I have a project in the final development stage of a prototype and I need to calculate how long my battery will keep everything on.
Someone know how to calculate this?
I'm using 6 AA batteries with 2000mAh and 1.2V each. My prototype drains about 70mA while it is processing sensor's data and 150mA during data exchange through wifi.
Note: I need to know the fundamentals and how to calculate the life time.
Battery lifetime in hours is given approximately by (capacity in mAh)/(average current in mA).
Note that manufacturers give capacity of a new battery after overcharging, at
the optimal temperature and discharge rate. Derate by a factor of two if you plan to
use the battery for some time - capacity decreases with age and abuse.
You'll need to know the relative times spent in data and wifi to calculate the average
current drain, then calculate the theoretical lifetime, then divide by two to get a
realistic lifetime that allows for battery aging, temperature variation and so forth.
It looks as though the average current is around 100mA, so you might expect about 20 hours battery life.
You could do a practical test, to find out. It will only take a day or two.
I believe a good idea would be to get some estimates on an upper and lower limit. However, it would be better to design around the assumption that you have maximum energy usage all the time.
If you have 6AA batteries in parallel, then your current capacity increases because your have more sources to sink current from. If they are in series you get no increase in current capacity. Your project is a little vague if they are in parallel or in series.
Anyway, in parallel you effectively have 2 Ahr * 6 = 12 Ahr. Assuming your maximum current draw of 0.15 amps 12Ahr / 0.15A = 80Hr. With the 0.07 A draw you could get 171 hours. I think you get the idea for doing the series one as its only 2Ahr total.
If you want to know for certain the best thing is to simply do a long term stability test. Hook up your project and devise a test to have it run at maximum capacity and minimum capacity and keep track of time before failure.
It's tricky to calculate if the current draw isn't relatively constant so I took the empirical approach with my project. I think it draws about 75mA (@5V) with occasional bursts during SD card writes.
The red line in the graph is the battery voltage with 6 AA alkaline.
I got about 30 hours before Vcc dropped below 5V and close to 40 before the SD card wouldn't write anymore. The blue line was using a switching regulator substituted for the stock Arduino linear regulator. It is an obvious improvement. After this test I removed the linear regulator and glued the switching regulator into the same place on my Uno board. It is a Pololu 300mA 500mA 5V regulator and cost about $5.
Temperature has an effect and can markedly reduce life of alkaline batteries. Since I am anticipating somewhat cool conditions (down to ~0°C) I also tested lithium AA batteries. In addition to cold tolerance they also have about 30% greater capacity as compared to alkaline. With 6 of those my project ran for 78 hours before Vcc fell below 5V and 79 hours before the voltage dropped too low for the SD card to write.
OP mentions 1.2volt AA. That could be NiMH rechargeables.
In my experience, the quality of those critters varies wildly.
I tested twelve 2500Ah brandless ones a while back. None of them were more than 700mAh, even after a few cycles.
Internal resistance is another problem that can make your Arduino reboot when the Wifi kicks in.
Field testing is the only option.
If battery life is important, do a "jboyton mod". A pololu micro buck converter (500mA).
In another post he has a picture of it.
Maybe he can post that here as well.
Leo..
I tested twelve 2500Ah brandless ones a while back. None of them were more than 700mAh, even after a few cycles.
Well, brandless says it all. I've done the same experiment with Everyready NiMH 2500 mAh AA cells, and using a resistor with 100 mA initial draw as the load, got better than 24 hours or over 2000 mAh in every case.
Got a few Energizers here. Maybe 2years old.
They still have the rated capacity, but they don't work anymore in a camera that draws ~1A at startup.
Don't know their charging history.
I suppose the type of charger used (e.g. delta-V) is equally important.
Leo..
Ive found a wide variation in the quality of AA, NIMH batteries, but one indicator of how likley they are
to meet their stated mah capacity is their weight.
The better better quality batteries Ive tried all weigh more than 25 grams, where the dodgy ones can be less than 20 g.
The Pololu mod on an Uno:
Rechargables: I have some AA Eneloop Pro batteries that I am hoping will be better than most. We'll see. For what it's worth I weighed a few and they averaged 30g. The Eneloop Pro AA is rated at 2550mAh vs about 2250 for the Alkaline and ~3000mAh for the lithium AA. The lithium batteries are nice in that they weigh about 15g.
Hey guys! Sorry about the delay... I keep waiting for an email notification that never comes and I end up forgetting about the topic. My bad. 
JohnLincoln:
It looks as though the average current is around 100mA, so you might expect about 20 hours battery life.
You could do a practical test, to find out. It will only take a day or two.
I was measuring the current drain using a Protek 506 (digital multimeter) connected on a USB port, but after almost 24h the multimeter's battery reaches the end (before my prototype) 
xeylode:
Dimensional Analysis (aka canceling labels through division)I believe a good idea would be to get some estimates on an upper and lower limit. However, it would be better to design around the assumption that you have maximum energy usage all the time.
If you have 6AA batteries in parallel, then your current capacity increases because your have more sources to sink current from. If they are in series you get no increase in current capacity. Your project is a little vague if they are in parallel or in series.
