How to choose battery capacity

08 Jan 2013350

To have a better usage of the solar system or backup power system, the right battery capacity select is important. Battery selection is according to the needs of the application, voltage, discharge time and temperature are the most basic parameters in the system (need to be determined).

The capacity is related to the battery charge and discharge current: the current increases, the lower capacity; smaller the current, the higher the capacity.

Battery discharge current and capacity has the relationships follow Peukert equation:

            C = I n · t
            C - Capacity (Ah)
            I - Current (A)
            t - Time (sec, min or hour)
            n – Constant value

Following is an example for battery capacity selection in constant current discharge condition:

Requirement:

Load current of 20A
Normal running voltage is DC 48V ± 15% 10% 0 ° C
Can work last 5hours (minimum).

The basic calculations are as follows:

The maximum DC bus voltage U max = 48 × 1.15 = 55.20V
The minimum DC bus voltage U min = 48 × 0.90 = 43.20V
If use the monomers voltage of 2volt battery, U max ÷ 2.25 (float voltage) = 55.20 ÷ 2.25Vpc, = 24.5, total need about 24pcs.
Minimum discharge voltage U min ÷ 24 = 43.20 ÷ 24 = 1.80Vpc only

If the temperature is 25 ℃, from the factory battery specification table shows that: when the discharge termination voltage of 1.8V, GFM-150 type battery 5 hour rate discharge current is 27A.

However, the required use of a temperature of 0°C! Should be carried out in accordance with the manual given formula and the correction factor correction:

The discharge capacity required by: Ct = 20A × 5h = 100Ah
Correction factor: K = 0.018
Capacity after correction: C e = 100 × [1 +0.018 × (0 - 25)] = 182Ah
Requirements of the discharge current (25 ° C, and 5-hour rate): I d = 182 ÷ 5 = 36A

Clearly, GFM-150 batteries can not meet the requirements. From the factory battery specification table shows that: GFM-200 battery 5 hour rate discharge current of 36A, it seems to meet the requirements.

But if need the battery can reliably support system in the entire life span should also be considered in the selection of the battery end-of-life provisions: 80% less than the rated capacity of the battery discharge when mean the end of life.

So the actual discharge capacity should be:

C = 182Ah × 120% = 218Ah
25° C when the discharge current (5 hour rate): I = 218/5 = 43.6A

From the factory battery specification table shows that: when the discharge termination voltage of 1.8V, GFM-250 type battery 5 hour rate discharge current of 45A, to meet the requirements.

Finial correct selection of the battery results: 24pcs GFM-250 type batteries.