question

dhearne avatar image
dhearne asked

Smart Shunt Chassis Earth

I've purchased a Smart Shunt, and looking to install shortly.

However my AUX battery is in the engine bay, and the bulk of the loads will be in the rear of the vehicle. I therefore have two questions.

One, given I need to ground the AUX battery to the chassis for a common grounding between Start, DC charger and AUX. Do I earth this after the shunt? I believe I need to run the DC charger ground to the shunt to ensure it is captured, so AUX and DC would be sharing a grounding on the load side of the shunt?

Two, given I'm going to be measuring the voltage of the start battery also using the AUX plug, I understand the Neg terminal of the start battery also needs to connect to the shunt. Given the common grounding between both batteries and charger is this already achieved without wiring direct? Therefore related to common ground, if it is common could I then ground my DC loads in the rear of the vehicle to the chassis, or would I need to explicitly run a negative to the load?

Unusure how the shunt would determine if the load was for the AUX battery if it was a common ground?

I've attached what I believe are the two alternatives of my questions. Advice appreciated

common-ground.png

individual-grounding.png

SmartShuntGrounding
common-ground.png (58.2 KiB)
individual-grounding.png (58.9 KiB)
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1 Answer
Mike Dorsett avatar image
Mike Dorsett answered ·

As long as the shunt is in the -ve circuit for the AUX battery, as in the top image, it will measure the current flowing in the battery - which is want you want it to measure. The loads and charger can be connected to the chassis, and loads connected to the vehicle battery won't upset the measurement - their current won't flow through the shunt.

Currents in the chassis will be a combination of the two currents. To work out where currents flow, the rule is that all currents flowing to a node must sum to zero - the chassis is a 'node' currents from the Aux battery positive must therefore return via the chassis and shunt - currents from the alternator, starter battery etc won't flow through this loop.

Yes, the negative of the Start battery is connected to the chassis, and the voltage can be measured by the aux plug. No additional -ve wiring will be needed for this.

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dhearne avatar image dhearne commented ·
Thanks, I was really hoping that was the answer, it simplifies install significantly! I'll give it a go when it arrives.
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