Of course there are cases where alternator can charge poorly and the actual battery voltage is 13.2-13.6V or can overcharge (more than 15V) but it happens quite rare. If your alternator fails and stops charging you will read values of about 11-11.5V or less. If you see anything over 12V-12.5V it charges and in most cases it charges OK. The main purpose of such a meter is to know does you alternator charge or not. I don't know the exact product, but most chinese meters may have difference of about +/-0.3V. Second - you can't be sure of this meter's accuracy. Any load (like stereo, GPS, dashboard illumination, etc.) drops a bit the voltage you will read on the cigarette lighter jack. There are ground losses thorough car's chassis, losses of "positive" as it goes through couple of relays, fuses and wiring. Sidenote, I use one in my car due to alternator issues and it's as accurate as my free harbor freight multimeter.įirst of all the voltage at cigarette lighter jack is always somewhat less than actual voltage on battery terminals. For the most part, this may mean it's off by a fraction of a volt. But the lower the quality of the item, the more likely that the resistor is not a very precise, and the Analog to Digital converter in the microcontroller they used will not be very accurate. It will have a resistor divider to bring the voltage down to the microcontroller voltage level. As mentioned, 13.7 is a typical voltage for a car under load while the alternator is running.Īs to the accuracy though, keep in mind that the trinket you got is a cheaply made mass produced item. In any case, aside from the negligible voltage drop across the fuse and wiring, is an accurate measuring point for the voltage in the car. Sometimes it is connected through the main switched power relay. A car cigarette lighter or 12V power outlet in a car is, like most everything else, connected directly to the battery positive/alternator output, through a fuse (typically 10 Amp).
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