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Thanks for that Rob. I have one of those meters for my PV total generation meter so despite the annoying bloke I found it very interesting.

It is a really nicely made meter and shows all sorts of useful stuff like power factor and per-phase current on the display.

cheers

Martin

Annoying indeed. What he failed to realise is the voltage inputs are there for when you're using the same basic meter with external current transformers - when you lift the link so that the c.t. isn't at line voltage. Nor did he know how to read a meter's spec. sheet!  Heigh-ho.

Pretty big phase errors on those CTs;  up around 5 degrees.  Although as he points out it's nice and linear so can be calibrated away.  That massive phase error seems to be down to the requirement that the CTs inside revenue meters have to be DC tolerant.  Anyone know why?   I wonder if the hackers found a way to saturate conventional CTs with some DC and use that to steal electricity?

Most likely the reason is people can and do (though shouldn't) connect half-wave rectifiers - I've seen it advocated in these forums as a means of reducing the power drawn by a water heater in order to better balance the PV generation (this was prior to Robin's Mk1 & Mk2).

So does that confuse lesser CTs?

Indeed - if it's got a standing current = flux = it's part-way up the B-H curve, it will hit saturation in the direction of the d.c. much earlier than it otherwise would, so the range over which it will measure accurately is reduced. Very much like biasing the Arduino a.c. input at 712 instead of 512 - you can only measure to 60% of what you could before.

I lasted 23 seconds.

So a DC-tolerant CT still measures the DC, but does so in a way that doesn't shift it up the B-H curve?

And speaking of DC, can I use the types of CTs everyone uses for energy monitoring in my car?  Can I wrap one around my battery lead for example and use it to determine if it's charging or discharging, and by how much?    Obviously subject to it being able to cope with the much higher amps.  Or would I need one of these DC-tolerant CTs to do that?

NO! How can a transformer - any transformer - pass d.c?  The primary current sets up a magnetic flux in the core, the output voltage is proportional to the rate of change of flux, so if the flux isn't changing, you get nothing out.  D.C tolerant means exactly what it says.

For your car, the best choice would probably be a Hall-effect device. Those can give an output when a direct current is being sensed.

Just go to eBay and type in car ammeter, they've been around years.

Just go to eBay and type in car ammeter, they've been around years. Moving magnet type no doubt. Rugged, not fantastically accurate but fit for purpose. An interesting project though to calculate (or do I mean estimate?) the energy in the battery taking starting current into account. No car ammeter measures that.

Yep, it's actually my auxiliary  (camping) battery I'm more interested in monitoring.  It never sees cranking amps, in fact probably never sees much more of a draw then about 10A (fridge, lights etc).   Currently, I guestimate it's remaining capacity by watching it's unloaded voltage:  12.6V full, 12V about 50% full etc.    I'm not sure watching amps in and amps out over time would produce a better result, but it'd be an interesting experiment.

There you are, then. Hall effect sensor - though if you're limited to 10 A a shunt followed by an op.amp with some gain (to reduce voltage you need to drop across the shunt, hence reduce the shunt resistance) might be better and cheaper, voltage input to cross-check against and determine "fully charged", maybe battery temperature as well and you could add a real-time clock to check the age of the battery (or maybe not! - this could get silly), an Arduino and a lot of experimentation and research to get the algorithm correct.

10A on the consumption side but the alternator can dump quite a bit more in when it first fires up, especially when the battery is low.   They are different wires, so I could potentially use different measuring technology on each, or maybe not even worry too much about the alternator feed.   When an alternator dumps 90A into a battery it all gets very non-linear anyway.  One day I might replace the alternator feed with a proper DC-DC smart battery charger.

Wh consumed (and Wh produced from the solar PV) since setting up camp might be more useful, in which case a 10A design is probably fine.