**Battery capacity and capacity design**

The capacity of a battery refers to the amount of electricity that a fully charged battery can release under specified discharge conditions, and is represented by “C”. The capacity of a battery is the most important indicator to mark the external discharge capacity of the battery, to measure the quality of the battery, and to select the battery. The capacity of the battery is measured by A · h (Ampere hour), that is, the capacity is equal to the product of the discharge current and the continuous discharge time. As the electrolyte density increases, the electromotive force of the battery increases, the active materials participating in the reaction increase, and the battery capacity increases. However, the electrolyte density is too high, the viscosity increases, the internal resistance increases, the plate vulcanization tendency increases, and the battery capacity decreases. Therefore, an appropriate density must be selected. The temperature also has a great influence on the battery. The temperature drops and the viscosity increases. It is difficult for the electrolyte to penetrate into the electrode plate, the utilization rate of the active material is low, the internal resistance increases, and the capacity decreases.

The basic steps of battery capacity design calculation are as follows. The first step is to multiply the daily power consumption required by the load by the number of self-sufficient days determined according to the actual situation of the customer to get the preliminary battery capacity. The second step is to divide the battery capacity obtained in the first step by the maximum allowable depth of discharge of the battery. Because the battery cannot be completely discharged in self-sufficient days, it is necessary to divide by the maximum depth of discharge to get the required battery capacity. The selection of the maximum depth of discharge needs to refer to the performance parameters of the battery selected for use in the photovoltaic system. Under normal circumstances, if you are using a deep-cycle battery, it is recommended to use 80% depth of discharge (DOD); if you are using a shallow-cycle battery, it is recommended to use 50% of DOD. The basic formula for designing battery capacity is: battery capacity = (self-sufficient days x daily average load) ÷ maximum depth of discharge.

If the battery voltage does not meet the requirements, you can use the series method; if the battery current does not meet the requirements, you can use the parallel method. The number of batteries in series = the nominal voltage of the load/the nominal voltage of the battery, where the power supply voltage of the battery is called its nominal voltage, and the working voltage of the load is called its nominal voltage. For example, if the voltage of a photovoltaic power supply system is 24V, if a battery with a nominal voltage of 12V is selected, 2 sets of batteries are required to be connected in series. The load of the photovoltaic power supply system is 20A·h/day, and the number of self-sufficiency days is 4 days. If a low-cost shallow-cycle battery is used and the maximum discharge depth allowed by the battery is 50%, then the battery capacity = 4 days x (20A·h/day) ÷ 0.5=160A·h. If you choose a 12V/100A·h battery, then you need to connect 2 batteries in series x 2 in parallel = 4 batteries.

**The naming method and model composition of the battery**

The name of the battery is composed of the number, model, rated capacity, function or shape of the battery. The nominal voltage of a single battery is 2V. The introduction of the models of products of different companies is different, but the meaning of the basic models remains the same. The meaning of the letters in the model is as follows:

G—Fixed type

F—Valve control

M—Sealed

J—Colloid

D—Power type

N—For diesel locomotives

T—For railway passenger cars

D—For electric locomotives