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Portable Fridge Power

DaveInDenver

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I have a couple of fridges at the house so I've been doing some testing with them. Thought it would be an interesting thread to post results.
 

DaveInDenver

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Testing inrush on ARB 47L (P/N 10800472) 12V portable fridge/freezer

Current probe is Fluke Y8100
Oscilloscope is Tektronix TDS2014B

Scope setup as follows.
  • Channel 1 is a current probe, range is set to the 20A maximum value. Output is 100mV/A so full 2V signal would represent 20 amps. Scope configured as current probe so display is amps.
  • Channel 2 is the voltage at the battery terminals.
Current measurement location is taken at battery end of cable.

Figure 1 shows the initial power up inrush. The fridge is connected to the 12V source and the inrush occurs when holding the power button down for a few seconds, bringing up the display for the internal temperature. The peak current is 37.4 amps with no appreciable voltage drop on the battery. The inrush lasts 2.5ms and bounced. The compressor started immediately, presumably after the microprocessor stabilized.

F0004TEK.JPG

Figure 2 shows a relatively long timebase indication of the compressor starting normally as the controller cycles. At 100ms/div the current appears as a step with a peak of 11 amps concurrent with the compressor motor falling into normal operation. Motor looks to take about 60ms to start before falling into regular operation. There’s an initial plateau prior to the motor starting discussed on the next captures.

F0009TEK.JPG

Figure 3 zooms in to the inrush. You can start to see the peak uncontrolled current. The peak is significant but still obscured in the timebase. There’s a period of 180ms where approximately 3 amps is flowing before the compressor is started. The reason for this is unknown, perhaps capacitor pre-charging.

F0010TEK.JPG

Figure 4 shows the initial compressor inush. This peaks at 26.2A for approximately 2ms.

F0011TEK.JPG

Figures 5 and 6 are the compressor during normal run, which averages about 4.5 amps. Vertical scale is 2A/div thus each tick is 400mA. The ripple is therefore about 800mA. Figure 5 trace is using a 4 sample averaging to limit noise while Figure 6 is no averaging, no bandwidth limiting.

F0008TEK.JPG

F0007TEK.JPG
 
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DaveInDenver

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Testing inrush on Engel MT45 12V portable fridge/freezer

Current probe is Fluke Y8100
Oscilloscope is Tektronix TDS2014B

Scope setup as follows.

  • Channel 1 is a current probe, range is set to the 20A maximum value. Output is 100mV/A so full 2V signal would represent 20 amps. Scope was configured as voltage probe, so display is in volts.
  • Channel 2 is the voltage at the battery terminals.

Current measurement location is taken at battery end of cable.

Figure 1 shows voltage and current powering on the fridge. So there's an inrush that's probably a control circuit or inverter capacitors charging. It's 15 minor ticks so 3 divisions at 200mV/division. That 600mV peak represents a 6A initial inrush. It lasts about 3 time divisions at 25 milliseconds per division, so about 75ms before dropping back to nearly zero current drawn.

F0001TEK.JPG


Figure 2 shows inrush as the compressor starts. That shows a peak of 1.26V and lasts about one division plus a tick or two before settling to the running current. So that's an inrush of 12.6A for, let's say, 35ms.

F0002TEK.JPG


Figure 3 shows running voltage and current, mean is 273mV indicates 2.73A with a 47Hz ripple. I didn't have min/max measurement on but we can approximate just by sight that the ripple is varying between about 2.5A and 3A to arrive at that mean.

I haven't done anything in depth to understand the ripple. I'm running from a big battery so it's not a power supply artifact. Must be something to do with the fridge. Whether it's the compressor itself, a control circuit or inverter I can't really say. When I measure the sound 47Hz is peak audio frequency (42dB is the SPL at about a foot away, if anyone's interested) so I assume that's the compressor speed.

F0003TEK.JPG


Looks like there is significant inrush with the Engel fridges. The choice of a 15A fuse doesn't seem off. You could go down one step to a 10A but then you may have to be careful with the time-current game. The time is short enough that a fast acting probably will be fine since the inrushes all seem to be less than 100ms. You'd have to start worrying at about 1 second, at least with an ATC fast type.

Screen Shot 2023-09-13 at 3.00.57 PM.png
 
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DaveInDenver

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One take away comparing the ARB to the Engel is inrush profile. A lot of people complain about random shutdown or similar issues with their Danfoss fridges. I've often speculated that inrush would be a concern and this seems to support that assumption.

Why?

Let's say you power your fridge using factory wiring to a back outlet. That's probably rated for 10 A so let's assume Toyota ran a 16 AWG wire for this and it's 20 feet from the fuse block distribution point to the outlet. For the sake of argument assume that the chassis is the return so there's only appreciable resistance in the one hot wire.

A 20 feet length of 16 AWG is 83 mΩ. So if my measurements of inrush is correct, that 26.2 A will yield a 2.17 V drop. Voltage drop is resistance times current, e.g. 0.083 Ω x 26.2 A = 2.17 V.

Let's then say you set you low voltage cut-off to be 11.5 V on the fridge.

So running your alternator is sitting at 13.5 V. But once stopped it's more like 12.5V (or there about). So a 2.17 V drop means the fridge sees 12.5 V - 2.17 V = 10.33 V. That inrush is 2 ms, though, so it's very doubtful the low voltage protection is that fast.

Then the next plateau I saw was about 3 A for 180 ms. That drop is 249 mV. That's not much, 12.5 V - 0.249 V = 12.251 V at the fridge.

