Why do automotive batteries measure more than 12 Volts? I think most newer ones measure closer to 12.8 V. Why??
Thanks.
To begin with a cell produces a certain voltage dependent on the chemical reaction. I think my battery has 8 cells, so each one must be producing about 1.5 volts, but if it is actually 1.6, then you get 12.8 volts. Second, as the battery discharges (and also as it ages), its internal resistance rises and the voltage drops. It also drops in very cold weather. So the 12 volts is only nominal.
Actually, I recall that so-called 6 volt vacuum tubes were actually supposed to be heated with 6.3 volts. Same thing; the standard cells were a bit over 1.5 V and 4 of them produced 6.3 when new. When the voltage fell below that, the tubes lost a little gain and if it was above, their life was shortened. I don’t think either happens with transistors until the voltage disparity is extreme. I once took a 240 V radio made for Europe, put an adaptor on the plug and plugged it into 110 (which actually averages about 117) and it worked fine. Didn’t even notice low volume. I still use it that way.
Few batteries have exactly the nonimal rated voltage at full charge. Part of that is rounding error. A fully charged lead acid cell has greater than 2v but that’s a nice round number to remember. All batteries have decreased voltage as they discharge and lead-acid has a failry steep slope. Fudging with the numbers is a way to make sure that the battery can deliver rated voltage even after a certain amount of discharge.
12 volt automotive batteries have six cells rated at 2.1 volts each. The standard rest voltage for the battery is 12.6. This rest voltage will vary minimally depending on the condition of the battery and charging system, but usually not more than one tenth of a volt for the battery as a whole.
The “twelve volt” nomenclature represents a nominal rating, and is not meant to be precise.
Why these batteries are made from 2.1v cells rather than 2.0 or 2.2v cells, I don’t know.
The voltage of the cell is fixed by the chemical reaction going on inside. The reaction inside a lead-acid battery produces about 2.1 volts, and there’s nothing they can do about that short of choosing different chemicals for the reaction.
This is why lead-acid produces about 2.1 volts per cell, while nickle-cadnium produces about 1.2 per cell. It’s just what the internal chemistry generates.
It is also not unusual for batteries which have just been fast-chaarged to show a voltage up to 15% higher than the nominal voltage. This is more prevalent with some chemistries than others.
Battery chemistry determines voltage, it isn’t adjustable.
that is a nominal voltage. Tha battery is of no exact voltage of any kind and would have a curve showing rates of charge/discharge at different stages of charge. A battery which is taking a charge will be closer to 14 than to 12 volts. A battery which is discharging to turn the starting motor may go down to 8 volts. 12 volt is nominal because it is easier than saying “a battery composed of six lead-acid cells”.
To clarify a bit, I was referring to rest voltage of a fully-charged battery that has had time to stabilize and dissipate surface-charge after charging. As others have pointed out, the actual voltage during use or charging can vary considerably.