Over the past couple of years we've had trouble keeping batteries charged at competitions. Part of the problem has been knowing which batteries are charged and which need charging- this has been solved in part by using color-coded tags, but if there's no tag on the battery, we have to dig a multimeter out of the electrical box to determine the charge. Here's how we made a compact voltmeter that goes connects right to the Anderson connector on the battery.
Mini Volt Meter (this is the one we purchased) - any 2 wire model that will run off the same supply as it is testing will work.
Sugru (one of our sponsors) - adhesive we used to affix the meter to the Anderson connector.
Begin by seeing how you will place the voltmeter on the connector. Our meter had a couple plastic tabs on the bottom that made it more bulky, so we cut them off. When you place the connector, make sure the wires are long enough to be attached to the terminals.
Next, attach the voltmeter to the connector with the Sugru (You could use hot glue or another adhesive, but Sugru is really awesome and you should go buy some.)
Now, if the wires are much too long for what we need, cut them down to size. Make sure they'll still be long enough to connect to the terminals.
Solder the wires to the terminals. The terminals are big chunks of metal, so if you have an adjustable temperature soldering iron, set it high.
Next fit the terminals into the Anderson connector, making sure that they click into place and won't pull out. It's important that you get the polarity right here, so triple-check that it works before you slot in the connectors.
Next, we want to give the wires some strain relief. Push any excess wire into the cavity at the back of the connector, then apply hot glue into the cavity. Lots of it. Make sure that the hot glue doesn't leak down into the front of the connector- we didn't have a problem with this, but it might be an issue for other styles of the connector.
Wait for the hot glue to cool, and you're done!