I've had a Harbor Freight solar electric fence charger for a couple of years now—not very powerful, but cheap, and it's been effective in discouraging my dog from escaping on jaunts around the neighborhood; which around here is likely to get her shot. However, checking the battery is an awkward job. Living with solar electricity has made me very aware of the importance of keeping batteries happy. Overcharging them or allowing them to go flat will shorten their lives considerably, and replacing them is expensive and usually inconvenient.

But the opening to the battery compartment on these units is on the bottom of the fence charger, held in place with two tiny screws just waiting for a chance to slip from your grasp to bury themselves in the sand (and which will in the nature of things eventually strip out the plastic casing). Once the screws are secured (gripped between the teeth, usually) and the cover set aside, hopefully where the ever-present wind won't blow it away, you've got to manage both the battery (dangling from its leads) and the multimeter, applying the probes of the latter to the posts of the former. Not hard, exactly, but frustrating.

So I decided to make it easier by adding test leads which would be accessible without opening the case. Once I got the few materials I didn't already have, the job took about an easy half hour.

Step 1: Tools & Materials

Materials:
-6” red electrical wire (14-16 gauge)
-6” black electrical wire (14-16 gauge)
-2 male spade connectors (14-16 gauge; not the kind that look like a 2-tined fork, but ones with a solid blade)
-2 female spade connectors (14-16 gauge)
-2 butt connectors (14-16 gauge)

Tools:
-drill w/ 5/32” bit (or whatever fits the diameter of your wire)
-crimping pliers
-wire-strippers
-wire cutters
-screwdriver
-multimeter or voltmeter

Step 2: Assemble Test Leads

The purpose here is to add an extra wire to both the positive and negative battery leads. These extra wires—the test leads—will be accessible from the outside of the case, without the necessity of opening the battery compartment. While there's more than one way to do this, I chose a method that wouldn't require cutting the original leads. [My apologies for the poor photo. It was a bit too much of a closeup for my camera to handle.]
-Cut 1½” lengths of the black and red wires. These add a bit of extra length to the main battery leads.
-Cut 4½” lengths of the black and red wires. These are the test leads.
-Strip ¼” off the ends of the wires (8 ends in all).
-Set aside the red wires for the moment.
-Fit an end of both black wires into the tube portion of the male spade connector. You may well find that they won't both quite fit. If so, you'll need to reduce the diameter of the end of the long piece of black wire (you don’t need much current simply to read the battery voltage). To do this, snip off roughly half of the copper filaments down to the insulation with your wire cutters. Try to insert them again. If necessary, trim a few more of the filaments (always from the longer piece!) until both fit.
-Crimp the tube portion of the connector to finish the connection.
-Test by tugging the wires to be sure you have a good connection.
-Repeat these steps for the red wires.
-Insert the free end of the short black wire into a female spade connector, and crimp to complete the connection.
-Repeat for the short red wire.

Step 3: Test the Connections

-Cover the solar panel or turn it so it faces down. No point in getting a shock if you don't have to, and your reading will be more accurate.
-Open the fence charger battery compartment.
-Check the battery voltage with the multimeter. Note the reading.
-Remove the battery and set it aside.
-Connect the dual leads you constructed in the last step to the original battery leads, red to red, and black to black (or to black striped with red).
-Connect the battery to the female spade connectors (again, red to red and black to black).
-Test the battery voltage at the ends of the long test lead wires. An extra pair of hands will be very helpful here! Your reading should be the same (or within a few hundredths) as the earlier reading, or you've got a loose connection somewhere. Correct and try again.
-Remove the battery and set it aside.

Step 4: Install Leads

-Drill 2 holes into the bottom of your fence charger box, one for each wire. The holes need to be just big enough to allow the wire (with insulation) to pass through.
-Assuming your test leads (the longer ones) are still connected, thread them out through the holes, one to each. Keep in mind that you need the extra connectors and wiring to fit into the bit of extra space at the left side of the charger casing, out of the way of the battery.
-When you are sure the battery will fit back into its compartment, slip the butt connectors onto the protruding stripped ends of the test leads, one to each, and crimp them into place.
-Tug on the butt connectors to make sure they are secure.

Step 5: Close It Up

Reconnect the battery. Replace it in the battery box and close the compartment. Test the battery voltage at the lead ends once more, just to be sure. With the butt connectors to hold the multimeter probes in place, you'll find that the job is an easy one. I managed it one-handed (though only to be showing off. (Note, though, that the multimeter reading is a bit too high for a 6 volt battery.).