ReVIVE PowerUP DV2 Dual USB Car Charger w/ Voltmeter Gauge $10 at Amazon $10 $15 Amazon with 2nd Street Electronics has the ReVIVE PowerUP DV2 3.1A Dual USB Car Charger USB Diagnostic/Voltmeter Gauge with Battery & Charging System Monitor LED Display for $14.99 - $4.95 off with code USBDV233 at checkout = $10.04 with free shipping on $25+ or with Prime. $10 retail: $15 Report Posted by Sharik12/12/2017 at 10:10 AM PT
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https://www.ebay.com/itm/12V-24-Dual-USB-Port-Car-Cigarette-Lighter-Socket-Plug-LED-Voltmeter-Waterproof/142593188763
Either way, keep in mind that some vehicle lighter outlets remain powered with the ignition off so there will be a few mA (guessing around 10mA) drain any time this is plugged in (just to power the voltmeter) unless you rewire the outlet.
Thank you!