Battery voltage and capacity

Lithium-ion batteries have a very useful feature for electricity metering. When discharging, the battery voltage gradually decreases with the passage of electricity, and there is a considerable slope. This provides us with another approximate method of electricity metering. The method of taking the battery voltage it is as if measuring the water level in the water tank can roughly estimate the amount of water remaining. But in fact the voltage of the lithium battery is much more complicated than the calm water level measurement inside the tank. Using voltage to estimate the remaining capacity of the battery has the following instability: the same battery, in the case of the same remaining capacity, the voltage value changes due to the magnitude of the discharge current. The higher the discharge current is the lower the voltage. In the absence of current, the voltage is the highest; the influence of ambient temperature on the battery voltage, the lower the temperature, the lower the battery voltage of the same capacity; the influence of the cycle on the battery discharge platform, as the cycle progresses, the discharge platform of the lithium ion battery tends to Deteriorating. The discharge platform is lowered. Therefore, the capacity represented by the same voltage also changes accordingly; different manufacturers, different capacity lithium-ion batteries, the discharge platform is slightly different.

Lithium-ion batteries of different types of electrode materials have large differences in discharge platforms. The discharge platforms of cobalt lithium batteries and manganese lithium batteries are completely different. All of the above will cause voltage fluctuations and voltage differences, making the battery capacity display unstable. When using a voltage to measure the battery capacity on a mobile phone, it is impossible for the mobile phone to remain in a standby state with a small current. Temporary loss of high current, such as turning on the backlight, ringing the ring, especially through, will cause the battery voltage to drop quickly. At this point, the capacity displayed by the phone is reduced more than the actual capacity. When the large current is removed, the voltage of the battery will rise. This will cause the unreasonable phenomenon that the capacity display of the mobile phone will rise instead.

The voltage of the battery has been decreasing during the discharge process. For example, the capacity of the battery 3.6v, 19ah, 19ah is not put into 0v, but 2 or 3. When the discharge capacity is 19ah, if it is put into 0v, the capacity will be 19 A little more, if it is over-discharged, it will damage the battery life.

If the design is excellent, the cut-off voltage of the meter and the voltage of the battery are basically the same. When the voltage reaches a certain value, such as 3V, the battery is discharged, or when the battery is almost discharged, the meter will reach the lowest working voltage. The minimum working voltage is relatively high. For example, if it is lower than 3.6v, it will not work. Then there is a situation where the battery still has electricity and the meter cannot work. In this case, the external power supply voltage should be increased.

Battery capacity is one of the important performance indicators to measure battery performance. It indicates the amount of electricity discharged by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.) (discharge test can be performed with JS-150D), that is, the capacity of the battery, usually Ampere·hour is the unit (abbreviated as A·H, 1A·h=3600C).

The battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions. The calculation formula of battery capacity C is C=∫t0It1dt (integration of current I during t0 to t1), and the battery is divided into positive and negative poles.

The battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions.

The minimum capacity to be discharged at a certain discharge rate at 25 ° C to the termination voltage is the capacity of the specified battery at the time of design and production. This is called the rated capacity of a certain discharge rate RH.

Square lithium ion battery

Square lithium ion battery

The battery capacity is generally calculated in AH (ampere-hours), and the other is calculated in CELL (unit plate) several watts (W). (W/CELL)

1. Ah (ampere hour) calculation, discharge current (constant current) I × discharge time (hour) T. For example, if the 7AH battery has a continuous discharge current of 0.35A, the time can be continuous for 20 hours.

2. The charging time is based on 15 hours, and the charging current is 1/10 of the battery capacity. Fast charging will reduce battery life.

Battery capacity refers to the amount of battery storage. The unit of battery capacity is “mAh”, and the Chinese name is mAh (when measuring large-capacity batteries such as lead storage batteries, for the sake of convenience, it is generally indicated by “Ah”, the Chinese name is Anshi, 1Ah=1000mAh). If the rated capacity of the battery is 1300mAh, that is, the current of 130mA discharges the battery, the battery can work for 10 hours (1300mAh/130mA=10h); if the discharge current is 1300mA, the power supply time is only about 1 hour (actual working time) There are some differences due to individual differences in the actual capacity of the battery). This is an analysis under ideal conditions. The current of a digital device in actual operation cannot always be constant at a certain value. (In the case of a digital camera, the operating current will be large due to the opening or closing of components such as the LCD display and flash. Change), so the battery can only provide a certain value for the power supply time of a device, and this value can only be estimated through practical experience.

Usually we say that the battery capacity is in ampere-hours, which is based on a certain battery that has been determined.

For example, we say the battery capacity of this mobile phone; the battery capacity of this battery car is different for different batteries. For the battery voltage has been determined, without considering the actual voltage, it is only necessary to say that the battery capacity can be represented.

However, for batteries of different voltages, we can't simply use Anshi to represent the capacity, such as a 12V20AH battery, a 15V20AH battery, even 20AH, supply the same power load, the equipment can work normally, but the duration is Not the same, so the standard capacity should be in work.

