My New Favorite Multimeter, and Measuring Inverter and Charging Efficiency

A little while back, I posted a short writeup on a plug-in ammeter I’ve used to measure current on circuits with standard ATO blade-style fuses.  When running through trying to find out which circuits have loads on them, it’s still as easy as it gets, especially when you don’t have good access to the wiring.

Before I go any further, let me point out that I have a degree in electrical engineering.  If you’ve done any reading at all on here, you know that I mess with electrical stuff more than any sane person should.  My “needs” in the tool department are more extensive than most, but I’m going to talk today about a tool that’s affordable and useful enough that every RVer should carry one.

What is that tool?  A multimeter, but not just any multimeter.  There’s a lot out there to choose from, varying in price from very cheap (~$10) to very expensive (>$500).  If you walk in to your local hardware store, Sears, or Wal-Mart even, you’ll be able to lay your hands on a digital multimeter that will do quite a bit for around $10.  Something that will look kind of like this (pictures link to Amazon listings):

Cheap $10 Multimeter

It’ll read AC and DC voltage, DC current, resistance, and a few other things.  But I know that dial and the three connections can be a scary sight.  You have to set the right scale for whatever you’re measuring, and you have to know which hole the red lead goes into (if you pick up the meter to measure voltage and are connected to the 10A current spot, bad things will happen).  They do work just fine for a lot of things though–in a pinch, they’re quite useful, but require a little more care than some of the others I’ll talk about.

I strongly suggest getting an auto-ranging multimeter though.  It’s simpler to use, with a lot fewer settings on the dial to do the same measurements.  If you go to Lowes or Home Depot, you can spend less than $50 and get one like this:

Extech MN16A Auto-Ranging Multimeter

Now, instead of selecting a range for voltage measurements, the meter will do it for you.  This one will also measure frequency and capacitance–the former useful in checking out your inverter, the latter lets you check the capacitors on your air conditioners.  With a type-K thermocouple instead of the test leads, you can measure temperature, and when testing for continuity, it has an audible feedback so both hands can be used to hold (and watch) test leads.

Ok, so that’s better.  But that only lets us measure current where we can wire the leads inline with the circuit, only up to 10A, and for a maximum of 30 seconds (these specs are almost universal in this type of meter).  We know that we have RVs with 50A hookups, DC circuits with lots of fuses bigger than 10A, and that we don’t always want to be unwiring stuff to connect the meter inline.

The type of meter that makes that a lot easier is a clamp on meter.  Most of the ones you’ll come across will only measure AC current, not DC current.  That’s still useful, and I’ve had one like that for quite a while.  Clamp-on meters that could measure DC current were usually cost prohibitive–take this Fluke meter at nearly $300.  Is is a really nice meter? Sure.  Is it overkill for most RVers? Yep.

That’s not what I want either, even as often as I use one.  It’ll eventually get left out in the rain, loaned out, lost, etc.  Every crook knows what Fluke’s yellow color means in terms of value.  I don’t need a NIST calibration certificate, and it’ll be pretty rare that I need to measure more than 100A.

So instead of any of the above meters, what would I recommend?  This guy:

Uni-T UT210E Clamp-On True RMS AC DC Meter with Capacitance Tester

For less than $40 (click on picture for Amazon listing), this meter does essentially everything that the previously discussed meters will do.  It’s true RMS, which means you can get an accurate reading when you’re using a modified sine wave inverter.  Lesser meters will just measure peak voltage and divide by the square root of two to estimate RMS voltage (which is what the 120V measurement is–peak voltage is normally 170V).  It’ll measure AC and DC current with the clamp, along with AC and DC voltage, resistance, capacitance, diodes, and even has a built-in non-contact voltage sensor.  What’s that?  It’s a mode where the meter will beep faster as it’s moved closer to a live wire–a good confirmation check as you get ready to start to work on something.

Red lead with protective sleeve on, black with it removed.
Red lead with protective sleeve on, black with it removed.

I also like that the leads come with a removable plastic sleeve, such that only the very tip is exposed with it on.  When I’m reaching in to measure cell voltages on lithium batteries, for example, I would normally have to be extra careful to make sure my leads didn’t touch each other.  Like the Fluke meter, the display has a backlight, so I can read it in all of those RV nooks and crannies that would otherwise shadow my meter.

With just this one tool, I can learn a lot about the efficiency of my charging setup and inverter.  This post is already quite long, so I’m going to just preview for now what will be a more in-depth discussion in a few days.

Measuring current into the batteries, mostly charged, I can see what it takes to essentially “float” the batteries at a given voltage.  This is only about 37mA per kW of usable battery capacity, or about 23W for the full Volt bank.  On the old 12V house batteries, that was more like 60W, and keep in mind that was about 1/4 the capacity.  Big difference.

Measuring DC current into the inverter and AC current out, I can also measure voltages and figure out an effective inverter efficiency.  With a few things on, like a couple of TVs, computer, Xbox, kitchen lights (which are AC), and a number of little items I don’t really keep track of, the inverter was putting out about 2.17A at 120V, or about 260W.  On the DC side, at 48V, it was taking in about 6A or 288W.  If we divide the output wattage by the input wattage, we can figure the inverter’s efficiency, which comes out to 90%.  Now, after I ran around turning stuff off, the input wattage didn’t really change much–I’m not really much above the idle consumption for the inverter.

The charger draws about 175W with almost no load, and at full load is about 80% efficient–actually better than I expected.  But how does that compare to my old 12V setup?  The 12V charger generally maintained about a 400W draw when I wasn’t using anything on the DC system.  Remember it’s putting in about 5 amps just to maintain the batteries at their current state of charge.  I haven’t taken the measurements yet, but I’m willing to bet that charging a fully discharged Volt battery is quite a bit more efficient than similarly charging a lead acid battery. Don’t worry though–I’ll do the test and collect the data!

Next time, when I talk more in-depth about system efficiencies, I’ll also include pictures and video of the measurement process.  Whether your intentions are to check a campground pedestal, figure out what’s causing a battery to go dead, or something more sophisticated, I’m going to present a series of troubleshooting walk-throughs to help a non-EE safely figure out the basics of what’s going on before deciding whether to hire some help or buy your neighbor some beer.

Next: Line Splitter to go with Clamp-On Meter