measuring duty cycles of 12v fridges

Submitted: Friday, Feb 16, 2007 at 22:14
ThreadID: 42396 Views:6631 Replies:6 FollowUps:11
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For anyone pondering how to measure the amps consumed running a fridge over a given period, I have worked out a simple solution. I simply installed a 12v analogue clock in parallel with the compressor of the fridge. If the clock is set at 12, it then acts as an hour counter showing the amount of time the compressor has been running. The duty cycle does not matter as the clock is only powered up when the compressor is on and stops when the compressor is of. This means that at any time the runtime of the compressor can be seen by looking at the clock. It is then just a matter of multiplying the runtime by the power consumption of the fridge typically approx 4 - 5 amps to calculate the power drawn from your battery. I have also placed a switch on the clock so that I can turn it off when running the fridge on 240v or car alternator thus preserving the battery only runtime. Cost was minimal, in my case an old vdo 12v clock, switch and wiring. The clock draws negligible current, in my case less than a computer fan.
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Reply By: mrbasilbrush - Friday, Feb 16, 2007 at 22:28

Friday, Feb 16, 2007 at 22:28
Awsome Idea.
It is also good to have a built in digital voltage meter so you can check your voltage at a glance.
AnswerID: 222203

Follow Up By: khavers - Friday, Feb 16, 2007 at 22:37

Friday, Feb 16, 2007 at 22:37
thanks mrbasilbrush. I searched the net for a meter to measure amps used with out much success and decided this was a simple and cheaper option
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Follow Up By: mrbasilbrush - Friday, Feb 16, 2007 at 23:13

Friday, Feb 16, 2007 at 23:13
This site might be what your looking for.
cheers.
www.fridge-and-solar.net/specials.htm
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Reply By: V8 Troopie - Friday, Feb 16, 2007 at 23:13

Friday, Feb 16, 2007 at 23:13
Interesting idea. There are, of course, little digital hour meters available to do the same job. I have one permanently installed with the fridge on my trailer sailer and also fitted a reset button. I can either see the running time over 24 hours by resetting it every day or for longer periods. The fridge is powered via solar battery recharging only. The digital hour meter also was inexpensive but, perhaps, not as minimal as an old clock would be.
Klaus
AnswerID: 222217

Follow Up By: Member - Brian A (NSW) - Saturday, Feb 17, 2007 at 15:22

Saturday, Feb 17, 2007 at 15:22
Klaus,

that sounds like a great idea. Where did you purchase the digital hour meter from?

Thanks in advance,
Brian
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Follow Up By: V8 Troopie - Saturday, Feb 17, 2007 at 21:28

Saturday, Feb 17, 2007 at 21:28
Brian, I got my meter from RS components, a large electronic dealer with outlets in each capital city.
The meter has a part No. 185-6107 and costs $36.80 + GST if applicable.
You need to mount it in a little plastic box or in a panel cutout.
They were quite a bit cheaper when I bought mine a few years ago.
Klaus
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Follow Up By: Member - Brian A (NSW) - Saturday, Feb 17, 2007 at 21:32

Saturday, Feb 17, 2007 at 21:32
thanks heaps for that Klaus.

I'll check-out 'tricky dicky' tomorrow.

I spoke to Jaycar today, but they appear not to have anything similar.

Brian
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Follow Up By: Member - Brian A (NSW) - Saturday, Feb 17, 2007 at 21:33

Saturday, Feb 17, 2007 at 21:33
Doh!

that should have read - "I'll check out RS"

Brian
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Follow Up By: khavers - Saturday, Feb 17, 2007 at 22:22

Saturday, Feb 17, 2007 at 22:22
Hi Klaus and Brian. this sounds promising. I went to the RS website but couldn't download the details. Klaus how have you got this wired up to your frig, in parallel with the compressor so it only counts when the fridge is running? How did you wire it to reset. I wonder if these are also available through dick smith as there isn't a RS where I live
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Follow Up By: Member - Geordie C (WA) - Saturday, Feb 17, 2007 at 22:57

Saturday, Feb 17, 2007 at 22:57
Great I might have a look at the hourmeter.

I have got a small neat digital volt meter in my dash with a switch to check each of my batteries. You can get them with no power supply wire needed, so there is no risk of cross wiring from an isolated battery and it only consumes a tiny amount when switched on.

Geordie
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Follow Up By: V8 Troopie - Sunday, Feb 18, 2007 at 00:44

Sunday, Feb 18, 2007 at 00:44
Khavers, I think you need to be registered with RS to access their online catalogue.
The info I quoted is from their catalogue which expires in March 2007, I have no idea if and how much the price will differ in this years catalogue. I have not seen these meters at DS nor in the DS catalogue.

Re the wiring up, The boat fridge I use the hour meter for has a fancy compressor not suitable to connect anything in parallel. Actually, I would NOT connect anything in parallel with any 12V fridge compressor, they not neccessarily run on 12V DC, even if that's the voltage fed to the fridge.

