The knowledge and detailed explanation of lithium battery in parallel and series

Connecting power lithium batteries in series and in parallel, it sounds simple, but with some simple rules, you can avoid unnecessary problems. The lithium battery pack consists of two parts: a lithium battery and a lithium battery protection circuit.

In a lithium battery pack, a plurality of lithium batteries is connected in series to obtain a required operating voltage. If you need higher capacity and more current, you should connect the power lithium batteries in parallel. The high voltage and high capacity standards can be achieved by combining the two methods of series and parallel. For example, the battery of a 36V10AH electric vehicle is to connect 50 2,000V lithium 3.6V lithium-ion batteries in parallel so that the capacity can reach 10AH; then, 10 sets of parallel batteries are connected in series; after connecting, the battery voltage can reach 36V or more. .

This connection is called "10 strings and 5", which means that 10 groups of lithium batteries connected in parallel by 5 sections are connected in series; the nominal voltage of lithium ion batteries is 3.6V. Using lithium ion polymers and other types of lithium batteries, it is rated at 3.7V. Another problem with using a high-voltage battery pack is that it is possible to experience a failure of a certain battery in the battery pack. It's like a chain, the more batteries are connected in series, the higher the chance of this happening. As long as there is a problem with one of the batteries, its voltage will decrease. At the end of the day, a “disconnected” battery may interrupt the delivery of current. It is also easy to replace the "bad" battery, because the old and new batteries do not match each other. In general, the capacity of new batteries is much higher than that of older batteries. As the operating voltage drops, it reaches the critical point of discharge end faster than the normal battery pack, and its use time is also drastically shortened. Once the device is powered off due to low voltage, the remaining cells are still intact and the stored battery cannot be sent out. At this time, the bad battery cell also exhibits a large internal resistance. If the load is also present at this time, the output voltage of the entire battery chain will be greatly reduced. In a series of batteries, a poorly performing battery, like a plug that blocks a water pipe, creates a huge amount of resistance that prevents current from flowing. Other batteries will also be shorted, which will reduce the terminal voltage to 3.6V, or disconnect the battery pack link and cut off the current. The performance of a battery pack depends on the performance of the worst battery in the battery pack.

Parallel Connections In order to get more power, multiple batteries can be connected in parallel. In addition to paralleling the batteries, another option is to use a larger battery because of the limitations of the battery that can be used. This method is not suitable for all applications. In addition, large-sized batteries are not suitable for the form factor required for a dedicated battery. Most chemical batteries can be used in parallel, while lithium-ion batteries are best suited for parallel use. For example, the battery voltage of a parallel connection of five batteries is kept at 3.6V, and the current and running time are increased by five times. High impedance or "open" batteries have less impact in battery parallel circuits than in series with batteries. However, parallel battery packs reduce load capacity and reduce run time. This is like an engine that only starts three cylinders. The damage caused by the short circuit of the circuit will be greater because the battery that fails during the short circuit will quickly exhaust the power in the other battery, thus causing damage to the entire battery life.

When using a series-parallel connection method, it is flexible in design and can achieve the required rated voltage and current with a standard battery size. It should be noted that the total power will not change due to the different connection methods of the battery. The power is equal to the voltage multiplied by the current. For lithium-ion batteries, series-parallel connection methods are common. One of the most commonly used battery packs is the 18650 lithium battery , which has a protective circuit, a lithium battery protection board. The lithium battery protection board monitors each battery connected in series, so its maximum actual voltage is 42V. This lithium battery protection circuit (ie, a lithium battery protection board) can also be used to monitor the state of each battery connected in series. When using the 18650 lithium battery in series, the following basic requirements must be followed: the voltage should be consistent, the internal resistance should not exceed 5 milliohms, and the difference in capacity should not exceed 10 mA. The other is to keep the connection point of the battery clean. There is a certain resistance at each connection point. If the connection point is not clean or that the connection point is increased, the internal resistance may be high, which may affect the performance of the entire battery pack. Do not mix lithium batteries from different manufacturers. It is necessary to select lithium batteries with high performance consistency. The battery packs made from lithium batteries in accordance with the above standards are basically standard.