How to Correctly Connect Batteries in Series and Parallel Formula
Understanding how to properly connect batteries in series and parallel formula might seem straightforward, but by adhering to a few simple guidelines, one can avoid unnecessary issues. In a battery pack, multiple batteries are connected in series to achieve the desired operating voltage. If higher capacity and greater current are required, batteries should be connected in parallel. There are also configurations that combine both series and parallel methods.
Connect in Series or Parallel Based on the Actual Operating Voltage
For instance, a laptop battery may consist of four 3.6V lithium-ion cells connected in series, reaching a total voltage of 14.4V. Then, two sets of these series-connected cells are linked in parallel. This increases the total capacity of the battery pack from 2000 milliamp-hours to 4000 milliamp-hours. Such a configuration is referred to as "four-series two-parallel," meaning two groups of four series-connected cells are paralleled.
In devices like watches, backup memory, and cell phones, typically a single battery is used. The nominal voltage of a nickel-based cell is 1.2V, an alkaline cell is 1.5V, a silver oxide cell is 1.6V, a lead-acid cell is 2V, a lithium cell is 3V, and a lithium-ion cell has a nominal voltage of 3.6V. Lithium-ion polymer and other types of lithium cells generally have a rated voltage of 3.7V.
To obtain uncommon voltages, such as 11.1V, three of these batteries would need to be connected in series circuit. Thanks to advancements in modern microelectronics, we can now power cell phones and low-power portable communication products with a single 3.6V lithium-ion battery types. Mercury cells, once widely used in light meters during the 1960s, have been phased out due to environmental concerns.
What is Series Connection and How to Connect in Series?
Series connection of batteries refers to linking multiple batteries end-to-end, where the positive terminal of one battery connects to the negative terminal of the next. Consequently, the positive terminal of the entire battery pack is the positive terminal of the first battery, and the negative terminal is that of the last battery. The main characteristics of this configuration are as follows:
1. Voltage Increase: The total voltage of the series-connected battery pack equals the sum of the voltages of each individual battery. For instance, two 3.7V lithium batteries connected in series will result in a total voltage of 7.4V.
2. Unchanged Capacity: The total capacity (mAh) of the series-connected battery pack remains the same as that of a single battery.
3. Unchanged Current: The maximum output current of the series-connected battery pack is the same as the maximum output current of a single battery.
Portable devices requiring high power are typically powered by battery packs composed of two or more cells connected in series combination. When using batteries with a higher voltage, the size of conductors and switches can be minimized. Medium-priced industrial power tools generally operate on batteries with voltages ranging from 12V to 19.2V, while high-end power tools utilize batteries with voltages from 24V to 36V for greater power output.
In the automotive industry, the ignition battery voltage for starters has eventually been increased from 12V (actually 14V) to 36V, and even up to 42V. These battery packs consist of 18 lead-acid batteries connected in series.The 42V automotive batteries are costly and tend to generate more arcing across switches compared to 12v batteries. Another challenge with high-voltage battery packs is the potential failure of individual cells within the pack. Like a chain, the more cells connected in series, the higher the likelihood of this occurring. If one cell becomes faulty, its voltage drops, potentially disrupting current flow . Replacing a “bad” cell is no easy task either, because new and old cells are incompatible. Typically, new cells have a significantly higher capacity than their older counterparts.
What is Parallel Connection and How to Connect Batteries in Parallel Formula
When batteries are connected in parallel, it means that the positive terminals of multiple batteries are linked together, and the negative terminals are also connected. Ultimately, the positive and negative terminals of the entire battery pack correspond to the positive and negative terminals of all the individual batteries. The main characteristics of parallel configuration are:
1. Voltage remains unchanged: The total voltage of the battery pack in parallel equals the voltage of a single battery.
2. Increased capacity: The total capacity of the battery pack in parallel is the sum of the capacities of each individual battery. For example, if two 3.7V/3000mAh lithium batteries are connected in parallel, the total capacity becomes 6000mAh.
