Performance Loss As A Result Of Series Connections

Everyone knows that it's better to build a lithium ion battery with copper. There are so many benefits to using copper. Copper has 4.16 times lower resistance than nickel. Copper is 4.4 times as thermally conductive as nickel. But really, how much better is a copper battery compared to a nickel battery. 

What specific things about a copper battery are better than a nickel battery? Will it perform better under load? Will I feel additional power and acceleration as a result of using copper? Will my battery have a higher range if I build it with copper? Will my copper battery be able to endure more charge and discharge cycles than my nickel battery?

In this deep dive article we are going to examine all of these things and give you the exact specific answers that you need to be more empowered as a battery builder.

Spoiler alert: The reason why we're using such a ridiculous battery as this example is because that's what's required to even be able to produce a difference between copper and nickel worth discussing. This boils down to the fact that series conductors have a stunningly low resistance compared to cells and other ohmic losses. 

The Example Battery.

Cells: EVE 40PL 20S4P

Series Conductor A: 8mm x 0.15mm pure nickel - 4 Links

Series Conductor B: 8mm x 0.15mm pure copper - 4 Links

BMS: JK 100A

Load Range: 0A - 100A 

How Is This A Ridiculous Battery?

The battery is ridiculous for several reasons. E40PL cells have extremely low resistance. It's just 3 milliohms. That is so hard to believe that before starting this article, I had to contact a trusted battery cell expert friend of mine and ask him if that was actually the real resistance, and he confirmed that it was. Thanks, Nelvick. 

The ridiculous part is using these batteries in a 4P configuration with just 4x 8mm links to connect the series groups. Nobody is realistically going to do that. Yes, using four simple links is very common, but the common batteries that are built using that construction method do not have cells like these.

They don't have cells anywhere near these. The resistance of the batteries that are built with this construction method is somewhere around 20 to 35 milliohms because these are going to be low quality, standard issue 18650 cells. Sometimes they'll be 21700. You might see a MalaCell P42A or something in there every once in a while. But ultimately, you have 20 or 30 milliohms of resistance per cell. And most batteries you're not drawing 100 amps from. Most batteries, even good batteries that you can get a lot of power from, you're drawing 40 to 60 amps from those batteries.

But a battery like that, even a high performance battery, won't be able to show any difference between copper and nickel. So we need to build this super ridiculous scenario so we can show the difference between the two more effectively.

How Much Power Is Lost In The Cells?

EVE 40PL 21700 Battery cells use an NMC chemistry and a tabless design, resulting in a comically low resistance of just 3mOhms. For a 4P battery, we need to divide the resistance by 4 to find the resistance of each cell group. This comes out to 750 microOhms. That is a very low resistance battery cell group for sure, but now we have to put 20 of those in series, which results in 15mOhms. 

The low resistance of these cells is simply amazing. After adding them all up, the total resistance of the battery pack is less than the resistance of most individual mid range battery cells. That is definitely impressive. But they aren't super conducting, so while the resistance is very low, it's not zero. That means some amount of power will be lost. But how much?

This alone is super impressive. At 100 amps, so 7.2 kilowatts, the entire battery pack is only losing 150 watts to heat. That is absolutely insane. But that's not the entire story for the cells. As it turns out, lithium ion cells run better when they are hot. Not just a little better, but a lot better. 

At any appreciable current, the heating inside of the cell happens nearly instantly, in less than a second. It may seem like it takes some time for the batteries to get hot, but that's simply because it takes some time for the cell to saturate with heat enough for it to make its way to the surface, where you can feel it. If you pull a heavy current from a cell for any amount of time, it instantly heats to these temperatures in the core of the chemistry, where it matters. So essentially, in the context of electric vehicles, right away, as soon as you start using your battery in any kind of way in which performance is going to matter, it warms up and its resistance drops by a significant margin. 

This effect varies cell model to cell model, as there are many factors involved, but most experiments result in a 40% to 60% resistance reduction when comparing room temperature to 60C. For this article, we will use the low end of that. 

How Much Power Is Lost In The Nickel?

