Capacity Rating Factor Kt in Battery Sizing - IEEE 485

To determine the amp-hour (Ah) capacity of a lead-acid battery, you typically find it directly on the battery itself or in the manufacturer's specifications. You can also estimate it by dividing the battery's watt-hour (Wh) capacity by its voltage. ...

Ah = Wh / Voltage.

• Understanding the 20-hour rate Lead-acid batteries are typically tested and rated at a 20-hour rate, meaning they are discharged at a current that will take 20 hours to fully discharge the battery.
• Practical Considerations
• Depth of Discharge (DOD): Lead-acid batteries, especially deep cycle batteries, should not be discharged to 100%. A common rule of thumb is to discharge them to 50% to extend their lifespan.
• Discharge Rate: The higher the discharge rate (the more current you draw), the lower the effective Ah capacity.
• Reserve Capacity (RC): Some batteries are rated with a reserve capacity (RC), which is the number of minutes at a 25-amp load that the battery can supply. You can estimate the Ah capacity from RC by dividing the RC (in minutes) by 60 to get hours, then multiplying by 25 amps.

Hello Solar Eagle,

I am trying to size a lead-acid battery for a DC charger to supply DC loads using the IEEE 485 method. I have created a duty cycle diagram and started filling out the cell sizing worksheet provided by IEEE 485.

According to the worksheet instructions, I need to record the capacity factors (Rt or Kt, from the manufacturer’s literature) in Column (6) for each discharge time calculated in Column (5). However, Rt or Kt values are provided in the manufacturer’s documentation for specific battery capacities (e.g., 150Ah, 200Ah, 250Ah, etc.). Since I have not yet determined the battery capacity, I am unsure how to select the appropriate Rt or Kt values.

Could you advise on the best approach for choosing Rt or Kt when the battery capacity is still unknown?

Best regards,

You must determine the load you will require to meet specs....then select the equipment that will fulfill that obligation taking into account all the losses associated with the system...How many watts you need for how long accounting for temperature and other variables involved....

...In battery calculations, "Kt" and "Rt" factors are used to account for the capacity of a battery at different discharge rates and temperatures, respectively, with Kt based on performance per rated ampere-hour and Rt based on performance per positive plate.

Here's a breakdown of each factor: Kt Factor:

• Definition: Kt is a capacity rating factor that accounts for the effect of different discharge rates on the available capacity of a battery.
• Calculation: Kt is calculated by dividing the rated ampere-hours (Ah) of the battery by the available amperes for a specific time (t).
• Example: For a 160 Ah battery discharged at 20 amperes for 8 hours, Kt = 160 Ah / 20 amps = 8.
• Application: Kt factors are used as multipliers in battery sizing calculations, particularly in IEEE worksheets.

Rt Factor:

• Definition: Rt is a capacity rating factor based on the performance of each positive plate in a lead-acid battery, used primarily for lead-acid cells.
• Calculation: Rt is calculated by dividing the available amperes for a specific time (t) by the number of positive plates.
• Application: Rt factors are used as divisors in battery sizing calculations, particularly in IEEE worksheets.
• Note: Not all manufacturers publish Rt factors, so Kt may be used instead.
• Lead Acid Battery Plates: Most lead-acid battery model numbers indicate the number of plates per cell (e.g., 3CC7 = seven plates per cell).

https://web.engr.oregonstate.edu/~webbky/ESE471_files/Section%206%20Battery%20Sizing.pdf

Typically we would consider at least 20% safety factor for lead acid battery backup systems...so you need to add that to the load requirement after determination of fully loaded operation in worst case operating conditions...

Something to be aware of if you find an AH rating have regard if the rating is at a 10Hr or 20Hr discharge rate, it makes a big difference.