How To Wire Lithium Batteries In Series to Increase Voltage

If you want to do anything practical with lithium-ion batteries, then you are going to need to know how to wire lithium batteries in series. Lithium-ion batteries have become part of everyday life. They are all around us, in our phones, tablets, laptops, electronic cigarettes, cars, and just about any other portable device you can think of. 

Wiring lithium-ion batteries in series is simple. It’s as simple as connecting the positive connection of the first cell to the negative connection of the next cell. Some configurations will require just 3 cells in series, other configurations require 20 or more. Either way, once you wire a set of lithium-ion batteries in series, it will form an open-ended chain. At the ends of the chain, you will find your main negative and positive connections. When battery cells are wired in series, their voltages are added but their amp hours are not. Overall capacity is added because power is measured in watts- and watts is volts multiplied by amp hours. Putting lithium batteries in series increases the overall voltage, which increases overall power. 

In this article, we will explain why you would want to wire lithium-ion batteries in series. We will also explain if it's even possible and if the battery cells can be charged while in series. 

Why Wire Lithium Batteries In Series?

Lithium-ion batteries are extremely power dense and over the last 10 years, the cost of a given amount of lithium-ion energy has come down about 10-fold. There is, however, a major shortcoming when it comes to lithium-ion battery cells. The voltage of a single lithium-ion battery is quite low, so using multiple cells in certain configurations is needed to build a battery pack. A single cell or parallel group of cells has a maximum voltage of just 4.2 volts. This is not a high enough voltage to power most things. 

For example, most inverters need at least 11 volts to turn on, and 12 or 13 to operate efficiently. Single LEDs can run at single-cell lithium-ion voltages, but if you plan on running a more sophisticated lighting setup, you are going to need a higher voltage. Most motors will work at just about any voltage you give them, but if you give a motor the voltage from a single lithium-ion cell, it will spin very, very slowly. 

So then, of course, the question comes to: ‘How do you increase the voltage of a lithium-ion battery?’ If you want to start effectively using lithium-ion batteries at reasonable voltage levels and have them power the vast majority of electrical devices that require a voltage level higher than that which a single lithium-ion cell can provide, you will first have to learn a little bit about series and parallel cell connections in general. 

Wiring Batteries In Series Increases Voltage

The primary reason for wiring battery cells wired in series is to increase overall voltage. Voltage is all about difference. When a battery is at 3.7 volts, that means that the positive side is 3.7 volts higher than the negative side. So, if that battery is the only power source, then the negative side is 0 volts because its 3.7 volts less than the positive side, which is 3.7 volts. 

Battery cells provide their own power. So if you add several battery cells together end to end, the voltage will increase. It’s important to keep in mind that when wiring batteries in series, the amp hours are not added in the same way that voltage is. Overall power is increased because watts is voltage multiplied by amp hours.

For example, if you combine two 3.7 volts 2ah cells in series, the end result will be a 7.4-volt 2ah battery. This means 2 cells in series will contain twice the amount of watts as a single cell.

How Series Connections Work

Series connections work based on the principle that voltage is based on a difference in potential. When you put something in series, the voltages are added because you are shifting the ground level of the second cell in series to whatever the positive level of the first cell is. 

Voltage can be thought of as the speed of electricity and amps can be thought of as the amount of electricity. Added power sources in series, whether they are battery cells or not, increases the speed of the electricity by shifting each subsequent series component’s ground level up.

Whether you are wiring capacitors, battery cells, power supplies, or any combination of the two in series, the end result voltage will be the sum of all power sources in series. When you know how to wire lithium batteries in series to increase voltage, you can do a lot more with lithium-ion batteries.

How To Wire Lithium Cells In Series

If you have been wondering how to wire battery cells in series, the good news is that it’s a simple concept to understand. All you have to do is connect the positive of one cell to the negative of the next cell. Regardless of how many lithium batteries you are connecting in series, you will always be left with one free negative end and one free positive end.  These are your main + and - connections. You will often use nickel and/or copper strips by spot-welding them to the positive and negative ends of the cells. 

Consider an example where you have three battery cells that are each charged to 3.7 volts. The first cell’s negative side is 0 volts and its positive side is 4.2 volts. If you connect the first cell’s 3.7-volt side with the second cell's 0v side, then the second cell's 0-volt side gets raised to 3.7 volts. 

NEVER connect the + and - from the same cell together. Also, never connect the - connection of a set of series-connected cells to the + connection on the other end. Doing either of these things will more than likely lead to an immediate fire. 

