As technology advances, auto manufacturers have replaced dash gauges with "idiot lights". The problem with lights is that they come on once the failure happens (usually).

I have a 2012 Jeep with such lights and would like to add a voltage gauge. It should read voltages as follows:

1. When the key is in the accessory position, provide a voltage reading on the battery.

2. When starting the vehicle, provide a voltage reading to display the draw on the battery.

3. When running, provide a voltage reading on the alternator(charging).

I believe the first two readings can be taken across the battery posts. The third (charging) would require the use of a relay, wouldn't it?

Lyman

 

I get all those on my Olds.
Voltage, Oil pressure, distance to go on remaining fuel, low tire pressure, oil life, etc.
At switch on the battery voltage is displayed 11v-12v, as soon as you start the engine, the charging voltage is displayed (14.5v).
The voltage across the battery is the charging voltage once started.
Max.

 

For all three you can read the voltage across the battery (the alternator voltage is the same as the battery voltage, differing only by any small voltage drop in the wire connecting the two.
But you will likely need to disconnect the meter when the ignition is off to avoid discharging the battery, unless the meter takes a very low current. You can do this with an automotive relay controlled by the ignition switch.

If you want to see both the starting voltage, running voltage, and accessory voltage, then you will need to power the relay coil from both the normal ignition and accessory voltages through two diodes for isolation (one diode from each output to the relay coil).

 

For all three you can read the voltage across the battery (the alternator voltage is the same as the battery voltage, differing only by any small voltage drop in the wire connecting the two.
But you will likely need to disconnect the meter when the ignition is off to avoid discharging the battery, unless the meter takes a very low current.
).

A common trick for adding a standard meter for voltage in an auto, is to put a 9.1V zener in series with the multiplier resistor. That way the whole sweep of the full scale deflection only shows the voltage range of interest - and it won't discharge the battery lower than 9.1V if you leave it on.

 

A common trick for adding a standard meter for voltage in an auto, is to put a 9.1V zener in series with the multiplier resistor. That way the whole sweep of the full scale deflection only shows the voltage range of interest - and it won't discharge the battery lower than 9.1V if you leave it on.

9.1V is pretty well a dead 12V battery.

 

9.1V is pretty well a dead 12V battery.

I agree. 10v is barely enough to crank the motor.

 

I agree. 10v is barely enough to crank the motor.

According to the Yuasa Little red book of batteries - 10.8V is officially the end point voltage.

 

Typically, it's 10.8V under a 0.1C load. If your reading 10.8V with a DMM, you can rest assured it's well under 10.8V under load.

 

Typically, it's 10.8V under a 0.1C load. If your reading 10.8V with a DMM, you can rest assured it's well under 10.8V under load.

A charged car battery can typically drop to about half of the stated nominal voltage under cranking current, what's your point?

 

The point is at .1C load ... and it's not 0.1 CCA load, the voltage will drop from the 12.6V down to 12 or so before slowly decreasing. The DVM, with a typical 1 Megaohm impedance, draws NOTHING with respect to the capability of the automotive battery.

CCA, as I understand, is a 30 second standard.

 

A charged car battery can typically drop to about half of the stated nominal voltage under cranking current, what's your point?

It can, but if it drops under 9V, you have a nearly dead battery with high internal resistance. If it still starts your car ok, you are generally lucky.

 

A centre-zero ammeter, to measure current to and from the battery, would be of more use as an indicator of impending alternator or battery failure.

 

Hi

Cigar lighter plug-in? £4 on ebay with USB 5V out.......

 

Hi

Cigar lighter plug-in? £4 on ebay with USB 5V out.......

Sometimes they have those in the £1 store.

 

I would say the simplest tie in point for your volt meter would to find one of the circuits in the fuse panel that is always on when the key is in Acc Run and Start positions.

All vehicle shave at least one circuit that' always live when the key is in any position but off.

 

A charged car battery can typically drop to about half of the stated nominal voltage under cranking current, what's your point?

You must be using some really crappy batteries.

Our big diesel tractors have dual 800 CA batteries and the starters can pull up near 1000 amps during cold cranking and I have never seen them drop much below 11 volts at the battery terminals when testing them at load.

Too be honest with our tractors if the battery voltage goes below 10 volts at load they usually crank too slow to start.