Individual lithium battery cells are usually 3.7 V nominal (for li-ion cells) or 3.2 V nominal (for LiFePO4 cells). This voltage is acceptable for some low power devices, such as cellphones and other small electronics, but it can’t provide enough power for anything more substantial. For bigger projects, including anything from an electric skateboard to an electric car, multiple lithium cells are wired in series to increase the voltage of the battery.
In a series connection, the positive terminal of one battery cell is connected to the negative terminal of the next battery. If you’ve ever slid more than one battery in a row into a flashlight tube, that is a series connection. The positive terminal of one cell always connects to the negative terminal of the next cell.
These series connections can combine just two cells or hundreds of cells.
The number of cells wired in series depends on what voltage is required. To calculate the voltage of a set of battery cells connected in series, simply multiply the voltage of one cell by the number of cells in the series connection.
Total voltage of cells in series = nominal voltage of a single cell × # of cells in series
If we are using 3.7 V nominal li-ion cells and we connect two cells in series, we’ll end up with a 7.4 V nominal battery calculated as follows:
Total voltage = 3.7 V × 2 cells = 7.4 V total
If we added one more cell into that series connection for a total of three cells, we’d have an 11.1 V battery. Ten cells in series would give us 37 V nominal. Fifteen cells in series would give us 55.5 V, and I think you get the idea from there.
An important thing to remember is that the nominal voltage of a battery cell or a larger battery pack is just that - “nominal”, which comes from the same root as the word “name”. Basically, these cells are “named” 3.7 V cells, but in reality they span a much wider voltage range during use.
A single 3.7 V nominal lithium-ion cell can be charged up to 4.2 V and discharged as low as 2.5 V, which is a very big range. Image that we connect 10 of these cells in series to create a 37 V nominal battery. That battery’s voltage will actually range from a fully charged voltage of 42 V down to a minimum of 25 V if discharged to 0% state of charge.
If we have a device that requires at least 30 V to operate, then we would stop discharging at 3.0 V per cell in this 10 cell battery, even though the battery could have kept discharging down to 25 V. That equates to not using about 5% of the pack’s total capacity. You might not think 5% is a big deal, but what if that device required 35 V? We’d stop discharging at 3.5 V per cell in this 10 cell battery, which would equate to leaving about 40% of the battery’s capacity unused. This is why it is important to consider the entire range of the voltage of a battery when calculating how many cells in series are required for your project.
Many electronics such as inverters, electric motors and other DC devices are designed for voltages in 12 V increments, such as a 12 V headlamp or a 48 V electric bicycle. This is a holdover from the many years when lead acid batteries were used to power these types of devices. Lead acid batteries use cells that have a nominal voltage of 2 V, and six are usually
connected together in series to create 12 V lead acid batteries. Those 12 V batteries are then easily connected in series to create any other size battery with 12 V increments.
The problem this old system has created for us is that most lithium batteries don’t conform well to this arbitrary 12 V increment.
Most electronics (but not all!) are capable of handling a small range of voltages above and below their rated voltage. For example, a 12 V LED headlamp can likely function with a voltage of between 9 V - 15 V, though more sensitive electronics will have smaller permissible voltage ranges.
This voltage range allows us to use a lithium battery voltage that is close to the 12 V increments that many electronics are rated for, even if it isn’t exact.
For example, electric bicycles are usually designed for either 24 V, 36 V or 48 V batteries. Again, this is because most of the ebike components were originally designed for lead acid batteries and the nomenclature in the industry just stuck.
The most commonly accepted lithium-ion battery for 24 V ebike systems is 7 cells in series, which creates a 25.9 V nominal battery that actually ranges from approximately 21 V - 29 V during use. For 36 V lithium batteries, nearly all ebike manufacturers use 10 cells in series to create a 37 V nominal battery that ranges from approximately 30 V - 42 V during use.
When it comes to 48 V batteries though, the industry is fairly split. Batteries with 13 cells in series used to be the most popular configuration for a 48 V battery. This resulted in a nominal rating of 48.1 V and a voltage range under use of approximately 39 V - 54 V. However, with voltage sag, the battery would actually spend the majority of its time below 48V, which results in less power.
For this reason, many 48 V batteries for ebikes are now made with 14 cells in series which gives a nominal rating of 51.8 V and has a higher voltage range of approximately 42 V - 58.8 V. These batteries are often referred to as 52 V batteries instead of 48 V batteries to signify that they are indeed of a higher voltage than “standard” 48 V lithium batteries.
Other industries don’t always have this 12 V increment issue and can
essentially use any voltage that they design their devices for. Battery powered tools are a great example. Many power drills use 11.1 V nominal batteries which consist of three lithium-ion cells in series, though many tool manufacturers still call these 12 V batteries. This isn’t really fair, as they spend very little time above 12 V. However, because they charge up to 12.6 V, the 12 V badge isn’t technically untrue. Rather, it’s just misleading. The next step up in power tools is usually an 18 V nominal battery, which consists of five lithium-ion cells in series.
One thing to note is that all of the above examples I gave used lithium-ion cells, as these are the most commonly used cells in these applications.
However, LiFePO4 cells actually lend themselves more easily to 12 V increments. With 3.2 V nominal cells, combining four LiFePO4 cells will create a 12.8 V nominal battery, which is pretty darn close to 12 V. LiFePO4
cells are fairly popular for DIY electric vehicle conversions, largely due to a combination of high cycle life, excellent safety and only moderate space restrictions (who needs that trunk space anyways?). As many electric vehicle components were originally designed for lead acid batteries, they are also often rated in 12 V increments, which makes using LiFePO4 cells a bit easier when you’re aiming for a specific voltage in a 12 V increment.