Originally Posted by JABS
My point is that a fully charged battery is 12.6 to 12.8 volts, an almost discharged battery is 12.3.
A 0 to 12 v voltmeter isn't going to tell you a lot.
As mentioned earlier, it will tell you is it is being charged, but doesn't tell you whether the battery is accepting that charge.
If you want to know what is in your battery, you need a more sophisticated piece of kit!
On the contrary an accurate voltmeter can tell you almost everything you need to know whilst underway! for example while you state the voltages for fully charged battery are correct, what if your voltmeter reads 12.5 volts when underway, well you know your charging system isnt working, likewise if it goes over 14.8 its likely overcharging, also while at rest you can use it as an indication of wether the engine will start after leaving lights or GPS on. also in conjunction with a hydrometer for flooded wet cells can be a good indication of a failing cell before it becomes a problem,
sure its a small voltage change but thats why voltmeters for batteries tend to have expanded scales (analogue ones) that start reading at 12 volts and FSD is 15 volts, with the green band indicating a healthy alternator between 13.5-14.4 volts. For a deep cycle battery the on load voltage change between charged and flat is 25%.
also theyre good for filling up holes in the console where you used to have a useless yamaha trim gauge