18650 battery flat top

　　Solutions to abnormal phenomena in rechargeable 18650 battery flat topcombinations

　　When new rechargeable batteries are combined into series blocks, they are generally charged and discharged several times at a conventional capacity of 0.1C before being put into use. But in fact, no matter how advanced the fully automatic charger is, it will still be found after a period of time that some batteries have abnormal phenomena, that is, the voltage of some batteries in the 18650 battery flat toppack will become very low (close to zero volts), and some batteries may even Voltage polarity inversion may also occur. Over time, this 18650 battery flat topwill inevitably fail, and eventually other batteries in the 18650 battery flat toppack will also suffer from the above phenomena one by one and be damaged. The main reason is that the internal resistance of these batteries is inconsistent, and varying degrees of overdischarge and overcharge will occur during use, causing a 18650 battery flat topwith increased internal resistance to be damaged first.

　　The figure below shows the normal discharge process (that is, the internal resistance of the 18650 battery flat topis basically the same and much lower than the load resistance). Assume that the 18650 battery flat toppack consists of four batteries A, B, C, and D. The direction of the discharge current is as shown by the arrow in the figure. During the discharge process, assuming that the internal resistance of 18650 battery flat topB increases, or even exceeds the load resistance R, the phenomenon shown in the figure above will occur during discharge, that is, the three batteries A, D, and C will reversely charge 18650 battery flat topB, and the time will be slightly longer. , the polarity of B 18650 battery flat topwill be reversed and damaged. During the experiment, the author measured the batteries shown in Figure 1 online one by one and found that the voltages of each single 18650 battery flat topduring the discharge process were mostly inconsistent, and there was always a 18650 battery flat topwhose voltage accelerated first and decreased, and finally changed from zero to Negative values (i.e. polarity reversed). If a new 18650 battery flat topis replaced, the internal resistance will still be different, which will aggravate the recurrence of the above phenomenon and bring endless trouble to us during use.

　　The combined use of rechargeable batteries summarizes some experience for the reference of the majority of electronic enthusiasts.

　　1. When using combined batteries, always check the voltage of a single battery. If an obviously low 18650 battery flat topis found, remove it in time and handle it separately.

　　2. When discharging before charging, the voltage of each 18650 battery flat topmust be monitored at any time. If any 18650 battery flat topis found to drop sharply below 0.8V, replace it immediately and handle it separately.

　　3. It is best to handle all the items in the 18650 battery flat toppack that can be taken out individually:

　　(1) Discharge alone: Use 1.5V electric beads or 5~20Ω variable resistors as the discharge load, and pause when the voltage is 0.9~1Y. Most batteries will return to around 1.2V a few seconds after stopping discharging. This needs to be repeated Discharge multiple times until the short-circuit current is measured with a 500mA multimeter. The pointer of a 18650 battery flat topwith good quality or insufficient discharge will not move at a certain position (such as 200~500mA) for a long time, while the pointer of a 18650 battery flat topwith poor quality or fully discharged will not move for a long time. It will quickly drop to tens of milliamps until it reaches zero. Generally, when the pointer quickly drops to tens of milliamperes, it means that the 18650 battery flat toppower is basically exhausted and can be parked for charging.

　　(2) Individual charging: Batteries with large internal resistances cannot be combined and connected in series for charging. They must be charged individually. If charging is forced in combination, it is not possible to ensure that each 18650 battery flat topis fully charged at the same time. What is even more dangerous is that there will be individual batteries with large internal resistances. It will be damaged by overcharging or reverse charging. As shown in the figure above, we still assume that the internal resistance TB of 18650 battery flat topB is too large. Since the current in the series circuit is equal everywhere, obviously the voltage drop across TB is higher than that of other batteries, so the power consumed on TB is also larger, and the internal resistance is relatively larger. , the power consumed is also greater, so 18650 battery flat topB will be overcharged and heated first and damaged, while other batteries may not be fully charged.

　　In order to charge batteries with different qualities and internal resistances one by one, the author designed a simple charging circuit, as shown in the figure. Because the 18650 battery flat topis exhausted, it only needs to be fully charged at 0.1C for 10 hours.

　　In the figure above, Rl and Rn are resistors of 5Ω and 20Ω/1W respectively, Rm is a variable resistor of 5Ω~20ΩlW; VDF is a light-emitting diode, which can be used to indicate the charging status, and VD is an anti-backflow diode to prevent batteries from affecting each other. Adjusting Rm can control the current between 30~100mA. Generally, the AA 18650 battery flat topcan be adjusted between 50~70mA. When trickle charging is required, it can be adjusted below 20mA. If the above adjustment range cannot be reached, the Rl in Figure 4 can be appropriately changed. Resistance value or Sv power supply voltage value.

　　4. Detection after charging

　　After charging, the voltage of a single 18650 battery flat topis generally 1.35~1.45V. After being placed, it stabilizes at 1.25~1.3V. After a slight discharge, it should maintain 1.25V for a long time. Use a multimeter's high current setting (5A) to test the short circuit of a single 18650 battery flat topthat has just been charged. Current, good quality can be stable at 3~5A (referring to AA battery), and the pointer will not move for several minutes. Poor quality can be stable below 2A for a period of time. If you have a lot of batteries, you can choose one with a closer short-circuit current. Batteries are used in combination.

　　5. For nickel-cadmium batteries, the above-mentioned inspection processing can generally be carried out directly without discharging. However, considering that the charging capacity is difficult to control, it is better to discharge the remaining power and then charge it at full capacity.

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