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How to Repair 18650 Lithium Battery Pack?
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How to Repair 18650 Lithium Battery Pack?

18650 lithium-ion batteries, cylindrical batteries with a diameter of 18mm and length of 65mm, have become a standard specification for lithium-ion batteries. One common problem with lithium-ion batteries is degradation, or colloquially, simply breaking down. Generally, battery packs are obtained by connecting numerous18650 batteries in series and parallel. What should you do if your 18650 battery pack breaks down? How do you repair it?


Testing and Analysis of 18650 Lithium-Ion Batteries

Generally, 18650 lithium-ion batteries increase their capacity by mixing and matching them in parallel, such as in a 20,000mAh mobile power bank that is usually constructed from eight 2600mAh batteries, or six 3400mAh 18650 batteries. Parallel connection increases the capacity by 3.7V, and then a series connection boosts the voltage. For example, 2 in series is 3.7*2=7.4V, 3 is 12.6V and so on.

Typically, laptop batteries require multiple battery series connections to increase the voltage to 12V, 36V, 48V, 72V, etc. However, if the batteries in the series are not of equal capacity or quality, one battery will always degrade faster. This is known as the principle of the weakest link in a chain. One battery or one group of parallel batteries can affect the overall voltage and capacity of the entire battery pack.

Some examples of how this affects devices include laptop batteries, which generally consist of two paired parallel batteries and three pairs of series batteries to produce a 10.8V~12.6V battery. If one of the pair of two parallel batteries degrades more quickly than the others, the overall battery capacity is reduced, resulting in shorter standby time, faster charging, and accelerated discharge. Electric vehicles and other devices share similar issues. At this point, the batteries should be replaced to avoid the effect of the weakest link in the chain. This ensures the balanced use of the batteries and prolongs the useful life of the battery pack.


Detection of 18650 Lithium-Ion Battery Degradation

1. Symptoms of battery degradation: inability of the battery voltage to rise and reduction of battery capacity.

2. Measure the battery voltage directly. If the voltage at both ends of the 18650 battery is lower than 2.7V, or if there is no voltage, the battery or battery pack is damaged. Normal voltage is 3.0V~4.2V (generally, a 3.0V battery's voltage will be cut off, 4.2V battery's voltage will be fully charged, and a few others also have 4.35V).

3. If the battery voltage is lower than 2.7V, it can be charged with a charger (4.2V) for ten minutes. If the battery voltage recovers, continue charging it until the charger indicates that the battery is full, and then check the full battery voltage.

When the full charge voltage is 4.2V, it indicates that the battery is normal and was cut off due to overuse during its last cycle. If the full charge voltage is much lower than 4.2V, it indicates that the battery is damaged. When a battery has been used for a long time, you can determine that it has reached the end of its service life, and the capacity has been almost depleted. Therefore, you should replace it. Lithium-ion batteries have a finite lifespan and cannot be repaired indefinitely.

4. If the battery has no voltage when measured, there are two possibilities. One is that the battery was originally good but was stored for an extended period and lost its charge. This battery can recover with a certain probability, usually achieved by using a lithium-ion battery pulse activation device (a battery charger and discharger) to charge the battery multiple times in a short period. There is a chance that it can be repaired. Generally, the cost of repair is not low, so it is better to buy a new one. The other possibility is that the battery has been thoroughly degraded or has ruptured its membrane and shorted its positive and negative electrodes. This problem cannot be repaired, and the only option is to buy a new battery.

Repairing 18650 Lithium-Ion Battery Packs

1. Lithium-ion battery packs often become unusable due to a damaged battery or group of batteries, but you can measure the voltage and capacity of each battery in the pack and remove batteries that do not have a voltage or with very low voltage, or with capacities or discharge times that differ significantly from those of the other batteries (as described for a single battery). Replace the removed batteries with those with similar capacities and internal resistances.

2. Generally, after the replacement, the lithium-ion battery pack can continue to be used until the other batteries in the pack become too degraded. This type of partial replacement is generally low-cost, and is currently the most commonly used method in the market.

3. Lithium-ion batteries must be used with a protective board that limits overcharging, over-discharging, short-circuits, etc., which helps to extend the life of the lithium-ion battery pack.

4. Lithium-ion batteries should not be shorted, and care must be taken during detection. Charging must not exceed the rated voltage and current, and the charging time should not be too long.

5. 18650 lithium-ion batteries must be used with a matching protective board, and a high-quality lithium-ion battery charger should be selected to extend the battery's service life.

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