Equus 3721 Battery and Charging System Monitor
This is a voltmeter that you can plug into your car's 12V outlet. I keep this in my glove compartment and use it from time to time to read the voltage while driving. Unfortunately, it does not have an on off switch, so leaving it in is not an option (unless it's garaged, or something). It's already saved me once, when my alternator went, I simply plugged it in an watched my voltage drop as I drove home. On another occasion, it helped me diagnose the bad voltmeter in my car's dash on another car (a standard 2" aftermarket gauge that I had moved from car to car over a period of 28 years that finally went bad but was intermittent for some time before I replaced it). It's also good to use a a diagnostic tool if you want to help somebody else out.
Besides not having an on off switch, the other thing I don't like about it is the LED's on it are probably best ignored, since it's normal to have lower voltage at idle, especially with accessories on so I know my alternator is fine even the the LED's indicate otherwise. I'd rather not have them.
This voltmeter gets good reviews on Amazon, but if you don't happen to be ordering something else at Amazon anytime soon, some WalMarts have them (that is where I got mine).
Amazon currently shows
$12.95 & eligible for FREE Super Saver Shipping on orders over $25.
It's a few bucks more at WalMart in the store, and it's online price is $17