To know how much power is lost in the series conductor, we must first learn how to properly measure the effective distance that the current has to travel. This distance is much shorter than you may expect. 

At first (or even second or third) thought, you may think that you need to calculate the resistance of the series conductor along the path of current using its known material resistance. Sadly, this is only part of the story. 

When welded to a cell, the series conductor forms a composite conductor with the top of the cell. This makes sense because even though the cells are made of steel, the ends are relatively thick.

Whether energy is leaving the chemistry and ending up at one of the ends, or coming in through one of the ends and heading towards the chemistry, when it's traveling along the series conductor plane and the current is over the cell, it is traveling through both the series conductor and the top of the cell.

In this way, the series conductor and the top of the cell form an emergent composite conductor that dramatically lowers the resistance of the series conductor directly above the cell. The resistance of the series conductor on either end measures around 110 microohms each, for a total added resistance of just 220 micro ohms per series link.

So we take 220 and then we divide that by 4 because it's 4 links for the series conductor. And then we multiply that by 19 because we have to put 19 of those in series. Remember, if we have 20 cell groups, we use 19 series connections.

This figure is tiny. For this battery, this would be a total of just 1.045 mOhms of resistance for 100% of the nickel in the entire battery that is directly above the cells. 

So, while it is true that current does indeed flow over the entire length of the series conductor, the series conductor no longer has a uniform resistance due to the cross-sectional area of the top of the cell itself being added to the series conductor. 

So Wait..
You’re telling me that the cell itself forms part of the series conductor and supports a significant amount of current?

Yes.

The nickel placement was not done by a machine, neither was the welding, and the multimeter probes certainly aren't being held by a machine, so none of that part of the process is going to be perfect. The actual value is more than likely somewhere between 2.1mv and 2.4mv, so we can just take the average of 2.2mv. 

That's for the outside edges of the series conductor. For the inside of the series conductor, we're getting 14.4 millivolts of voltage drop. This results in a resistance of 720 micro-ohms. Doing the same divide by 4 and multiply by 19, we get 3.42mOhms.

All together the nickel comes out to 4.465 milli-ohms of resistance for the entire battery which results in 44.65 watts of power loss at room temperature and at 60 degrees Celsius the power loss figures are 54.04 watts.

Unlike the cells, which lower in resistance by 40% at 60C, the nickel actually increases in resistance by 21%. While it's never ideal for resistance to increase, the fact that it does in this way is ideal for performance. This is due to the fact that the cells have a very high resistance and that very high resistance is being lowered by a large margin due to heat. In contrast, the nickel, which has an already small resistance, experiences a smaller increase in that smaller resistance compared to the decrease of resistance seen in the cells.

If the situation were reversed, with the nickel having a high resistance and the cells having a low resistance, and the nickels resistance increasing by a lot and the cells only decreasing in resistance by a little, then the battery would lower in performance as it heated up. 

How Much Power Is Lost In The Copper?

The resistance of copper will increase with temperature just like it does for nickel, but it does it at a slower rate. By the time you get to 60 degrees Celsius, copper only increases in resistance by 15%.

Copper is 4.16 times less resistive than nickel, so all we have to do is scale our values accordingly, and that shows that the copper will lose 13.1 watts to heat 11.41 watts to heat at room temperature and at 60 degrees Celsius it will lose 13.1 watts of heat. 

How Much Power Is Lost In The Main Negative Connection?

I didn't spend a lot of time on this, but what I did was I took some 8-gauge wire and I soldered it down really good to a piece of 25-millimeter copper. I then placed one multimeter probe on the copper as close as possible to the soldered connection and one multimeter probe on the wire as close as possible to the soldered connection to measure the junction itself.

I got a voltage drop of 1.4 millivolts with a current of 20 amps flowing. So that means that this connection is 70 micro-ohms. And really, I'm measuring more than just the connection. I'm measuring a pretty decent chunk of the solder and the wire itself, but this is the best I can do. So all that really matters is that we can see that it's such a low figure that we don't really need to consider it.

How Much Power Is Lost In The Main B- Wire?