There is an easy way around that though. Get a reed switch with a N/O contact - these can be got from DS or by removing one from a magnet door security switch.
Wind a small coil around the glass switch case ( its fragile! - use a similar diameter dowel former for the winding) with an enameled wire suitable to carry the fridge current - I used 1mm diameter wire.
Connect this coil in series with one of the fridge power leads. When the fridge comes on the current through the coil sets up a magnetic field which in turn closes the reed switch inside. I used the positive lead which allows me to connect one reed contact to one coil end and the other reed contact supplies the switched 12V to the hour meter.
If it does not turn on you need more turns, add a second layer to the coil, though mine works fine with one.
Use the reed switch to turn the hour meter on and off, the meter requires a 12V DC supply, same as the fridge.
The hour meter keeps its reading even if it gets disconnected.
The reset is wired from the meter module, it comes with a little circuit diagram that explains how. I just fitted a tiny pushbutton switch to the plastic jiffy box I mounted the hour meter in. Pushing the button briefly resets the display.
Hope that explains it.
Klaus
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Reply By: Robin - Friday, Feb 16, 2007 at 23:24

Friday, Feb 16, 2007 at 23:24
Good Idea Khavers

Check out current Silicon chip issue for an easy to build project based on switching on a bicycle computer to directly measure and read out amp-hours.

Robin Miller
AnswerID: 222223

Reply By: Member - andrew B (Kununurra) - Saturday, Feb 17, 2007 at 06:16

Saturday, Feb 17, 2007 at 06:16
I was a bit curious as to how the 80l waeco handled the heat, so I put it int the back of the car, put 8 litres of bottled 35 degree water in the freezer section, and a cold block of cans in the frige section (still in the box, fits perfectly....coincidence?) and set it to -20

Well after 20 hours the freezer got down to -8, the battery was down to 11.6v (under load - I did do a few 15 min trips during the days which would have given the batt. a bit of a top up), and the frige hadn't cycled once! I was disapointed....so I put a charger on the battery and hooked up the frige to 240 v, and but a digital themomoter in the back on the car.....and had a beer out of the frige. The digital screen on the thrmo went black with the heat, so I put it in the frige and the wired 'outside' sensor into the space in the car - 62 degres. So I figured whhile its sucking the power, it got the freezer section to 70 degrees below ambient, and the cans were at 6 degrees, 56 degrees below ambient.

I am now happy with the friges performance, although I think iit sucks about 65 watt, which is about 5.5 amps....20 hours =110 amps, I used an exide extreme (80Ah).

Not the mmost scientifi experiment, but it would be the maximum you'd expect from a frige I would think.

Second night freezer got down to -19, but didn't cycle, and during the heat of the day warmed back up to -12 (with about 10 openings of the fridge to "check its performance LOL). Last night I set it to -15, so it would ccle, and the battery was reading 12.45 volts at 16:00, so I will update volts an temperature later....

Cheers Andrew
AnswerID: 222240

Follow Up By: Member - andrew B (Kununurra) - Saturday, Feb 17, 2007 at 11:44

Saturday, Feb 17, 2007 at 11:44
OK, at 9.30, fridge freezer was a -15, block (half) was at -1. frige had cycled at least once !....battery voltage was on 11.6 under load.....probably as low as I would want it to get.

From what I have done, I can rely on getting at least 12 hrs at full noise out of the one battery, at a guess 24 out of 2, but then it is time to charge. On the few small trips I did, the ''loaded" voltage went up to 13.8 on the aux, and took about half an hour to get back down to about 12.6 after the car was switched off, which would Probably be the 'surface charge' being used.

I only have a 4.7 amp charger, which struggles to maintain the charge in the battery with the frige running off it, so its time to end the experiment and run it off mains.....but I may need to go for a drive, might leave it hooked up for the day & se what happens.

Cheers Andrew
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Reply By: Member - Matt M (ACT) - Saturday, Feb 17, 2007 at 22:10

Saturday, Feb 17, 2007 at 22:10
A good simple solution, but when you fit a gauge to your fridge, you would be surprised at how much the current draw varies. I have a Waeco CF50AC which will typically run at 4.5-4.7 Amps. On a stinking day when the thing is trying to cool warm water, etc, it can draw up to 7.8 Amps. A fair variation.

Your technique sounds like a good simple way to get an idea though. What if you wired the compressor to your dusters? Just remember to start your stopwatch every time you scream and turn it off when the pain stops.

Matt.
AnswerID: 222365

Follow Up By: Im.away - Sunday, Feb 18, 2007 at 01:16

Sunday, Feb 18, 2007 at 01:16
That's an interesting observation you make. I have a plasmatronic Solar regulator and it enables me to watch the current draw of my Waeco CF-50. When it's running it draws 3.9 Amps. Day in and day out. The current only varies by a really small amount whether the fridge is in full sun or whether it is cool and in the shade.

What does change, however, is the amount of time, or duty cycle, it runs for. Obviously the fridge runs longer when the ambient is hotter.

I'm not a fridgey, so I can't explain why yours draws more current and mine just runs for longer, but it is curious isn't it?
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Reply By: khavers - Wednesday, Feb 21, 2007 at 21:19

Wednesday, Feb 21, 2007 at 21:19
I must say I'm also intrigued by the current change unless it is because of an economy/normal/turbo type setup. l have checked power consumption on a trailblaza and bushman fridge and found both to be quite stable at 5A and 4.2A respectively after the initial startup (say 2-3 minutes). Ambient temperature does not seem to effect current draw only cycle time. Perhaps someone smarter than me knows why the difference.
AnswerID: 223248

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