How To Connect Batteries in Series and Parallel: Series vs. Parallel
Batteries can be connected in series and parallel in order to create battery packs of larger voltage and/or capacity.
While in theory, batteries can be almost freely connected in parallel and series, following just a few simple rules, in real life, things are not that simple.
But, battery packs consisting of several batteries connected in parallel and/or series are used every day, and it is good to learn how to do it properly.
Updated: January 17, 2023.
Series and Parallel Connections: Little Bit Of Theory
For short, batteries are connected in:
Series
Batteries of the same capacities but different voltages may be connected in series in order to create a battery pack that features a capacity that is equal to the capacity of the individual batteries, but total voltage is the sum of all voltage of used batteries.
For example, if we connect 6V 10Ah and 12V 10Ah batteries in series, the new battery (battery pack) will have a nominal capacity of 10Ah, but the total voltage will be 18V.
Parallel
Batteries of the same voltages but different capacities may be connected in parallel in order to create a battery pack that features voltage that is equal to the voltage of individual batteries, but the total capacity is a sum of all capacities of used batteries.
For example, if we connect 12V 5Ah and 12V 10Ah batteries in parallel, the new battery (battery pack) will have a nominal voltage of 12V, but the capacity will be 15Ah.
Real Life
In real life, it is highly recommended to use not only batteries of the same voltage and capacity, but it is recommended to use the same battery model from the same manufacturer, preferably from the same batch, of the same age, and that they are new!
That means that these batteries will have the very same chemistry and as similar as possible nominal voltage, capacity, internal resistance, etc.
Only such similar batteries can minimize the imbalances in the battery pack, prolonging its operating lifetime - since some battery packs consist of 8 or even more batteries, it is very important to use the very same batteries.
Note: while lead-acid batteries can be freely connected in series and parallel, lithium batteries should be connected in series and/or parallel only if allowed by the manufacturer - lithium batteries feature a built-in Battery Management System (BMS) that can cause issues when batteries are connected in parallel/series in the wrong way. And lithium batteries without BMS? Stay away from such batteries...
Before connecting the batteries in parallel/series, it is important to fully recharge the batteries using the same battery charger(s) at the same temperature and to connect the batteries in parallel for the batteries to equalize their voltage and charge level.
How To Connect Two 12V Batteries in Parallel
When two 12V batteries are connected in parallel, the newly created battery pack features a nominal voltage of 12V, and the total capacity is the sum of the capacities of the used batteries.
Since the very same batteries are used, that means that if we use two 12V 50Ah batteries and connect them in parallel, the new battery pack will have 12V voltage and 100Ah capacity.
This is a very simple example, used very often. However, there is one very important detail to look for.
If the negative terminal of the battery pack is taken from the negative terminal of the "A" battery, then the positive terminal of the battery pack MUST be taken from the positive terminal of the "B" battery.
What would happen if take both positive and negative from the battery "A" terminals? Well, in that case, battery "A" will work harder than battery "B," and it would most probably fail first, destroying the entire battery pack.
Why would battery "A" work harder? Well, if we take both positive and negative from the battery "A" terminals, then the battery "B" external resistance is the resistance shared by the battery "A" AND two wires connecting batteries "A" and "B" - these wires regardless how thick are have some resistance which can be of great importance in the long run for batteries that are drained using very strong currents. Thus, battery "B" will be less loaded than battery "A."
Similarly, if we connect three batteries in parallel (batteries "A," "B," and "C"), the negative is taken from the negative terminal of battery "A," while the positive is taken from the positive terminal of the battery "C," etc.
How To Connect Two 12V Batteries In Series
When two 12V batteries of the same capacities are connected in series, the newly created battery pack features a nominal voltage of 24V, and the total capacity equals the capacity of each battery.
Since the very same batteries are used, that means that if we use two 12V 50Ah batteries and connect them in series, the new battery pack will have 24V voltage and 50Ah capacity.
As one can see, this is a very simple example, often used to create 24V systems with no hidden dangers or issues - just be sure to use the very same batteries from the same manufacturer, preferably of the same batch, and that they are both charged using the same battery charger at the same temperature.
How To Connect Four (4) 12V Batteries to Make a 24V Battery Pack
Four 12V batteries can be used to make a 24V battery pack by connecting them in a so-called 2S2P configuration (two in series, two in parallel).
This battery pack is a little bit more complicated than the battery packs with just two batteries, but it is very logical and has just one very important detail to look for:
Again, the location of the positive and negative terminals of the newly created battery pack is very important.
In the previous picture, if the negative is taken from the negative terminal of battery "A," and the positive is taken from the positive terminal of battery "B," the batteries "C" and "D" will work less than the batteries "A" and "B," leading to the premature failure of the battery "A" or "B."
