Waeco current draw question?

[atcmatt]

Hi guys,

Anyone here that is electrically minded or familiar with waecos care to share their opinion.

So I have a brand new 82ah AC Delco deep cycle battery and I have an old cdf35 Waeco and a brand new cfx50 (well impressive the new cfx). I also have a matson 20amp dc to dc charger (don't ever buy anything matson, this one is about to go in for its second warranty replacement)

I charged up my battery last night to full capacity at about 6pm. It was at 12.7v I then connected both the cfx50 and the cdf35 to see what sort of run time I would get. Both units were still happily buzzing away at 6am this morning at 2degrees respectively, so about 12 hours. Battery was still on 12.3v. When I got home from work this afternoon the battery was at 11.7v, the little Waeco was still buzzing away at 2degress but the cfx50 had shut itself off, must have been off for some hours as it was warm inside the unit.

Now my question is, I had both the units set to low battery protection. 10.2v for the 35 and 10.1 for the 50. The battery still had 11.7 volts, which I know is quite flat. Why would of this happened? You would think with that voltage both fridges would still be going? Maybe to much current draw with both of the compressors on?

Anyone able to shed some light, sorry for the long winded post but wanted to explain it all.

I think my battery is definitely too small for this application. I intend to get a 110ah one and then will link in the 80ah for when I go camping.

Thanks.

 

[andrew0]

At my guess the combined load of both would have dropped the battery voltage to low.

What is the cut voltage? Maybe grab a go pro or similar and put it on a time laps.

 

[75 GRC]

Watching with interest.
I'm sure OldTony will come along and see this and help out!

I believe it's going to come down to the draw of your fridge verses the amp hours available (from your current battery)


TONY!!!!
Help !!!

 

[atcmatt]

Yep was waiting for old tony to chime in! I'm feeling like the combined current draw (would have to be a maximum of 8amps, surely) for both fridges at full hilt must be getting that battery real low which in turn cuts the larger Waeco off. I just don't see why the small Waeco stays running? Really doing my head in here!

Thanks for the replies.

 

[abouttogobush]

Depends on the cycle of both fridges that will vary with the load and the temp they are working in. You say you charged your battery fully to 12.7v, that would be a resting voltage after 30mins. A healthy wet deep cycle needs to be charged to at least 14.4 to be fully charged, agm a tad more.

Ideally you should measure the draw over a 12 hour period, if you are getting the 12.7 at full charge then the battery is not fully charged correctly or the means of charging is inadequate. That battery is small given the load upon it and it may be at the end of it's useful life.

 

[Old.Tony]

It's not just the combined draw, it's the thickness of the conductor between the battery and the CFX50 combined with what is probably an over-sensitive voltage detection circuit in the CFX50.

Waeco do have one flaw that irks me rather badly and when it bites, it bites so bad you lose your food. When the detected voltage on the cable is high enough, the fridge circuit sees that all is good and brings the compressor on. Under load (on a light cable) the voltage will drop below the threshold even if the battery is still reading a decent voltage. The compressor will shut down, the fridge will go into 'pause' mode for about 30 seconds, and as you'd expect, with no load on the cable, the voltage rises back to acceptable levels. The cycle repeats until the battery truly is dead flat.

There are two handy tricks here. First, don't measure the voltage at the battery, measure it at the fridge. Do it under load and when it's not, so you can see the difference. If the difference is large enough, replace the cable with something heavier (and that's the second trick).

With 12V systems, you don't just want a cable that won't act like a fuse when getting the load you're installing it for. If the circuit is sensitive to voltage (like a fridge) then going several times above in cable size is the way to do it.

Now with all that out of the way, if that Waeco's circuit board was made in 2011-2012 it may have one of the 'bad' components on its board (it was a ceramic fuse that had a large voltage drop across it, fooling its own circuit). Waeco were replacing this fuse undr warranty.

 

[atcmatt]

Cheers for the replies. Makes sense. I tested it under no load Tony and it's not losing any voltage through the cable, will try it under load this afternoon. Any ideas how to tell when the fridge was made?

Matt.

 

[Old.Tony]

Apart from the labelling or notices fixed to it ... you can get a rough idea by looking at the chips on the circuit board, they will have a 4 digit number like 2614 which means it was manufactured in week 26 of 2014. You know the fridge was supplied some time after that.

 

[atcmatt]

Ok no worries. May have to have a look at it over the weekend. Think I'm going to be better off running a 110 ah battery. Got another quick one for you Tony while I've got your attention.

With my dc to dc charger all connected up as it should be. If I connect a 7amp 240volt charger to my auxiliary battery, will it short circuit my dc charger and cause it to fail? I think that I have blown my DC charger due to this. I guess in hindsight I should really charge from my cranking battery with it all hooked up as its meant to be.

Thanks Mate.

 

[Old.Tony]

That question can be "will 15V on the output of my DC-DC charger cause any problems to my DC-DC charger".

Difficult to answer without testing the chargers themselves. It might be a problem but as usual there are ways around it.

Get your hands on a double-throw (single pole) relay - underneath it will have 5 pins, labelled 30, 85, 86, 87 and 87a. Now we're going to work a little magic.

Splice in to the positive line coming from the battery and feeding power to the DC-DC charger. Don't cut it, just add to it. Connect a wire (can be thin, won't take a large load) from this to pin 85 of the relay. Connect pin 86 of the relay to the negative line that's coming in. Now, when the car is turned on, this relay will turn on and off.

Now to hook up the power - what we're about to do looks like it's done backwards (and it is) but it's very effective. Take the positive line going to the aux battery and connect it to pin 30 instead of either charger. From now on, this is where the battery will get its charge power from.

Grab the output power of the DC-DC charger and connect it to pin 87 of the relay. Pin 87 is only ever active when the relay is turned on. Connect the 240V charger's positive to pin 87a. You can connect all of the earths (black leads) together, no drama at all, power won't flow at all unless there's a complete circuit on both sides.

Job done. If you are connected to 240V and turn on the ignition, the relay will disconnect the 240V charger from the battery and connect the DC-DC charger - and vice versa. No chance of any damage now!

I recommend using a relay socket in case the relay blows. Makes changing the relays much easier. I've had some bad experiences with cheap Chinese relays (they melt drawing just under 40A through an 80A relay, go figure).

 

[atcmatt]

What a whiz! Thanks Tony you definatley know your stuff!!