Calculation for Determining Life of 9v Battery Given Current Draw?

[natron_mn]

I was wondering if there is a simple formula to determine how long a 9V battery would last/power a stomp box given the current draw of said box?

Example - I've got a Chase Bliss Gravitas that can be powered by a 9V, and in the manual it says the current draw is 20mA. So, I'm wondering how long a 9V could power that pedal?

[GibsonGM]

You would look up the ampacity of your type of 9V battery.  Ampacity is the ability to deliver current.   That is expressed in "ampere hours" or "milliampere hours". Ah  or mAh.    That means how many amps (or mA) the battery can provide over time.

A rating of 5 amp hours can be taken as the battery can deliver 5 amps for one hour, or 1 A for 5 hours...and so on.


I just found that for an alkaline, the avg. typical ampacity is 550mA hours. https://en.wikipedia.org/wiki/Nine-volt_battery#Technical_specifications

So a circuit drawing 20mA should go 550/20 = 27.5 hours.    That would be average, so the real-world duration of the battery is not going to be that certain; but this should get you close enough.

Thus, the simple formula is;  Ampacity /Circuit draw = Hours   with the current expressed in same units.

[natron_mn]

Very helpful, thank you!

[antonis]

Unfortunately, there are many specs about rechargeable batteries but almost nothing for primary/dry cells (single use).. 

Even if battery's manufacturer gives capacity data (I never met one..  ) the depth of discharge in which battery should be considered "empty" is unknown..
(everybody has to set it's own standard for a dead-empty battery..)

Open circuit voltage measurement (using a high impedane multimeter) is an indicator for bad only battery..
(you can't be sure what will happen under an even light load..)

[GibsonGM]

Very helpful, thank you!

You're welcome!  Yes, it is very inexact; what I showed you is the textbook approach.  I have never tried to compute this and then relied on it in the real world.   It probably would be safer to just say "oh, probably 15 to 20 hours of use" rather than try to "time" it, LOL!   

Hey Tony!  Pssst!! http://data.energizer.com/pdfs/522.pdf 
Some do, some don't.  But I think you are correct, the manufacturer surely will say the battery is still good, when WE would say it has discharged too much to use!        The graphs they give assume down to 4.8V - WAY below what we would think it ok!!

[PRR]

> typical ampacity is 550mA hours

...to an end point of typically 5.4 V.

Will your gizmo work on 5.4V?

Some won't be happy.

Duracell has capacity versus voltage charts.

https://d2ei442zrkqy2u.cloudfront.net/wp-content/uploads/2016/03/PC1604US12141.pdf

10mA to 8V seems to be 20 hours, 200mAH. 2mA to 8V is 130 hours, 260mAH. This may not be their best grade. However I found the "CopperTop" data, and it is nearly the same.

[antonis]

Hey Tony!  Pssst!!

I couldn't visualize a blue blood gentleman Psssitting but et discere et vivere.. 

So Sir, haven't you ever measured a brand new battery, put in on duty for a while, stored it in a friendly enviroment and measured it after some time and found that two measurements are almost identical..??
(electrical cells tend to "recover" their nominal voltage - not wasted energy, of course - so they follow Voltage/Current capacity graphs only when in continous use..)

Batteries are considered perfect voltage sources but only for a very narrow working margin..

[GibsonGM]

Hey Tony!  Pssst!!

I couldn't visualize a blue blood gentleman Psssitting but et discere et vivere.. 

So Sir, haven't you ever measured a brand new battery, put in on duty for a while, stored it in a friendly enviroment and measured it after some time and found that two measurements are almost identical..??
(electrical cells tend to "recover" their nominal voltage - not wasted energy, of course - so they follow Voltage/Current capacity graphs only when in continous use..)

Batteries are considered perfect voltage sources but only for a very narrow working margin..

Yes sir!  I understand it won't be something you can set a clock to.  But for the sake of sanity, don't we need to be able to at least express SOME lifetime?  For the above example, could we not say "oh, expect this to go at least 20 hours of continuous use on a 9V"?   

Doesn't it depend on the level of accuracy you desire?  The results of powering a Mosfet boost on 9V battery would be much more pleasant than trying to power an Easyvibe on one...