Thus, in order to avoid that, the positive should be taken from the positive terminal of battery "D":
This "little detail" is often forgotten by many manufacturers of lithium batteries when they draw hypothetical battery packs using their lithium batteries - and that is not good.
For example, if we use four 12V 100Ah lithium batteries to create a 24V 200Ah battery pack and take negative from the negative terminal of battery "A" and positive from the positive terminal of battery "B" and use 2/0 AWG 0.5 meters wires to connect the batteries between themselves, then the resistance of each 2/0 AWG 0.5m wire will be 0.12785 mΩ which is really not much.
But, if batteries "C" and "D" are providing, let's say, 50A, then the voltage drop between the negative terminals of the batteries "A" and "C," and positive terminals of the batteries "B" and "D" is:
UDrop = 0.12785 mΩ * 50A = 6.3925 mV
With the complete voltage difference being double that: 12.785 mV for a current of 50A - the maximum continuous discharge current of the lithium batteries is usually around 1C.
At first, this is not a big difference, but in the long run, if the battery pack is heavily loaded and if the battery pack cycles a lot, in the long run, the batteries "A" and "B" may fail much sooner than the batteries "C" and "D."
How To Connect Eight (8) 12V Batteries To Make a 24V Battery Pack
If we want to connect 4, 6, 8, or even more 12V batteries to make a 24V battery pack, they are connected as 2S2P, 2S3P, 2S4P, etc., battery packs.
2S4P battery pack means that the eight (8) batteries are connected as:
Thus, if we use eight (8) 12V 100Ah batteries to create a 2S4P battery pack, the newly created battery pack will have a 24V nominal voltage and 400Ah nominal capacity.
When creating such a battery pack, use the same batteries from the same manufacturer of the same age and use the very same connecting wires, as thick as possible, to keep the energy losses to a minimum.
Also, if the negative is taken from the negative terminal of battery "A," the positive MUST be taken from the positive terminal of battery "H."
How To Connect Four (4) 12V Batteries To Make 48V Battery Pack
If we want to create a 48V battery pack using four (4) 12V batteries, these batteries must be connected in series:
This is the so-called 4S1P configuration - total voltage is a sum of individual battery voltages (4 * 12 = 48), while the capacity of the battery pack equals the capacity of each battery (batteries are of the same voltage and capacity - again, the very same battery model from the same manufacturer of the same age. This cannot be emphasized enough!).
So, if we have four 12V 50Ah starting batteries that are able to provide, let's say, 500 CCA, the newly created battery pack will feature 48V nominal voltage, 50Ah capacity, and 500 CCA.
How To Connect Eight (8) 12V Batteries To Make 48V Battery Pack
If we want to create a 48V battery pack using eight (8) 12V batteries, these batteries must be connected in 4S2P configuration:
Again, this is very straightforward schematics, just be sure to take negative and positive from the batteries "A" and "H" and NOT from the batteries "A" and "D."
For example, if we have eight 12V 50Ah 500 CCA batteries, the newly created battery pack will have 48V (4 * 12 = 48), 100Ah capacity (2 * 50Ah = 100Ah), and 1000 CCA (2 * 500 CCA = 1000 CCA).
How To Connect Sixteen (16) 12V Batteries To Make 48V Battery Pack
If we want to create a 48V battery pack using sixteen (16) 12V batteries, these batteries must be connected in 4S4P configuration:
As one can see, negative is taken from battery "A" and positive from battery "P" - if one takes positive from battery "D," that would mean that batteries A, B, C, and D have to work harder than the other batteries, leading to premature battery pack failure.
For example, if we have sixteen 12V 50Ah 500 CCA batteries, the newly created battery pack will have 48V (4 * 12 = 48), 200Ah capacity (4 * 50Ah = 200Ah), and 2000 CCA (4 * 500 CCA = 1000 CCA).
4S4P configuration is usually the maximum for high-capacity commercial lithium batteries, but using, for example, sixteen Ampere Time (now LiTime) 12V 400Ah batteries, one may create a battery pack with a nominal voltage of 48V, the actual voltage of 51.2V, and capacity of 1600Ah(!), which is able to store up to ~82 kWh of energy and can power large RVs, entire homes and apartments and similar loads for many hours...
Few Final Words
12V batteries are able to store plenty of energy and are able to deliver it in very strong surges causing damages and causing injuries or worse.
Battery packs created using such batteries are even stronger.
So, when working with 12V or any other battery, safety first. Also, if unsure, always contact local certified electrician or company, ask for advice, or preferably, let them do work for You - in the long run, that is the safest and the cheapest method.
