Having been jealous of non-turbo cars having a voltmeter for years, I came across this cigarette lighter USB charger that also has a voltmeter function for about $5 delivered. In the photos you can see the voltage before starting at 12.5 V and after starting at 14.3 V. This indicates that the alternator is functioning to charge the battery. When you plug-in a device though it reverts to current draw in amps for that device being charged. It can output up to 3.1 A at 5 V. I usually leave it in the glove box as it might be pulling some current when not in use but it's handy to check the state of charge of the battery and that the alternator is functioning. When you get a dead cell in the battery it's going to be 10 V and crank very slowly. Here's a link to eBay for this item.
https://www.ebay.com/sch/i.html?_fro...ighter+charger