Sizing a Battery Charger

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Battery Chargers should be large enough to power the normal system loads while recharging a discharged battery within a reasonable amount of time. Manufacturers recommend a recharge time of 8 to 12 hours. Shorter recharge times require larger battery chargers and might result in excessive current flow into the battery during the recharge process. For this reason, 8 hours is usually the minimum recharge time for a discharged battery. On the high end, 12 hours is often recommended for an upper limit; however, this recharge time is somewhat arbitrary and 14 hours or 16 hours might be acceptable, depending on the application and how the recharge is controlled. The primary consideration is that the charger should be sized to recharge the battery within a reasonable amount of time.

Size the charger to be large enough to supply the normal continuous loads while also recharging the battery within a reasonable time period. The charger sizing formula is as follows:

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Where:
A = Output rating of the charger in amperes.
k = Efficiency factor to return 100 percent of ampere hours removed.  Use 1.1 for lead-acid batteries and 1.4 for nickel-cadmium batteries.
C = Calculated number of ampere hours discharged from the battery  (calculated based on duty cycle).
H = Recharge time to approximately 95 percent of capacity in hours. A recharge time of 8 to 12 hours is usually recommended.
Lc = Continuous load (amperes).

The above sizing method is recommended, but tends to provide an optimistic recharge time. The actual recharge time is usually longer than indicated above because the charging current tends to decrease as the battery voltage increases during recharge.

 

Temperature Compensation

If the ambient temperature varies by 5 Deg C or more Battery voltage-temperature compensation should be considered. Typically charge voltage compensation is somewhere in the region of 3-6mV/Cell/Deg C change.

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Voltage Regulation

If the temperature of the battery room is allowed to drift either higher or lower after installation, it is critical to compensate the charge voltage. A flooded, lead-calcium battery that was originally sized for 77°F and is later kept at 87°F, should have its charge voltage reduced by 0.028 volts per cell, or 2.8 millivolts per degree F per cell (or 5mV/°C per cell). A VRLA cell should be compensated in the same manner by 0.002 volts per degree F per cell (or 3.6mV/°C per cell). For a total battery, this actual voltage change can be significant.

For Maximum service life it is a general rule of thumb that the charge current should not exceed 10% of the C10 rated capacity (commonly written as 0.1C10). In practice the recharge current should not exceed 0.3C10. If the application guarantees a depth of discharge of >40% of the C10 capacity, through the use of a battery Load disconnect device, then the charge current limits itself and the charge limitation may be removed.

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Higher Temperature operation
During high temperature conditions, float current increases. This causes more heat and gassing, and therefore more release of hydrogen and increased water loss. In addition, positive grid corrosion rate is accelerated and is the basis of temperature derating (for example, 50% reduction in life for each 15 F increase in temperature over 77 F). Loss of water due to high-temperature operation accelerates the drying out and further shortens the life of both AGM and Gel VRLA products.

Much of today’s newer charging equipment includes temperature compensation and even fold-back capability to reduce the effects of higher temperatures, excess gassing and thermal runaway. Provisions for airflow between cells allows for more uniform thermal distribution from cell to cell.

Thermal Runaway
The result of not compensating the float voltage on flooded batteries that operate at high temperatures is excessive gas evolution, greatly increased water loss, excessive shedding of positive active material and increased positive grid corrosion – all factors in low capacity and early life failures. VRLA batteries that are not charge-compensated for high temperatures are at an even greater risk of failure. VRLA batteries normally operate at higher temperatures internally than a flooded battery due to the internal heat of gas recombination, the lower thermal mass of the acid-starved element and the reduced heat transfer (as compared to a flooded battery container). Should temperature rise without the voltage being lowered to compensate, the battery will be forced to generate additional gas, which will recombine and add more heat to an already warm battery. Should the battery reach the point where more heat is generated internally than can be dissipated to the surrounding air, the batteries will reach a point of thermal runaway. At this point, the batteries will almost certainly be damaged beyond repair, and may generate enough heat to damage surrounding equipment. This is an extreme example, but it is important to consider the ramifications of allowing a battery room’s temperature to climb, while not compensating the float voltage.