These are the power loss figures for 9 inches of 8 gauge wire at 20 amps. And it also shows the increased resistance that you will get at 60 degrees Celsius. You probably wouldn't want to use just this one wire for 100 amps, but it still serves the purpose of this article. If you would like to divide the results by two for the case of using thicker cables, then that's just fine.

How Much Power Is Lost In The BMS?

According to the documentation, this JK Smart Active Balance BMS has a resistance of 1.3 milliohms. That's very low. The BMS claims a maximum discharge current of 200 amps and it's rated for a continuous current of 100 amps. JK makes absolutely fantastic BMS. And they have the best app for BMS on the market.

With such a small resistance, even at 100 amps, this BMS will only dissipate around 12.5 watts of power. Now, don't get me wrong. 12.5 watts of power is a lot in a small space, but these BMS are actually rather large and considerably wide and made of metal and feature lots of cooling.

How Much Power Is Lost In The P- Wire?

For this wire, we'll just assume the same 9-inch length as the B-minus wire. Of course, they could be a little shorter or a little longer. I'm just trying to give some realistic lengths to be able to do this article.

How Much Power Is Lost In The XT90 Connection?

For the XT90 connectors, I probed the back of the male and the back of the female while they were connected. And I saw 6mv millivolts of voltage drop at 20 amps which results in a resistance of 300 micro ohms.

It's important to remember though that the circuit is indeed a circuit and not a one-way street down one leg. So that means we have to multiply by two because it's going to have to come back through that connector the same way. That gives this XT90 to XT90 junction a total resistance of 600 micro-ohms, which results in 6 watts of waste heat at 100 amps. That increases to just south of 7 watts at 60 C.

Even before considering the lowered resistance caused by the cells heating up, with just a 33.92 Watt difference between nickel and copper, there is absolutely no way a human could feel that on a scale of 7200W. 

This calculation considers just about everything. It accounts for the fact that nickel increases in resistance by 21% when compared between room temperature and 60°C. The series conductors in the cells decrease in resistance by 40% at 60°C. The main negative connection is soldered — I did not calculate that, but it's only losing 0.7 watts anyway, so I don't think it's a big deal.

Material Properties and Thermal Effects

The main negative cable is made of copper, so its resistance increases by 15% at 60°C. The BMS is a composite, complex device made of silicon, copper, and some other materials. I didn’t take the time to calculate those effects. It could account for a few watts or could be insignificant when the temperature changes — I’m not sure. Overall, it’s very small compared to the total, but it is the missing piece here.

Resistance in Connections and Measurement Method

The positive wire is also made of copper, so its resistance will increase by 15%. The XT90 connections are made of copper as well, so they also go up by 15%. The resistance measurement for the XT90 connections is made by probing the closest piece of wire that isn’t soldered on the back of the connection — and then doing that on both sides while it’s connected. This captures as little of the wire’s resistance as possible while including 100% of the contact resistance from the soldering, 100% of the resistance from each plug, and all of the resistance at the junction between those plugs.

That result is then multiplied by two, since it includes both the positive and negative sides. I didn’t multiply the wire resistance by two because I measured the negative and positive wires separately.

Nickel Battery Power Loss Behavior

With the nickel battery, you start with a resistance of 23.19 milliohms, which would cause 231.9 watts of power loss, but by the time you get out of the driveway, down the street, and punch the throttle, your resistance has dropped enough to reduce your power losses to 184.84 watts of heat.

Sure, the cells don't heat up instantly, but they heat up extremely quickly. And before they heat up, they're fully charged. So they're going to have the highest possible voltage. So any delay in onset for the cells to lower their resistance, that's more than compensated for by the fact that your voltages are higher and all of that before you punch it anyway. So there's a trade-off where you would normally lose all of that power, but you're not losing it because the cells are heating up.

Dynamic Resistance and Performance Behavior

So you lose your top-end off-the-line power immediately because it's a battery. It's no longer being charged and you're depleting it. But in that process, in just a few seconds, the resistance of the batteries are dramatically dropping. So essentially, you never will see those static power loss figures. It will pretty much always be some level of at-temperature figures, especially if you're doing any performance riding, and that's the only time this matters anyway.

