish to build the ultimate offgrid solar power system, look no further: All-in-one units make setup a breeze. Each unit has it's own Inverter, MPPT, Transfer Switch and Battery Charger. Budget...

www.mobile-solarpower.com

Nader DC Circuit Breaker | 60V 200Amp

Signature Solar provides solar panels, off-grid solar systems, grid-tie, and hybrid systems. Quality solar inverters, bifacial solar panels, complete solar kits, solar batteries. Featuring brands such as EG4 Electronics with their solar battery, LifePower4 and EG4 LLifePower4 and EG4 LL

signaturesolar.com

So. yes.. I agree I need to add a circuit breaker , it does seem to be slight overkill as the cable connections on the Sol-ark also have circuit breakers , but I can live with two in series.

Thanks again

CPU