battery capacity
Battery capacity is one of the important performance indicators to measure battery performance. It represents the amount of power released by the battery under certain conditions (discharge rate, temperature, termination voltage, etc.) Ampere·hour is the unit (referred to 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 the time from t0 to t1), and the battery is divided into positive and negative poles.
Classification
Battery capacity is divided into actual capacity, theoretical capacity and rated capacity according to different conditions.
The minimum capacity mentioned at a certain discharge rate at 25°C to the final voltage is the specified capacity of the battery at the time of design and production, which is called the rated capacity of a certain discharge rate RH.
The battery capacity is generally calculated in AH (ampere hours), and the other is calculated in watts (W) of CELL (unit plate). (W/CELL)
1. Calculated in Ah (ampere hour), discharge current (constant current) I × discharge time (hour) T . For example, if a 7AH battery has a continuous discharge current of 0.35A, the time can last for 20 hours.
2. The charging time is 15 hours, and the charging current is 1/10 of the battery capacity. Fast charging will reduce the battery life.
Battery capacity refers to the amount of power a battery can store. The unit of battery capacity is 'mAh', and the Chinese name is mAh (when measuring large-capacity batteries such as lead-acid batteries, for convenience, 'Ah' is generally used, and the Chinese name is ampere-hour, 1Ah=1000mAh). If the rated capacity of the battery is 1300mAh, that is, the current of 130mA discharges the battery, then the battery can continue to 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 in an ideal state. The actual current of a digital device cannot always be constant at a certain value (taking a digital camera as an example, the working current will be larger due to the opening or closing of the LCD display screen, flash and other components. change), so the time the battery can power a device can only be an approximate value, and this value can only be estimated through actual operating experience.
capacity unit
Usually we talk about battery capacity in ampere-hours, which is based on a certain battery that has been identified. For example, let's say what is the battery capacity of this mobile phone; what is the capacity of this battery car battery, they are all for different batteries. The battery voltage has been determined without considering the actual voltage. It only needs to say that the ampere hour can represent the capacity of the battery.
However, for batteries of different voltages, we cannot simply use ampere-hours to represent the capacity. For example, a 12V 20AH battery, a 15V20AH battery, even if they are both 20AH, supply the same power load, the device can work normally, but the duration are not the same, so the standard capacity should be measured in work.
For another example, a device can support 12V and 24V, and it can be powered by a 12V (20AH) battery for one hour, then using two batteries in series will become 24V (20AH). The ampere-hour does not increase, but continues The time will be doubled, so the capacity should be considered as the work contained in the battery at this time, not simply as the ampere hour.
W(work)=P(power)*T(time)=I(current)*U(voltage)*T(time)
It is meaningful to discuss battery capacity in this way, and it must be realistic, otherwise there may be a claim that a mobile phone battery has a larger capacity than a battery car battery, which is obviously unscientific.
test method
Charge a battery with constant current and constant voltage, and then discharge it with constant current. The amount of power released is the capacity of the battery, storage battery, nickel-metal hydride battery, etc., but lithium batteries are not good, it has a minimum discharge voltage, that is, the discharge voltage cannot be low. At 2.75V, usually 3.0V is the lower limit protection voltage. For example, if the capacity of a lithium battery is 1000mAh, then the charge and discharge current is 1000mA, and when the maximum voltage of the battery is 4.2V, it is placed at 3.0V, and the released capacity is the real capacity of the battery.
Influencing factors
The capacity of the battery is an important indicator to measure the performance of the battery. It is generally expressed in ampere hours. The general term for discharge time (hours) and discharge current (amperes), that is, 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 material. The more the amount of active material, the higher the utilization rate of the active material, and the greater the capacity of the battery. On the contrary, the smaller the capacity, the more factors that affect the capacity of the battery. Common There are the following:
(1) Influence of discharge rate on battery capacity
The capacity of the lead battery decreases with the increase of the discharge rate, that is to say, the larger the discharge current is, the smaller the capacity of the battery is calculated. For example, a 10Ah battery can be discharged for 2 hours with 5A discharge, that is, 5×2=10 ; Then only 47.4 minutes of electricity can be discharged with 10A discharge, which is 0.79 hours. Its capacity is only 10×0.79=7.9Ah. So for a given battery discharged at different time rates, there will be different capacities. We are talking about When the capacity is reached, the time rate or rate of discharge must be known. Simply speaking, how much current is used to discharge.
(2) Influence of temperature on battery capacity
Temperature has a great influence on the capacity of lead-acid batteries. Generally, with the decrease of temperature, the capacity decreases. The relationship between capacity and temperature is as follows: Ct1= Ct2/1+k(t1-t2 ). t1t2 is the temperature of the electrolyte, and k is The temperature coefficient of capacity, Ct1 is the capacity (Ah) when the temperature is t1, and Ct2 is the capacity (Ah) when the temperature is t2. In the battery production standard, a temperature is generally specified as the rated standard temperature. For example, t1 is specified as the actual temperature, t2 It is the standard temperature, (usually 25 degrees Celsius). The negative plate is more sensitive to the influence of low temperature than the positive plate. When the temperature of the electrolyte decreases, the viscosity of the electrolyte increases, the ions are subjected to greater resistance, the diffusion capacity decreases, and the resistance of the electrolyte also decreases. Increase, the electrochemical reaction resistance increases, part of the lead sulfate cannot be converted normally. The charging acceptance capacity decreases, resulting in a decrease in battery capacity.
(3) Influence 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, and the cut-off discharge voltage It is called the discharge end voltage. Setting the discharge end voltage is of great significance to prolong the service life of the battery. Generally, the discharge end voltage of the electric motorcycle battery that we repair is 1.75 volts per grid, that is to say, a 12-volt battery is 6 grid, the discharge termination voltage is 6 × 1.75 = 10.5 volts.
(4) Influence of the geometric size of the electrode plate on the battery capacity
When the amount of active material is constant, the geometric area of the electrode plate in direct contact with the electrolyte increases, and the battery capacity increases. Therefore, the influence of the geometric size of the electrode plate on the battery capacity cannot be ignored.
①Influence of plate thickness on capacity
The amount of active material is constant, and the battery capacity decreases with the increase of the thickness of the electrode plate.
②Influence of plate height on capacity
In the battery, there is a big difference in the utilization rate of active materials in the upper and lower parts of the electrode plate. Experiments have confirmed that the current density in the upper part of the electrode plate is about 2 times ~ 2.5 times higher than that in the lower part in the early stage of discharge. The time of discharge decreases gradually, but the current density in the upper part is larger than that in the lower part.
③ Influence of plate area on capacity
The amount of active material is certain, the larger the geometric 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 shell and the same amount of active material, the thin plate is used to increase the number of plates. That is to say, the effective reaction area of the plate is increased, thereby improving the utilization rate of active materials and increasing the capacity of the battery.