BU-302: Series and Parallel Battery Configurations

BU-302: Configuraciones de Baterías en Serie y Paralelo (Español)

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Batteries achieve the desired operating voltage by connecting several cells in series; each cell adds its voltage potential to derive at the total terminal voltage. Parallel connection attains higher capacity by adding up the total ampere-hour (Ah).

Some packs may consist of a combination of series and parallel connections. Laptop batteries commonly have four 3.6V Li-ion cells in series to achieve a nominal voltage 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 series and parallel connection. It is important to use the same battery type with equal voltage and capacity (Ah) and never to mix different makes and sizes. A weaker cell would cause an imbalance. This is especially critical in a series configuration because a battery is only as strong as the weakest link in the chain. An analogy is a chain in which the links represent the cells of a battery connected in series (Figure 1).

A weak cell may not fail immediately but will get exhausted more quickly than the strong ones when on a load. On charge, the low cell fills up before the strong ones because there is less to fill and it remains in over-charge longer than the others. On discharge, the weak cell empties first and gets hammered by the stronger brothers. Cells in multi-packs must be matched, especially when used under heavy loads. (See BU-803a: Cell Mismatch, Balancing).

Single Cell Applications

The single-cell configuration is the simplest 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 1.2V, alkaline is 1.5V; silver-oxide is 1.6V and lead acid is 2.0V. Primary lithium batteries range between 3.0V and 3.9V. Li-ion is 3.6V; Li-phosphate is 3.2V and Li-titanate is 2.4V.

Li-manganese and other lithium-based systems often use cell voltages of 3.7V and higher. This has less to do with chemistry than promoting a higher watt-hour (Wh), which is made possible with a higher voltage. The argument goes that a low internal cell resistance keeps the voltage high under load. For operational purposes these cells go as 3.6V candidates. (See BU-303 Confusion with Voltages)

Series Connection

Portable equipment needing higher voltages use battery packs with two or more cells connected in series. Figure 2 shows a battery pack with four 3.6V Li-ion cells in series, also known as 4S, to produce 14.4V nominal. In comparison, a six-cell lead acid string with 2V/cell will generate 12V, and four alkaline with 1.5V/cell will give 6V.

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; the end-of-discharge voltage must be respected, however.

High voltage batteries keep the conductor size small. 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 the electrical components and arcing problems on mechanical switches derailed the move.

Some mild hybrid cars run on 48V Li-ion and use DC-DC conversion to 12V for the electrical system. Starting the engine is often done by a separate 12V lead acid battery. Early hybrid cars ran on a 148V battery; electric vehicles are typically 450–500V. Such a battery needs more than 100 Li-ion cells connected 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 cause a failure. To prevent this from happening, a solid state switch in some large packs bypasses the failing cell to allow continued current flow, albeit at a lower string voltage.

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 is why battery packs are commonly replaced as a unit.

High-voltage batteries in electric vehicles, in which a full replacement would be prohibitive, divide the pack into modules, each consisting of a specific number of cells. If one cell fails, only the affected module is replaced. A slight imbalance might occur if the new module is fitted with new cells. (See BU-910: How to Repair a Battery Pack)

Figure 3 illustrates a battery pack in which “cell 3” produces only 2.8V instead of the full nominal 3.6V. 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 a “Low Battery” message.

Batteries in drones and remote controls for hobbyist requiring high load current often exhibit an unexpected voltage drop if one cell in a string is weak. Drawing maximum current stresses frail cells, leading to a possible crash. Reading the voltage after a charge does not identify this anomaly; examining the cell-balance or checking the capacity with a battery analyzer will.

Tapping into a Series String

There is a common practice to tap into the series string of a lead acid array to obtain a lower voltage. Heavy duty equipment running on a 24V battery bank may need a 12V supply for an auxiliary operation and this voltage is conveniently available at the half-way point.

Tapping is not recommended because it creates a cell imbalance as one side of the battery bank is loaded more than the other. Unless the disparity can be corrected by a special charger, the side effect is a shorter battery life. Here is why:

When charging an imbalanced lead acid battery bank with a regular charger, the undercharged section tends to develop sulfation as the cells never receive a full charge. The high voltage section of the battery that does not receive the extra load tends to get overcharged and this leads to corrosion and loss of water due to gassing. Please note that the charger charging the entire string looks at the average voltage and terminates the charge accordingly.

Tapping is also common on Li-ion and nickel-based batteries and the results are similar to lead acid: reduced cycle life. (See BU-803a: Cell Matching and Balancing) Newer devices use a DC-DC converter to deliver the correct voltage. Electric and hybrid vehicles, alternatively, use a separate low-voltage battery for the auxiliary system.

