Best Configuration for Wiring Multiple Batteries

[SSTraveler]

FitRV recently did a video showing the installation of 5 Lithionics GTX12V315A (320ah) Batteries and he wired them using the Diagonal Takeoff Method.


I found this interesting because Lithionics recommends the Battery to Bus Bar method for optimal battery balancing. They really only recommend the Diagonal Takeoff method for their smaller batteries like the 12V130A-G31 and even then they don't recommend it for more than 3 or 4 batteries.


I contacted Lithionics to get clarification on the best method and they told me that the FitRV installation was unprecedented. They said, "Diagonal takeoff is the most practical and easiest to manage. It is perfectly acceptable for all applications where batteries are placed together in one space. When parts of the same bank are scattered in multiple places, then a different approach is needed to ensure symmetry between positive and negative sides." They said didn't have any test data, but that one of the Marine Engineers they work with did his own testing of a 5 battery configuration using Firefly 116ah AGM batteries and Reliance 100ah LifePO4 Lithium batteries and here are his results.


In looking at the results it is clear that Common Point Connection using Equal Length Cables (or Battery to Bus Bar) is the best method. What was very interesting to me is that the Split Feed Takeoff (or Diagonal Takeoff) and the Both Connections on Middle Battery Takeoff give you really about the same characteristics just in opposite ways. One performance curve peaks while the other valleys, but overall essentially equal. If the Relion Battery test that Rob Warren did can be considered representative of LiFePO4 batteries in general then I find it very helpful.

Here is the FitRV video if anyone is interested.

 

[TampaSteve]

Very interesting thanks for posting. I have no interest in the nuclear sized battery builds for myself, but the optimal wiring is highly relevant. My new modest build is 3 100 amp batteries. Even for that small build, using equal length cables produces a lot of extra cable draping around, and I think often people default to the diagonal method just because it is so tidy. I still haven't decided as I await my batteries, but if I end up doing diagonal maybe I'll rotate them seasonally. One of the reasons I was obsessed with getting batteries with integral switches.

 

[EnerGT]

I honestly don't know what FitRV was thinking with this setup. I normally like his blog because he's not afraid to try anything and he's very creative. But he has $15k in batteries there and not wiring per manufacturer recommendations, not to mention all the time and effort he has put into this project.

I think I would have mounted those Victron busbars on top of the batteries so I could run even cable lengths from each battery to the busbars. It wouldn't be as pretty, but nobody is going to see it anyway.

Just seems strange.

 

[99sport]

I have 2 Victron batteries wired in parallel with EXACTLY equal length cables. The batteries were ordered at the same time and have seen the same number of cycles - I have done everything possible to make sure these batteries see the same loads, age, cycling, etc. Despite this, one of my batteries is always perfectly balanced (all cells report the same voltage). The other battery always goes out of balance after a few charge / discharge cycles (without letting the internal BMS rebalance).

My take away from this is that even if you do your cabling perfectly the differences inside and between each battery are going to imbalance the cells / pack over time. As long as your BMS can restore cell balance it won't really matter.

So I think it probably isn't worth sweating too much for a van. Put the batteries next to each other and do your best and let the BMS fix the imbalance that occurs (and monitor the cells to make sure the BMS is indeed keeping them balanced - if not change something).

I use my van several days a month on average, and I don't think an occasional imbalance is going significantly reduce their useful life to me. However, if I was building a battery bank for household solar - where the batteries are going to see huge / charge discharge cycles every day for a decade or two, you bet I'd sweat every possible detail in matching the wiring.

Yes, the cable layout shown above is definitely not ideal, but I suspect it will work just fine for their (non full-time) usage.

 

[CraigA.]

I have 2 Victron batteries wired in parallel with EXACTLY equal length cables. The batteries were ordered at the same time and have seen the same number of cycles - I have done everything possible to make sure these batteries see the same loads, age, cycling, etc. Despite this, one of my batteries is always perfectly balanced (all cells report the same voltage). The other battery always goes out of balance after a few charge / discharge cycles (without letting the internal BMS rebalance).

My take away from this is that even if you do your cabling perfectly the differences inside and between each battery are going to imbalance the cells / pack over time. As long as your BMS can restore cell balance it won't really matter.

So I think it probably isn't worth sweating too much for a van. Put the batteries next to each other and do your best and let the BMS fix the imbalance that occurs (and monitor the cells to make sure the BMS is indeed keeping them balanced - if not change something).

I use my van several days a month on average, and I don't think an occasional imbalance is going significantly reduce their useful life to me. However, if I was building a battery bank for household solar - where the batteries are going to see huge / charge discharge cycles every day for a decade or two, you bet I'd sweat every possible detail in matching the wiring.

Yes, the cable layout shown above is definitely not ideal, but I suspect it will work just fine for their (non full-time) usage.

Newbie here. How do you monitor each battery individually?
Thanks

 

[99sport]

Newbie here. How do you monitor each battery individually?
Thanks

Victon batteries have built-in monitoring with bluetooth and an app that you can use to check individual cell status. I imagine other high-end systems have a similar means to get this data. A DIY system would also have an external BMS that would measure this (and you may or may not have easy access to the values depending on how you DIY). If you buy a "drop-in" battery such as Battle born you have no idea what is going on inside and have to trust (hope?) that whatever is going on inside the batteries to maintain balance is happening.

 

[CraigA.]

