Battery Capacity

Battery Capacity

Date:Apr 09, 2021

Generally, the capacity of power supply equipment is expressed in kV · a or kW. However, as a power supply, VRLA battery is more accurate to use a · h to indicate its capacity. The battery capacity is defined as ∫ t0tdt, theoretically "t" can tend to infinity, but in fact, when the battery discharge is lower than the termination voltage, it may damage the battery, so the T value is limited. In the battery industry, the hour (H) is used to represent the battery's sustainable discharge time, and it is aware that there are some obvious factors C24, C20, C10, C8, C3, C1 and other nominal capacity values.

The nominal capacity of small batteries is measured in milliampere hours (MA · h), while that of large batteries is measured in ampere hour (a · h), Ka · h. the telecom industry usually takes the nominal capacity values of C10 and C8. For example, the common Deka battery 12avr100sh is a 12V monomer with a capacity of 100 a · h, which can discharge continuously for 10h, the current is 10a, and the total discharge time is 10 * 10 = 100 a · H (in actual test, in order to keep the current value constant and stable, the external circuit load should be adjusted to measure when the voltage changes).


The capacity of the battery for electric vehicles is indicated by the following conditions:

Electrolyte ratio  1.280/20 ℃
Discharge current 5 hours
Discharge termination voltage 1.70v/cell
Electrolyte temperature in discharge30 ± 2 ℃

  1. the voltage drop in discharge the terminal voltage is lower than the load free voltage (open circuit voltage) before discharge, for the following reasons:

    1.V=E-I.R

    V: Terminal voltage (V)   

    I: discharge current (A)

    E: Open circuit voltage (V) 

    R: internal impedance (Ω)

2. when discharging, the specific gravity of electrolyte decreases and the voltage decreases.

3. when discharging, the internal impedance of the battery will be enhanced accordingly. If it is 1 times when fully charged, it will be increased by 2-3 times when fully discharged.

The reason why the voltage of the battery is lower than that used in walking is that the oil pressure motor used for lifting is more powerful than the driving motor used for walking, so the discharge current is larger, and the I.R of the above formula is also larger.


capacity representation of battery

In the capacity test, the relationship between discharge rate and capacity is as follows:

5HR1.7V/cell
3HR1.65V/cell
1HR1.55V/cell


It is strictly forbidden to discharge when the above voltage is reached. The deeper the discharge is, the higher the temperature in the battery will rise, the worse the deterioration of the active substances will be, and the battery life will be shortened.

Therefore, if the battery voltage of the reactor without lifting load reaches 1.75v/cell (42V of 24cell and 21V of 12cell), it shall be stopped and charged immediately.


battery temperature and capacity

When the battery temperature is reduced, the capacity of the battery will also decrease significantly for the following reasons.

(A) The electrolyte is not easy to spread, and the reaction rate of the two polar active substances is slow.

(B) The 5hr capacity of the battery will decrease with the decrease of battery temperature.

Therefore:

1. the winter is shorter than that in summer.

2. especially, the battery used in the freezer has a large discharge, which makes the actual use time of a day significantly shorter.

If you want to extend the service time, you should raise the temperature in winter or before entering the freezer.


discharge quantity and life

When charging and discharging repeatedly every day for use, the battery life will be affected by the depth of discharge.


discharge quantity and specific gravity

The electrolyte proportion of the battery is almost proportional to the discharge amount. Therefore, according to the specific gravity of the battery when fully discharged and the specific gravity at 10% discharge, the discharge capacity of the battery can be calculated.

The best way to know the discharge capacity is to determine the electrolyte specific gravity of lead-acid battery. Therefore, the specific gravity after use shall be measured periodically to avoid excessive discharge. While measuring the specific gravity, the temperature of electrolyte shall be measured. The specific gravity converted at 20 ℃ shall not be reduced to below 80% discharge.


discharge state and internal impedance

The internal impedance will increase due to the increase of discharge capacity, especially at the end of discharge. The main reason is that the discharge leads to the decrease of lead sulfate and electrolyte proportion, which leads to the increase of internal impedance. After discharge, it is necessary to charge the battery immediately. If the discharge is allowed to continue, lead sulfate will form stable white crystal (this is to say, in Literature) The said curing phenomenon), even if charged, the active substances of the plate can not be restored to the original state, and the service life of the battery will be shortened.


Lead white sulfuric acid

When the battery is discharged, lead sulfate (pbs04) will be produced simultaneously on the negative and anode plates. If it is allowed to discharge continuously and no charge is allowed, stable white lead sulfate crystal (even if recharged, it is difficult to recover the original active substance) will be formed. This state is called white sulfurization.


temperature in discharge

When the battery is over discharged, the internal impedance increases significantly, so the battery temperature will rise. The high temperature of discharge will increase the temperature when charging is completed. Therefore, it is the best to control the temperature below 40 ℃ at the end of discharge.

Theoretical capacity

The theoretical capacity is also called the calculated capacity determined by the amount of active substances contained in the battery plate. The electric equivalent of lead-acid battery is 0.517 a · h/g for Pb, and the second price is 0.259 a · h/g, for PB02, the price of 4 is 0.488 a · h/g, and the 2 price is 0.224 a · h/g. The capacity calculated according to the amount of electrified equivalent and active substance is called the theoretical capacity of the battery.



Actual capacity

The actual capacity refers to the capacity measured during battery discharge, which depends on the amount and utilization of active substances. The active substances are related to lead plates, but not equal to the weight of lead. It is related to the structure form, discharge current, temperature, termination voltage, raw material quality, manufacturing technology and use method of battery plate, and it is changing. Today, the current situation is that the capacity of the battery is not equal to the weight of lead, The maximum capacity of single plate is known to be 100 a · H / 2V.


Rated capacity

Rated capacity is also called nominal capacity, that is, the minimum working capacity that the battery can release under the conditions specified by the manufacturer. For example, 97 a · h battery is 100 a · h nominal, while some manufacturers' batteries reach or exceed the nominal capacity after several cycles of use.


power efficiency (safety efficiency)

The ratio between the output and the input is called the battery efficiency, also known as the safety efficiency.


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