Shunt location

MarkJ

Full Member
I’ve been staring at the attached drawing for about 30 minutes and wondering about it. (The drawing is a crop from a schematic on the Victron site, but the principle applies to other makes).

My schematic was subtly different: I had one logical negative busbar (actually two physical components, joined - one for low current loads and one for high current stuff) with the shunt the ’last’ thing before earth. Victron have a negative busbar dedicated to the batteries and another busbar for all other negatives; these busbars are connected by the shunt (I think). So all the negatives for loads and chargers are direct to chassis earth, on the earth side of the shunt.

Why do it this way? Is it that you get a purer measure of battery current? For example if you have the charging current from the mppt going through the shunt I guess that would be confusing? Most schematics you see don’t show the negative wiring very clearly, but one I was looking at elsewhere had a mix of negatives : some directly connected to chassis earth, others to the non-earth side of the shunt. Confusing, or am I being dense?
58F3C8FA-A413-4590-ACF4-17F468F27AB6.jpeg
 

wildebus

Full Member
That diagram makes logical sense :)

The key thing is that to measure current accurately with a monitor and shunt, the only connection the battery bank -ve/0V should have to the outside work is via the shunt.
On the other side of the shunt, it does not matter if the -ve's are on a common bus-bar, on a chassis ground point or anywhere else, as long as they are not on the battery side of the shunt. In practice, chances are you would have a mix of a bus bar and chassis grounds, just like with the vehicle wiring.

FWIW, this is my setup ....
1578089580637.png

Three Batteries in parallel (like the Victron example), connecting to a common busbar pre-shunt. Then I have a cable from that busbar to the shunt, and that one connection is the only ground connection from the battery bank -ve to the outside world. The other side of the shunt has a cable that goes to a ground bolt and then to another -ve busbar and a chassis point.
 
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MarkJ

Full Member
Right, so when you see schematics that have some negatives connected on the battery side, they are wrong - if there is a shunt involved. You often see people say things like “I took the negative right back to the battery”. I guess that’s fine so long as they are not using a shunt.

Or if you retrofit a shunt you might need to do a bit of extra rewiring, I suppose.
 

wildebus

Full Member
Right, so when you see schematics that have some negatives connected on the battery side, they are wrong - if there is a shunt involved. You often see people say things like “I took the negative right back to the battery”. I guess that’s fine so long as they are not using a shunt.
It is ok to do that IF they are not measuring power usage. But 'ok' is a relative term. It can get very messy adding loads of connections to a battery terminal and can actually lead to connection issues with terminals working loose.
Case in point is my own van and the Starter battery setup I inheritated.

LT-Battery-12V
by David, on Flickr
This is the +ve of course, but the same principle applies - a right mess!

This is an example of a much neater way to do this IMO and much easier to manage going forwards for both +ve and -ve connections ....

Horsebox Fusebox
by David, on Flickr
Here there are multiple power connections, but all have their own individual fused +ve connection and corresponding -ve connection, with a single +ve cable and -ve cable to the battery bank.

Or if you retrofit a shunt you might need to do a bit of extra rewiring, I suppose.
Should not be that awkward as just a matter of moving the -ve Battery post connection to the LOAD stud on the Shunt and then adding in an extra cable from the -ve Battery post to the BAT stud on the shunt.
Some Battery Monitors also come with a Hall Sensor instead of a physical shunt and with those you just push the cables through the sensor loop and connect as before so nothing extra to fit (more common for AC applications, but are used for DC as well).
 
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