[volt-nuts] Battery pack info for Fluke 731B

[J. L. Trantham]

I agree with Bill's approach but I chose the 'battery holder' approach to
avoid soldering the cells together.

Joe

-----Original Message-----
From: volt-nuts-bounces at febo.com [mailto:volt-nuts-bounces at febo.com]On
Behalf Of Bill Gold
Sent: Sunday, August 21, 2011 3:50 PM
To: volt-nuts at febo.com
Subject: Re: [volt-nuts] Battery pack info for Fluke 731B


David:

    Faced with the same problem with the battery pack, I disassembled an old
battery pack and found that there were indeed 12 NiCad cells in the pack.  I
then reasoned that replacing them with 12 AAA NiCads would probably just
have me replacing them a few years down the line.  So I got 12 AAA NiMh
cells right off the shelf at the local Home Depot, probably around 1000 mAh,
and arranged them in two rows of 6 cells each and stacked the rows so that
the second row of cells would fit into the "depression" or space between the
cells of the first row, because the cells are round.  This reduced the
overall thickness of the two rows.  I then wrapped this with Duct tape, a
little difficult to hold the shape, but not impossible.  Then I soldered
wires from the "+" and "-" terminals so that the cells were in series so I
got the 14.4 volts necessary (12 X 1.2V). I then took the two original wires
and plug from the original battery pack and connected them coming off of the
end of the string.  I covered the top and bottom of the pack to insure no
shorts occurred.  I made no changes to the charging circuitry and I simply
plugged it in and measured the current flow and voltage.  I saw no over
current or voltage problems.

    I then got two longer screws for the "U" bracket that holds the battery
pack so that I could clamp the new pack inside the 731B.  This keeps
everything just like normal from the outside.  And it fits very nicely
inside as there is plenty of room.

    Be careful when soldering the ends of the AAA batteries as they are not
intended to be soldered (if you can buy "solder tabs" then so much the
better) and don't get them too hot.  I make sure there is no corrosion on
the ends of the batteries by cleaning them up with some fine sandpaper, then
tin the ends, wait for the battery to cool down and then solder the wires on
quickly.

    I checked the 10 volt output before doing this against a Fluke 731B from
work, left the 731B plugged in while changing the batteries, and then
checked the 731B again after all was done and it had a chance to settle down
after being opened.  No change I could measure other than a possible room
temp change effect.

    I did this to my personal 731B and then to a 731B at work.  I then sent
the 731B at work into the local Fluke Cal lab, when Fluke had a lab around
here, and they returned it with no change noted.  The one at work had always
been returned with less than a 0.5 ppm change for about 10 years, and never
adjusted.

    I did this about 8 years ago and have had no problems since, including
having to change the batteries every few years.  And the batteries last
longer when not on line power.

    It looks to me like you have selected the correct ten turn dial
indicator.

I hopes this helps everyone.
Bill
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