If you've got a second battery in your 4WD, and you want to keep an eye on the voltage you can't go past the Projecta Dual Battery Monitor.
We had one installed in our 80 Series Land Cruiser for several years, and really rated it. I've just had another one installed in the back of the Dmax Canopy, to keep an eye on both batteries.
They are pretty simple, and they work well. There's not much else to say about them!
You hook the wires up to each battery, and it will display the voltages of each battery. Wire them up correctly though, or you'll end up with your battery voltage displaying the wrong battery first.
There are a couple of options when it comes to the display; you can have it only showing one battery, or showing both. There are alarms that can be set for different voltages, which beeps very annoyingly if they go outside of the limits, and it just uses a basic toggle switch to swap between modes.
What voltages do you want to see?
Most deep cycle batteries are full around the 14.3 to 14.5 volts, but it is manufacturer dependent, so look this up. Your cranking battery should get up to around the 14.2 - 14.4 volts while driving, if you have a vehicle that does't have a smart alternator. When stopped, it should sit around 12.6 to 12.8 volts.
I don't like letting my secondary, or deep cycle batteries get below 12.1 volts, for reasons which I go into below.
These days, you can get some pretty fancy little gauges. Some will read the power going in, power going out and then tell you how much longer your situation will run for. Victron, Redarc, Enervolt and several other companies make some amazing products.
On the other end of the scale, you can get cheap gauges off eBay that plug into cigarette lighter ports, and will tell you the voltage of wherever the power is coming from.
Why the Projecta unit?
For me, I've gone with an electrical setup that is overkill for our needs, and all I really care about is the battery volts staying over 12.1V (which is around 50 percent stage of charge). Of course, it takes some time for batteries to settle, but with our system I very rarely see the battery drop below 12.5 volts, even with the fridge running. Sure, sometimes it'd be nice to see exactly what the system is doing, but it comes at a cost that I really don't think is worth it.
What are they worth?
You can pick one of these up off eBay for about $100. I'm sure when I put the one in our cruiser they were about $140 or more, and this one was supplied as part of the labour and parts a local Auto Electrician did on our Dmax, and I don't have the break down of prices.
At the end of the day, I'm happy with it. They are a solid unit, they do what they are supposed to and I'd get one again.