for someone that is afraid to turn the key and hit the button without asking permission from the volt meter.. maybee..
You don't need the volt gauge to tell you that you have a bad or week battery.. your starter will tell you. That's part of paying attention to your machine, same with the amp gauge.
If I hit the start button on my tractor and the starter is spinning 20% slower than normal, I don't need to look at some volt meter to tell me the battery is low... maybee some do I guess.
Once you are started, you can then look at your ammeter. if the charge tapers off... your battery was low but probably ok.. ( IE.. you used the lights for a project for 30 minutes after shutoff and the battery drained a bit ). On the other hand, if the charge doesn't taper off, now you know that the high continous charge rate, coupled with the slower starter.. that you very likely have a problem with the battery. That very special and highly prized volt meter won't tell you that.
Again.. tools like this do rely on the user having some basic understanding of the world around him. Mindless sheep who just hop on press buttons and go won't benefit much or any from an ammeter.. and the volt meter just makes them feel special.. because they have a volt meter.. and that's the right thing to have.. right?
besides.. that last volt meter I say at orilies.. the cheap one.. that lots of tractor owners probably reach for has 3 voltages listed. 8, 12 and 18 usually.. though the middle number I have seen from 12-15 depending on what chinese company made the gauge. the rest of the gauge is in, sometimes graded, sometimes non graded hash marks that may or may not add up to voltage increments you can figure out. The last one of these I bought vibrated so badly, because it was a cheap undampened meter.. that you really couldn't tell if the needle was on 10 or 14 volts while driving thru a pasture cutting grass.
I'm trying to figure out how a meter that's hard to figure out a voltage helps anyone... if you can't tell if you are 10 or 14.. then you may be discharging, or may be charging. Also.. keep in mind my original post.. volts matter a little, amps matter alot. you can take a battery and deplete it past the point it will turn a starter, and still read 12v on it, hit the key and have it drop right on down to near nothing. Volt meters are virtually no load.. so you could be driving along during the day on your diesel tractor that basically uses virtually no electricity.. your almighty golden rule the world volt meter tells you no problem.. my needle is somewhere around 12.. so you are happy, sun is shining, life is good, and then you turnt he tractor off by pulling the kill cable, and find that the battery has 12v on it al right.. with no load.. but turing ont he headlamps makes it drop to 4v and it won't try to start.
Your amp meter ont he other hand would have been swung negative into the discharge area telling you.. hey dummy driving the tractor.. your battery is draining, do something, help, help.. then you might have actually gotten back home or to the trailer instead of needing a fresh battery out in the middle of your thousand acers.
remember.. volt meter said.. fine.. I see 12v under no load.. you are good. ammeter is telling you.. hey.. the volt meter is an itiot.. you are draining your battery.. do something.
It's straight electrical fact.... A volt meter on a unloaded week battery will lie to you every time.. the ammter will shouw you that charge rate.. and it don't matter if you can read the numbers on the gauge or not. if the needle is on the right side of the gauge, you are good, as long as it's not way right, and staying there for an extended time.. if it's on the left, you are bad.
But yeah.. you have to understand what your gauges are telling you.. your voltage gauge tells you one thing... voltage... and that's basically useless... it's nothing you can't figure out when you turn the key and hit the button. the amp gauge tells you stuff while you can still do something about it..
The sad part is.. most people aparently can't read gauges, including oil gauges.. and idiot lamps are well named.