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### Topic: Battery life running a motor (Read 1 time)previous topic - next topic

#### SouthernAtHeart

##### Jan 09, 2013, 10:39 pm
Going battery powered on my chicken coop door.
Any idea of how much power this motor would use in a year, running on 4 alkaline batteries?
http://www.ebay.com/itm/350681724570
It's 6 volt,  65 mA, and it will be opening/closing a door once a day. The motor is not heavily taxed.  The time to do so is about 5 seconds, so that's 10 seconds a day runtime.    I want enough battery power to last a year without worry.  I figured I'd need to go with D batteries, but my calculations show 4 AA batteries would last around 3 years!  Can someone verify this please.

After reading Nick G's informative article on power saving, it seems I can run my Arduino chip at such a low current (.1uA) that it won't have much bearing on the life of the batteries.

#### retrolefty

#1
##### Jan 09, 2013, 10:50 pm
Quote
It's 6 volt,  65 mA, and it will be opening/closing a door once a day.

Most likely that current draw spec is just of the motor/gear train without an external load attached. You might want to measure the current draw while it's opening the door with a digital multimeter is see if that is an accurate specification or not.

Lefty

#### SouthernAtHeart

#2
##### Jan 09, 2013, 11:55 pm
Your are right. 120-150 mA is the working current. It's not working hard, though, the specs show max efficiency to be 700 mA.
After redoing the math, I'm unsure...
Typical Arduino alkaline battery's are 2850mAh.
2850 mAh / 150 mA = 19 hours of run time
19 hours = 68400 seconds
68400 / 10 seconds runtime a day = 6840 days
6840 days = 18.74 years

That hardly seems possible?
Bottom line:  I don't like leaving batteries in something for more than year, so I what big enough batteries to safely operate for 2 years, then replacing them every year, I'll never have to worry.  I thought I'd need to go with C or D cells.  Will AA really last that long?

#### retrolefty

#3
##### Jan 10, 2013, 03:48 am

Your are right. 120-150 mA is the working current. It's not working hard, though, the specs show max efficiency to be 700 mA.
After redoing the math, I'm unsure...
Typical Arduino alkaline battery's are 2850mAh.
2850 mAh / 150 mA = 19 hours of run time
19 hours = 68400 seconds
68400 / 10 seconds runtime a day = 6840 days
6840 days = 18.74 years

That hardly seems possible?
Bottom line:  I don't like leaving batteries in something for more than year, so I what big enough batteries to safely operate for 2 years, then replacing them every year, I'll never have to worry.  I thought I'd need to go with C or D cells.  Will AA really last that long?

Well a couple of issues to be aware of. First front page published (read marketing department driven) mAH ratings for batteries is usually stated on a pretty small continuous current draw. When drawing higher currents then their mAH rating usually has to be 'de-rated'. Some better battery datasheets will show mAH ratings at various discharge amounts. So your actual mAH delived will be less then the 'advertised' amount. How much less can vary from insignificant to holy heck what kind of battery are they selling me.

Battery duration also suffers from 'self discharge' even when no current is being drawn from it, which might be a factor in your application or maybe not. Just be aware that oversimplified math for battery duration is almost always comes out too optimistic.

But all that aside it really would appear at first glance the good quality alkaline AA cells might reach your target of yearly replacement. Note that battery contact resistance/corrosion if not operating in a good environment may be a reliability factor to look into, but if it's good enough for the chickens it will probably be ok for the batteries.

Lefty

#### SouthernAtHeart

#4
##### Jan 10, 2013, 04:42 am
I always buy radio shack batteries so I know I've got good, healthy long life ones.

...seriously,
One scorecard shows AA Alkaline   Duracell Coppertop
100mA discharge rate: 2.2mAh
500mA discharge rate 1.3 mAh

Self discard only seems to be a few % for a year, so should change much.

If my motor ends up drawing 300ma
AND
If my batteries only produce 1.7mAh
AND
The door takes 10 seconds to open instead of 5
...
It'd still run for 2.8 years. I was hoping to get 2 years or better, so a yearly replacement plan should be fail proof.
I'll order some motors and report the current tests (I normally use 12 v motor, non battery operated projects).
If I go with a 3volt motor instead of 6 volt, I could run it on 3 AA batteries instead of 4, and skip the regulator for my chip.  It seems the voltage regulator take way more quiescent current than the chip!  I'll have to research this some--maybe it doesn't like being run off a non-regulated source like three batteries?  It'll be sleeping most of the time waking from the INT pin of a RTC DS3234.  Would I be able to run the atmega328P and DS3234 directly off (3) AA batteries?
Many thanks!

#### SouthernAtHeart

#5
##### Jan 10, 2013, 05:25 am
Edit:  3 volt motors aren't big enough.  Use considerable more current for the same amount of torque.
I wonder if its kosher to use (4) AA's for the motor, but just two of them for the 328P & DS3234?
Then I could use something like this:

#### retrolefty

#6
##### Jan 10, 2013, 05:29 pm
That should work, but I would run the chips at the 3 cell (4.5) tap and the motor at the 4 cell tap.

Lefty

#### MarkT

#7
##### Jan 11, 2013, 12:21 am
150mA for 10s per day is equivalent to 17 microamps continuously.  So if the rest of the circuitry takes more than
17uA it will be the dominant factor.  Using sleep mode?  Checked quiescent current consumption of all the devices?
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]

#### SouthernAtHeart

#8
##### Jan 11, 2013, 01:29 amLast Edit: Jan 11, 2013, 01:34 am by SouthernAtHeart Reason: 1
Looking at Nick Gammon's "detecting a key press while asleep" shows a power consumption of 0.1 uA, so I figure my chip will be comparable. However, looking at the DS3234 RTC, it might draw 140 uA!  That's a lot.  Can someone verify that?  It doesn't use the term quiescent, but it mentions the term standby current on page two, which seems to be the same thing.
I need an accurate RTC, maybe I'll look at alternatives.
http://www.sparkfun.com/datasheets/BreakoutBoards/DS3234.pdf

#### SouthernAtHeart

#9
##### Jan 11, 2013, 02:33 am
I think I've worked out the motor current.  So I will continue my RTC issue under an appropriate thread.  Thanks.

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