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Discussion Starter #1
There are a number of threads on battery meters and looking at the state of batteries.

How would I rig up a meter to measure current usage off the batteries? Like the units you get from electricity suppliers?

This would help understand drain on batteries with stereo/lights other components plugged in.

Yes, it may not be necessary but something of a boy project for the winter.

Please excuse lack of search - on mobilephone.
 

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The only way is to install an in line ammeter to physically measure the current being used. The clamp types issued by the electric companies only work on ac voltage, and even then are not that accurate.
 

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The only way is to install an in line ammeter to physically measure the current being used. The clamp types issued by the electric companies only work on ac voltage, and even then are not that accurate.
You can get DC clamp ones for 20, 200 and 2000A that work on Hall Effect...Fluke make them for their multimeters, but don't expect much change from an arm and a leg!
 

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An 'amp clamp' is the device you are talking about. They are basically a current transformer wired to a meter.
These units are find if you know the output which the item you are measuring should be producing (with relation to the calibration of the unit), but the numbers will be meaningless unless you calibrate the unit to the system you are measuring and re calibrate every time you add or remove an electrical device.
This is the problem with current transformers, the in and out values must be known by the designers for the ammeter measurements to have any relevance to the actual system current being used.

A really simple way of doing this would be to put an ammeter in line with the main power line you want to measure and have the display on the dash or something.
 

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Depends on the maximum expected amperage you are going to be measuring.
If this will pick up the charging current, that can be up to 80A, which would likely damage this meter.
I would PM some of the electrical gurus on here, as my theory is not good enough to advise you any further.
 

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Discussion Starter #9
Thanks. Will do.

I think the issue with in line would cause headaches with wiring as I would need the ammeter to be located with the battery prior to any wiring branching off.

The battery is under the bonnet and was thinking of ammeter under passenger seat. Would need too much heavy wiring running everywhere. That said, does this item install online with the meter at a remote location so the measuring is not co located with the unit, rather done where the shunt is installed?

Questions questions I know!
 

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the shunt is wired between the battery negative and earth return,the ammeter is connected to either side of the shunt,thereby measuring the draw across the shunt not actually carrying the current itself,so the ammeter only needs thin wires to it as it is only showing a reading...upspex will hopefully confirm this is correct
 

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An alternative to a 'proper' calibrated shunt ammeter is to measure the voltage drop from one end to the other of the main battery cable. It can be either the + or - terminal cable, it doesn't matter too much, just connect a volt meter (probably on millivolt range) with one lead connected to one end of the cable, and the other at the other. No need to disconnect anything on the van.

Current (I) = voltage drop on meter (V) divided by cable resistance (R). The tricky bit is estimating R, but it can be done by knowing the length of the cable, its cross-sectional area and the bulk resistivity of copper. Look it all up on Wiki, or send me the numbers and I can work it out. Your main problem might be that to measure low power things like stereos/lights/etc you will most likely need a very sensistive volt meter. Fine for measuring glow plug, starting current, etc.

Rob. (Electronic engineer)
 

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i would put the shunt in the neagative cable, ironically if you have a shunt you dont want it shorting to the metalwork as they dont look very impact protected to my eyes
 

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Discussion Starter #14
That's where I get confused as realise shunt to be in the earth return but if measuring total current draw from all appliances do they need an earth cable running back through the shunt as the rear amp for example is locally earthed.

Thanks for all the help.

Upspex - that looks perfect but being me, I'd rather spend less.
 

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An alternative to a 'proper' calibrated shunt ammeter is to measure the voltage drop from one end to the other of the main battery cable.
I've done a similar thing to this.
The problem with any in-line shunt system is that your adding a little bit more voltage drop into your charging system which will in turn slightly reduce the maximum attainable charge rate.
The way above is making use of the resistance already in your charge system, so in using it to make the measurement your aren't impeding it's performance.

I did mine a dirty way (not much calculation needed) but it's close enough me, and maybe more accurate as it is measuring instead of calculating.. it involves using 2 DMMs.

I pulled the main fuses to my leisure batteries and also the rear charging fuse, I then bridged the terminals of the charging fuse with one DMM set on the 10 amp current mode.
I then knocked up a wire to allow me to trigger the split charge relay while the engine is off - that gives you stable running voltage without needing to run the engine (and wait for the starter batteries voltage to stabilise).
Then I turned all the lights & TV on in the rear of my van and managed to set up an 8 amp constant drain.
Finally I then measure the voltage drop across my charging cable run.
After this I calculated that (in my case) each 0.1 volts drop is equivalent to 2.6 amps of charge current - not great but good enough for me).

So that takes care of charge current.
Usage current can be done the same way but TBH I'd just fit an ebay current meter as power used from the LB's isn't particularly affected by a small amount of extra voltage drop of an in-line shunt.

Rich (also electronics engineerT:)
 

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Usage current can be done the same way but TBH I'd just fit an ebay current meter as power used from the LB's isn't particularly affected by a small amount of extra voltage drop of an in-line shunt.
I agree - if it's only leisure batt current that you're interested in, a cheapo ebay current meter (inserted into the supply to the stereo/lights/etc, i.e. nowhere near alternator or starter!) will be fine. I think the original post was probably after state-of-charge, rather than instantaneous current. In which case, batt voltage is probably a better thing to measure. I use a CBE battery meter with four coloured LEDs to show state of charge. Works fine and a doddle to install... I:
 

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Discussion Starter #17
Almost - I have a couple of battery meters and will be using them for state of charge.

This was an idea to look at the real time current draw from the LB in order to know both what uses more juice (I know I could work this out by looking at the individual appliances) and at what times I am using most and why.

Therefore if you find yourself low then you could avoid the luxury items.

I don't really foresee a time when it will be an issue as we like to be hooked up and always have been quite mobile when camping so the van is never laid up in one spot for a long time - I just like fancy things in the van! This really was a bit of a boy project and the ebay meters could suffice but it's the where to put it for an effective reading that is the headache.

This is my set up as of once-I've-found-time-over-the-winter to get it all in place:



I can't see where I'd put an ammeter for the right reading.
 

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Just be aware that any ammeter that you put in line with the battery earth will need to be capable of handling up to 90 amps. Anything less and you will fry it when the alternator starts to charge a seriously discharged battery.
 

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Discussion Starter #20
Many Thanks

I may give this a go then so the shunt would need to run off the negative side of the leisure battery and that should give a reasonable indication of current draw at any point in time?

Noted regarding the requirements of the ammeter when put in this location.

Thanks again all

Dave
 
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