Installing an Ammeter in 12V circuit +ve or -ve?

Thread Starter


Joined May 29, 2011
I want to install an ammeter into the charging circuit of the system on my boat that charges a large bank of 12v batteries. I have 100Amp alternator & a Balmar "smart reg" that manages & delivers a high charge rate & sustains it for a longer period. The charge varies with the engine revs with full 100Amps being delivered at 1500 revs. However at some point the Reg cuts back the charge rate & at that point I can cut back the revs ( & the cost ) if I match Amp output to revs.
So I want to install a 100Amp Ammeter so I can manage this better.

My problem is that all the units on sale show the shunt being installed in the negative side - but on my boat I have so many wires going around ( I know little about electrics obviously ) I get confused every time I try to trace out where the negative lead from the batery goes & using a clampmeter it seems to read a lower level of amps than the positive lead that runs straight from the alternator to the battery.

My question:
After reading lots of comments on ammeters & circuits on this site, which note only that it must be installed into the circuit - can I just install the shunt & the meter into the positive side without blowing anything up?

All the directions that go with these if you buy them state "Must be installed in the -ve side of the circuit only.


Joined Nov 30, 2010
For people like us, this is usually simple. We would need to know more about the circuit to determine why the mfg would require this condition. Can you tell about whether your meter has electronics in it? Provide a drawing of its internals?


Joined Feb 5, 2010
The 'shunt' can be installed in either side of the circuit. The call for the negative side is a safety concern on the part of the manufacturer. By keeping the shunt at a 'ground' potential in relation to the metal around it, the chance of a dangerous short circuit is greatly reduced. The shunt is just a large piece of metal with a carefully crafted precision resistance. The meter merely reads the voltage dropped across this low precision resistance. It has to be a 'large' conductor due to the size of the current flow.

A skillfully cut plastic box can be made to cover the whole arrangement if you so choose. Leave small openings for air flow, since the large currents can cause significant heat build up which will change the resistance of the metal and affect the readings to some degree. The clamp on ammeter is better in the one respect, that it does not introduce any additional resistance in the charging circuit. The fact that it reads less than 100 amps is probably due to the fact that you are probably not actually delivering a full 100 amps to the battery from the alternator. The small resistance of the wires has a large effect(voltage drop) when delivering large amounts of current.

Read the wiki on current shunts for more in depth understanding.

Also be aware that should a short circuit occur which leads to fire, or other damage, your insurance will likely refuse to pay if the investigator discovers the shunt in the 'high' side of the circuit.


Joined Nov 30, 2010
There's a new one on me. Not being a boat person, I would not suspect that placing an amp meter on the positive side would be any kind of "dangerous".

I learn something every day on this site.


Joined Jul 7, 2009
Jaguarjoe's suggestion is a good one. But if it was me, I'd measure the resistance of an existing chunk of wire in the installation and use that for the shunt -- then just make two connections to each end of the wire and run these to a suitable analog meter (a center-zero meter). Then a small resistor would be needed to get the meter to read properly.

Total parts: the meter, a resistor, and some small wire.

The only slightly challenging part will be to measure the resistance of the wire accurately (I already have the equipment to do this, but you might not). If you want to go this route, lemme know and I'll make some suggestions.

Thread Starter


Joined May 29, 2011
Hey guys, thanks for the feedback.
Kermit, I've done a lot of reading & I think I have a general understanding of the basic idea of how a shunt & the meter work together. But your comment on the danger of install on the positive side & the insurance warning is exactly what I was looking for.

I also understand the idea of building something - but never held a soldering iron in my life & not sure I have enough yrs left to master that so I'll have to give that a miss- but thanks.

Jaguarjoe - I think the "hall effect" shunt substitute is a perfect answer. Not having to cut into the heavy cables & install a shunt deep in the engine bay is an absolute bonus & it looks like & can use a standard guage which don't cost much.

Just need to see if I can get one or two sent out or locate a local supplier.
Googling the "Hall effect" it doesn't look too hard to get one built here once you have the basic chip, by a local electronics service guy.

Low-cost & peace of mind solution.
Thanks heaps.

Laird Scooby

Joined May 31, 2011
Just read this thread with interest as i'm planning on building something similar to monitor the current flow from a self-regulating alternator to charge a battery that also provides power to lighting circuits etc.

My idea was to use an op amp with the -Ve and +Ve inputs connected to different ends of the battery cable then calibrate using a known load to drive a pair of LM3914 bargraph display drivers set up as a centre-zero meter.

I'm building a garage and there's no real possibility of mains power for lighting in there so the plan was to get a small engine (ex lawnmower or similar) to provide the charge to the battery to cover the lighting. The ammeter described above would be an indication for whether i need to increase the rpm of the engine or not, hence the LED bargraph displays - red for discharge and green for charge so i could see at a glance (and from a distance!) what the state of play is.

Might look into some of the suggestions on here first though.