Battery Balance Meter 9V Battery Replacement

As it turns out I got tired of replacing the 9V battery which powers the Battery Balance Monitor volt meter. I was going through them faster than my smoke detectors were "producing" them and the place I had them installed required a zip-tie each time I replaced the battery.

I decided to see if I could find an isolated DC-DC. I have some Tyco 1/2 brick DC-DC converters which would run from pack voltage but it seemed a bit overkill to use a 150W unit to power something which draws in the mA range of current. Not being an EE I didn't have much to go by as to where to look or what kinds of parts are available. I found that Jameco Electronics had a small isolated DC-DC converter which took 9-18V input and had 9V output. I ordered this, two 3.5mm terminal blocks, a small ABS case, a PCB which fit the case, and some stand-offs to mount it.

The DC-DC said it had filtering built in but I decided to install an inductor and capacitor on both the input side and the output side to smooth out the current. I didn't find out if I need an inductor in both the positive and negative leads. I only put the inductors in the positive leads. I used two small inductors out of a dead PC power supply. The fuse holder, 1/2A fuse and 22uF capacitors came from a dead digital mulit-meter. Here is a picture of the finished board.

I didn't realize that the 3.5mm terminal blocks were not spaced correctly for mounting in the holes of the board. As it turned out they mounted diagonally just fine which actually made it easier for the wires to make the bend to go out the hole in the bottom of the case. The block on the left is for input power and the right is for output. They are connected to match the order/placement of the markings on top of the DC-DC. On the back side I used leads cut off of LEDs used in other projects. They are plenty large for the minimal currents involved and they don't flop around with road vibrations. I only had to use one insulated wire which I decided to put on the top side of the board.

I mounted the box portion of the case to the underside of the dash in the Gizmo. There is a little fiberglass tab which was unused in my Gizmo. Looking at another Gizmo I see that this tab is where the fuse block used to be mounted. I put some soft wide weatherstripping on the underside of the box, drilled holes to match the holes in the fiberglass tab and then ran a large zip-tie through the holes to mount the box. I figured that mounting in this way would lessen the sharp jarring and vibration that Gizmos get on rough roads. I then mounted the PCB to the lid of the box and drilled a small hole in the lid for the wiring. This allows me to get to the unit by merely removing the lid to the box making installation/removal much easier.

So far, this has worked flawlessly.

My parts list:

• CONVERTER,DC-DC,5W,9V@0.556A 9-18Vin,REGULATED,ENCAP,FCC/CE Jameco PN:2107477
• CASE,ABS SPEEDY,3.125 x2 x.875 Jameco PN: 18922
• PROTOTYPE BUILDER,1.6 x2.7 Jameco PN: 105100
• MOUNTING HARDWARE KIT,CIRCUIT BOARD Jameco PN: 106551
• HEADER,3.5mm,TERM BLOCK,2 POS, TOP SCREW Jameco PN: 2094506
• Two 50V  22uF capacitors
• 250V Fast Acting 0.5A fuse and holders
• Two small inductors