Mr Pefhany is right on. The battery needs to be read under load to
determine its internal resistance.

What I usually do is to measure the battery under load. In one product
with 4 AA Alkaline cells, I did the measurement 4 times / day:

1. Normally the ADC is turned off, and no battery divider is in place.

2. At the time to take the samples, I:

    A. Place the battery divider into the circuit, and turn on the ADC.
I then take two samples, average, and store the values.

  {It is more accurate if there is a few minutes between steps A and B.}

     B. I then place a 100mA resistor load across the nominally 6V
battery. I quickly take two samples, average, store, then disconnect the
heavy load quickly. I am under load for about 10mS.

     C. I take everything back to normal.

     D. I then analyse the value without the load and with the load; if
     the two readings are different by more than 15%, the battery is now
flat. I usually consider the min. voltage as well under load.


--Bob


Spehro Pefhany wrote:

> At 12:14 PM 5/27/2004 -0400, you wrote:
>
>> Fred,
>>
>> The voltage you measure from a 9 Volt alkaline battery near the end of
>> its
>> life
>> depends upon the load placed on the battery at the time you are
>> measuring it.


Replier: Most attachments rejected
       --------------
         Bob Axtell
PIC Hardware & Firmware Dev
   http://beam.to/baxtell
       1-520-219-2363




--

Replier: Most attachments rejected
       --------------
         Bob Axtell
PIC Hardware & Firmware Dev
   http://beam.to/baxtell
       1-520-219-2363

--
http://www.piclist.com hint: The list server can filter out subtopics
(like ads or off topics) for you. See http://www.piclist.com/#topics