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12v fridge won't start with AC200 without secondary load?

bhosty

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Mar 30, 2021
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I have a weird issue with my bluetti AC200. If I plug my Dometic CRX50 12v fridge into the cigarette port (and have tried the 25amp aviation socket too) with nothing else using power the fridge lights come on but the compressor fails to start, and the red error light flashes once, indicating low voltage.

I've checked the voltage on the cables and its 13.4v.

If I then add 10-15w of drain to the dc outputs by turning on some lights for instance, the fridge compressor starts perfectly and no error lights show.

I've even tried plugging the same cable into my AC50S and the fridge runs fine off that (with nothing else powered by the AC50S).

I'm sure when I first got the AC200 several months ago this wasn't an issue, but I suspect that back then I had a couple of 12v parasitic drains in my system (usb socket always powered on, diesel heater display powered on etc) - I've since put switches on all of them to try to conserve power.

Could it be that the AC200 takes a split second or so to spring into life when a high drain is placed on it, and thats enough to prevent the fridge starting? With a load already on the system perhaps it's already ready to go...? Its very odd

Thanks
 
You can't see the 'compressor startup' Voltage Drop on most meters, because it lasts for only about a millisecond.

(The rest of this is all SWAG.) It does sound like the AC200 is somehow leaving the 12v sockets disconnected from the full current capability of the internal LFP battery, until after the socket Voltage has been "dragged down" by some load. It's really weird that they'd bother to build such a thing into the AC200, but your consistently good operation after adding a 15w load seems to be pretty conclusive evidence that they did.

The Danfoss compressor requires a minimum of 10.5V across that startup, and the OFF-->ON power circuit switc in the AC200 is reacting too slowly, or waiting for an even lower voltage drop.

If that is the case, then maybe you could a big "startup" capacitor downstream of AC200 and upstream of the Dometic and Danfoss. Older Danfoss control boards did not have "soft start" capacitors built into them, although they've changed that very recently. Your Dometic, even if fairly new, might contain with an older controller module. (Or maybe even a non-Danfoss, custom-procured module, I'm not familiar with the CRX-50. I own a very old CRX-1110s, and has no such 'load sensing' switch on the way to the battery terminals.) You could also add "secondary" smaller battery, serving as the capacitor, but that's much more expensive than a large Capacitor. You don't have to size the Startup Capacitor carefully, because the Danfoss already has start-versus-run circuitry on its control board. It just needs to be large enough to hold the Voltage above 10.5V during the Compressor Motor start.

You'd plug in the cord first, wait for the capacitor inrush to activate the "power is being consumed, and switch on the fridge. Between the Voltage protection provided by the Capacitor and the fact that the "weird switch" will turn on immediately after startup, I SWAG that you should have no further issues. Your other alternative is to dig into the AC200 and bypass any such "switching" on the 12v ports, but that would void your warranty.
 
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Thanks for the detailed reply.

I have just tried wiring this voltage regulator but it has made no difference, perhaps its not suitable for the task. https://www.amazon.co.uk/gp/product/B08KY3PJK8/ref=ppx_yo_dt_b_asin_title_o02_s00?ie=UTF8&psc=1

Could you advise/link to a suitable capacitor possibly?

For now I've got the fridge plugged in to the cigarette outlet on my ac50s, and have the cigarette output from the ac200 plugged into the input on the back of the ac50s, so the solar should keep everything charged up - not pretty though. The rest of the van (water pumps, lights, chinese diesel heater) are all powered from a fuse box attached to the 25amp aviation socket on the ac200.
 
Here's a bit more SWAG for you: This is not a normal "start capacitor" application, because we can simply try to hold up the 13.4 Volts "input" Voltage to a value above about 11.0 Volts, for the period of time it takes the AC200 to react (to the dropping Voltage, and to start applying the 13.4 Volts again). The Voltage restoration would both refill the capacitor and help to finish driving the Danfoss compressor start.

The Voltage decay curve of a capacitor is given by the formula (voltage decay): V = Vo * e raised to -(t/RC). If I assume that the Danfoss motor start time is longer than the "t" delay of the AC200 to react to the problem, than I may use the AC200 delay as the governing time factor "t". The resistance of the Danfoss will vary, and I have very little idea what to choose for that value. The natural log of 11 volts /13.4 volts is about -.198 (this is "-t/RC"). T might be as short as .03 seconds, but it might also be a lot longer - up to maybe 0.1 seconds. If I use the full 0.1 seconds as a worst-case for compressor motor startup, then I derive RC =

(We need to solve for C, given both t and r, with minimum "V" = 11.0, Vo = 13.4, and some time delay before the AC200 realizes that there is a problem.) I can do no better than simply assume both factors. T might be as short as .03 seconds, but it might also be a lot longer - up to maybe 0.1 seconds? The natural log of 11 volts /13.4 volts is about -.198 (this is "-t/RC"). RC (the time constant for this circuit) would need to be raised to about 0.5 seconds, in order to keep Voltage above 82% of the initial charge (instead of letting it degrade all the way to only 63% of initial charge).

We know that the Danfoss BD-35 pulls up to 6A when starting up, so the resistance of the controller board AND the downstream compressor must not be exceeding about 2.2 Ohms. .5 seconds / 2.2 ohms = .28 Farads.
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So: To make this work, you should get maybe a 4700uF capacitor (that's a standard and widely available size), and link it through a pretty big resistor (maybe 4000 ohms, ceramic, capable of 5 watts) connected to the ground wire of the fridge cord. When you plug in the cord, the capacitor slowly charges up, with only the 10K resistor path to ground causing it to store energy. After you have the capacitor charged (just a couple seconds, I think) then you turn on the CRX fridge. It should start OK for as long as it remains plugged in.

It will leak about .05 watts of power through the resistor, whenever it is plugged in. But even if you need to lower the resistor size, to make it charge more quickly, it still looks like a winner in comparison to the 10-15 watts which you spend in "assuring" that all of your CRX compressor starts are successful.
 
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