Ammeter

Marion, I presume you are wanting to measure the current draw on your auxilary batteries by fridges and accessories. I run two auxilary batteries in my Nissan Patrol, one under the bonnet and the other in the back. Both 90 A/H AGM's. I have made a power management system for them incorperating an ammeter. The only thing you have to remember is that for an ammetre to be of any value, all current being drawn by your accessories, must pass through the ammetre. This means you have to wire everything back centrally if you are like me and run two fridges, a computer, gps, phone and camera chargers, 12 volt lights etc. It can mean a bit of work. Also if you intend running an inverter through an analogue guage, you may need to install a shunt to reduce the ampage to the guage.

There are various types of guages depending on how clever you are and how much you want to spend. The best one I saw was at Carac RV supplies and is the type they put in vans - $300 plus. I opted for an analogue guage up to 15 amps so I doin't need to install a shunt. $14 at Jaycar. I'm no auto electrician so I hope my description is OK.

Cheers Mick

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If all you want to do is check current occasionaly get a clamp meter. DC Clamp Meter

If you want to monitor battery condition get a voltmeter and switch between batteries. Much simpler to wire.

Marion it depends what you want to measure and why.

For example do you want to measure the current draw form the AUX Battery? If so what is the maximum charge rate and maximum discharge rate.

Different scenarios will mean different things. Eg if you have the winch off the second battery then you might need a 400A ammeter which will be useless for anything else. And if you take the winch off a seperate lead you can get a smaller ammeter that makes sense for the accessories but wont take into account the winch.

The list of combinations are endless. If you give more info then it may be possibel to give advice. A good voltmeter, while not perfect may be the best bet.

Hi Marion

A meter will only tell you what is going on at that point in time. If you are serious about knowing the state of charge and how much you use then you really need to look at amps over time - or Amp hours.

You can get devices that read current through a shunt and log it over time telling you how many Ah you put into the battery on charge and how much you took out in using accessories.

As to wiring everything centrally, all ammeters need a shunt and that shunt need not be central in the vehicle - in fact there are good reasons why that's a really bad idea (long cable runs, unnecessary voltage drop etc.)

Moving coil meters and other devices measuring current read millivolts generally and will specify the number of millivolts required to get Full Scale Deflection or FSD. Typically on a panel meter as seen on one other photo here that's 50mV.

A 50mv FSD ammeter required to read say 20 amps maximum would need a shunt that drops 50mV across it when 20 Amps is passed through it. The meter reads the 0-50mV and displays a corresponding reading between 0 & 20 Amps.

You can mount the shunt under the bonnet near the battery and run all your accessories as you would normally - you don't really need one on the main battery as traditionally you would only have the vehicles normal electrics and maybe the winch running off that anyway.

Then you can run wiring from the shunt read out terminals to where ever you put your meter or logger.

If I was going to this much trouble I'd install a device that logs the information and gives a historical record of the past few days. An instantaneous reading is nearly useless apart from indicating imediate information which as has been suggested could be gained with a clamp meter. Low current flowing into a battery that should be charging doesn't necessarily tell you that it's nearly charged. It could also be nearly stuffed.

If you do decide to install an analog meter go for one that has a centre zero and is capable of showing current both in and out of the battery.

Just my 2 amps worth.

Dave

I prefer a digital voltmeter (with 2 decimal places) with measurement taken from either 1st battery pos or aux battery pos via an ON/OFF/ON switch.

The difference in the voltage readings is proportional to the current going into the aux battery. When the difference is zero, the aux battery has fully recharged. When the difference is great (say 0.5V in my setup) I know the aux battery is down and there is a heap of current recharging the second battery.

IMO, its a lot simpler than using ammeters and shunts and one gauge is giving you all the practical info you need.