Ammeter V Voltmeter

[Poida]

What is the prefered instrument to install, a ammeter or a voltmeter and why?

[Swimpy]

In my opinion it's the ammeter. It shows how hard your alternator is charging, and will instantly show an alternator, voltage reg., or belt failure. You can estimate how much charge is in battery by how many amps the system is charging.

A voltmeter only shows how many volts in the battery.

Terry

[Verytricky]

hmmmmn.

I always assume my equipment works, so a volt meter whould show my 'energy reserve' and indicate any problems by dropping. When the engine is running 9v = alternator not working, 14 volts says it is.

When at engine off, it allows me to see electrical drain.

[PAR]

I also prefer the volt meter, but will mount an amp meter and oil pressure, plus an ignition on light (usually tied to an hour meter) on the engine itself, so I don't have to keep looking back at the instrument panel, while working on an engine.

[Frosty]

It depends what you want to know. If you want to know if your charging , a light will tell you that. If you need to know how much you need an ammeter. If you need to know if the battery is charged then only a voltmeter will do that one . A digital voltmeter is far better to the point of a analouge voltmeter bieng pretty usless for judging 1/2 a volt which is all the difference between flat and charged.

If your starting from afresh, I would go digital volmeter and one of those battery management sytems that will tell you whats going in whats coming out and how much you have left.

On the other hand most intrumentation like depth sounders etc will give you a digital voltage-- if taken from a sound supply.

[Poida]

It would seem from my investigations and from the comments here that an ammeter is the way to go. A voltmeter does not tell you the condition of the battery. For example is one cell is working it will show 12 volts even if there is not enough current to start the engine.

This probably solves the problem I have with one battery, it shows the battery fully charged ie, 12 volts but won't start the engine.

If I install an ammeter it will show the available current.

Thanks for your help.

[Verytricky]

Originally Posted by Poida

If I install an ammeter it will show the available current.

Ummmn - no.

[longliner45]

ampmeter will be valuable ,,, it will tell you how many amps stored you have left in your battery banks,,for instance if you have 250 amp batterys,,and your running all bildge pumps and (bailing with 5 gallon buckets) and your down to 50 amps ,,,,,make sure you make radio contact with someone ,,longliner,,,,,been there done that.

[marshmat]

I would install both.
An ammeter tells you how much current is flowing in or out of the battery at that point in time.
A voltmeter tells you the voltage of the battery (duh). Depending what it's hooked to, it can tell you whether your alternator's working, whether the battery's running flat, etc.
Ideally a dedicated battery controller is the instrument of choice, being able to monitor all important parameters- including in some cases an estimate of the energy left in the battery, which neither an ammeter nor a voltmeter alone will tell you.

[Crag Cay]

If you need to know if the battery is charged then only a voltmeter will do that one . A digital voltmeter is far better to the point of a analouge voltmeter bieng pretty usless for judging 1/2 a volt which is all the difference between flat and charged.

This is so true. A digital voltmeter is essential, especially for liveabords who need to know the state of their batteries. 12.6V is fully charged, 12.2V is 50 percent discharged and the effective lower limit for a lead acid battery.

A hand held one will even tell you which cell is not working ina battery.

[luckettg]

Originally Posted by longliner45

ampmeter will be valuable ,,, it will tell you how many amps stored you have left in your battery banks,,for instance if you have 250 amp batterys,,and your running all bildge pumps and (bailing with 5 gallon buckets) and your down to 50 amps ,,,,,make sure you make radio contact with someone ,,longliner,,,,,been there done that.

I do not believe the ammeter will show you how many amps are stored in the batteries, but will only show what current is flowing through the circuit it is in. Voltage along with amperage does a better job of showing capacity, and I suggest using a watt meter for a better idea of power flow. This will give a better indication of system capacity and status.
Greg Luckett

[longliner45]

ok what gauge shows stored amps?

[stevel]

To judge the stored energy in the battery, the "gauge" would need to know the present voltage of the battery, the type of battery, the total storage capacity of the battery, and the "health" of the battery. Only something computerized could do that.

[marshmat]

Amps (short for amperes) are units of electric current- coulombs per second, to be precise. That's a measure of how much current is flowing through something, in our case the cable to the battery.

Volts are units of electric potential- the driving force that makes current flow. Think of this as the height of the water in a reservoir; more voltage is analogous to more water pressure.

Power is the amount of energy transferred per unit time. Units are watts (one watt = one joule of energy per second). Knowing voltage across a device, and current through it, P=I*V

No gauge can directly measure the amount of energy stored in a battery. With lead-acid cells, a hygrometer can tell you the condition of the battery, ie. what percentage of maximum capacity it's currently at. A sensitive voltmeter that is designed for measuring battery capacity can also provide an estimate of % capacity, if the battery's discharge curve is known (this is what many commercial battery monitor systems do). Or, if you're familiar with basic calculus, you can record current and voltage at regular time intervals, and integrate I*V dT (with respect to time) over the entire time interval since the battery was fully charged; the result is the energy you have drawn out of the battery in that time. Subtract this from the useable energy capacity of the battery to get the energy available. Or if you know the "amp-hour" rating of the battery, integrate I dT to get the amp-hours used. (amp-hours * battery voltage = energy capacity).

The calculations are easy to implement in any spreadsheet if you remember grade 12 math. But don't expect any one gauge to give you the answer on its own. If you want one gauge to say "I have X amount of energy left", you need a battery monitor system designed for the type of battery bank you use.

[luckettg]

That would work ok for a battery system that is not being recharged. Where is the math for the recharging?

Simply knowing what the capacity rating of the battery system is supposed to be, then subtracting the power used (Watts), and adding back in the charging capacity will give an approximate idea of the power stored...in a very crude way. For instance, batteries not being charged will draw down in a non-linear way, so the battery power curves would be needed to approximate the stored charge. Voltage is a good indicator of battery condition while the battery is either not being used or is being used, while amps being drawn will also show when capacity is dropping only while the battery is being drawn on. Toss in a charging system and all the voltage or amperage will tell is the the effect of the charging system, not the battery. I have had batteries with open cells on a running engine with the volt and ammeter showing good, until the engine was turned off. Restarting required a jump, then the engine system was up and running again, all lights working, and not any indication of battery problems.

Putting this another way using just a battery and no charging system in the circuit. The loads (lights, radios, etc.) will control the current flow being drawn off the battery. As the battery capacity drops, so will its voltage. This is why voltage is a good indicator of battery condition, and amps are not. Amps will stay constant until the battery voltage drops enough to allow the ammeter to indicate the loss of current due to reduced voltage at the battery.
Greg Luckett