[volt-nuts] Affordable batteries for Fluke 732A DC Reference Standard?

[Mark Sims]

OK,  I just got in a 732A with bad batteries...  hint,  SLA batteries should not rattle with a crunchy sound.

The original batteries were PowerSonic PS604's with a flat top and wire leads/molex type connector coming out the side of the battery at the top.  The battery "terminals" were under the sealed flat top.  This battery style was once used in about every emergency exit light ever made,  but it is no longer made...  bummer.

Your easiest solution is to use two PS1229 12V 2.9AH batteries.  They fit in the space with room to spare.  No problem.  You do have to use some rigid foam, etc to shim up the excess space.  You also have to bridge over the unused battery connectors on the PCB.  The biggest problem is you lose 25% of the capacity (or 33% of the capacity available with modern 4.5 AH capacity).

The problem with current production PS604 cells is their size.  Although their case is slightly shorter than the original battery,  the new ones have spade lug terminals that stick up out of the top of the battery and are bent over at a right angle.  I went to the local battery/light bulb store (www.evssupply.com - highly recommended) and checked out the current production PS604 cells.  The ones I looked at are made by PowerSonic.  Other brands may have slightly different case sizes... 


I was convinced that they would work.  I selected four cells that had the bend in the terminal closest to the battery case.  I also bought four 10" wire harnesses with the molex type connector on one end and insulated spade lugs on the other.  Total cost was about $50.  I modified the spade lugs by shaving part of the bottom of the spade lug insulation off.  I pressed down on the battery terminals as much as possible to lower their height.

I managed to shoehorn the new cells into the 732A case,  but,  alas,  when I tried to install the battery unit back into the 732A,  the top plate of the case where the PC board is was bowed up enough so that it would not mate with the connector in the 732A...  Leaving screws loose, etc made no difference..   crap...

Looking at the construction of the 732A battery case,  I noticed that the bottom plate of the case was a small rectangular dish about 1/4" deep that is mounted bottom side up.  The batteries sit on the flat, bottom part of the dish.  Hmmm,  the dish is held to the case with several screws/PEM nuts that are symmetrically spaced.  You can undo the dish and flip it over (you will have leave out the three screws on the front of the assembly where the dish is notched out for some nylon bushings or do some grinding on the bushings if you want those three screws to fit).

The battery cells are a tiny bit too big to drop down into the dish,  but (according to the Battery Expert) you can mount the cells upside down.  The top of the cells will rest on the lip of the dish and the battery terminals will sit in space between the lip and bottom of the dish.  There is enough space between the batteries to run the wires in the dish and up between the two middle cells to the PC board connectors.  Voila,  everything fits perfectly.  

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