Back-up Time
This is the time the battery will drive the load before needing to be recharged and can also be described as autonomy or discharge time.

Selection
Knowing the discharge current and discharge time, the battery "size" can be selected by using the manufacturer's selection graph. This will show the "size" of the battery in Amp Hours or Ah. This is known as the Capacity of the battery. Power-Sonic publishes such a chart, but, even better, we have a simple slide-rule type calculator covering Ah from 0.5 to 100Ah. This is available free of charge from Power-Sonic.

Battery Capacity
The industry standard for stating the capacity of VRLA/SLA batteries assumes a discharge time of 20 hours (20 hour rate). A 7.0Ah battery will discharge at 0.35 Amps down to a voltage of about 10.5V (for 12V battery) or 5.25V (for a 6V battery).

20 hours x 0.35 Amps = 7.0Ah

But be careful: as the discharge current is increased the Ah (capacity) of the battery is decreased. So, if you discharge a 12v 7.0Ah battery at 1.1 Amps it will reach 10.5 volts in 5 hours and so the capacity of the battery becomes:

5 hours x 1.1 Amps = 5.5Ah

This can be confusing and it is better to use the manufacturer 's charts (or a Power-Sonic slide rule) for selecting the capacity (size) of the battery needed. Also, allow a little bit of extra capacity, especially if wide temperature variations may occur.

Charging for Optimum Performance

General: Dependable performance and satisfactory service life of POWER-SONIC batteries depend upon correct charging. Faulty charging procedures or inadequate charging equipment result in decreased battery life and/or unsatisfactory performance. The selection of suitable charging circuits and methods is as important as choosing the right battery for the application. To charge a POWER- SONIC battery, a DC voltage higher then the open-circuit voltage of 2. I 5V/cell is applied to the terminals of the battery. POWER-SONIC batteries can be charged using any of the conventional charging techniques. To obtain maximum service life and capacity along with acceptable recharge times and economy, constant voltage current limited charging is recommended.

Overcharging: As a result of too high a charge voltage, excessive current will flow after reaching full charge causing decomposition of water in the electrolyte and, hence, premature ageing.

Undercharging: If too low a charge voltage is applied, the charger current output will essentially stop before the battery is fully charged. This allows some of the lead sulphate to remain on the plates which will eventually reduce capacity.

CHARGING CHARACTERISTICS
As the terminal voltage of the discharged battery rises, its current acceptance decreases. The battery is fully charged once the current stabilizes at a low level for a few hours. You will notice that there are two criteria for determining when a battery is fully charged — the final current level and the peak charging voltage while this current flows.

Charging Methods

Taper Charging
This is the simplest, most inexpensive charging method. Either quasi-constant voltage or quasi-constant current characteristics can be built into the charger through combination of transformer, diode and resistance. Of the two, constant potential charging is preferable.

Constant Current Charging
Constant current charging is suited for applications where discharged ampere-hours of the preceding discharge cycle are known. Monitoring of charge voltage or limiting of charge time is necessary, however, to avoid excessive overcharge. This method is effective for charging a battery that has been stored for an extended period of time or for occasional overcharging to equalize cell capacities.

Constant Voltage Charging
Constant voltage charging is the preferred method to charge POWER-SONIC batteries. Depending on the application, batteries may be charged either on a continuous or non- continuous basis.

Cycle Applications:
Limit initial current to 0.25C (C is the rated AH capacity of the battery). Charge until battery voltage (under charge) reaches 2.45 volts per cell at 20°C. Hold at 2.45 volts per cell until current drops to approximately 0.01 C ampere. Battery is fully charged under these conditions, and charger should either be disconnected or switched to “float” voltage.

“Float” or “Standby Service”:
Hold constant voltage source of 2.25 to 2.30 volts per cell continuously across the battery. At this voltage, the battery will seek its own current level and maintain itself in a fully charged condition.