CONSTRUCTION

An example of the construction of SMU series batteries is shown in Fig. 1. Lead-Calcium-alloy is used for the plates to minimize self-discharge. Separators made of fine glass fiber unwoven fabric, insulate between the positive and negative plates and serve to absorb and maintain the electrolyte not to flow. Oxygen gas generated in the positive plate during charge is absorbed into the negative plate through the separator. The size of glass fiber and the amount of electrolyte are determined so that the gas can permeate easily. The container and the cover are made of PP. resin. A safety valve and ceramic filter are installed in an exhaust outlet of the cover to release gas and keep the internal pressure to the optimum range of safety. The safety valve prevents also oxidation (discharge) of the negative plate due to immersion of air from the outside.

Fig. 1 - Construction

2. Charging Characteristics: Equalizing charge for SMU Series batteries is not necessary because of the low self-discharge rate, the low dispersion among cells and floating charge at sufficient charging voltage will keep the battery fully charged. Also, after the battery is discharged, charging at high voltage is not required and floating charge at 2.23 volt per cell will be sufficient enough. However, when the battery is overcharged exceeding the gas absorption capacity, the water will be consumed and the cells life span becomes shorter. Therefore, charging current of initial stage must be controlled not to exceed 0.3 C (A) and caution must be taken for over-voltage at the last stage of charging and also during the floating charge. The recovering charge characteristics after the fully discharged condition is shown in Fig. 6

Fig. 6 - Example of Recovering Charge Characteristics

3. Relation between Charging Current and Gas Recombination Efficiency The relation between charging current and gas recombination efficiency is shown in Fig. 7. A gas recombination efficiency is maintained at about 100% until charging current becomes around 0.02 C (A), and after passing this point, gas recombination efficiency decreases to 60% at charging current of 0.1 C (A). Since gas recombination on negative plate type battery allows the oxygen gas generated from the positive plate in the last stage of charging absorbed to the negative plate, characteristics between cell voltage and charging current in the fully charging state is different form those of a conventional (flooded) type battery. There is a special ration between this characteristics and gas recombination efficiency, the gas recombination efficiency staffs a reduction from the point that the charge voltage rises rapidly.

Fig. 7 - Charging Current and Gas Recombination Efficiency

4. Requirements for the charger

Requirements for the charger used with SMU Series battery are shown below.

(a) Use an auto constant voltage circuit having a drooping characteristics.

(b) Output voltage accuracy requirement within ± 2%.

(± l0% in the AC input voltage fluctuation and 0 to 100% in the DC output variation)

(c) If the ambient temperature in installation area is beyond the range (5°C - 35°C) (40°F - 100°F), a temperature correction device shall be equipped. The correction value: -3.5 mV/°C at the datum point of 25°C (77°F)

(d) DC output current: Approx. (Current of continuous operation) + 0.1 C (A)

(e) Particular consideration should be given to the designing of Load Voltage Compensator (SID) because of SMU Series battery requires higher floating charge voltage compared with conventional (flooded) type batteries.

5. Calorific Value at Charging: The calorific value of SMU Series battery is much higher than that of conventional (flooded) type batteries, since the oxygen gas absorption in the negative plate is an exothermic reaction and therefore particular care should be taken to the temperature rising. The calorific value of the valve regulated sealed type battery is shown as follows.

Q = 1 x V x I x N x 3.6

J

where, Q: Calorific value (Kcal/hr.)
J: heat equivalent of work (= 4.18)
V: Charging voltage per cell (= 2.23 V)
I: Charging current { = 1/1000 C (A)}
N: Number of cells

Battery built-in type charger and battery box should be designed to maintain the ambient temperature of battery below 35°C (100°F) even during the hot season in consideration of the calorific value of SMU Series battery as follows:

V = Q

r x Cp x (t₂ - t₁)

where, V: Ventilation value (m³/hr.)

Q. Calorific value (Kcal/hr.)

r: Air density (1 .2kg/m³)

Cp: Specific heat of air (1.24 Kcal/kg °C)

t₂: Allowable temperature (°C)

t₁: Open air temperature (°C)

6. Capacity Conservation Characteristics (Self Discharge Characteristics): The lead-acid batteries will discharge, even with no load applied during storage period. This phenomena is called self-discharge. Self-discharge rate depends on the temperature. The higher temperature is, the more discharge rate is. Lead-calcium alloy grids are used in "SMU Series" battery to minimize the self-discharge rate, which is ¹/₃ to ¼ of that of the conventional (flooded) type batteries and the capacity conservation characteristics is superb.

