Battery consumption calculation
Battery consumption calculation
(OP)
Hello
I have a device which consumes 83uA during idle, but once every 2 hours it consumes 247mA for 12s during transmission of a signal. The device is powered by a 6600mAh LiPo battery. I am trying to calculate how long the battery will last and have used the following formula:
% Of hour transmitting = (12s / 3600s)*0.5 = 0.17%
% Of hour idle = 100% - 0.17% = 99.83%
Total amp consumption per hour = (99.83% * 83uA) + (0.17% * 247mA) = 0.42mAh
On a 6600mAh battery, this would imply the device would last 15714 hours or around 1.8 years.
I know that LiPo batteries self discharge over time and various papers on the web suggest this to be 10% per year? How do I work this into my calculation and does anyone have any other sources about self discharge rates of LiPo batteries? Am I better off using another type of batteries ? (say NiHM?)
The device will operate outside in the elements, typically @ 0-30degC (during 9 months of the year) and 25-35 degC during summer (3 months of the year). Obviously not continuously at these temperatures as temperature varies over the course of a day.
I am trying to achieve 3-5 years battery life and can bump the battery capacity up to 52A/h if I have to (preferably not). It will be difficult to actually test this without having the device sitting out there for 5 years. Can anyone think of any types of tests I can do to gain some comfort?
I have a device which consumes 83uA during idle, but once every 2 hours it consumes 247mA for 12s during transmission of a signal. The device is powered by a 6600mAh LiPo battery. I am trying to calculate how long the battery will last and have used the following formula:
% Of hour transmitting = (12s / 3600s)*0.5 = 0.17%
% Of hour idle = 100% - 0.17% = 99.83%
Total amp consumption per hour = (99.83% * 83uA) + (0.17% * 247mA) = 0.42mAh
On a 6600mAh battery, this would imply the device would last 15714 hours or around 1.8 years.
I know that LiPo batteries self discharge over time and various papers on the web suggest this to be 10% per year? How do I work this into my calculation and does anyone have any other sources about self discharge rates of LiPo batteries? Am I better off using another type of batteries ? (say NiHM?)
The device will operate outside in the elements, typically @ 0-30degC (during 9 months of the year) and 25-35 degC during summer (3 months of the year). Obviously not continuously at these temperatures as temperature varies over the course of a day.
I am trying to achieve 3-5 years battery life and can bump the battery capacity up to 52A/h if I have to (preferably not). It will be difficult to actually test this without having the device sitting out there for 5 years. Can anyone think of any types of tests I can do to gain some comfort?
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RE: Battery consumption calculation
144s x 247mA = 35,570mAsec
35,570mAsec x (1hr/3600sec) = 9.88mAHr/day transmitting
Idle
24hrs x 60min x 60sec = 86400sec/day
86400sec - 144sec = 86,256sec/day
83uA = 0.00083mA
86,256sec x 0.00083mA = 72mAsec
72mAsec x (1hr/3600sec) = 0.02mAHr/day idling
Total AH/day = transmitting + idling = 9.88mAHr/day + 0.02mAHr/day = 9.9mAHr/day
3 years => 365 x 3 x 9.9mAHr/day = 10,840mA
4 years => 365 x 3 x 9.9mAHr/day = 14,454mA
5 years => 365 x 3 x 9.9mAHr/day = 18,067mA
6,600mAhr / 9.9mAhr/day = 667 days (1.83 years)
I concur with your answer.
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What battery depends on lots more than the temperature.
Lead Acid discharge at about 5% per month. Not gonna work.
NiMH discharge while you wait. They're damn near useless for anything and need to be tethered to a charger.
LiPo's are pretty decent at the 10% per year.
If you want to exceed more than a year rechargables of any and all kinds are not really an option. Forget them. You need to look at primary batteries.
There are two that would work for you. One is the Everyready L91 an AA Lithium Disulfide battery that puts out 1.5V. It has a 20 year shelf life with over 95% remaining so your couple of years are-a-walk-in-the-park. It's the only AA I now allow in my house or business since Duracell batteries have become absolute disastrous crap in the last 3 years. These are engineered batteries with full data sheets showing pulse responses and everything.
L91 Data Sheet
Application Manual for Lithium Disulfide cylindrical cells
Highly recommended.
If you want hardcore non-consumer quality millspec multi decade quality kick-ass batteries and are willing to pay for them then the only alternative is Tadiran Primary Lithium Batteries. They have dozens to select from that would do your job. They have single cells that are 4.9"L x 1.3"D with 35,000mAHr!
Tadiran Battery
Keith Cress
kcress - http://www.flaminsystems.com
RE: Battery consumption calculation
A couple of other considerations:
> Temperature -- The actual temperature profile and the ampacity at temperature need to be considered to get a more accurate EOL
> Discharge rate -- Rated ampacities are typically at very low discharge rates, 0.01xC or lower. The 247 mA discharge possibly reduces the rated ampacity. You could get around that by current limiting the battery discharge and use a secondary storage like a supercap, since you have nearly 2 hrs to charge it up.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
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RE: Battery consumption calculation
Well. Now we know, "who's checking the checker".
That's why I show all my work. LOL
Thanks IR.
Problem with supercaps is that they leak like a sieve. This can cause you to loose 2X the wake-up energy. They're also a bit of a temperature sissy. They're fine at keeping things going while one changes batteries, or to carry over on short power fails, or to store kinetic energy in a vehicle before the next pedal press. I reached for them a couple of times only to have them turn out to have pitfalls.
Keith Cress
kcress - http://www.flaminsystems.com
RE: Battery consumption calculation
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm
RE: Battery consumption calculation
It'll help us avoid these types of typos and let us concentrate on the more critical stuff.
TTFN (ta ta for now)
I can do absolutely anything. I'm an expert! https://www.youtube.com/watch?v=BKorP55Aqvg
FAQ731-376: Eng-Tips.com Forum Policies forum1529: Translation Assistance for Engineers Entire Forum list http://www.eng-tips.com/forumlist.cfm