# Power Consumption Estimation Method for Deciding Battery Capacity

#### amh_87

Joined Feb 23, 2019
1
Hi everyone,
I currently do a project in designing an electronics circuits about hardware tester. This hardware tester should be used for testing some/all peripherals of MCU. The aim why I would like to make it was I want to have device which can be used to check whether the MCU is still good OK or need a replacement.
I have already made the hardware and now I would like to make it portable by powering it with some batteries so then I can used it wherever I am. Since here I got a problem, I could not get a specific value of power consumption that my hardware tester needed. To get it answered, I have tried to measure the current which flow from my batteries to the circuit and I got a value in mA rating. I did it by wiring a multimeter and load (my hardware tester) in series with batteries like shown in image below.

My question is, What have I done lead me to get an exact result of power consumption or Did I miss something?
All of this test I did after the hardware is made, So, My other big question was, Is there a way or method that I can do/follow to estimate the power consumption of my hardware such a simulations before it send to manufacture?
best regards
Amh

#### MrAl

Joined Jun 17, 2014
7,466
Hi,

When you measure milliamperes or amperes that is good because the main issue is the ampere hour capacity of the battery, or sometimes in milliampere hours.

If the load draws 100ma and the battery is 1000mAh (that is 1000 milliampere hours). then the run time will be approximately:
time=1000/100=10 hours.

That came out to less than 20 hours so you will need a battery better than 1000mAh to get to 10 hours, but that is a first approximation.

So if you have a 200ma load and you want the run time to be 20 hours, you need a battery with capacity:
mAh=20*200=4000 milliampere hours (which is the same as 4 ampere hours).

If you only need 10 hours run time then increase the result by maybe 20 percent or even 50 percent to be sure.
For example, for a run time of 10 hours the first approximation is:
mAh=10*200=2000 mAh
and increased by 20 percent we get 2400mAh and by 40 percent gives us 2800mAh.
So something like that would be good.

You see how we went by the milliamps not the power.

If you want longer overall life time of the system maybe double the ampere hour rating calculated above.

#### Picbuster

Joined Dec 2, 2013
996
Hi everyone,
I currently do a project in designing an electronics circuits about hardware tester. This hardware tester should be used for testing some/all peripherals of MCU. The aim why I would like to make it was I want to have device which can be used to check whether the MCU is still good OK or need a replacement.
I have already made the hardware and now I would like to make it portable by powering it with some batteries so then I can used it wherever I am. Since here I got a problem, I could not get a specific value of power consumption that my hardware tester needed. To get it answered, I have tried to measure the current which flow from my batteries to the circuit and I got a value in mA rating. I did it by wiring a multimeter and load (my hardware tester) in series with batteries like shown in image below.
View attachment 199203

My question is, What have I done lead me to get an exact result of power consumption or Did I miss something?
All of this test I did after the hardware is made, So, My other big question was, Is there a way or method that I can do/follow to estimate the power consumption of my hardware such a simulations before it send to manufacture?
best regards
Amh
MrAl is correct however;
The battery could be discharged to a point lower than the MCU operates.

The best thing to do is the following;
Check the discharge curve for the battery/cell used.
e.q manufacturer spec's 10V and 5A/H do not discharge more than 1000mA/H
Produce a load of 1 Amp keep this constant.
When a battery goes empty the internal resistance will go up, result a lower output voltage however; your load remains 1A until the voltage is to low.

Take let say 4 point at the discharge curve as supplied by the manufacturer.
Make a program to sample over time and compare the values with the pr-programmed values fount in the specs.
Last point is valid if you use always the same battery.

Picbuster

#### KeithWalker

Joined Jul 10, 2017
796
If you want to measure the total power from the battery, start with a fully charged battery. Measure and note the current and the voltage across the battery at regular intervals until the voltage is at cut-off.
The current capacity of the battery will be the average of the current readings (sum of current readings/Number of samples) times the total time. This is normally stated as MaH
The power capacity of the battery is calculated by multiplying the current capacity by the average of the voltage readings. This is stated in Wh.

#### MrAl

Joined Jun 17, 2014
7,466
Hi,

Yes there are some other issues that should be taken into account when doing a project like this. We could discuss them further.