12v Fridge experiment.
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Topic: 12v Fridge experiment.
Posted By: furpod
Subject: 12v Fridge experiment.
Date Posted: 06 Sep 2011 at 4:42pm
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Was a discussion about towing/charging and battery life when running the fridge on 12v... OK, so we got home from camping, fridge still loaded. Parked and leveled camper. Fridge had been on 12v for 5+ hours while towing home. Dual batteries, only a couple months old. All measurements were taken in the following order, batt A, batt B, batt A+ batt B-, battB+ battA-. At no time, out of 52 individual measurements was there a deviation to the .000 point. I only recorded to .00, but they were always the same to three decimals.. Measurements taken at 1 hour intervals, =/- a minute or two. Ambient temperatures were in the mid 80's trailer was closed up, vent open a crack, no fan. Full sun. Start: 12.78 1) 12.63 2) 12.54 3) 12.50 4) 12.47 5) 12.40 6) 12.34 7) 12.28 8) 12.21 9) 12.12 10) 12.02 11) 11.98 12) 11.91 So.. 12 hours for my batteries to go to just above 40%. An extrapolation is that a single battery system should be able to run the fridge 5 hours pretty easy. And it's important to note my batteries were fully charged (at least 12.6v = 100% charge) at the end of the tow. My take on it is it should be absolutely fine to tow with the fridge in 12v, as long as you don't stop for a 5 hour lunch. |
Replies:
Posted By: secretbard
Date Posted: 06 Sep 2011 at 7:24pm
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Thanks for the time and energy (literally) that you expended providing us with this data. Your findings answer a lot of questions I've had about running the frig. on battery power. That explains why my battery (when I only had one battery) went dead on me after a long 5.5 hour trip. I now have 2 batteries, but I am still going to travel with the frig. on propane power... my longer trips necessitate the use of propane. ------------- 2010 Rpod 172 :) 2004 Chevy Trailblazer |
Posted By: furpod
Date Posted: 06 Sep 2011 at 7:51pm
You should maybe check your charging circuit.. You really shouldn't find your battery running down when towing.. Start your truck and use a DMM to check the poles at 1 and 7 o'clock in the connector.. you should see +13v.. The only reason I can think of (beyond a bad charging circuit), is if you didn't pre-cool your fridge.. and thus needing more power than the circuit could push back there... ------------- |
Posted By: TerryM
Date Posted: 06 Sep 2011 at 8:10pm
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Furpod is absolutely correct about your vehicle charging your batteries. I lived on the road full time for 6 years in a motorhome and always used 12v on the road for the fridge. Never a problem. Terry ------------- RP-175 W/Lift Kit 2011 Ford F-150 4X4 Saint Augustine, FL: The first permanent European settlement in the USA: 1565 |
Posted By: techntrek
Date Posted: 06 Sep 2011 at 10:04pm
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Make sure your +12 line is at least 10 gauge, and even better would be 8 gauge. Most places install 12 or 14 gauge and it is impossible to push 10+ amps (what the fridge uses in 12 volt mode) through that. Add in a battery that is partially discharged and so is also demanding many amps, and a small charge line is hopeless. Oh, and make sure the negative line is either attached to the frame within 6-12 inches or that it also uses 10 or 8 gauge line back to the battery.
------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual 
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Posted By: secretbard
Date Posted: 06 Sep 2011 at 10:15pm
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Actually, furpod is correct. The one time my battery did drain completely was when I didn't pre-cool the frig. and it ran a lot to achieve the desired temp inside. The frig. basically had to cool from the start, powered by the battery alone, so that probably did it. All the other electrical issues are fine (i.e. at the hitch end, the battery end and the Pod charger end). I should have considered the draw of power that it would have taken, or more precisely, the continuous draw of power it would take to lower the frig. temp from scratch. When I added the second battery, I went ahead and replaced both batteries with new batteries, so I should be fine on that end as well. ------------- 2010 Rpod 172 :) 2004 Chevy Trailblazer |
Posted By: TerryM
Date Posted: 07 Sep 2011 at 4:27am
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I never gave precooling a thought but it would make a big difference. I always plug in the day before I leave so the fridge is cold when I leave. Sometimes its easy to forget something when you automatically do it. Terry ------------- RP-175 W/Lift Kit 2011 Ford F-150 4X4 Saint Augustine, FL: The first permanent European settlement in the USA: 1565 |
Posted By: TerryM
Date Posted: 07 Sep 2011 at 4:37am
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Does anyone remember the point at which a 12v battery is considered dead? Ten volts or something like that? I know it isn't really that much of a drop. Terry ------------- RP-175 W/Lift Kit 2011 Ford F-150 4X4 Saint Augustine, FL: The first permanent European settlement in the USA: 1565 |
Posted By: techntrek
Date Posted: 07 Sep 2011 at 7:33am
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If the wiring is sized correctly it won't matter if you precool or not. 10 amps is 10 amps whether its steady for 3 hours during prechill or intermittent for 10 minutes three times every hour once its cooled down. Either way the 10 amp load will cause a voltage drop on the charge line below the voltage potential of the pod's battery if the wire is too small, which will force your pod's battery to handle the load. The only difference with an already-cold fridge is it will discharge and then slowly recharge your battery between cycles. Possibly not all the way, either. With 10 gauge wire and a 10 amp load you should still see about 13.2 volts at the Bargman which is above the 12.7 resting voltage potential of the pod's battery, so it won't discharge.
furpod, thanks for the test results, very interesting! One thing to consider when interpreting your results is the battery needs to rest for at least 3 hours to get an accurate SOC reading based on voltage. The voltage will be lower under load and will rise a bit with rest. At the end of your 12 hours if you had let it sit and then took another reading you probably were closer to 45 or 50%.
Terry, 11.75 volts at rest equals 30% SOC, 12.7 volts is 100% SOC, so there's less than a volt difference between keeping the lights on for the weekend and damaging your battery. Since most of the time you don't want to go below 50%, 12.06 volts is the magic number. Take a look at the chart about 2/3 of the way down on this page: http://www.windsun.com/Batteries/Battery_FAQ.htm - http://www.windsun.com/Batteries/Battery_FAQ.htm ------------- Doug ~ '10 171 (2009-2015) ~ 2008 Salem ~ http://www.rpod-owners.com/forum_posts.asp?TID=1723 - Pod instruction manual
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Posted By: furpod
Date Posted: 07 Sep 2011 at 8:24am
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Another good resource is http://www.marxrv.com/12volt/12volt.htm - http://www.marxrv.com/12volt/12volt.htm A couple hours after the experiment, I plugged in and let the batteries recharge, didn't think to see what they rebounded to.. But my next ongoing experiment is to track them, with just the parasitic loads until we camp again.. Just to see what the discharge rate is.. Unplugged yesterday morning. Voltage this morning is 12.80.. Has anybody used one of those plug-in voltmeters for a car? and if so did you find it accurate? I can only find them that go to 2 decimal places.. OK I guess, but mostly, I could plug it in and just open the camper door and check it, rather than having the DMM laying around for a month or two.. ------------- |