3. Increased current: The maximum output current of the battery pack in parallel equals the sum of the maximum output currents of each individual battery.
The purpose of connecting two or more batteries in parallel is to obtain a greater amount of power.A battery pack consisting of four identical cells connected in parallel combination maintains a voltage of 1.2V, while the current capacity and runtime are increased fourfold.Besides this method, another approach for more power is to use larger-sized batteries. However, due to limitations in the available battery sizes, this option is not suitable for all situations. Moreover, large batteries may not conform to the specific shape requirements of custom-made batteries. Most chemical batteries can be used in parallel battery configurations, with lithium-ion batteries being particularly well-suited for this purpose.
In contrast to series combination batteries, the impact of a high-impedance or "open-circuit" battery is less pronounced in parallel circuits. However, parallel battery packs can reduce load capacity and shorten operational duration. It's akin to an engine running on just three cylinders, where a short simple circuit can cause more damage. This is because, in the event of a short circuit, the faulty battery can rapidly deplete the charge from the other batteries, potentially leading to a fire.
Case Illustration
Let's consider an example of a battery pack where the third cell only produces a voltage of 0.6V, instead of the normal 1.2V. As the operating voltage decreases, it reaches the critical point of discharge termination faster than a normal battery pack, and its usage time is drastically reduced. Once a device shuts down due to insufficient voltage, the remaining three cells, still in good condition, are unable to deliver their stored energy. At this stage, the third cell exhibits significant internal resistance, and if a load is still applied, it causes a considerable drop in the output voltage of the entire battery chain. In a series circuit, one underperforming cell acts like a blockage in a water pipe, creating substantial resistance and obstructing current flow. The third cell might also short circuit, reducing the terminal voltage to 3.6V, or cause the battery capacity to disconnect, cutting off the current. The performance of a battery pack depends on the performance of its weakest cell.
The top illustration shows a parallel connection (where capacity is added, and voltage remains unchanged).
The bottom illustration depicts a series connection (where voltage is added, and capacity remains unchanged).
What is series-parallel connection?
The series-parallel connection method offers great design flexibility, allowing the use of standard battery sizes to achieve the required voltage and current ratings (as shown in Figure 3). It is important to note that the total power remains unchanged by different connection of batteries. Power is defined as the product of voltage and current. For lithium-ion batteries, series-parallel connection of batteries are quite common. One of the most commonly used battery packs is the 18650 (with a diameter of 18mm and a length of 65mm). This type of battery pack includes a protection circuit that can monitor each cell in series circuit, thus its maximum practical voltage is 14.4 V. This protection circuit can also be used to monitor the status of each cell connected in parallel.
Precautions for Series and Parallel Battery Connections
The previously discussed methods for connecting batteries in series or parallel pertain to rechargeable battery packs, where the batteries are permanently soldered together. When using multiple batteries in series or parallel, it is essential to follow these basic guidelines:
● Keep the battery connection points clean. When four batteries are connected in series, there are a total of eight connection points (the connection between each battery and the battery compartment, and from one battery compartment to the next). Each connection point has a certain resistance, which can affect the performance of the entire battery pack if additional connection points are introduced.
● Do not mix different types of batteries. Replace all batteries when their charge is low. When using batteries in series, ensure they are all of the same type.
● Never attempt to recharge non-rechargeable batteries. Charging such batteries generates hydrogen, which may lead to an explosion.
● Pay attention to battery capacity. If one battery is inserted with reversed polarity, it will decrease the total voltage of the battery series instead of increasing it.
● Remove fully discharged batteries from devices that are not in use. Old batteries are more prone to leakage and corrosion. Alkaline batteries are generally less problematic compared to carbon-zinc batteries.
● Avoid placing all batteries in a single container to prevent short circuits. A short entire circuit can cause overheating and potentially lead to a fire. Dispose of used batteries by placing them in small plastic bags, insulating them from the environment.
● Primary cells like alkaline batteries can be disposed of in regular trash bins. However, it is preferable to send used batteries for recycling to support environmental sustainability.
