Force Torque Battery Selection

Computing battery capacity for a given user requirement can be difficult as modern CPUs use complex power management both for itself and all it's included peripherals, such as Bluetooth, SPI communication, and in the Force Torque case the load cell electronics.

A first step is to acertain the use case required.

Here are several questions:

• Is the use continuous or intermittent
• If intermittent then what is the duty cycle, i.e. once per hour, once per week.
• Will it be turned off during no use or should it just stay on but on 'standby'
• How many measurements would be desired per battery charge
• What is the percentage of Bluetooth usage
• What is the maximum charging time desired
• What is the maximum time between chargings

Simple Example

Note that this is an example. With the answers to the above questions a better analysis could be done.

• Let's say that 10 measurements are required per hour all requiring Bluetooth.
• Let's assume a current utilization for each measurement is 60ma. This is typical, but a guess which can be refined later.

A typical power usage might be for a 40ma hour battery....

import Text.Printf

baseCurrent = 20 -- in nilliamps
blueToothCurrent = 40 -- in milliamps
measurementTime = 10 / 3600 -- in hours
batteryCapacity = 40 -- in ma-hrs

-- The total number of measurements would be
mahPerMeasurement = (baseCurrent + blueToothCurrent) * measurementTime
totalMeasurements = batteryCapacity / mahPerMeasurement
printf "Total Measurements -> %5.2f" totalMeasurements

Total Measurements -> 240.00

If 100 measurements per day were required then,

capacityInDays = totalMeasurements / 100
printf "Days before charging required -> %5.2f" capacityInDays

Days before charging required ->  2.40

Charging time is a tradeoff between extended battery life and charging speed. Faster charging generally means more heat which can reduce the battery overall lifetime. A good compromise is about 0.3 C .. where C is the battery capacity in milliamp-hours. Which would be about 4-5 hours. Faster is possible as short as one hour.

Note that the milliamp hour rating of batteries can depend on many things, most importantly age of the battery, temperature, and discharge rate. So the above numbers should be derated by at least 25%. Thus to summarize, for the scenario above, 40mah battery with the given use case recharge interval would be,

printf "Practical Recharge Interval = %5.2f" (capacityInDays * 0.75)

Practical Recharge Interval =  1.80

Again this is an example to give some perspective of the issues involved. Refiningthe power usage profile will help, and increasing the battery capacity limited by size and cost restrictions.

Availability

For more perspective here are some sample LiPoly batteries. The LiPoly has varied form factors, and a high energy to volume ratio. Below is an image of some LiPoly batteries for perspective.

Here is a link to Chinese LiPoly Batteries appropriate for our use. There are images, and specs for a mind numbing selection of batteries. Anything above 40mah would be suitable based on the example above.

Cost

Cost for LiPoly batteries varies depending on capacity and purchase volume but for 200 units would be from \$2 to \$3.