From: jon_sree@world.std.com (Jon Sreekanth)
Subject: Re: How to estimate 9V Alkaline Battery life ?y
Organization: The World
	<1992Apr10.015016.1816@memstvx1.memst.edu>
	<92102.120544LEEK@QUCDN.QueensU.CA>
Distribution: sci
Date: Wed, 15 Apr 1992 04:20:50 GMT


Thanks to everyone who replied to my question. Here's a summary of 
various email and other responses : 

-------- original question: 

I'm using a 9V alkaline (Duracell, or some similar store brand) battery
in a circuit which has a fairly low current drain. I estimate 100uA 
constant drain, and about 0.5 mA for maybe 6 hours per day. Also,
the circuit will be functional down to about 4V from the battery. 

Is there some magic formula for estimating, from your experience, or 
any  published specs (where do I find them ?) battery life ? Battery 
life is defined in this case as # of days between buying an average-aged
battery from a drugstore to voltage dropping below 4V ? 

(Right now, I have a 500maH estimate from Panasonic, and I've looked at
Horowitz and Hill. I'm hoping I can get more maH because of the low
drain, and because the circuit can go down to about 4V at least.)

-------------

	The amount of energy available when the battery hits
7V or so is negligible; can you stack some low-voltage circuitry
in series to use the power-supply voltage latitude?  Typical
tricks here include putting LEDs in series with a current
source, and shorting the ones you want to turn off.

	Approximate battery capacity (alkaline cells)

D cells:  10000 mAH
C cells:   5000 mAH
AA cells:  2000 mAH

	NiCad C cells are good for 1200 mAH to 1800 mAH.

-------------

   My dad got into determining the charge left in batteries when trying 
to accurately adjust digital watches.  There are books from Eveready, and
prob. other manufacturers as well, giving all kinds of detailed specs. 
for all of their batteries.  You can make good estimates with their 
published discharge curves.  Sorry, got no details, but try giving them
a call and getting those books.  As I remember, they were quite detailed.

-------------

I think it is a fair bet that the 500 maH figure was measured at
a fairly low current drain, especially since 9V batteries are
mainly intended for use in transistor radios.  There shouldn't
be much difference in available energy between 5 mA and 0.5 mA
discharge for an alkaline battery.  They are much more linear
than zinc-carbon batteries, though not as much so as nicads.

-------------

Batteries from different mfgrs are not much different from one
another with a few exceptions.  I don't think it is a good idea
to use alkaline batteries if your application depends on a battery
running *out* at a particular time, as mfgrs are constantly
changing their formulations (usually improving them) and there
is some variation from one battery to another even in the same model.
If you need predictable behavior, use mercury batteries or have
some kind of timer in your application to keep track of battery use.

-------------

   Battery engineering manuals (available from the manufacturers)
   usually are full of graphs showing voltage vs. time at different loads.
   My current Digi-Key catalog says Panasonic 9V alkaline gives 500 maH
   to 5.4 volts (after which I expect it would drop near zero pretty quick).
   Most other brands should be comparable.

Radio Shack had a small book giving data about their batteries.  It had
charts giving time vs voltage for various drains on the batteries.  I don't
know if they still sell it, or whether it has enough info. for you, but it
is a place to look.

-------------

>I'm using a 9V alkaline (Duracell, or some similar store brand) battery
>in a circuit which has a fairly low current drain. I estimate 100uA 
>constant drain, and about 0.5 mA for maybe 6 hours per day. Also,
>the circuit will be functional down to about 4V from the battery. 
	Hence your average ma-hr rate is (.5*6 + .1*18)/24 = 0.2.
..
>(Right now, I have a 500maH estimate from Panasonic, and I've looked at
>Horowitz and Hill. I'm hoping I can get more maH because of the low
>drain, and because the circuit can go down to about 4V at least.)
	Technical data sheets are available for alkaline batteries.  I have 
before me one from DURACELL.  The MN1604 9V transistor battery is rated
at 565 ma-hr to 4.8 volt cutoff.  At 4.8 volts the voltage dropoff vs. time
is fairly steep, i.e., 4 volts is not far off, hr-wise. 
	Based on your consumption rate, such a battery would serve around 565/
0.2 ~= 2825 hrs, or 117 days, probably a bit more for the 4 v cutoff and the
fact that alkalines offer highest energy density at low discharge rates- but
not dramatically so in this case.   In short, you could probably expect 4
months from a fresh battery.

-------------

According to The Art of Electronics by Horowitz & Hill....
               Rint  Voc   Cap to 1V/cell                Comment
9V "1604"
Le Clanche      35    9    300mAH @ 1mA   160mAH @ 10mA
Heavy Duty      35    9    400mAH @ 1mA   180mAH @ 10mA
Alkaline         2    9    500mAH @ 1mA   470mAH @ 10mA
Lithium         18    9   1000mAH @ 1mA   950mAH @ 80mA  Kodak Li-MnO2

You can see that Alkaline & Lithium are pretty good with Lithium at
2X capacity and 2X price depending on where you get it.

>> (Right now, I have a 500maH estimate from Panasonic, and I've looked at
>> Horowitz and Hill. I'm hoping I can get more maH because of the low
>> drain, and because the circuit can go down to about 4V at least.)

Do watch out when you try to drain the cell down to 4V.  The cell
internal resistance goes higher near the end.
Also do watch out - cheap batteries rust & leak whatever's left in them
as years goes by.

You can probably use the Discharge curve (fig 14.1) in Horowitz&Hill
to estimate the battery capacity from the 1V/cell to 0.66V/cell.
From the graph, I would say 20 to 25% for Alkaline.

In article <1992Apr10.015016.1816@memstvx1.memst.edu>,
sowellrm@memstvx1.memst.edu says:
>
>Kodac used to sell a 9V lithium battery that would probably last longer than
>the alkaline.  It costs around $5 though.

Yeap.  2x life at 2x price.  Not bad.

-------------end of summarized responses.

Again, thanks for all the information.

/ Jon Sreekanth

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