The comparison between SMU series and conventional (flooded) type on capacity conservation characteristics is shown in Fig. 8. Please note when the battery is stored for a long period, the capacity may not recover to the initial fully charged state. In such a case by repeating charge and discharge cycles, the capacity can be recovered to initial state.

Fig. 8 - Capacity Conservation Characteristics

7. Life Characteristics

The life of storage battery varies depending on floating charge voltage, ambient temperature, discharge cycles and depth of discharge.

The following are considered as main factors, which cause short life of the battery.

(1) Unsuitable floating charge voltage

Suitable charge voltage is 2.23 volt per cell. Floating charge at high voltage causes overcharge and the gas generated from the plates inside the battery will not be absorbed which will result in the discharge of gas through the safety valve and eventually will reduce the water content in electrolyte. When such a condition continues for a long period, it will result in the life of the battery due to lack of electrolyte. Continued overcharge by extra charge current and temperature rise will result in shortening of the life of the battery. On the contrary, low floating charge voltage for a long period leads to under charge state and this is also a cause of the life shortening.

(2) Ambient temperature:

Battery used in high temperature will accelerate the corrosion of the grid of plate rapidly and will cause the deterioration of the battery as shown in Fig. 9. Life expectancy of SMU series is approximately 20 years under proper maintenance and normal operation at proper temperature and floating charge voltage.

Fig. 9 - Temperature Life Characteristics

CAPACITY CALCULATIONS

Determining the Capacity of batteries

Battery capacity calculations are based on the Standard of Battery Association SBA S 0601

An overview of the calculations method is presented below.

1. General Formula for Calculation of Capacity

C = [K₁1₁ + K₂ (1₂-I₁) + K₃ (I₂-I₁) + K₃ (I₃ - I₂) +

... + K_n (I_n - I_n-1)]

where C: Rated capacity at 25°C (77°F) Ah

L: Maintenance factor

K: Coefficient determined by discharge time T, minimum battery temperature and final voltage (Refer to the appended figures.)

I: Discharge current (A)

Suffixes 1, 2, 3...n: the turn number of discharge current (Refer to Fig. 10.)

Fig. 10 - Example of Load Characteristics

When applying this general formula to a discharge pattern which includes current drop, as shown in Fig. 11, the required battery capacity (C) should be determined by reference to the calculated capacities before current drop.

The maximum value obtained through the above calculations satisfies the capacity for the total load requirement.

For a discharge pattern, for example, with current drop points A, B and C, as shown in Fig. 11 the maximum value among the calculated capacities from point 0 to points A, B and C, which are represented as C_A, C_B and C_C respectively, provides the most suitable capacity for total load.

Fig. 11 - Example of Load Characteristic and Calculation Methods

2. Requirements for calculating battery capacity

Before calculation, the following four conditions should be considered regarding the capacity:

(a) Maintenance factor: Because the battery capacity changes gradually during service, the maintenance factor (L = 0.8) is necessary in order to compensate for the battery capacity change.

(b) Discharge time and current: The expected maximum load time is used for the discharge time. For current changes during discharge, the capacity should be calculated for the discharge pattern so that the larger discharge current occurs, as much as possible, at the end of discharge. By so doing, the capacity should be enough to satisfy the total load when a large current flow occurs at the end of discharge.

(c) Final voltage: Final voltage is the total value of the voltage drop between the battery and the load, and the maximum value of the required lower limit voltages of the various equipment. The final voltage used for determination of the K value in the appended figures is given as the cell voltage, which includes the value of the voltage drop at the connector. The final voltage is shown by the following formula:

	Va + Vc
V =	-----------------
	N

where V_a        = the required voltage of the load (V)

V_c        = the voltage drop by connecting wires, between batteries and load, including jumping wire drop (V)

n          = the number of cells connected in series

(d)   The lowest temperature of the battery: The lowest temperature of the battery should be determined by the estimated condition of the battery location.  If the room temperature is always kept constant by air-conditioner; the lowest temperature is equal to the controlled temperature

Requirements for calculating battery capacity (cont')