Reddit reviews RioRand Waterproof 12 V Blue LED Digital Car/Auto Voltmeter Motorcycle Battery Monitor

With an older bike like that, who knows if any manufacturers rated amperage output is going to be accurate anymore. This is what I did on my 95 CB1000 when I got it all wired up.

The first thing you're going to want to do, before you start adding any electronics at all, is make sure that your rectifier/regulator is doing what it's supposed to. The best way to test that is use a voltmeter to measure the voltage across the battery terminals when the bike is off. Ideally, you should see a voltage greater than or equal to 12.6 volts. Next, start the bike, and measure voltage at the terminals again. You should see a voltage somewhere right around 14 volts plus or minus a couple tenths of a volt. If your "running" voltage is lower than this, you may want to look at the health of your rectifier/regulator.

At this point, if everything looks good, you can start adding on to the bike! To start, you will need a "sense" wire to tell when the bike is running, a fused connection back to the battery terminals so that if something fails, your bike doesn't catch on fire, and a way to distribute the power to multiple devices. There are a bunch of great aftermarket devices that do all this in one package, but they are generally a bit expensive. If you make something yourself, please make sure you do lots of research so you don't burn your bike down. Hell, you should probably do research anyways.

Just make sure that whatever you are adding, you're not causing the running voltage of the bike to drop too far below 14, which would signify the regulator isn't able to keep you fully supplied. Even if everything looks good at first, extended riding may expose failures or weakness as circuits heat up, heatsinks saturate, and old, soft wiring insulation decides it doesn't want to insulate quite so well anymore. Just make sure that you check your system voltage every once in a while, and you should be fine. You could probably mount something like this on your handlebars if you're particularly paranoid

Happy farkling, and feel free to ask questions!