I've got 7 of those and keep one in each car. Also, get their cig lighter LED voltmeter, or if you really splurge a digital display one (Amazon $15).
In the old days, people did the opposite. They would run with headlights on in the daytime to "recharge the battery". At least that's what everyone said when you saw a car on the interstate with lights on. Probably had to do with voltage drops in the harness stimulating the Vreg, or maybe bogus.