# Design of battery capacity and protection circuit

1. Design of battery capacity and its charging characteristics

The capacity of the battery refers to the storage capacity of the battery. It is usually expressed as the total energy released by the battery when the fully charged battery is discharged to the specified final voltage.

(1) Design of battery capacity

BC=A×QL×T0/CC

In the formula, A is the safety factor, which is between 1.1 and 1.4; QL is the daily average power consumption of the load, which is the working current multiplied by the daily working hours; T0 is the temperature correction coefficient, generally 1 above 0°C, 1.1 above -10°C, and 1.2 below -10°C; CC is the depth of discharge of the battery, generally 0.75 for lead-acid batteries.

(2) Charging characteristics of the battery

An acceptable charge profile can be made that presupposes a minimum outgassing rate for the battery. Experiments show that if the charging current changes according to this curve, the charging time can be greatly shortened, and the capacity and life of the battery will not be affected. In principle, this curve is called the optimal charging curve. The initial charge current is large, but decays quickly. The main reason is that the polarization phenomenon occurs during the charging process, that is, resistance polarization, concentration polarization, and electrochemical polarization, which increases the terminal voltage of the battery and hinders the charging of the battery, and the outgassing rate and temperature rise of the battery are significantly improved.

When charging the battery, high current will cause energy loss, and a large amount of gas will be generated in the later stage of charging, which will shorten the life of the battery. And the charging current is different, the recovery of the battery capacity is also different. The higher the charging current, the earlier the “outgassing” process will occur, and the smaller the capacity of the battery will be recovered. The study found that the battery charging process has the greatest impact on battery life, and the discharge process has less impact. That is to say, the vast majority of batteries are not worn out, but “charged bad”. It can be seen that choosing an appropriate charging method plays a pivotal role in the service life of the battery. In addition, in the previous article, we also introduced the charging and discharging characteristics of lead-acid batteries in detail.

(3) MPPT control charging

For photovoltaic power generation systems, the charging power source comes from photovoltaic cells, and the charging current is related to the output characteristics of photovoltaic cells. In order to maximize the use of solar energy resources, the photovoltaic cells can output the maximum power to ensure that the battery can obtain the maximum energy, the MPPT controller is constantly calculating and looking for this maximum power point, and changing the duty cycle of the PWM wave through the output to control the current given of the charger to achieve the purpose of MPPT.

2. Design of battery protection circuit

The battery has its rated capacity, and if it continues to be seriously overcharged after it is fully charged, the battery will be damaged. At the same time, the battery can only allow a certain depth of discharge when discharging, so for the battery, it is required to charge and discharge as evenly as possible. However, due to the influence of the weather, its charging voltage and charging current are not constant values, but appear in an intermittent process.

Sometimes the voltage will be very high, sometimes it will be very low, sometimes there will be no sun for several days in a row, and sometimes the sky will be clear for several days in a row, if the charging and discharging process cannot be well controlled, it is likely to cause overcharge and overdischarge of the battery and reduce the service life of the battery. For this reason, the charging and discharging of the battery needs to be controlled. When the charging voltage is higher than the rated upper limit voltage of the battery, the charging is stopped; when the discharging voltage is lower than the rated lower limit voltage of the battery, the discharging is stopped.

(1) Design of battery overcharge protection circuit

Overcharging of the battery means that the single cell voltage of the battery exceeds a certain level, and the battery cannot fully recombine the oxygen produced at this time. If the overcharge voltage is 2.35V/single cell, the overcharge protection voltage of the 48V battery is 56.4V. The overcharge warning point is set to 56V. When the voltage of the battery reaches 56V or more, the PWM wave stops outputting and the charging circuit is cut off to prevent the battery from being damaged by overcharge.

(2) Design of battery over-discharge protection circuit

The battery discharge termination voltage is usually set to 1.80V/single cell. When the over-discharge termination voltage is lower than the low-voltage warning point, the over-discharge protection circuit works, the contacts of the relay act, the load is cut off, and the battery stops discharging, thereby preventing the occurrence of over-discharge.

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