Anyway, in parallel you effectively have 2 Ahr * 6 = 12 Ahr. Assuming your maximum current draw of 0.15 amps 12Ahr / 0.15A = 80Hr. With the 0.07 A draw you could get 171 hours. I think you get the idea for doing the series one as its only 2Ahr total.
If you want to know for certain the best thing is to simply do a long term stability test. Hook up your project and devise a test to have it run at maximum capacity and minimum capacity and keep track of time before failure.
I'm using 6AA batterys in series. Each battery has 1.2V and I need more then this to feed everything. Thank you for your explanation, very helpfull. ;D
jboyton:
(...)I got about 30 hours before Vcc dropped below 5V and close to 40 before the SD card wouldn't write anymore. The blue line was using a switching regulator substituted for the stock Arduino linear regulator. It is an obvious improvement. After this test I removed the linear regulator and glued the switching regulator into the same place on my Uno board. It is a Pololu
300mA500mA 5V regulator and cost about $5.Temperature has an effect and can markedly reduce life of alkaline batteries. Since I am anticipating somewhat cool conditions (down to ~0°C) I also tested lithium AA batteries. In addition to cold tolerance they also have about 30% greater capacity as compared to alkaline. With 6 of those my project ran for 78 hours before Vcc fell below 5V and 79 hours before the voltage dropped too low for the SD card to write.
My intention is to move every piece of code into an ESP8266, I'll be using an LD1117 regulator to feed my project with around 3.3V and a maximum current of 1A (ESP8266 drains a lot of current sometimes). Still don't know wich battery type I'll be using...
Wawa:
OP mentions 1.2volt AA. That could be NiMH rechargeables.In my experience, the quality of those critters varies wildly.
I tested twelve 2500Ah brandless ones a while back. None of them were more than 700mAh, even after a few cycles.
Internal resistance is another problem that can make your Arduino reboot when the Wifi kicks in.
Field testing is the only option.If battery life is important, do a "jboyton mod". A pololu micro buck converter (500mA).
In another post he has a picture of it.
Maybe he can post that here as well.
Leo..
jremington:
Well, brandless says it all. I've done the same experiment with Everyready NiMH 2500 mAh AA cells, and using a resistor with 100 mA initial draw as the load, got better than 24 hours or over 2000 mAh in every case.
Battery life is CRITICAL for my project. If I can't reduce battery consumption drasticaly (it needs to keep at least 2 years with the same battery) my hole project will be turned into nothing D: That's why I'll change to ESP8266. I'm also thinking in perform some duty-cycle optimizations to reach this bettery life of 2years.
leandrogs:
...I'll be using an LD1117 regulator to feed my project with around 3.3V and a maximum current of 1A......Battery life is CRITICAL for my project.
So then wouldn't it make sense to use a regulator with a smaller drop out voltage? That was the lesson I learned recently.
leandrogs:
Battery life is CRITICAL for my project. If I can't reduce battery consumption drasticaly (it needs to keep at least 2 years with the same battery)
But it wasn't critical to include this specification of exactly how long when you first asked for help?
It's not impossible. Lots of things run this long on tiny batteries. Remote controls are an example that most people are familiar with.
2 years is 17520 hours. IF your battery is 2000mAh then your average consumption needs to be 114uA or less. Note that the battery has its own self-discharge consumption and this will probably dominate your total power budget. NiMh batteries don't last longer than a few months sitting on my shelf, so they aren't going to work for your application. Good akalines from Duracell or Energizer will have a shelf life of 10 years or more, so they are most likely the best choice.
The next tent-pole in your power budget is going to be the power used in sleep mode. How low can you go? Nick Gammon's power saving page has some good numbers on squeezing an Arduino chip down to nano-amps. You don't need to apply all of those techniques to get it down to something like 10uA.
Next tent-pole is the voltage regulator. I don't have the numbers on top of my head but I suspect that you will have to use Nick's suggestion of no voltage regulator, under "other power saving techniques."
OK, so with maybe 80uA lost to the battery, maybe 10uA in sleep mode, you have 24uA to do useful things like powering a sensor and communicating. Work out how many mA it takes in "wake" mode, how long you need to be awake to get a reading off the sensor and send it, then multiply that out to get milliamp-seconds. You might need 100mA for 1/10th of a second, which is 10mAs (1000uAs) to send one reading. So that defines how often you can send one of those messages within your 24uA budget. 1000/24 gives me 41.7 seconds. Does that suit your project?
leandrogs:
I have a project in the final development stage of a prototype and I need to calculate how long my battery will keep everything on.Someone know how to calculate this?
I'm using 6 AA batteries with 2000mAh and 1.2V each. My prototype drains about 70mA while it is processing sensor's data and 150mA during data exchange through wifi.
Note: I need to know the fundamentals and how to calculate the life time.
this is the political for AA batteries connected in serial:
Voltage(t) =+ (25.7422E-2 • CRNT • t)⁴ - (41.51947E-2 • CRNT • t)³ + (50.22063E-2 • CRNT • t)² - (41.28597E-2 • CRNT • t)+ 16.08757E-1 •N
t= hours (use two time ,one to find the 0.8 V mull by n,second time to gate the graph)
CRNT -current in mA
N numbers of batteries in serial
for more details just send email
You realize you've just posted to a 5-year old thread?