Copper Battery Performance Comparison

You can see the performance figures for the copper battery are much better. With the same cells and the same bike and controller and motor and all of that, after the cells heat up and their resistance lowers, you end up getting only 143.12 watts of power loss. Compared to the 184.84 watts of power loss, that's a 41.72-watt difference.

So that means if you didn't build your battery out of copper and you chose nickel instead, that's 41.72 watts you're just leaving on the table. You're wasting them. That's range you're not going to get. That's performance you're not going to see. But really, how much range are you going to lose and how much performance are you going to lose?

Real-World Impact and Practical Difference

You have to consider this is at 100 amps at 72 volts. So that's 7.2 kilowatts. And your power loss with nickel is 184 watts, and with copper is 143 watts. So that 41-watt difference is definitely worth saving. But could you really feel that on a scale of 7.2 kilowatts? Absolutely not. Is having a nickel battery with all other things being equal going to be a slower battery? A lower performance battery than a copper battery? Absolutely not.

Context and Construction Quality

And remember, this is an extreme example, doing 100 amps at 4P with these super tiny 8 mm wide pieces. Whether it's copper or nickel, we're still using super tiny 8 mm wide 0.15 mm thick material.

Whether you are building a 100 amp battery out of nickel or copper, you should not be using a simple four-link system like this. The type of system that we used for this test in this article is the lowest quality type of lithium-ion battery construction there is. It's the type of construction that you will see when you buy a $400 e-bike and take apart the battery and look at the battery like a Jetson Bolt Pro or something like that.

It's going to have this simple basic construction. You will not see that on a 100 amp battery whether it's built with nickel or copper. You're not going to see that. You're going to see solid pieces of nickel or solid pieces of copper. So this is an extreme example showing that the only difference even in this extreme example is about 41 watts — 41 watts of difference at 7.2 kilowatts.

So that means instead of four 8mm wide links connecting each cell group, we would really have a single 61.9mm wide length. I used a 2mm overhang on each side because the bus bars are usually larger than the smallest possible polygon that can be formed by connecting the cell positions. So that's going to change the math a lot for both the copper and the nickel batteries.

And Then Suddenly, The Math Changes

When building a battery that has a solid conductor like this, it won't always be able to be 61.9 millimeters. You might be using a linear layout instead of an offset layout. You could have dimensional constraints that make it so you can't put all your cells perfectly like that. But this data shows more than well enough that as long as you use a solid piece, you're going to have a dramatic performance difference compared to a set of links. You can see as you use more and more material to connect your series groups, the resistance of that material matters less and less.

In this more realistic example, we're using a more realistic series conductor size. But it's still just 0.15 millimeters thick. We could double the thickness of both of these things. And then whether one is copper or one is nickel will matter even less.

With this realistically constructed battery, you can see the difference between copper and nickel is negligible. Once things heat up, there is only a 5.4 watt difference between the two, and even if you don’t consider the cells lowering in resistance as they heat up, there is only a 4.4 watt difference. 

Then Why Build A Copper Battery?

While it's true that copper is 4.16 times as conductive as nickel and therefore is more efficient and produces less voltage drop, the main benefit of a copper battery in this range of current is its thermal properties. Copper can radiate away about 4.5 times as much heat per second as nickel. This helps the copper dissipate its own heat effectively.

Thermal Efficiency and Longevity

Copper also produces less heat per amp than nickel does. While the differences are small, over hundreds of charge and discharge cycles, this will provide some measure of extended battery life as heat is the primary enemy of NMC chemistry.

Think about it. 

While you definitely won't feel a 40 watt difference at 7.2 kilowatts, your battery over time most certainly will. It's certainly not the case that all of those waste watts are going into the cells, but most of them are. 

Experience and Skill in Copper Battery Building

If somebody is building a battery using copper then chances are they have a lot more experience building batteries than someone that just bought their first spot welder. If they don't have more experience, they're at least more prepared for a more involved process that is multi-layer and all of that. It also generally means that they're going to use the right size wires, they're probably going to be decent at soldering, and they're going to know what cells to use for the best battery performance.

