Can multiple 18650 batteries be connected in series to obtain 12V voltage and be used as an emergency starting power source for cars?
Can multiple 18650 batteries be connected in series to obtain 12V voltage and be used as an emergency starting power source for cars? From the question, it seems that most of the cars we use have a 12V voltage, so there seems to be no problem. Therefore, it can be concluded that connecting multiple batteries in series can meet the starting needs of the car? For example, we use a Nanfu battery with a single cell voltage of 1.5V and 8 cells connected in series at 12V, which is equivalent to the voltage of a car battery.
But there is another important condition that cannot be ignored, which is the magnitude of the discharge current. The power of an electrical appliance is the product of voltage and current, P＝UI。 Therefore, when the voltage meets the requirements of the electrical appliance, the current must also meet the requirements. The working current of appliances with different powers is also different, for example, the working current of a light bulb and a starter with the same working voltage of 12V is different. For example, if the power of a brake light bulb is 21W, the working current is 21W/12V=1.75A. If the power supply cannot provide 1.75A of current, the brightness of the light bulb will be insufficient.
The power of the starter motor for household cars is above 1.1kw, and the typical working current is 1100w/12V=91.7A. If the 12V power supply cannot provide a current of 91.7A or more, the starter motor cannot work. If the current is insufficient, the electromagnetic switch cannot be closed, and the starter motor may make a noise. In actual use, the starting current of the engine is determined by the resistance of the engine, and the starting current of a cold car may increase several times.
The common 18650 battery is an alkaline battery, suitable for low current and long-term discharge, but not suitable for short-term and high current discharge. Therefore, even if the voltage is sufficient, it cannot be used as an emergency starting power source to start the vehicle. Let's take a look at the capacity of the 5th Nanfu battery:
When discharged with a current of 500mA, the capacity is about 1148ma. 500ma current discharge, its discharge power is 12V × 0.5A=6W. Alkaline batteries do not support high rate discharge, and the maximum discharge current will not exceed 2A. The maximum discharge power is 12V × 2A=24W, and it can only light up one brake light. If used to start a vehicle, a low internal resistance of the starter motor relative to the battery is considered an artificial short circuit. There are many examples of explosions during the use of alkaline batteries, and short circuits are a factor in battery rupture, leakage, and explosion, so doing so carries certain risks. Moreover, the current of 2A is much smaller than the working current of the starter motor (100A), making it impossible to start the car.
Finally, let's talk about the compact car emergency power supply: the emergency starting power supplies sold worldwide mostly use lithium batteries as their internal cells. These lithium batteries have low internal resistance and support high rate discharge, for example, they can support up to 20-30 ° C discharge.
That is to say, a lithium battery with a capacity of 5000mAh can achieve an instantaneous discharge current of 150A. Some high-capacity batteries have an instantaneous discharge current exceeding 200A. This current can start the car, and it can be started multiple times. Although the starting current is high, the starting time is short, only a few seconds each time, and the power consumption is not high. The first time the vehicle starts, the current is high, and then the current will decrease and the starting time will be shortened.
