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Old 21-04-2014, 18:57   #1
F51
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Engle duty cycle calc

I'm very happy with our new Engle MT-80 fridge/freezer. It uses less 12vdc than our old Waeco 45qt. A lot less. Here is what I have observed: Ambient temp about 85 degrees, interior temp about 38. The compressor comes on for 3:00 minutes out of 14:12 minutes. When it is running, the unit draws 3.4 amps. 3.0 / 14.2 = .2112 for a duty cycle of 21.12%, right? Now I get amazed. 3.4a x 24hrs = 81.6 amp hours. If I multiply this by 0.2112 I get 17.3 ah/day. Somebody please check my math! This seems too good to be true!
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Old 22-04-2014, 21:08   #2
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Re: Engle duty cycle calc

Your math is fine.
Spec's on the units web site show variable input power 0.7 to 3.6A @ 12V and 1-2 a.hr/hr typical.
So, your 3.4*0.21= 0.7 sounds low. Measurement errors?
Here's a ballpark check:
Heat flow out of the box is area times temperature difference divided by resistance of insulation.
So from the manufacturers site, the interior surface area of the box is 12.2 ft2.
Your temperature difference is 84-38=47F
A guess at the insulation - say 3 inches of R7 gives R21 (very optimistic, could be R10)
Compressor efficiency should be 40-60 percent. Let be generous and say 60%.
So heat flow = 12.2 x 47 / 21 = 27.3 btu/hr or 8 watts or .67 A @12V. (3.412 btu/hr is 1 W )
But at 60% eff. you will use 1.1 amps.
So, with these very optimistic calculations, I think you are light.
Maybe take more measurements? Is your system voltage 12V or 13.8V?
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Old 22-04-2014, 22:39   #3
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Re: Engle duty cycle calc

14 mins is a pretty small test time... most probably cycle more then 14 mins apart. try timing over like 12h.
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Old 23-04-2014, 20:06   #4
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Re: Engle duty cycle calc

My Engle MT 80 Combo uses around 27-28 AHr each day. I liveaboard and have 450 watts of solar and have not been pluged into the shore power for 5 weeks now. Freezer was at -9C when I left for work today, Battery bank was at 100% at 10am and the charge controllers were only allowing 1-2 amps to go into the battery bank. Sun was out and not a cloud in the sky at 10am. So far the most I have seen the pannels put out was 16 Amps and that was under similar conditions as today but I had been on the SSB for about 3 hours chatting on the Ham bands during the previous night so the battery bank was down to about 84% at 10am that time.
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Old 23-04-2014, 23:55   #5
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Re: Engle duty cycle calc

I just checked and I have a MT-60 Combo not the MT-80.
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