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How to estimate battery capacity?

by Siddharth Mistry on May 13, 2015
How to estimate battery capacity?

More and more hardware designs are powered from battery with the rise of portable devices, hand held devices, IoT devices and wearable devices. As the battery selection directly affects back-up time and form factor of these devices, it is very important to find out right battery for these devices in the beginning of design process. This article illustrates the procedure on how to estimate required battery power. It is a simple three step process.

Information required to begin with estimating battery capacity

  • Device Use cases
  • Device architecture
  • List of Major components
  • Information about power consumption of these major components
  • Back-up time required for device or intended battery to be used in battery capacity estimation

Step-1: Prepare a State Diagram

First of all prepare a state diagram of different states of the devices depending on the use cases. These states provide information about components in active state and components in standby or power save mode. Clear understanding of user cases and architecture are required to chart out state diagram.

Step-2: Estimating average power for each state

Further divide each state into different sub-states or events. Find out power required, occurrences & duration of each of the events. Duration & occurrences of event provide information about the duty cycle of that event. Consider 1 hour as the repetitive cycle for calculating duty cycle. Multiplying active power required for each of the event with its duty cycle provides information about the average active power of that event. Multiplying standby or power save mode power with (1 – duty cycle) provides information about the average standby power. Adding all these active and standby power of all the events of a state provides average power for that state.

Step-3: Estimating battery capacity or battery back-up time

Adding average power of each of the states provides information about the average power requirements of the hardware.

Using appropriate battery voltage (battery voltage depends on battery chemistry), find out average current required from battery. Consideration of power efficiencies of power regulators need to be consider while deriving average current.

Total Average Current (mA) = Average power (mW) / [Battery voltage (Volt) x Efficiency]

Multiplying these total average current (mA) with required back-up time (hours) provides information about required mAH. 

Battery capacity (mAH) = [Total average current (mA) * Back-up time (hours)] / [Battery Degradation Factor].

For better battery life, batteries should not be discharged completely. It should be left with 20% of capacity while discharging. And discharge rate should be lesser than 50% capacity of battery for some of the battery chemistry types (e.g. lead acid battery). In this case, battery degradation factor is 0.4.

In general, for other types of lithium batteries, Battery degradation factor can be approximated as 0.7. This factor is different in different literatures & different battery chemistry types. More accurate estimates can be derived by considering Degradation factor based on the manufacturer’s recommendations of selected battery for better battery life.

Similarly if battery is decided and need to find out back-up time following equation can be used:

Back-up time (hours) = [Battery capacity (mAH) * Battery Degradation Factor] / [Required mAH]

Apart from estimating right battery capacity, it is also important to optimize the whole hardware design for better power efficiency. This will also help in increasing the backup time or reducing the size of battery and intern help in reducing the form factor of the device.

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