Battery consumption calculation

[bspace123]

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?

 

[itsmoked]

12 x per day 12sec @ 247mA =>
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.

=====================================================================

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

 

[IRstuff]

OK, you both need to get new calculators ;-), in Keith's case, I can see that 83 uA does not equal 0.0083 mA, it equals 0.083 mA, making the discharge rate 494.53 uA, not 420uA, resulting in 1.5 yr EOL. Can't tell what the OP did, but possibly the same thing

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!

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[itsmoked]

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

 

[IRstuff]

I meant them to be be essentially online and trickle-charged as needed, or, they could be preemptively charged in the minutes before the 12-second data burst, 247 mA for 12 s, could be 4.9 mA for 600+ seconds

TTFN (ta ta for now)
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[IRstuff]

btw, I'll just toss in another plug for SMath Studio here, which is free, and handles units and conversions on the fly... ;-)

It'll help us avoid these types of typos and let us concentrate on the more critical stuff.

TTFN (ta ta for now)
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