The Red Cross division I am with recently purchased a new ambulance. It comes with a touchscreen display in the cockpit where we can control lights, sirens, check the temperature in the back, control airconditioning and some other stuff. Part of this display shows the voltage of both batteries in the car, both the regular car battery and the secondary battery that powers lights, sirens and all other things when the engine isn't running. This is an actual measurement as I can see this drop a little as time goes by when we draw on the battery without the engine running. With the engine running, I see both values go up to 14V, otherwise this starts at 13V and very gradually goes down to 12.7V

I'm actually curious if this measurement is useful in any way. We occasionally do long stretches of time where we are on duty and in our ambulance (things like music festivals, sporting events...). We have the radio on at times but need to remember to start the car every few hours and have it idle a bit to make sure we don't drain the battery completely. So naturally I'm curious if the little numbers on our display would warn us of impending drained-battery doom or not...

  • Does the display show independent battery operation? IOW, do you see voltages for both batteries or is it an average of the two? Something I'd check on if I were you is if the secondary battery is isolated from the primary battery. If your additional electrics pull from both, it could leave the your ambulance stranded (at least unable to start) if you were to run the additional stuff too long. Just a thought. – Pᴀᴜʟsᴛᴇʀ2 Sep 25 '15 at 11:53
  • Voltages for both, and isolated in the sense that the second can't draw from the first but we have a button we can use to bridge the second into the first. So if the first battery dies, we can use the charge in the second to hopefully start. It's been built by a company that does these sorts of things on a daily basis, but they didn't really explain all the doohickeys on the display:) – JDT Sep 26 '15 at 20:13
  • That's exactly what I'd look for in this type of system. I figured it would be like this, but thought I'd throw it out there for you. Glad to see some companies want to do things right ...whether they actually accomplish this goal is another story all together! – Pᴀᴜʟsᴛᴇʀ2 Sep 26 '15 at 20:48

The Voltage levels let you know how much charge is present in the battery thus warning you before the entire thing drains out. For example the below information is for the standard car battery which starts the car, works the steoreo etc , not the additional equipments like on the ambulance.

 12.66v . . .  100% Charge
 12.45v . . .  75%
 12.24v . . .  50%
 12.06v . . .  25%
 11.89v . . .  0% Charge

As you can see the display is basically telling you how much charge you have in your battery.

As stated by Mauro it also lets you know if the charging process is working as expected.

14v = normal alternator output.
Engine running and no change in voltage = insufficient charging.
Engine running + > 14.5v = potentially overcharging.
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    Also lets you know the charging circuit is working. Engine running and 14v = normal alternator output, Engine running and no change in voltage = insufficient charging, Engine running + > 14.5v = potentially overcharging – Mauro Sep 25 '15 at 10:36
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    I don't know where you got the above chart for % of charge, but it isn't right. Unless, of course, you are saying at 11.89v there is 0% usable charge left in the battery. If there is a voltage reading present, there is a charge in the battery. It may not be usable, but there is a charge. Also, as anecdotal evidence, I recently changed the battery out in my truck. It read at 10.1vdc right after running. While quite sluggish, the charge was enough to start the engine. Saying 11.89v is 0% charge would be incorrect from this standpoint. – Pᴀᴜʟsᴛᴇʀ2 Sep 25 '15 at 12:14
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    @Paulster2 I am pretty sure 10.1v will not last long. The figures which I mention are for reference purpose meaning its better to start diagnosing battery for fault if charge drops below 11V , The voltage output may vary with the age and condition of the battery so there is no concrete way of figuring out that , the values which i gave are arbitrary. – Shobin P Sep 25 '15 at 12:24
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    Thank you for clarifying ... "arbitrary" makes more sense, even to a head made of stone like mine :D – Pᴀᴜʟsᴛᴇʀ2 Sep 25 '15 at 12:30
  • @Paulster2 looks like these are open-circuit voltages. A lead-acid battery that reads 11.9V open-circuit does indeed have less than 1% of its original energy in it, so you might as well call it 0%. As soon as you apply a load like a car starter to it, it will be reading something more like 1V :) – hobbs Sep 25 '15 at 20:21

There are three different ways to measure electrical stuff and they are all inter-related. Volts, Amps and Watts. Watts = Volts * Amps They are the equivalent of pressure, flow and work. Watts is how much work is done, Amps is how much current is drawn and volts is how hard the current can be pushed.

Knowing the volts on its own doesn't tell you much, but knowing the volts and the context can tell you a lot. With the engine off, things are simple. The voltage tells you how much 'pressure' is available from the battery. The more things you turn on, the more current they draw, the less push that is left. They longer they stay on, the more they discharge the battery and again, the less push that is left. If the volts get too low, you might not be able to do something you need to do (such as start the engine).

A second thing to remember is the current (Amps) is directly related to heat, more Amps means more heat and too much heat damages stuff. But... the lower the voltage the more amps that are needed to do the same job. Say you have a fully charged battery with 13.6Volts (12Volt batteries are never 12Volts unless they are in trouble). To turn your headlights on (two 55watt bulbs) needs 110Watts / 13.6 volts = 8amps. Leave them until until your battery drains down to 10Volts, and now you are using 11Amps to produce the same light.

When the engine is running, the alternator is providing voltage to charge the battery and run everything else. Typically it will be about 14.4 Volts (which reduces the current needed and hence the heat) but there is a limit to the current the alternator can produce. Again, the more things you turn on, the more current they will draw and the lower the voltage will become. If the volts drop below about 13Volts with the engine running, then you are no longer charging your battery. If they drop below 12.5 Volts, then you are actually draining your battery and if left too long, it will go flat.

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