The positive side of the second cell is 3.7 volts higher than its negative side, so if you measure the voltage between the negative of the first cell and the positive of the second cell you will get 7.4 volts. 

The process is the same when adding the third cell. Simply connect the positive from the second cell to the negative of the third cell. This will raise the third cell’s negative side to 7.4 volts. Just like the second cell, the third cell’s positive side will be 3.7 volts higher than its negative side. This means that if you measure the voltage between the negative side of the first cell and the positive side of the third cell, you will get 12.6 volts.

How To Wire Lithium Batteries In Series

The connections needed to wire batteries in series are the same for wiring cells in series. It’s a matter of connecting positive to negative in a chain whereas attaching cells in parallel is + to + and - to -. There are, however, some additional things that need to be taken into consideration when wiring batteries in series. 

This is because complete batteries have a BMS installed. The BMS, or battery management system, is responsible for controlling when a battery can be charged or discharged. In order to provide this control, a BMS uses a component called a MOSFET (Metal Oxide Semiconductor Field Effect Transistor) to control the flow of current into and out of the battery. 

MOSFETs come in a wide variety and are rated for a particular amount of voltage and current. When two batteries with BMS are put in series, as long as both batteries are not dead, everything will work fine. Each set of BMS MOSFETs will see their normal voltages and when you put their outputs in series, you will see their voltages added and you can run higher voltage equipment with no problem. 

The moment either one of those batteries dies, however, you are going to run into an issue. While a MOSFET is active, it has an extremely low resistance and is basically just a wire. This allows current to freely flow, so the higher voltage coming in from the first battery goes into the second battery without a problem. When either one of the batteries dies, though, its MOSFETs will be switched off and they will go into a high resistance state, which prevents current from flowing. 

When this occurs, the MOSFETs on the BMS of the dead battery will experience the full voltage of the battery that is not dead plus the voltage of the cells in the dead battery. If the MOSFETs cannot handle this higher voltage, they will be damaged. Alternatively, you could choose the right BMS for the voltage of the two packs in series, remove each pack's individual BMS and rewire with the new higher voltage BMS.

So, in review, wiring lithium batteries in series is just as simple as wiring lithium cells in series. The difference is that lithium batteries have a BMS which contains MOSFETs that might not be able to handle the higher voltage that they would experience when one battery dies. This means that as long as you make sure neither battery dies during operation, it's fine to use lithium-ion batteries in series.

Alternatively, if you can verify that the MOSFETs in your BMS are able to handle the higher voltage, then you have nothing to worry about at all. 

How To Charge Lithium Batteries In Series

Charging lithium battery cells while they are in a series configuration is not only possible but very common. It’s how ebike, laptops, and just about any other battery chargers work. When charging lithium batteries in series, the charge voltage is divided among the number of cells in series. As long as each cell has about the same resistance, then the voltage will be split equally. 

An NMC lithium-ion battery cell has a max charge voltage of 4.2 volts. If 3 of those cells are placed in series, they can be charged in series by attaching a 12.6-volt battery charger to the main negative and main positive connection of the series group. 

If the cells are not in balance or if one is damaged or otherwise not performing as expected, the amount of voltage that each cell gets can become very uneven. If the situation is extreme enough, the cell groups that don't have any problems can be overcharged past 4.2 volts. This is why using a BMS is crucial to lithium battery safety and longevity. 

For example, if a charger is providing 12.6 volts and one cell is damaged and won't go above 2 volts, then the other 2 cells will be seeing 5.2 volts each!

Conclusion

Lithium-ion batteries are not very useful on their own but are extremely useful when added in series to increase voltage. Connecting battery cells in series is a pretty straightforward process, but there are some key elements that should be understood before doing so. 

To connect lithium-ion batteries in series, all you have to do is connect the positive connection of the first cell to the negative connection of the next one. An infinite number of cells can be put in series, and common series configurations are between 3 and 20 cell groups in series. When connecting lithium-ion batteries in series, an open-ended chain is formed that will have a free connection on either end. These end connections are the battery’s main negative and main positive connections. Adding battery cells in series adds their voltages together while not changing the amp hours. It’s important to consider, however, that because power is a measure of volts multiplied by amp hours, putting lithium batteries in series increases the overall power by increasing the overall voltage. 

We hope this article helped you learn more about how to wire lithium-ion batteries in series. Thanks for reading!