Of course, this isn't always the case. You could have somebody building a copper battery that has absolutely no idea what they're doing or has the very intention of cutting corners or using cheap garbage cells or something like that. Just like how you can have a car that has leather seats and a nice infotainment package and heated mirrors and all of that, and it could have a totally garbage engine. But chances are, if it has all of that stuff, it's probably a premium high-end car. So it probably has a premium high-end engine and it was probably built well.

That's definitely not always the case either, but I think you get the point.

So that 40 watts can add up over time. I'm just going to use totally random figures here off the top of my head, but I think it'll be enough for you to get the point. Let's say I ride my bike three hours every day. And let's say, because it's a lot of stop and go, I'm at 30% on average in terms of current draw. So instead of around 44 watts, it would be 13 watts. And then we take that 13 watts and we multiply that, or 13.2 watts rather, we multiply that by three hours a day. And we get 39.6 watt hours of heat that we're just cooking our battery with. And that's just in one day. 

Let's say I'm an avid rider and I ride 300 days a year. That's 11.8 kilowatt hours of wasted energy that you're burning just in the series conductors. Now you can't switch from nickel to a superconductor, but you can switch to copper. When you switch to copper, that's going to reduce that by a lot. You'll end up with about 2,800 watt hours of waste heat pumped into the battery over the course of a year.

How much damage does that actually cause? How many months or cycles are you going to lose from that? Question mark. I have no idea. That is going to take a lot of testing and a lot of math to figure out. But it's definitely something.

Comparing Copper and Nickel

So copper is better than nickel to use for lithium-ion batteries, but nickel's not as terrible as we all thought. Also, series conductors don't have to be as big as we all thought. For a battery over 100 amps — if you have several hundred amps — then yeah, you're going to want to use copper because if you don't, you'll have to have like six or ten layers of nickel and it just gets ridiculous, even though the electrical performance would be the same if you did that.

Aesthetic Appeal

Copper batteries look better in pictures. If you want to take a picture of your work, copper batteries are the best kind of batteries to build to show people. It's not just because they think it provides this huge performance benefit that it doesn't. It's not just a matter of playing on people's misconceptions — when it comes down to it, copper is pretty and people like the way it looks. It almost looks like gold, and humans have a natural fascination with that color.

Also, copper is cheaper than nickel believe it or not and it's arguably easier to work with nickel is very hard and rigid copper is soft and malleable you still need some nickel to put on top of the copper but just tiny little squares or strips if you prefer you don't have to actually make your entire series conductor out of them so that's another good reason.

So What Does This All Mean?

The findings in this article mean so many things.

It means that for just about all batteries in existence that are built with copper.. if you were to sneak into that person's house.. take all the copper out of the battery and then weld nickel in its place… even if they went down to a drag strip, they wouldn't notice the difference.

It means that you don't need 0.3 millimeter thick copper for a 100 amp battery. It means you don't need 0.4 millimeter copper for a 200 amp battery. 

It means you need much less series conductors than you realize for any battery.

It means everybody is wildly wrong about this concept and is erring to the side of caution and prestige.

It means that you don't need a welder nearly as powerful as you thought you did to build the battery that you want to build. Of course you still need a high-quality welder and you still need a powerful one and you definitely want one that can weld copper, even if the battery you're planning isn’t going to need it. 

It also means that a battery built with copper is not necessarily higher performance than a nickel battery. There could be many situations in which a nickel built battery, because it has better cells, better wiring, and stuff like that, will actually perform better and have a lower voltage drop than a copper battery.

Why Did You Write This Article?

This is a battery knowledge website. This is not a battery sales website. This is not a battery lore website. This is a battery knowledge website. This is a website where you come to learn about the nitty gritty deep details regarding battery technology. This isn't the place you go to get marketing material to make more sales in batteries.

This is the place you go when you're building a battery and you're concerned about doing it the right way or over building or under building or something like that. You come here to get all your ducts in a row. So that's why I have to tell you the truth. Copper batteries are great. They are better than nickel batteries, but they are not putting you further back in your seat when you press that throttle. Point blank.

We hope this article helped you learn everything you needed to know about the performance loss as a result of the series connections. Thank you for reading.