Current Shunt?

Hey! Have any of you folks ever installed a current shunt between your negative battery cable and the battery so you could infer the amount of current being used by the car’s electrical system? I think I’d like to have an ammeter in the car, but I don’t know what kind of current the system uses on a regular basis, so I don’t know how big a shunt to use. I’m pretty sure 100A ones I keep finding places won’t be enough given that the battery is designed to provide upwards of 350A for starting. I did run across a 1500A shunt, but that seems massive overkill.

 

Really, I think it’d be cool if I could find a metering system actually designed for automotive applications rather than having to roll something myself. It’s been a long time since my electrical engineering courses in college and my spark-fu is no longer that strong. I just haven’t run across anything yet. Thought it was kind of weird that nobody seems to care to install an ammeter.

 

 

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I'm not going to claim to have looked that closely at the battery cables recently enough to be sure about this, but I think there's probably a way to put a meter in there without also being inline with the starter. You wouldn't be able to put it right between the battery post and the main cable lug, but as long as you're in parallel with the starter motor, you should be safe. I think you could put a meter downstream of the power lug on the alternator and be safe, but that would only show net current generated by the alternator, and wouldn't show you what's going to (or from) the battery.

 

The other option would be to look for some sort of an inductive DC ammeter, which would just go around the cable, rather than having to be inserted inline with it. Most of what I'm seeing on Amazon in the DIY realm looks to be 0-100A, but I'm not sure if that's 100A max reading, or if more than 100A draw will cook something. Here's one I found: https://www.amazon.com/bayite-Digital-Current-Voltage-Transformer/dp/B01DDQM6Z4

Current clamp meters generally only work on AC circuits. That was why I wanted to use a current shunt, which is basically a low resistance, but precision resistor between the load and the ground that has enough current carrying capacity to suffer the maximum amount the load will ever draw. Knowing the resistance of the shunt, you can measure the voltage drop across the shunt and calculate the current flowing through it based on that via Ohm’s Law, which you might remember from Physics in high school. You can get (or design with something like an Arduino) a meter that does the math for you and drives a display or a needle or something. I recently saw a friend of mine with one installed on the ground for the heavily modified Power Wheels truck he built for his kid.

 

Texas Instruments does sell a shunt rated to 1500A, but that seems like it’s massive overkill. Though, given that it’s basically just a large conductor, it might not even be very expensive. As such, overkill isn’t a big deal.

 

I don’t know if this is something I’ll ever actually do; it’s more something I’ve been kicking around in my head in the shower for the last few days. I *really* dig instrumentation in my cars. Like, maybe a little too much.

 

 

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Current clamp meters generally only work on AC circuits.

 

That's not true at all; look at the link cww516 provided. Hall effect DC ammeters are nothing new. The only real difference is that shunts tend to be more accurate, but I don't think you need precision accuracy in this application.

 

Here's one that does 400A with a more informative display...

 

https://www.amazon.com/AiLi-Battery-Voltmeter-Ammeter-Transformer/dp/B07D2X2JSH

I'll be honest, until I did a little searching on Amazon, I wasn't sure if DC amp-clamps existed, so I make no guarantees as to their accuracy. Slapping one of those over the negative cable just upstream of the battery post sure would be easier than trying to find the right place to put a shunt, though.

That's not true at all; look at the link cww516 provided. Hall effect DC ammeters are nothing new. The only real difference is that shunts tend to be more accurate, but I don't think you need precision accuracy in this application.

 

Here's one that does 400A with a more informative display...

 

https://www.amazon.com/AiLi-Battery-Voltmeter-Ammeter-Transformer/dp/B07D2X2JSH

 

 

Huh. Would ya look at that. I’ve never even heard about that before. Hell, if I stuck the negative battery cable through that, wrapped it all in that weatherproof electrical tape, then routed the sensing wires into the dash, I could mount that thing someplace and have a nice without having to screw around! It wouldn’t be able to drive a needle gauge like I was hoping for, but I can’t really complain for $40...

 

 

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