Parallel Connection

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 in a P4 arrangement. The nominal voltage of the illustrated pack remains at 3.60V, but the capacity (Ah) and runtime are increased fourfold.

A cell that develops high resistance or opens is less critical in a parallel circuit than in a series configuration, but a failing cell will reduce the total load capability. It’s like an engine only firing on three cylinders instead of on 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.

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A total short can occur through reverse polarization or dendrite growth. 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.

A weak cell will not affect the voltage but provide a low runtime due to reduced capacity. A shorted cell could cause excessive heat and become a fire hazard. On larger packs a fuse prevents high current by isolating the cell.

Series/parallel Connection

The series/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 sum of voltage times current; a 3.6V (nominal) cell multiplied by 3,400mAh produces 12.24Wh. Four Energy Cells of 3,400mAh each can be connected in series and parallel as shown to get 7.2V nominal and a total of 48.96Wh. A combination with 8 cells would produce 97.92Wh, the allowable limit for carry on an aircraft or shipped without Class 9 hazardous material. (See BU-704a: Shipping Lithium-based Batteries by Air) The slim cell allows flexible pack design but a protection circuit is needed.

Li-ion lends itself well to series/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 applies to e-bike batteries, hybrid cars and the Tesla Model 85 that devours over cells to make up the 90kWh pack.

Terminology to describe Series and Parallel Connection

The battery industry specifies the number of cells in series first, followed by the cells placed in parallel. An example is 2s2p. With Li-ion, the parallel strings are always made first; the completed parallel units are then placed in series. Li-ion is a voltage based system that lends itself well for parallel formation. Combining several cells into a parallel and then adding the units serially reduces complexity in terms of voltages control for pack protection.

Building series strings first and then placing them in in parallel may be more common with NiCd packs to satisfy the chemical shuttle mechanism that balances charge at the top of charge. “2s2p” is common; white papers have been issued that refer to 2p2s when a serial string is paralleled.

Safety devices in Series and Parallel Connection

Positive Temperature Coefficient Switches (PTC) and Charge Interrupt Devices (CID) protect the battery from overcurrent and excessive pressure. While recommended for safety in a smaller 2- or 3-cell pack with serial and parallel configuration, these protection devices are often being omitted in larger multi-cell batteries, such as those for power tool. The PTC and CID work as expected to switch of the cell on excessive current and internal cell pressure; however the shutdown occurs in cascade format. While some cells may go offline early, the load current causes excess current on the remaining cells. Such overload condition could lead to a thermal runaway before the remaining safety devices activate.

Some cells have built-in PCT and CID; these protection devices can also be added retroactively. The design engineer must be aware than any safety device is subject to failure. In addition, the PTC induces a small internal resistance that reduces the load current. (See also BU-304b: Making Lithium-ion Safe)

Simple Guidelines for Using Household Primary Batteries

• Keep the battery contacts clean. A four-cell configuration has eight contacts and each contact adds resistance (cell to holder and holder to next cell).
• Never mix batteries; replace all cells when weak. The overall performance is only as good as the weakest link in the chain.
• Observe polarity. A reversed cell subtracts rather than adds to the cell voltage.
• Remove batteries from the equipment when no longer in use to prevent leakage and corrosion. This is especially important with zinc-carbon primary cells.
• Do not store loose cells in a metal box. Place individual cells in small plastic bags to prevent an electrical short. Do not carry loose cells in your pockets.
• Keep batteries away from small children. In addition to being a choking hazard, the current-flow of the battery can ulcerate the stomach wall if swallowed. The battery can also rupture and cause poisoning. (See BU-703: Health Concerns with Batteries)
• Do not recharge non-rechargeable batteries; hydrogen buildup can lead to an explosion. Perform experimental charging only under supervision.

Simple Guidelines for Using Secondary Batteries

• Observe polarity when charging a secondary cell. Reversed polarity can cause an electrical short, leading to a hazardous condition.
• Remove fully charged batteries from the charger. A consumer charger may not apply the correct trickle charge when fully charged and the cell can overheat.
• Charge only at room temperature.

References

Different Types of Batteries and Why They Matter

In our mobile and on-the-go digital world, batteries are a necessity. When you want to take electronics on the go, they need batteries to work when not plugged in. Portable electronics offer convenience, but the right battery for the right device is key. Some batteries have been around for over a century and others have just recently joined the ranks. Some are single-use and others are rechargeable and all come in their own unique packaging.

So how can you determine what type of battery to use for your electronic devices and your own original electronic projects? The following information will help shed some light on the advantages, disadvantages, and unique capabilities of the common types of batteries available.

