Question about grounding an ammeter (1 Viewer)

[JCruse]

That should be "Questions about WIRING an ammeter", not just grounding.

I got an onboard multimeter with shunt for measuring amps consumed from the Aux battery. My question is where to put the shunt. Some of the juice from that battery comes off an aux fuse box (fridge, fan, phone charging) and the rest from a fused 1 awg line run from the aux battery to the audio amp in the rear, but all of these things are grounded in place without lines run back to the battery.

So would I have to run all new grounds back to the battery? Or can I place the shunt between the fender and the aux battery? And if so would that confuse the reading with things wired to the starter battery too?

 

[Cruiserdrew]

One way or the other you need to capture all of the current, but hot leg or ground leg should not matter. Most shunts get installed on the ground leg, because generally there fewer connections.

 

[JCruse]

I didn't know that. If you look below you'll see that there's a lot of electronics connected to this shunt, including a BT transmitter, so I'm a bit leery of not following instructions and hooking 12V to it. But what about going from the fender? Would the body act as a ground return and only pass through what came from this one battery?
I've been measuring voltages and wiring things into cars for a long time but I guess amperage was never an issue before, so I don't know enough about it. I know that when electrons run out of the + they run in the - , and vice-versa. But does the one battery only receive the little guys that ran out of it's own + even in a dual battery system?
My head hurts now.








The shunt in question:

It's connected wirelessly to this thingy:

 

[ozcrusier]

I would not trust the ground from any bolt on panels as age will play a big part in there condition so best run a heavy ground wire back to the battery.

 

[e9999]

The electrons flow from the - to the + electrodes if the battery is discharging.
One difference between putting the shunt on the ground side lead vs the hot one is that the absolute voltages will be lower, which may be safer.
Regardless of where you put it, the shunt will only measure current (voltage drop actually) in the leads in or out of the device itself, not through other paths, so if there is leakage to ground someplace else you will not measure the total current coming out of the battery, say, only the current going through that particular wire.

 

[JCruse]

So if I put the shunt between the auxiliary battery negative post and a body ground the ammeter will show me current used by all loads fed by that battery and grounded to the body? And question #2: It will ignore all loads connected to the starter battery?

I think I'm finally starting to get a handle on this

 

[e9999]

1: only if that's the only lead between the negative of the aux battery and the ground and if it's not connected to the other battery in parallel.
2: yes if the batteries are not connected together in parallel, not necessarily if they are

I think.... some real EE check this...

 

[george_tlc]

Think of your wires as pipes carrying water (electrons). Put the flow meter (shunt) where you want to measure the flow, it'll only measure what is flowing through the particular pipe that you put your flow meter on.

Draw pictures of your battery and your pipes. Should clear things up...

cheers,
george.

 

[JCruse]

So I would have to lift the block ground on that battery AND the Neg lead that's connecting the two batteries. I don't know if it's wise to do both of those things.
Looks like I may have to run some grounds back to the battery. It's the heavy gauge lead that's the PITA. There's just no room in the channels for another 1awg line, and I'd really hate to have a big fat cable running across the floor after so much effort to keep wiring neat.

Think of your wires as pipes carrying water (electrons). Put the flow meter (shunt) where you want to measure the flow, it'll only measure what is flowing through the particular pipe that you put your flow meter on.

Draw pictures of your battery and your pipes. Should clear things up...

cheers,
george.

Well yeah but since there are two batteries and both "ground" to the body, I didn't/don't know if they "talked" to each other, if electron flow out of one would flow in to the other - screwing up my readings.

Again this is all about - do I HAVE TO run multiple grounds back to the Aux battery in order to watch my amp hours, or can I just use the body as my "pipe"?

Capiche?


I thought this would be a simple thing to google up an answer to, but it's just not out there. However, though I of it a while back, I think e9999 has got it.




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[george_tlc]

Not sure why you think you need to run grounds back to the battery from all your stuff. But then I'm not sure why you want to be measuring current flow from you aux battery anyway (all the time)... I'd measure voltage as that gives a pretty good indication of state of charge and way easier to do without any shunt mucking around.

If you are married to the shunt/current measuring approach, then:

I'd say the easiest place for you to put the shunt is on the +12V lug on the aux battery that feeds all your 'stuff'. i.e. take a 1g (or whatever you have as you 'big' lead) from the battery +ve to your shunt. Then 1g from the shunt to your distribution point where all your 'stuff' gets the aux battery +12V. Hopefully you have a big fuse/breaker somewhere on that 1g cable to protect the wiring in case of shorts...

Keep the grounding simple. One heavy ground to the block and a thinner ground to the body (at the PS fender - there's a threaded hole already there just like there is on the DS - mimic what Toyota did for the main battery).

Connect your +12 charging power directly to the aux +12V lug. Now your shunt will only show current flowing from the aux battery +12V lug to your stuff. Obviously that current is coming from the aux battery when not charging and coming from aux battery/charging source when charging/engine running. For charging/engine running that aux battery load better be less than the (alternator output - running loads), e.g. EFI, headlights etc etc).

cheers,
george.

 

[Bogo]

Personally I'd put a shunt on both battery banks. That way you know what is going in and out of each of them. If only one shunt, then the house battery.

For those that don't know, the frame ground isn't that reliable. Running ground wires back to the battery is a good idea. When you have two or more batteries, linking their grounds is a good idea. Also all heavy loads must have a ground back to the battery.

 

[TrickyT]

Also keep in mind that your shunt has a maximum current rating that should not be exceeded. If it's like the picture you show in post #3 then it's probably somewhere around 20A-50A. Exceed that and you'll burn up the shunt.

The other way to measure DC current is with a Hall effect current sensor, then there's no shunt involved. That's what GM uses in it's vehicles. Then you just run all the wires from, say, the negative battery post through the round hole in the sensor and the sensor figures out how much current is flowing by measuring the associated magnetic field. There's no danger of burning up the sensor this way since there is no resistive shunt that is subject to overheating. Maybe someone has adapted a GM sensor for what you're trying to do. The sensors cost ~$15, but you'll need additional electronics since they output a PWM waveform intended for input to one of the vehicle's ECUs.

The Hall effect sensor is also the technology used in clamp-on DC ammeters like this UNI-T UT210E:

 

[JCruse]

The shunt in the online pic is probably as you say but they sell a range. The one I selected is 100A because I knew I'd never pull more than that out of my house battery. 30 for the stereo, and only at full blast, 3 for the fridge, 2 for the fan, maybe 5 for LED lights, and phone charging.

The screen device in the cab is linked to the shunt by Bluetooth and monitors voltage, current, power, charge and discharge capacity in amp hours. I particularly like knowing exactly what was taken out of the battery, and then put back in. It's a pretty cool little gadget. And besides, it's 28 degrees out, what else am I going to do..?

After discussing this with everyone I think I'm going to do what I was trying to avoid... I'm gonna run all small load grounds back to the aux fuse box that feeds them, run the heavy line back to a post I'll put next to it, and install the shunt there.

Thanks everyone.





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