Determining battery capacity for applications where load current is known at an operating voltage other then the battery voltage
This guide will help you to determine what the necessary battery capacity you need is for your particular application. It will be especially useful for people who are planning out their application and don’t have it completely built yet. Otherwise you could use a multi meter to determine the amount of current which is necessary.
Calculating the battery capacity would be straight forward if the voltage that your load runs on was the same as the battery voltage. However, most circuits today use some type of linear or switching regulator to provide Vcc to the application thus changing the voltage.
Therefore, in order to determine what AH capacity battery we need when the voltages are different. We have to express the application’s parameters in terms of power. See the steps below to determine the proper capacity.
Step 1: Determine the total current your application will need at Vcc. Typically this is done by looking at the datasheets for the typical operating current.
Step 2: Determine the Power you’re device consumes by P=IV (Amps * Volts = Watts)
Step 3: Decide how long you want your application to run off battery power for.
Step 4: Determine the Watt-Hour of your device by multiplying step 2 and step 3.
Step 5: If you’re using rechargeable batteries it is recommended that you never let the battery charge drop below 50%. So multipy the Watt-Hour number from step 4 by 2.
Step 6: BattAh = Wh / BattV
NOTE: The steps outlined above don’t take into consideration the power lost in voltage regulation. Linear voltage regulators such as the 7805 5V regulator can only be 50% efficient in transforming battery power to system power. For more information see this application note: http://pdfserv.maxim-ic.com/en/an/AN751.pdf.
Example:
Vcc = 5V
VBatt = 12V
Step 1: If i had a PIC microcontroller and a two other IC’s and the PIC draws 20mA @ 5V and the other two IC’s each draw 15mA @ 5V then my total current is 50mA.
Step 2: Power = 50mA * 5V = 250mW = 0.250W
Step 3: For sake of our example lets say i want to power my application for 20 hours.
Step 4: Wh = 0.250W * 20Hrs = 5Wh
Step 5: In my application i’m using sealed lead acid batteries so i’m going to multiply by 2 so that my batteries last longer. Wh = 5WH * 2 = 10Wh
Step 6: Since i plan on using a 12V battery the required capacity in Amp-Hours is Ah = 10Wh / 12V = .833Ah