Most common types:

Alkaline: One of the most common types, alkaline batteries contain an alkaline electrolyte, usually potassium hydroxide. Alkaline batteries are best used in low drain electronics, such as LED flashlights, clocks, radios, toys, and remote control devices. Moderate drain items, such as incandescent light bulbs, can also operate effectively with alkaline batteries. High drain electronics, such as digital cameras, will operate using alkaline batteries, but the battery life is greatly reduced.

Lead Acid: The oldest rechargeable battery, the lead acid type was the first commercial use rechargeable battery. These are still in use today in cars, golf carts, scooters, and wheelchairs because there are no affordable alternatives yet to replace them. Lead acid are also a tried and true choice for off-grid solar systems. Lead acid batteries can last a long time, but they have to be constantly charged. When sitting in a discharged state they lose power and cannot be recharged if unused for a long period of time.

Lithium: The advantage of lithium batteries is that lithium is very light, making it possible to hold more power in the same size casing as other batteries. Lithium batteries have a long shelf life and work well in all temperatures, but should only be used in high-power devices. Using a lithium battery in a device that isn’t designed for that much power can cause damage to the circuitry. The only other disadvantage is that they are more expensive than alkaline batteries.

Lithium Ion: These batteries are usually rechargeable and used in items like smart phones, laptop computers, and sport watches. They are most often found in block, slab, or battery pack form. Lithium Ion batteries can be recharged over and over, but they will gradually lose capacity to hold a charge over time. Not only is their shelf life short, but when they sit unused for long they lose their ability to be recharged.

Nickel Cadmium: These are rechargeable batteries containing nickel oxide hydroxide, metallic cadmium, and an alkaline electrolyte of potassium hydroxide. They are primarily used for portable items, such as computers, drills, camcorders, and other items that require an even power discharge.

Nickel Metal Hydride:These batteries contain a combination of nickel, an alloy, and potassium hydroxide as an electrolyte. Most often found in rechargeable form, these are good for high drain devices and prolonged energy usage, such as digital cameras, flash units, radio controlled cars, and GPS devices. Nickel Metal Hydride batteries deliver more current and a steady voltage, which makes them ideal for sourcing devices that require a lot of power. The disadvantages include fast loss of power when not in use and a decrease in capacity after more than 100 charges. They are also a bit fragile, as their performance suffers if they are dropped or roughly handled.

Most common packaging:

AA, AAA, C, D, 9V: These batteries are the typical household use batteries you will most likely recognize. A-D are typically cylindrical with a positive end and a negative end. 9Volt batteries are rectangular with a positive and negative terminal side by side on the same end, most often used in smoke detectors. Dont forget to check out our availability of AA, AAA, C, D, and 9 Volt batteries.

Button or Coil Cell:These are small, round, and flat. The most common types are CR series batteries, such as the CR, CR, CR, CR, CR and CR to name a few, that are used in garage door openers, car key fobs, book lights, and any small devices that need a very compact battery. Some are lithium and others are alkaline.

Camera Battery:Camera batteries are usually made in rechargeable units or battery packs because they last longer than individual AA or AAA alkaline batteries. A few of common camera batteries are the CR2, CR123A and 2CR5.

Custom Battery Packs: Custom battery packs are created for specific devices to provide the right amount of power. Let us know if you have needs for custom battery packs, we can assist with designing in and supply any type of battery pack you might need for your application.

How Do Batteries Differ from Capacitors?

While they may look similar, batteries and capacitors are not the same thing. The battery provides the energy from its own internal chemical energy stores. The capacitor then takes energy from the battery and stores it in a magnetic field, as opposed to a chemical storage system. The capacitor is able to charge and discharge faster due to the lack of chemical reaction required. Batteries can store a higher electrical charge and capacitors can handle higher voltage.

Tips for Maximizing Battery Life

• Store batteries in moderate temperatures whenever possible.
• Remove batteries from devices before long term storage (more than a month).
• Remove batteries from any device before plugging it in.
• Don’t try to charge different brands or types of batteries at the same time with the same charger.
• Keep batteries out of hot vehicles, attics, garages, or other areas where extreme heat would be an issue.
• Don’t store batteries where they may come in contact with other metals, such as coins, paper clips, or metal hardware.

Get the Right Batteries for your Electronics

Whether you’re looking for batteries for a premade device, a do-it-yourself project, or a large commercial endeavor, you can find the right batteries at Quest Components. Call (626) 333- to place an order or ask questions about batteries and other available products. You can also order online at your convenience. Contact Quest Components today at 626-333- for all your battery needs!

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