EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: AArdvark on September 08, 2020, 12:31:25 pm
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Hi,
i understand the basics of calculating current and watts using ohms law etc but lets say for example i am designing a circuit with a micro controller that is in sleep mode most of the time and draws 10uA and lets say it wakes up once a minute and does something that draws lets say 20mA for 10ms
how would i calculate the best battery capacity to last for 12 hours in mAh
my confusion is converting current in mA to mAh
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my confusion is converting current in mA to mAh
1 mA for 1 hour = 1 mAh
50 mA for 6 minutes, repeated every hour = 50 x (6 / 60) = 5 mAh per hour it is switched on (on for 6 minutes, sleep for other 54 minutes).
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Another approach is to first work out the average current before multiplying by the number of hours:
AverageCurrent = (Current1 * Period1 + Current2 * Period2 + ... + CurrentN * PeriodN) / TotalPeriod
where TotalPeriod is the sum of all periods: Period1 to PeriodN
Using your (O.P.) numbers:
AverageCurrent = (10uA * (60s - 10ms) + 20mA * 10ms)/60s
I calculate that as near enough 13.3uA. Multiply by 12 hours and you'll get 0.16mAH.
Therefore the total battery capacity required is tiny. The limit will be how long the battery can supply the 20mA pulse, as its internal resistance will increase as it discharges. This will almost certainly force you to use a larger battery than the capacity calculation suggested, or a large low ESR capacitor across the battery as a reservoir to supply the 20mA pulses without excessive voltage droop.
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Thanks for the explanation it makes sense to me up to when you convert it to mAh
i followed your calculations and also got 13.3uA but i could not seem to calculate this into the 0.16mAh could you please share you calculation steps? i probably missed something really obvious.
also the first calculation you divide everything by 60s so does that mean the 13.3uA is per second or per minute?
:-DD :-DD :-DD forget that my calculator was in the wrong mode! :palm: :palm: :palm:
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If you're going for very long runtimes (months, years), you might want to also factor in battery self-discharge. If you were to go only by the theoretical 0.32mAH/day figure, you might think a standard 2500mAH cell would keep your microcontroller going for 20 years. The cold hard reality being that if it's a crap NiMH cell, it will drop dead in less than 3 months.
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The average current calculation gives you a current. For your sanity, work in SI units, and only scale to mA or uA when the calculation is complete, after saving the result for further calculations as its all to easy to slip a decimal place and be out by an order of magnitude.
Once you have an average current (in A), multiply by the number of hours to get a result in AH (Ampere Hours)#. It will have a lot of zeros after the decimal point, and you want mAH so shift the decimal point three places right.
As atmfjstc notes, the self discharge becomes important for low average currents and long runtimes. It can be difficult to find the equivalent current to the self discharge as its normally not quoted that way. However if the battery's datasheet says that after 10 years it will have 50% of its capacity left, you can calculate the capacity loss in AH or mAH, then work back to an equivalent current using the number of hours in a year * no. of years. Generally if your application uses less than 10% of the equivalent self-discharge current. you can ignore its usage and take the battery life to be the shelf life.
# AH are *NOT* a SI unit. 1 AH = 3600 Coulombs as there are 3600 seconds in an hour.