Here’s another topic you hear battery nerds talk about a lot (and electrical engineers in general): Series and parallel connections.
In the context of battery building, you don’t just buy one cell of the desired voltage and capacity and attach powerpoles to it. (Wouldn’t that be nice?) Cells and capacities need to be combined and calculated to total 12-ish volts and have enough runtime to power your ham radios and 12V devices. You can use a variety of battery chemistries to try to arrive at your desired voltage and capacity, but in the end it will almost always involve putting the smaller cells into series connections, and optionally into parallel connections.
To describe this combination of series and parallel connections, we use abbreviations like “4S3P”.
The “4S” in 4S3P indicates 4 cells in series. A series connection involves connecting the positive terminal of one battery to the negative terminal of another battery, creating a chain of batteries. The voltage of the battery bank is the sum of the individual voltages of each battery, while the capacity and current remain unchanged. This means that the total energy output is increased, as the voltage is increased, but the runtime is not increased.
For example, if you connect four 3.2V LiFePO4 cells in series, the total voltage of the pack will be 12.8V (3.2V + 3.2V + 3.2V + 3.2V). However, the current remains constant, meaning that the capacity of the battery bank remains the same as that of a single battery. The disadvantage of a series connection is that if one battery fails or is disconnected, the entire chain is broken, and the voltage drops to zero.
For lithium iron phosphate (LiFePO4) cells, this “4S” won’t change much from battery to battery. Because the cells are conveniently 3.2V, you will have multiples of 4 to create 12V, 24V, 48V packs depending on the desired use. A 24V battery is normally 8S. A 48V battery is normally 16S. Nearly every ham radio LiFePO4 battery is 4S.
Contrast this with other chemistries such as Nickel Metal Hydride (NiMH), or lithium-ion chemistries (Li-ion): NiMH provides 1.2V nominal so creating a 12V pack might mean 10S. When a device asks for eight 1.5V alkaline batteries (often not included), you are creating an 8S (12V) pack to power your device. Lithium ion batteries have a nominal 3.7V, so creating a true 12V pack is not achievable; you either have to create a 11.1V pack or a 14.8V pack (3S or 4S). Most Li-ion “12V” batteries I see are in a 3S configuration.
The “3P” in 4S3P indicates 3 cells in parallel. A parallel connection involves connecting the positive terminals of two or more cells together and the negative terminals together, creating a group of cells with a higher overall capacity. The voltage of the group remains the same as that of a single battery, but the capacity and runtime are increased. The total capacity is the sum of the individual capacities (amp-hours) of each battery.
For example, if you connect three 3.2V 6Ah cells in parallel, the total voltage of the battery bank will be 3.2V, but the capacity will be three times that of a single cell, or in this case 18Ah. The advantage of a parallel connection is that if one battery fails or is disconnected, the other cells continue to supply power, and the voltage remains constant.
A series-parallel connection is a combination of series and parallel connections, where two or more cells are connected in parallel, and each parallel group is connected in series. In our previous example “4S3P” this abbreviation indicates a series-parallel battery configuration. This configuration provides both an increase in voltage and capacity. By using a combination of series and parallel connections, the battery bank can be customized to meet specific voltage and capacity requirements.
For example, if you need a 12V battery bank with a capacity of 20Ah, you can connect four 3.2V 5Ah cells in parallel, and then connect four of these parallel groupings in series. This will give you a 12.8V (3.2V * 4) battery pack with a capacity of 20Ah (4 * 5Ah). The resulting pack is 4S4P.
Where It Gets Scary in Battery Building
When putting together a battery pack, we are dealing with stores of immense power. One thing a battery builder never wants to do is close the circuit between the positive and negative terminals of a battery, whether an individual cell, a parallel pack, or a series pack. Doing so would release all the stored power as fast as the circuit can handle (a short circuit), producing heat, sparks, flame, and overloading the cells to the point of venting their internal chemicals.
Parallel batteries can be connected positive to positive and negative to negative with a degree of confidence because no short circuits will occur. However, we should remember that parallel cell connections equalize voltage across all cells. If you have cells of different states of charge, where some have lower voltage and others are at the top of their charge voltage, and you connect all the positives together, then separately all the negatives together, a massive inrush of current will occur as the cells charge each other to arrive at the same overall voltage. This isn’t a short, but if the charges are different enough, it is a large movement of current that could damage cells. Remedy this by individually charging each cell to the same voltage before connecting them in parallel (also known as “top balancing”).
