How To ~

BUILD A PORTABLE BATTERY BOX

By Bill Castro
August 1999
More scopes and accessories are available using 12 VDC auto accessory outlets. This is great for portability but inconvenient for running down the car battery or tripping over extension cords.

For around $50 and 2 hours of time you can build a battery box using a plastic toolbox holding a 12V lawn tractor battery wired with fused panel mounted 12V accessory outlets.   The parts can be easily obtained from Wal-Mart and Radio Shack.

SELECTING THE BATTERY AND THE TOOLBOX

I first selected a 12V lawn tractor battery.  I used a maintenance free Exide GT-235 battery rated at 15 Amp-Hours with a maximum recharge rate of 6 Amps.  A plastic toolbox is needed to prevent the possibility of shorting the battery terminals together and making construction easier.  The toolbox was selected by placing the battery inside it and making sure there was enough height for the lid to close as well as space on each side of the battery for the panel mounted outlets.  The outlets intrude 2 inches into the toolbox. Picking up the toolbox with the heavy battery inside tested its strength.  I ended up with a Popular Mechanics brand 19 inch toolbox with tray. Although I couldn't fit the tray in place with the battery, the tray was later modified and proved to be useful in holding the battery in place.
 

LOCATION OF PARTS

The battery was placed in the middle of the toolbox and slightly forward toA front view with the lid open and tray in place. My dew removal gun (front left) and LX200 power cord (front right) are also shown. distribute the weight evenly.  (I refer to the side of the toolbox with the latches as forward.) Styrofoam was used to keep the battery from sliding.  I used two layers of 1/2 inch thick Styrofoam cut to fit snugly around the battery and rest on the bottom of the toolbox. Next the tray was cut out to help hold the battery in place. The tray also adds rigidity to the toolbox and is convenient to hold other accessories like power cords and extra fuses. With the battery and Styrofoam in place mark the location of the battery on the bottom of the tray.  Cut out the rectangular section of the tray so that the tray can rest in its normal position with the battery sticking through it. This should be a close fit with no more than 1/8 inch clearance between the battery and the tray. A 1/2 inch notch was made along the back side of the cutout to allow wires to come through.

Next the holes were located on the front to mount the accessory outlets. Viewed from the side with the tray removed (right) shows the parts placement and wire routing. There is still room to carry accessories and to add more outlets.The outlets were Radio Shack panel mount 12VDC accessory outlet (270-1539).  With the battery and tray still in place, the location of the outlets on the front side were chosen to allow for the connectors 2 inch intrusion. This had to be located above the Styrofoam and below the tray.  I used 2 outlets, one on each side of the battery.  I also left space for additional outlets for future growth. Follow the manufacturers directions for mounting the outlets. The tray, battery and Styrofoam were removed for the drilling process. The back end of the outlet and 1/2 inch of its wires were wrapped with electrical tape to add strain relief to the wires.

Vent holes are needed to prevent any gasses from being built up inside the box.   These vent holes are 3/8 inch diameter.  There are 3 holes in the left, 3 holes in the right and 3 holes in the back sides of the box. The holes in the left and right sides should be below the tray to allow air flow under the tray.  The holes in the back side should be above the tray to allow air flow over the top of the tray. 

HOOK UP

For this step the Styrofoam and the battery need to be in place.  Use 14 AWG wire for additional wire length as needed. All spliced connections are soldered and insulated with electrical tape.  The Radio Shack outlet is rated at 15 amps. A Radio Shack heavy duty in line fuse holder (270-1217) with a 20 amp rating is used with each outlet. My maximum current draw is 13 amps for a dew removal gun and within the current ratings of all the components. 

First the red wire from the outlet is spliced with the wire from one side of the fuse holder.  The other side of the fuse holder is spliced to 12 inches of red 14 AWG wire. This wire is then connected to the positive terminal of the battery with the automotive battery side terminal connector and the 1/4-28 hardware. The accessory outlet comes with a ring terminal connected to the black wire.  The ring terminal is removed and discarded. The black wire from the outlet is spliced to 12 inches of black 14 AWG wire. This wire is then connected to the negative terminal of the battery with the automotive battery side terminal connector and the 1/4-28 hardware. Lead battery connectors minimize corrosion buildup with the lead battery terminal.  Other outlets are connected the same way.  The 12 inches of additional wire allows for future work on the battery box.  The wires are routed along the battery sides, up the back side of the battery, through the notch in the tray to the top of the battery. Radio shack self-adhesive utility wire clips (278-1647) are used to hold the wires in place. These clips can be unlocked any time by pushing a tab. 

THE BATTERY BOX IN USE

The maximum fuse rating for this type of circuit is 15 amps. For the dew remover gun I use a 15 amp slow blow fuse. For my 8 inch Meade LX200 I use a 3 amp slow blow fuse. I labeled the outlets to remind myself of the fuse rating. To help maintain battery life I never completely discharge the battery.  With my setup I can easily observe for 13 hours before recharging. 

For safety concerns the battery box lid should be open when recharging the battery. This prevents the battery from overheating and allows gasses to dissipate. Follow the battery manufacturer's recommended charge rate.  For lead acid batteries this current is typically less than or equal to one-fourth of the amp-hour rating.  The battery should be recharged at least once a month to prevent self discharge and maintain battery life. Monitor the battery temperature while charging. It should only be a few degrees above room temperature. If it is too hot then lower the charge rate and/or remove the battery from the box. If it doesn't hold a charge then its time for a new one. My Exide GT-235 battery can be recharged at a 6 amp rate and still not get very warm.

For the Exide GT-235 maintenance free battery the manufacturer recommends the following:

Maximum charge rate 6 amps.
Trickle charge rate 1.5A or less.
Charging voltage 15V to 16V maximum.

BATTERY VOLTAGE READING AND CHARGE STATUS

CHARGE 
STATUS VOLTAGE

100% 13.0V
 75% 12.8V
 50% 12.5V
 25% 12.2V
  0% 12.0V OR LESS

Note that after charging, the battery voltage  has to stabilize for about 1 hour prior to checking the charge status.

PARTS LIST

1   19 inch plastic toolbox, Popular Mechanics series.
1   Lead acid lawn tractor battery, Exide GT-235, maintenance free 
2   Automotive battery side terminal connectors.
1   Styrofoam sheet, 1/2 x 16 x 16 inches.
2   Panel mount 12VDC accessory outlet, Radio Shack 270-1539
2   Heavy duty in-line fuse holder, Radio Shack 270-1217
1   3A slow blow glass fuse, 1 1/4 x 1/4 inch.
1   15A slow blow glass fuse, 1 1/4 x 1/4 inch.
1   Self-adhesive utility wire clip package, Radio Shack 278-1647
2ft Red 14 AWG hook up wire.
2ft Black 14 AWG hook up wire.
2   Screw, 1/4-28 x 3/4 inch
2   Nut, 1/4-28.
2   Lock washers 1/4.

By profession Bill Castro is an electrical engineer and a member of the Institute of Electrical and Electronics Engineers Power Electronics Society. 

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