Learn how to arrange batteries to increase voltage or gain higher load currents.
Battery packs achieve the desired operating voltage by connecting several cells in series; each cell adds its voltage to the total terminal voltage. Parallel connection attains higher capacity for increased current handling; each cell adds to the ampere/hour (Ah) count.
Some packs may consist of a combination of serial and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve 14.4V and two in parallel to boost the capacity from 2,400mAh to 4,800mAh. Such a configuration is called 4S2P, meaning four cells in series and two in parallel. Insulating foil between the cells prevents the conductive metallic skin from causing an electrical short.
Most battery chemistries lend themselves to serial and parallel connection. It is important to use the same battery type with equal voltage and capacity (Ah) and never mix different makes and sizes. A weaker cell would cause an imbalance. This is especially critical in a serial configuration because a battery is only as strong as the weakest link in a chain. An analogy is a chain in which the links represent the cells of a battery connected in series (Figure 1).
Figure 1: Comparing a battery with a chain
Chain links represent cells in series to increase voltage, doubling a link denote parallel connection to boost current loading.
A weak cell may not quit immediately but will get exhausted more quickly than the strong ones when in continued use. On charge, the low cell fills up before the strong ones because there is less to fill and remains in over-charge longer than the others. On discharge, the weak cell is empty first and gets hammered by the stronger brothers. Cells in multi-packs must be matched, especially when used for demanding industrial applications. (See BU-803a: Cell Mismatch, Balancing).
The single-cell configuration is the most simplistic battery pack; the cell does not need matching and the protection circuit on a small Li-ion cell can be kept simple. Typical examples are mobile phones and tablets with one 3.60V Li-ion cell. Other uses of a single cell are wall clocks, which typically use a 1.5V alkaline cell, wristwatches and memory backup, most of which are very low power applications.
The nominal cell voltage for a nickel-based battery is 1.20V, alkaline 1.50V; silver-oxide 1.60V and lead acid 2.00V. Primary lithium batteries range between 3.00V and 3.90V. Li-ion is 3.60V; Li-phosphate 3.20V and Li-titanate 2.40V.
Li-manganese and other lithium-based systems often use voltages of 3.70V and higher. This has less to do with chemistry than marketing. For operational purposes they serve as 3.60V cells. Low internal resistance keeps the voltage high under load and this advantage is utilized to increase the voltage and watt-hour (Wh). (See BU-303 Confusion with Voltages)
Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 2 shows a battery pack with four 1.20V nickel-based cells in series to produce 4.8V. In comparison, a six-cell lead acid string with 2V/cell will generate 12V, and four Li-ion with 3.60V/cell will give 14.40V.
Figure 2: Serial connection of four cells (4S)
Adding cells in a string increases the voltage; the current remains the same.
Courtesy of Cadex
If you need an odd voltage of, say, 9.50 volts, connect five lead acid, eight NiMH or NiCd, or three Li-ion in series. The end battery voltage does not need to be exact as long as it is higher than what the device specifies. A 12V supply might work in lieu of 9.50V; most battery-operated devices can tolerate some over-voltage but quit on low voltage. Higher voltage applications often have less tolerance, however.
Portable devices using higher voltages have the advantage of keeping the conductor size down. Consumer cordless power tools run on 12V and 18V batteries; high-end models use 24V and 36V. Most e-bikes come with 36V Li-ion, some are 48V.
The car industry wanted to increase the starter battery from 12V (14V) to 36V, better known as 42V, by placing 18 lead acid cells in series. Logistics of changing electrical components and arcing problems on mechanical switches derailed the move. Mild hybrid cars experiment with 48V Li-ion and use DC-DC conversion to 12V to feed the electrical system. Early hybrid cars run on a 148V battery; electric vehicles have packs with 450–500V. Such a battery needs more than 100 Li-ion cells in series.
High-voltage batteries require careful cell matching, especially when drawing heavy loads or when operating at cold temperatures. With multiple cells connected in a string, the possibility of one cell failing is real and this would break the circuit. A solid state switch that bypasses the failing cell would continue current flow, albeit at a lower sting voltage. Such a bypass is seldom used because of cost.
Cell matching is a challenge when replacing a faulty cell in an aging pack. A new cell has a higher capacity than the others, causing an imbalance. Welded construction adds to the complexity of the repair and this gives reason why battery packs are replaced as a unit. High-voltage batteries in electric vehicles, in which a full replacement would be prohibitive, divide the pack into blocks, each consisting of a specific number of cells. If one cell fails, the affected block is replaced. A slight imbalance will also occur here.
Figure 3 illustrates a battery pack in which “cell 3” produces only 0.6V instead of the full 1.20V. With depressed operating voltage, this battery reaches the end-of-discharge point sooner than a normal pack. The voltage collapses and the device turns off with “Low Battery” message.
Figure 3: Serial connection with a faulty cell
Faulty cell 3 lowers the voltage and cuts the equipment off prematurely.
Courtesy of Cadex
If higher currents are needed and larger cells are not available or do not fit the design constraint, one or more cells can be connected in parallel. Most battery chemistries allow parallel configurations with little side effect. Figure 4 illustrates four cells connected in parallel. The voltage of the illustrated pack remains at 1.20V, but the current handling and runtime are increased fourfold.
Figure 4: Parallel connection of four cells (4P)
With parallel cells, current handling and runtime increases while the voltage stays the same.
A cell that develops a high-resistance or opens is less critical in a parallel circuit than in serial configuration but this will reduce the total load capability. It’s like an engine only firing on three cylinders instead of all four. An electrical short, on the other hand, is more serious as the faulty cell drains energy from the other cells, causing a fire hazard. Most so-called electrical shorts are mild and manifest themselves as elevated self-discharge.
A total short can occur and this happens when a cell receives reverse polarization. Large packs often include a fuse that disconnects the failing cell from the parallel circuit if it were to short. Figure 5 illustrates a parallel configuration with one faulty cell.
Figure 5: Parallel/connection with one faulty cell
A weak cell will not affect the voltage but will provide a low runtime due to reduced current handling. A shorted cell could cause excessive heat and become a fire hazard.
Courtesy of Cadex
The serial/parallel configuration shown in Figure 6 enables design flexibility and achieves the desired voltage and current ratings with a standard cell size. The total power is the product of voltage-times-current; four 1.20V cells multiplied with 1000mAh produce 4.8Wh. High energy-dense Li-ion has an advantage as four 18650 cells with 3,000mAh produce 12Wh.
Figure 6: Serial/ parallel connection of four cells (2S2P)
This configuration provides maximum design flexibility.
Li-ion lends well to serial/parallel configurations but the cells need monitoring to stay within voltage and current limits. Integrated circuits (ICs) for various cell combinations are available to supervise up to 13 Li-ion cells. Larger packs need custom circuits, and this also applies to the Tesla Model 85 that devours over 7000 18650 cells to make up the 85kWh pack.
Last updated 2015-09-16
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