The next step was the what looks like the motor actually starting. I saw a 11 A peak for that. On that 16 AWG wire the drop is 911 mV, about 0.91 V, so your 12.5 V battery gets you 11.59 V at the fridge. That lasted for about 60 ms. So whether that's long enough to trip the under voltage protection seems unlikely but not all designers are going to do the same thing. I'd personally window the voltage to stretch out to seconds at least. Ideally you'd want to see some sampling and perhaps a value held across more than one cycle to develop a trend. But that all depends on how much horsepower their microprocessor has. I'd guess it's pretty minimal so holding more than a few samples over time is asking a lot.

Then it settled into running. I saw about 4.5 A for that. On the 16 AWG wire that's 373 mV of drop. On a fresh battery that's no problem, 12.5 V - 0.373 V = 12.13 V. But your low voltage trip is 11.5 V + 0.373 V = 11.87 V is the lowest your battery can reliably go in that case.
 
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DaveInDenver

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So to fix it people recommend bumping up and doing a proper run. Now in this case you go to 12 AWG but let's say you do a complete out and back run instead of using the chassis for the return. There's draw backs and benefits but doing the full run is generally more reliable and safer.

Now a 20 feet distance is actually a 40 foot loop, so 65.5 mΩ. Better, but might not solve it completely. If you just do a hot side run and use the chassis it would be closer to 32.75 mΩ. Next, some do a really good run of 8 AWG. That is 12.95 mΩ for the 20 foot run, 25.9 mΩ for a full out-and-back.

Let's say you did 12 AWG and the loop is 40 feet and it's running current that trips the low voltage. At 4.5 A your battery minimum is 11.5 V + (4.5 A x 0.0655 Ω) = 11.79 V. That's a pretty deep discharge that you're unlikely to hit all that often.

As a comparison you can see why you don't hear as many complaints about the Engel for this. The draw is quite a bit lower. Even running on a factory harness it's going to take a fairly deeply discharged battery to hit 11.5 V.
 
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Inukshuk

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Very nice deep dive.
Often once people go to the prong and screw type ARB plug (or another type that is not cig lighter style with spring loaded tip) the ARB shutoff issue goes away. Can that voltage drop be tested?
Overall, I am shocked by the 37.5 amp ARB 47qt inrush.
 

DaveInDenver

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Very nice deep dive.
Often once people go to the prong and screw type ARB plug (or another type that is not cig lighter style with spring loaded tip) the ARB shutoff issue goes away. Can that voltage drop be tested?
It can. I know Romer and Corbet were just talking about that so it would be an interesting test. I don't know if I've ever seen a spec for what it's supposed to be, though. It would take very sensitive test equipment, a DMM probably would not read it. My current probes would not, although on mA a DMM shunt may get a rough value of it.

For comparison Anderson does give a spec for a standard PowerPole, which for the smallest terminals (15A for 16 to 20 AWG wire) is 875 μΩ. Note the prefix is micro-Ohm so it's 0.875 mΩ. The voltage drop at 15 amps for this would be 13.125 mV (it's small, 0.013125 V)

I wouldn't think a properly functioning cigarette plug is actually going to be all that much worse to be honest. They're rated for 10 amps in most cases and that would imply they'd expect tens of mΩ otherwise they'd be melting all the time.

So if I was to hazard a guess the main problem is one of two.

First is "properly functioning" is a major assumption. Those things are prone to falling out from vibration and the springs losing their flex.

Second is corrosion. Legitimate PowerPoles are silver plated so fairly immune to most things. Cigarette plugs may be chrome, nickel (such as Blue Sea uses) or most anything shiny. Quality is going to vary I imagine.

Overall, I am shocked by the 37.5 amp ARB 47qt inrush.

Don't read too much into that. Inrush is a matter of resistance (and impedance, strictly speaking), current and time.

More important, though, is impedance. Impedance is resistance of a device that operates on alternating current. The total current that flows is a function of both DC and AC.

A motor winding (an inductor) has both resistance and impedance. So do electronic devices like capacitors and transistors. So the inrush on a circuit may have a very low initial DC resistance but once an electric or magnetic field builds up the impedance completely changes the amount of current flowing. Doesn't always mean the current goes down but usually that's the case.

So look at the time for that inrush. It's very short, 2/1000th of second. A fuse reacts in hundreds of milliseconds up to tens or hundreds of seconds, perhaps minutes or hours. For a fuse to trip in 2 millisecond would take probably thousands of amps. This is due to heating and heat is a function of current and time, exponentially.

Just to belabor the point. Think about your laptop power supply. When you plug it in after it's been sitting for a while you may get a small arc on the plug. That's inrush to the big caps in it. That doesn't hurt anything for the most part. That's basically what is happening with that ~37 amps when I turned on the fridge. All the "stuff" inside getting it's first power.

I was a little surprised by the ~26 amps when the compressor was going to turn on. The 11 amp surge when the motor starts I expected.

The Engel did that, too. It was lower but still significant. When you turn it on there's about 6 amps of inrush to the control circuit and each time the compressor starts it's about 12 amps of surge. If I was looking at this from my perspective as a designer I'd just say whomever did the Engel paid more attention to cap charging or perhaps mitigating FET inrush.

I don't want to suggest it's not important, cumulative or immediate damage can occur with any inrush. But it's also not surprising. Anecdotally, you know how some people say their whatever is left running forever while some people who turn them off seem to get less life? This is partially why. Turning on a TV or computer or fridge, or indeed starting your engine, is by far the most damaging time for it.
 
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