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'Formula'
1999\01\30@133554 by

Hi
Can somebody tell me the formula to know how long
will a battery last supplying a 20 ma load.

Thanks

At 01:35 PM 1/31/99 -0500, Darwin Reynoso wrote:
>Hi
>Can somebody tell me the formula to know how long
>will a battery last supplying a 20 ma load.
>
>Thanks

'snot that simple.  You'd think a 20mah battery would last one hour, but it
will typically give you about half that.
The Amp/Hour rating of a battery is called "C". You need a battery with a
rating of at least 2C in order to get an hour's runtime.  Depending on the
chemistry, this can get a lot worse. Zinc-Air cells can give you a huge
amp-hour rating for their size, but they only like to supply tiny currents.

First, settle on a battery type, (Nicad, Gell, Alkaline..... ) then get
with the manufacturer for current/runtime curves.

Okey
Using a 9volts Duracell battery
supplying a 20 ma load how long will it lasts

Eduardo R. wrote:

{Quote hidden}

At 05:24 PM 1/31/99 -0500, Darwin Reynoso wrote:
>Okey
>Using a 9volts Duracell battery
>supplying a 20 ma load how long will it lasts
>
>Eduardo R. wrote:

Well, I'd reccomend a resistor, DVM and a stopwatch.  Apply load, measure
battery every 10 minutes, plot a graph.

You also have to define what you mean by "dead battery". If you need 9V,
your battery will be dead a lot sooner than if you only need 6V.

1999\01\31@145857 by
Even that's not fully answerable but it's closer.
An ALKALINE 9v "transistor" battery (PP3 etc) has a capacity of
around 500 mAH.
This means that NOMINALLY it can provide about 1 mA for 500 hours or
10mA for 50 hours etc.
For your 20 mA you would expect to get APPROXIMATELY 500/20 = 25
hours. This will depend on temperature, whether you do it in one
burst or eg 10 minutes every hour (battery chemistry gets a chance to
do things while its resting) and very importantly, how many hours you
want to discharge it in (20 in this case) how you define the end
point. An Alkaline battery starts off at about 1.5 volts per cell
(there are 6 cells in a 9 volt battery) drops a few tenths of a volt
fairly quickly then settles into a fairly flat decline until it gets
to around 1 volt per cell and then plunges quite rapidly thereafter.
Below 0.9 to 0.8 volts its fully gone and drops very rapidly to zero.

Manufacturers publish curves of expected performance under various
"standard" conditions.

Rough guide.
For discharge currents of 0.1 x amp-hour capacity or less:

Time ~ capacity/time

For more rapid discharges

Time ~ capacity / (2 x time)

For very fast discharges - much less than you would expect.

Here are a few TYPICAL capacities for modernish batteries.
All these are in milliamp  hours.

Alkaline    AA          500
Alkaline    C           7500
Alkaline D            15000
NiCd AA                 500
NiCd C                1800
NiCd D                 4000

Tjaart noted a while ago (6/1998) that a Duracell D rated at 15000
mAH nominal will actually provide 12000 mAH at 300ma at 50 to 70
degrees Fahrenheit. (strange new fangled temperature units we don't

From: Darwin Reynoso <darwinfuse.net>

{Quote hidden}

current (20mA)
>> from.
>>
>>  For Rechargable type bateries: AA penlight   = 500mA/h
>>                                   for C  type = 1500mA/h
>>
>> Then  we have that AA will last   500mA/20mA = 25 hours when fully
charged.
>>
>>   for C type 1500mA/20mA = 75 hours
>>
>>  You need to know the mA/h rate for the batery you are planning to
use, or
>> calculate it from the above values in case you are using those or
similar ones.
{Quote hidden}

An ALKALINE 9v "transistor" battery (PP3 etc) has a capacity of
around 500 mAH.
:
Here are a few TYPICAL capacities for modernish batteries.
All these are in milliamp  hours.

Alkaline    AA          500
NiCd AA                 500

Those don't corrolate.  A typical 9V alkaline battery contains 6 AAAA
cells, so would of necessity have a much lower mAH rating than an AA.
Also, a NiCd usually has significantly less energy than an alkaline
(although a higher current capability) (as with the C/D cell numbers.)
I therefore think that the quoted Alkaline AA capacity is incorrect.

NiCd C                1800
NiCd D                 4000

Another thing to watch out for is that many of the "consumer" D cells
are actually C cells in a bigger package (keeps prices lower.)

IIRC, the duracell web site includes a fair amount of technical
information, including the discharge curves for their common batteries.

BillW

>Here are a few TYPICAL capacities for modernish batteries.
>All these are in milliamp  hours.
>
>Alkaline    AA          500
>Alkaline    C           7500
>Alkaline D            15000
>NiCd AA                 500
>NiCd C                1800
>NiCd D                 4000

For comparison, from The Art of Electronics, I get these:

Alkaline:
D         10000 - 8000
C         4500 - 3200
AA        1400 - 1000
AAA       600 - 400

These are *measured* capacities at a continuous drain down to 1V/cell.
The first number is at 10 mA, the second at 100mA. Note that the
capacities go down significantly, especially with the smaller cells.

So yes, a LOT depends on what kind of current you're drawing and whether
it's continuous or intermittent.

Dave Johnson

'Formula'
1999\02\01@021150 by
My table of typical battery capacities gave a wrong value for the
Alkaline AA cell.

All values in mAH

>Alkaline    AA        2250
>Alkaline    C           7500
>Alkaline D            15000
>NiCd AA                 500
>NiCd C                1800
>NiCd D                 4000
>

Darwin Reynoso wrote:
>
> Hi
> Can somebody tell me the formula to know how long
> will a battery last supplying a 20 ma load.
>
There's bound to be something here:
http://www.duracell.com/OEM/

--Matt

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