Victon batteries have built-in monitoring with bluetooth and an app that you can use to check individual cell status. I imagine other high-end systems have a similar means to get this data. A DIY system would also have an external BMS that would measure this (and you may or may not have easy access to the values depending on how you DIY). If you buy a "drop-in" battery such as Battle born you have know idea what is going on inside and have to trust (hope?) that whatever is going on inside the batteries to maintain balance is happening.

View attachment 196729
View attachment 196730

Thanks for the info. I didn't realize Victron batteries broke out each individual cell like this. That's pretty cool.

 

[firebat45]

Center tap and Diagonal being similar but opposite makes perfect sense. With diagonal, you're "reaching in" up to 3 batteries away from the (electrical) edge of the pack. With center tap, you're "reaching out" up to 3 batteries away from the (electrical) edge of the pack.

I would only recommend diagonal in a 2P setup. Equal length/bus bar is obviously the way to go with systems with more than 2 batteries.

The real question is what do you do when you start wiring in series too. Let's say you have 6x 12V batteries in a 24V system. Do you connect the first 3 batteries in parallel, and the other 3 batteries in parallel, and then put the two groups in series? Or are you better off connecting 3 pairs in series first, then paralleling them all together?

 

[SSTraveler]

In speaking with the Lithionics Chief Engineer it is clear to me that his main message is don't worry about being to precise. Even though the Diagonal Takeoff method does not lend itself to be equally power-sharing, the batteries will tend to balance out at the top-end charge. The best wiring method for any individual setup boils down to practicality. But I thought the test results of the various wiring methods was very interesting and gives one insight into the trade offs they might be making when choosing a particular multiple battery wiring scheme. Lithionics has an advanced internal Battery monitoring system with Bluetooth connectivity. You can monitor the cell balancing and get lots of other data. Here is a video on the Lithionics App.

 

[SSTraveler]

Some 2022 Leisure Wonder Owners are reporting that their dual 100ah Dragonfly Lithium batteries were wired incorrectly by Leisure. As pictured below, Leisure has connected the 2 Dragonfly 100ah battery together in parallel but then connected the coach to both terminals of just one battery vs the Battleborn recommended Diagonal Takeoff method, which is connecting the coach to the positive of one battery and the negative of the other battery. I notified Mike Elias, Leisure Marketing Manager of this issue as a heads up that Leisure Owners maybe calling them about this. If I found my new Leisure Lithium batteries wired incorrectly I would correct it immediately because as you can see from the testing chart that the second battery in an array where the connections points are made on just the first battery, that the draw and charging is not balanced, you may see a reduction in capacity by as much as 50%. If I paid Leisure nearly $4000 for their 200ah Lithium battery option, I certainly would want to get the full 200ah use out of them, versus maybe only 150ah, and I definitely would be concerned that the battery life of the second battery may be greatly reduced since it is not be used in a balanced manner and may never be getting fully charged. I'm very surprised that Leisure started making a mistake in the installation of the Dragonfly batteries. It is certainly something new Leisure Owners might want to check.
2022 Wonder Rear Twin Bed Model

 

[mikeme]

Couple thoughts to share here. (you are still overstating the impact of this wiring)

Looking at test 4 in your previous post, one would think the wiring variability has been taken care of, yet there is a 25% difference in battery current due to differences between batteries. looks like the lithium battery 1 pulls 20 amps while battery 4 pulls 25 amps.(assuming the test was carefully conducted with each battery brought to a similar state of charge before the test)

The case of five batteries in parallel is quite different than two batteries in parallel. (you cannot ignore the impact of the three additional batteries)

It would be more appropriate to compare batteries 4 and 5 rather than 1 and 2. In this test case, it looks like the 5th battery gets around 75% of the current the fourth gets. (which, remember, is of the same order as the variability between individual batteries). Details matter here. Do I remember correctly that this was a charging current test? Are you willing to share or link to the whole test report?

The thickness of the cables used is important here and it seems like LTV uses pretty hefty jumpers. Not sure how this compares with the test configuration you reference. The battery internals are also important; batteries with lower internal resistance will be less impacted by the impedance of the jumper wires.

But this test point is just that, a point in time. Follow this up and consider what happens in real life. After an initial operating period, the first battery has less capacity (charge) remaining than the second battery. Because of this, the second battery (with a higher charge level), will have a higher discharge voltage at that state of charge, increasing the current share provided by the second battery. By the time you get to the low voltage cutoff for the first battery, the second will not be far behind.

It is true that for modest discharge rates, one could imaging a disparate number of fractional discharge cycles in the first battery. (gets worked a little harder). These batteries come with a ten year warranty, so it does not seem worth getting all excited about.

Reference the quote from your previous post, attributed to the Lithionics engineer "..don't worry about being to(sic) precise. ...... the batteries will tend to balance out at the top-end charge "

Remember that all these lithium Ion batteries with internal BMS need to be fully charged on a regular basis, to let the management system perform housekeeping. This means that the second battery will be fully charged, at a time a little after the first battery.

I engaged in an e-mail exchange with dragonfly. The person who responded had this to say about hooking up a pair of 100 AH batteries in parallel:

"You can measure differences in amperages(sic) across cables using an amp clamp to see the differences in power distribution with all connections on one battery vs diagonal connectivity. There are multiple ways to wire up batteries and everyone does it differently. If you prefer having connections on a single battery, then use this configuration."