Introduction: How to Size Your BATTERY: Beginner to Engineer

Battery cells are electrochemical accumulators, which means they allow you to store electrical energy in chemical form. The most common type of battery nowadays are Li-Ion, being used on almost any portable device, cordless power tool and electric vehicle such as skateboards, bikes and cars.

Step 1: Form Factors

Li-Ion cells come in various form factors, which means simply the shape of the cell. In the photos you can see the two most common ones:

  • Steel cilinder with the cell rollend insidie. Come in various sizes, the most common by far is 18650 -> 18mm diameter X 65mm high
  • Pouch, a soft wrapping with the cell folded inside. It's the type of cell used in cell phones

Step 2: Knowing Your Cell

Wether you already have a particular cell or are looking to buy one, you need to know some important parameters, which determine whether the cell fits your particular application or not.

All of the information you can possibly need for a DIY application is available on the datasheet of the cell. In the video I'm showing the Samsung INR18650-25R cell, for which you can find the datasheet here INR18650-25R

The most important parameters are the cell's nominal voltage, nominal capacity and maximum sustained discharge, which you can see highlighted in the photo.

Note that the nominal voltage reported on the datasheet (in this case 3.6V) is measured at 50% State Of Charge (SOC). The actual voltage will be higher when the cell is full and lower when empty

Step 3: C-rate, V, Ah, Wh, J

Units and their meaning:

  • Ah, is a charghe capacity -> Ah = (C/s)h = (C/s)3600s = 3600 Coulomb
  • Wh, energy -> Wh = (J/s)h = (J/s)3600s = 3600 Joule
  • C, current normalized on battery capacity. For a 2.5Ah battery, 1C means a current of 2.5A. If you discharge that battery at 1C, it's gonna last 1h. If you discharge it at 2C, it will be empty in 30 minutes.

In the photos you can see some examples relative to the same INR18650-25R cell.

Step 4: Scalin of Units in Battery Packs

The arrangement of the cells inside a battery pack is usually reported like 10s2p, for example, where 10 is the number of series cells (10s) and 2 the number of cells in parallel (2p). This means that the battery contains a total of 20 cells, as shown in the drawing above.

The C-rate, in this case, is calculated from the capacity of the whole pack. If for example I use the usual cell to make this 10s2p pack, I'll get a nominal voltage of 36V, a capacity of 5Ah and a maximum sustained discharge of 40A.
A discharge of 1C now means 5A but the battery is still gonna last 1h.

Step 5: Assembling Battery Pack

When assembling a battery pack you should use just one type of cell and balance them before assembling. Note that wiring in parallel cells which are not at the same voltage may make the cells blow up in your face. Not nice.

For the assembly itself you have essentially two options:

  • Soldering: Cheaper and easyer for sure, but also a bit dangerous and likely to ruin your cells. The problem is the heating, which you should avoid on any cell beyond 50/60°C. The exact value depends on the specific cell, but soldering is performed at 200+ °C, clearly too high.
  • Spot welding: This is the right way to do it, but a spot welding machine is way more expensive and you can use it essentially only for soldering cells. Also, making a nice spot weld takes some practice and patience, but the result is superior on many levels, including contact resistance.

Step 6: A Note on Safety

As stated a couple of times before in this Instructable, Lithium cells have the tendency to blow up. For real.
Underestimating that possibility is an error that you don't want to make. It's even shown on the datasheet! Check the photo.

The main causes for blowing cells are:

  • Overcharge and overdischarge. Not very likely to explode but defenitely possible
  • Short circuit, which overheats the cell, makes the electrolite boil and eventually a thermal runaway
  • External overheating, like putting the cells near a motor or under the summer sun.
  • Puncture, happens when the cells are not mechanically protected inside a rigid case. It's easyer to happen on pouch cells which have a soft wrapping, but can happen on any cell.

It has happened on many cell phones (Samsung is maybe the most notorious) and some Tesla cars, which use the 18650 form factor for they batteries.

Take precautions! The flame is very high temperature!

Step 7: What's Your Project?

I enjoy building my own equipment (when it makes sense to do so), thus I made a simple spot welder machine. In case you are curious and don't know how spot welding works, you should check my last video and the Instrucables on that topic here ->

Tell me in the comments, on what project do you plan to use Li-Ion cells? In what configuration?

Did you ever have a cell blow up in your face? Hope your face is fine. What provoked the cell to explode? How can you avoid that? Let us know!