The differences between parallel and series connection of 18650 lithium batteries
When it comes to 18650 batteries, many people merely consider them as simple power sources, but overlook the internal connection logic. In fact, series connection and parallel connection are not just different wiring methods; they are the "genes" that determine the character of the entire battery pack. Simply put, series connection is for higher voltage, while parallel connection is for longer battery life. Understanding the differences between these two is the first step for DIY or purchasing custom battery packs.
1. Series Boost, Parallel Expansion
Series connection is like a relay race, with the current flowing through each cell one after another. The most obvious effect of this connection method is "addition": the total voltage is the sum of the voltages of all individual cells, while the capacity (Ah) remains unchanged. For example, connecting three cells with a nominal voltage of 3.7V and a capacity of 2000mAh in series results in a battery pack with a total voltage of 11.1V and a capacity of 2000mAh. This is very common in devices that require high voltage to generate sufficient power, such as power tools or certain laptops, as they need a higher voltage to produce sufficient power.
Parallel connection is more like a large reservoir, with the positive terminals of all cells connected together and the negative terminals also connected together. The logic of parallel connection is "addition" in terms of capacity: the total capacity is the sum of the capacities of each cell, but the voltage remains at the level of a single cell (approximately 3.7V). If two 2000mAh cells are connected in parallel, a battery pack with a total voltage of 3.7V and a capacity of 4000mAh can be obtained. This structure is suitable for scenarios where the voltage requirement is not high but long-term power supply is needed, such as large-capacity power banks or low-power sensor devices.
2. Consistency and Protection Mechanism
In practical applications, regardless of the connection method, the consistency of the cells is the key indicator determining the lifespan and safety. For series circuits, the short-circuit effect is extremely obvious: any significant deviation in capacity or internal resistance of a single cell will cause the performance of the entire battery pack to be lowered, and even lead to overcharging or overdischarging risks. Therefore, series connection must be combined with BMS (Battery Management System) for balanced management to ensure that each cell is in a healthy state.
Parallel circuits, although have a slightly higher tolerance for consistency, also have hidden dangers. If new and old cells are mixed or cells from different brands are directly connected in parallel, due to the existence of voltage differences, a huge circulating current will be generated, causing the cells to heat up rapidly and even damage. In addition, whether in series or parallel, it is necessary to pay attention to preventing reverse connection. Incorrect polarity connection will not only burn the circuit board but also may cause cell leakage or fire. It is recommended to add a reverse connection diode or use a PCB board with protection to ensure safety, which is the bottom-line operation for safety.
In summary, choose series connection for voltage requirements, and choose parallel connection for battery life duration, and always ensure the selection of cells and safety protection.
