Battery pack cell configuration calculator

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DIY battery pack building has become very popular lately, and many beginners run into the question of how best to arrange the cells in a pack to get the maximum capacity.
Today I’ll show you a handy calculator for working this out.

A while ago, a friend sent me a link to a handy calculator that, as it turned out, was little known. I thought it over and decided not only to add it to the blog, but also to translate the interface into Russian to make it easier to use.

Some of you may have noticed the link to it in the sidebar. Today I’ll explain how to use it, and along the way, why it’s important to match cell capacities in the first place.

Let’s start with a pack of four cells connected in series. This is straightforward: in this configuration, the pack’s capacity will always equal that of the weakest cell, which in this case is 800 mAh.


Of course, you can improve things a little with active balancers. I’ve covered them in detail several times, for example here, here or here. I highly recommend checking them out.


These balancers can redistribute energy between cells, charging weaker ones using energy from cells with a higher state of charge.


But battery packs are often built with cells connected in series-parallel, and in these packs the situation can be partly corrected.
Let’s take the same set of cells and connect them in a 4S2P configuration, where S means cells connected in series and P means cells connected in parallel. This notation can describe any pack. For example, the battery shown in the photo heading consists of two 15S2P modules, so its voltage is 15 × 3.7 V and its capacity is 2 × 2500 mAh.
If you connect the cells any old way, you won’t get anything good, of course.


And as they say, “that’s not how we do things”—although, actually, it is how we do things. But why waste valuable battery capacity, shuffle charge around, or limit yourself when you can simply arrange the cells in the pack more efficiently?
Open the calculator and start by entering all the capacity values shown above.


Click “Add cells” and go to the tab where you set the battery configuration and cell voltage. In this example, that’s 4S2P and 3.7 volts.


Click “Generate packs” to get a calculated arrangement, with the cells grouped in pairs. Each pair is a parallel section of the pack; you then connect these pairs in series.
The total battery capacity in Ah and Wh, as well as its voltage, is also shown here.


But this layout isn’t very convenient to work with, so you can switch the display to “compact mode.”
It also shows how far the capacity of each parallel group deviates from the overall average.


Build the pack according to the calculation. The order in which you connect the resulting parallel groups doesn’t matter; what matters is the parallel sections.
The result is a 2200 mAh pack made from the same cells that gave us just 1600 mAh above. As they say, profit!

It’s worth noting that it’s always best to use cells that are as identical as possible. But for low discharge currents, cells with a much wider spread in capacity are perfectly acceptable—as long as you arrange them so that the capacity of each parallel section is as close as possible.


But let’s look at an example closer to real-world use: building a battery pack for a power tool.
We buy a nice case with a protection board. I recommend this one for 10 18650 cells. Packs for 10 × 21700 cells are becoming popular now, for example these, but the protection board in those is heavily stripped down.


Prepare the cells. For example, buy 12 “to be on the safe side,” measure their capacities, and enter them in the table. These values are just examples; I don’t remember the actual cell capacities anymore.


Set the battery configuration—in this case, 5S2P. By the way, I’m often asked about the maximum current of batteries. Here I explained this in detail. In short, the larger the pack—for example, 5S3P—the less demanding the requirements for maximum current.


The battery calculation is complete. The calculator has set aside a couple of cells that won’t help achieve maximum efficiency in this case.
The battery capacity in this example is 5.867 Ah, or 108 Wh.


Connect the cells by spot welding or soldering, following the calculation.


You can also mark cells you’ve already found and set aside—for example, if you have a whole lot of them.


You can also export the results as a CSV file (although that seems to work a bit wonkily), or simply copy them to the clipboard and print them to a TXT file. Very handy.


Calculate, assemble, stick on the labels, and you’ve got the most efficient battery possible. Here’s a tip: you can use the same method to match cells by internal resistance—just enter the impedance value instead of capacity :)


This calculator will be especially useful for anyone building a battery pack for electric vehicles, where packs tend to be much larger.
Also, remember that when measuring battery capacity, it’s important to keep the test conditions consistent, since even the air temperature can have a significant effect on the result.

That’s all. I hope this was useful, and may your batteries last as long as possible :)