Electrical Engineering/Calculating battery life
QUESTION: Hi Cleggsan,
Genki desu ka? I hope you are well.
I am trying to calculate how long a battery is going to last powering a component I have. The component is a piezoelectric buzzer with the following specs -
Rated voltage(Vp-p) 5
Operating Voltage(Vp-p) 1~25
Rated Current(mA) ≤3
The buzzer is switched on for 0.25 seconds, twice a second, for two minutes - so the buzzer is switched on 240 times in two minutes. The total time it is switched on equals one minute.
I want to try to work out how long my 6 Volt, 240 mAh battery is going to last under these conditions.
I do not know exactly how much current is required to switch on the buzzer. As the rated current is given as (mA) ≤3 can I use 3mA in these calculations?
If so, can I say that the buzzer consumes 3 milliamp minutes per operation?
3 milliamp minute = 0.05 milliamp hours.
240 mAh divided by 0.05 mAh = 4,800 operations
This seems like a lot.
Am I going about this the right way? (I suspect not!) A while ago now, I had another question about calculating battery life and you suggested using joules in the calculations, but that situation involved switching on large numbers of devices at the same time, so I am wondering if this current situation can be calculated using the numbers given above.
Am I on the right track, or off on the wrong one again?
Thanks always for your sage advice!
ANSWER: Genki Desu.
Let's take a look. Under ideal conditions the power required to turn on the buzzer is 6v x 3ma = 18mw. That power will be required for each 1/4 sec for a total of 240 ONs.
Since battery energy is 240mAh you can get 240mAh/18ma = 13.3 hours run time.
It will be switched X many times in one hour. Total is 13.3 times that amount.
you can do the rest.... Your homework assignment.
---------- FOLLOW-UP ----------
QUESTION: Thanks Cleggsan,
So, if the total on time for each operation is one minute, can I just divide that 13.3 hours by 1 minute?
13.3 hours x 60 minutes = 798 minutes
Therefore, 798 operations for one battery?
The device will be switched on once a day for a total of one minute. Can I say the battery will last for 798 days?
Sorry to be dense, I just want to make sure!
I am assuming it doesn't make any difference here that the device is pulsed on and off, is that correct?
A good engineer does real test to verify the calculations. In your case the current is unknown and there will be voltage droop from the battery near its end all of which make your calculations only rough estimates.
You asked about the pulse duty cycle; Yes switching off and on stresses the battery internal organs and my cause some deterioration its run time. Again, experimental results is a must for circumstances of this kind.