The other side of this is when performing series connections. In parallel connections all the terminals are connected. But in series connections, there is always an open remaining negative terminal and positive terminal, that you will use to connect any load. Covering all terminals of a series connection will close the circuit and create heat.
As you can imagine, series-parallel configurations can get confusing where a battery builder might inadvertently close a series connection when thinking they were completing a parallel connection. With small builds this is more about burnt fingers and possibly fire extinguishers. But with high capacities this could mean explosions and severe injury. Always pay attention, especially when getting creative with parallel pack layout and bridging them to series, to what is what so you don’t make this mistake.
Can I Connect Different Voltage Cells in Series?
Someone might understand these concepts and say, “Cool! Let’s build a 12V battery out of four 1.5V batteries and two 3V batteries. That will be 6S of (1.5 * 4) + (3 * 2) = 6 + 6 = 12V, right?”
Well… yes. But this is a bad idea. While technically they will work in a series chain, during charging, the lower voltage cells will charge faster before the higher voltage cells. If the charge is continued, there will be damage from overcharging. During discharge the lower voltage batteries, since lower voltage often means smaller in size and therefore less capacity, could run out of juice first while the higher voltage batteries continue to run. As this keeps going, the lower volt cells should have stopped discharging by now, but they continue to get drained in the chain, damaging them. Also, when the weaker cells are almost completely drained, the stronger ones will attempt to recharge the weaker ones in order to keep the circuit going. After prolonged use like this the lower voltage cells will truly be dead and won’t recharge at all.
This also presents challenges for battery management systems since they would have to be designed to monitor individual series cell parameters instead of a common expected voltage: For our example it would require one set of protection parameters for 1.5V cells, and another for 3V cells. As far as I know, no BMS like this exists. It would have to be a smart and programmable BMS to handle varying cells voltages almost to the point of being impractical. Knowing when to cut off charge or discharge to prevent cell damage would be challenging.
How About Different Cell Capacities in Parallel?
Say you want to mix a 20Ah cell with a 6Ah cell to get an overall capacity of 26Ah. This will work, but is generally not recommended. It is safer than putting different voltage cells in series, and some people do mix different capacity batteries into parallel groups. The higher capacity battery will drain at the same rate as the lower capacity one, though the current flowing into and out of the larger cell will be proportionally higher than the lower capacity cell. Some commercial systems now exist to mix different capacity batteries in a power system, but these deal on a macro scale with parallel battery packs instead of parallel cells of different capacity that are made into a pack. It is a contentious subject among battery builders. In actuality, many battery builders are unknowingly doing this, especially if they repurpose used cells. These “second life” cells are almost guaranteed to be of different capacity due to age and degradation, and yet they seem to work fine when used together for a DIY battery. Here is a good explanation on the topic.
Well, What About Different Cell Capacities in Series?
This happens a lot with used cells, and BMS’s exist to help remedy the difference in capacities. The overall pack will always be limited by the lowest capacity cell in series, and if the BMS is working right, the battery will simply shut off when one of the series cells is drained. To get more out of a used pack, an Active Balancer is used, which uses high frequency switched parallel and series relays to charge and distribute voltage via capacitors across the series chain. This is in essence “charging the weakest link” during the discharge phase, or “discharging the weakest link” during the charge phase, in order to normalize the whole pack. It’s not ideal, but the best you can do with mismatched capacities in series.
And Different Voltages in Parallel?
No, no, NO!!!
Do not do this. The higher voltage cells will immediately charge the lower voltage cells with a huge inrush current and potentially blow them up. Just don’t even think about it.
So what is 4S1P?
4S1P means 4 single cells in series and “1 parallel”. Technically a 4S1P pack is a misnomer and could also be called a 4S0P pack, none in parallel. I believe we see this inaccurate abbreviation to maintain the series-parallel naming convention. Parallel can only be two or more cells, but many batteries only use a chain of single cells.
Series multiplies voltage. Parallel multiplies capacity and current. Now you know what designers did when they tell you the battery is a 16S8P or a 4S2P. Knowing the cell specifications, you can reverse-engineer what it took to make the battery and what performance characteristics you can expect out of it.