For another example, a device can support 12V and 24V. It can be powered by a 12V (20AH) battery and can provide one hour. Then two series will become 24V (20AH). The time will be doubled, so the capacity should be considered as the work contained in the battery at this time, and it should not be considered simply.

W (work) = P (power) * T (time) = I (current) * U (voltage) * T (time)

This discussion of battery capacity has practical significance, and must be realistic. Otherwise, there may be a saying that a mobile phone battery is larger than the battery capacity of a battery car. This is obviously unscientific.

Constant current and constant voltage charging of a battery, and then discharging with a constant current, how much power is discharged is the capacity of the battery, battery, nickel-metal hydride battery, etc., but the lithium battery is not good, it has a minimum discharge voltage, that is, the discharge voltage cannot be low At 2.75V, the voltage is usually protected with a lower limit of 3.0V. For example, if the capacity of the lithium battery is 1000 mAh, the charge and discharge current is 1000 mA, and it is placed at 3.0 V in the maximum voltage of the battery at 4.2 V. The discharged capacity is the most realistic capacity of the battery.

The capacity of the battery is an important indicator to measure the performance of the battery. It is generally expressed in ampere-hour. The general term for discharge time (hours) and discharge current (amperes), capacity = discharge time × discharge current. The actual capacity of the battery depends on the amount of active material in the battery and the utilization rate of the active substance. The more the active substance is, the higher the utilization rate of the active material, and the larger the capacity of the battery. On the contrary, the smaller the capacity, the many factors affecting the battery capacity, common there are the following:

(1) Effect of discharge rate on battery capacity

The lead battery capacity decreases with the increase of the discharge rate, that is to say, the larger the discharge current, the smaller the battery capacity is calculated. For example, a 10Ah battery can be discharged for 2 hours with 5A discharge, ie 5×2=10 Then, with 10A discharge, only 47.4 minutes of electricity can be released, which is 0.79 hours. Its capacity is only 10×0.79=7.9 ampere. So for a given battery to discharge at different rates there will be different capacity. We are talking about when you reach capacity, you must know the rate or rate of discharge. Simply put, how much current is used to discharge.

(2) The effect of temperature on battery capacity

The temperature has a great influence on the capacity of the lead-acid battery. Generally, the temperature decreases, the capacity decreases, and the relationship between the capacity and the temperature as follows:

Ct1=Ct2/1+k(t1-t2).t1t2 is the temperature of the electrolyte, k is the temperature coefficient of capacity, Ct1 is the capacity (Ah) when t1, and Ct2 is the capacity (Ah) when the temperature is t2. In the battery production standard, it is generally required to set a temperature to the rated standard temperature. For example, t1 is the actual temperature t2 is the standard temperature, (generally 25 degrees Celsius). The negative plate is more sensitive to the low temperature than the positive plate. When the electrolyte temperature is lowered when the viscosity of the electrolyte increases, the ions are subjected to a large resistance, the diffusion capacity is decreased, the electrolyte resistance is also increased, the electrochemical reaction resistance is increased, and a part of the lead sulfate cannot be normally converted. The charge acceptance is decreased, resulting in a decrease in the battery capacity. .

(3) The effect of termination voltage on battery capacity

When the battery is discharged to a certain voltage value, the voltage drops sharply. In fact, the energy obtained is very small. If the battery is deeply discharged for a long time, the damage to the battery is quite large. Therefore, the discharge must be terminated at a certain voltage value calling the discharge termination voltage. Setting the discharge termination voltage is of great significance for prolonging the battery life. Generally, the electric vehicle battery that we are repairing has a discharge termination voltage of 1.75 volts per grid, which means that a 12 volt battery is 6 grids the discharge termination voltage is 6 × 1.75 = 10.5 volts. [2]

(4) The influence of the geometry of the plate on the battery capacity

When the amount of active material is constant, the geometric area of the plate directly contacting the electrolyte increases, and the battery capacity increases. Therefore, the geometrical size of the plate and the influence on the battery capacity cannot be ignored.

1 plate thickness on the capacity

The amount of active material is constant the battery capacity decreases with the increase of the thickness of the plate, and the thicker the plate, the smaller the contact surface of sulfuric acid with the active material, the lower the utilization rate of the active material, and the smaller the battery capacity.

2 plate height impact on capacity

In the battery, there is a big difference in the utilization ratio of the active materials in the upper and lower parts of the plate. Experiments have confirmed that in the initial stage of discharge, the current density on the upper part of the plate is about 2 times to 2.5 times higher than that in the lower part. The change in discharge is gradually reduced, but the upper portion is larger than the lower current density.

3 plate area impact on capacity

The amount of active material is certain, the larger the geometrical area of the plate, the higher the utilization rate of the active material and the larger the capacity of the battery. In the case of the same battery case and the same active material quality, the thin plate is used to increase the plate piece. The number, that is, increases the effective reaction area of the plates, thereby increasing the utilization of the active material and increasing the capacity of the battery.

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