I might not keep this vehicle that long, so I'm no sure I want to go to the trouble and expense of installing a wired in system like the National Luna to power my fridge. As an option I thought of putting a low voltage alarm/ shutoff on the main battery (suggestions for a source for one?) and having an unconnected backup battery in a battery box with a cigarette lighter connector wired to it and topped off with a small 5-15 watt solar charger. If the alarm goes off on the main, switch the fridge to the backup battery and switch the solar charger to the main battery. After starting the vehicle, switch the solar charger back to the backup battery. Sound feasible/advisable?
Dual Battery Alternative
- Thread starter cnskate
- Start date
wrcsixeight
Adventurer
A 5 to 15 watt panel will do very little in offsetting any fridge usage.
It's my opinion that the time and money you spend on wiring in a low voltage alarm, you could just buy a cheaper Solenoid isolator from Napa and some more cabling. National Luna is a nice product, but you can get a dumb solenoid for 30$ and spend 15 on cabling and have a true isolated battery that charges anytime the key is turned on.
To activate the solenoid, you just find a fuse which is only hot with the key on, or engine running, and run a wire from that fuse to the contact on the solenoid. When the solenoid sees 12 volts from this wire, your 2 batteries are now 1 big battery.
It's my opinion that the time and money you spend on wiring in a low voltage alarm, you could just buy a cheaper Solenoid isolator from Napa and some more cabling. National Luna is a nice product, but you can get a dumb solenoid for 30$ and spend 15 on cabling and have a true isolated battery that charges anytime the key is turned on.
To activate the solenoid, you just find a fuse which is only hot with the key on, or engine running, and run a wire from that fuse to the contact on the solenoid. When the solenoid sees 12 volts from this wire, your 2 batteries are now 1 big battery.
LandCruiserPhil
Expedition Leader
I agree with wrcsixeight 5 to 15w will not do much. I use a 55watt panel and it is whats needed to keep up with my frig. Most of the new frigs have voltage cut off incorporated into the controls. Have you considered the purchase of a jumper box? You can use it to jump your vehicle if you have a dead battery, power other small accessories, and you can charge it with your truck when driving.
This kind of sounds like what you are looking for:
http://www.batterybrain.com/batterybrain.pl
They have different options to choose from, are fairly inexpensive and you don't have to really add extra wires.
Salue
http://www.batterybrain.com/batterybrain.pl
They have different options to choose from, are fairly inexpensive and you don't have to really add extra wires.
Salue
Thanks for the tips. I always travel with a battery jumper in the outback, but I don't trust it a 100%. I was thinking of the second battery as sort of an emergency backup, and using the solar charger over a few days to recharge 3-4 hours of fridge usage. Would 15 watts still be to little for that?
That Battery Brain thing looks cool. National Luna makes an audible alarm too.
Maybe I'll go with the solenoid setup anyways, thanks.
That Battery Brain thing looks cool. National Luna makes an audible alarm too.
Maybe I'll go with the solenoid setup anyways, thanks.
whatcharterboat
Supporting Sponsor, Overland Certified OC0018
A fridge with a Danfoss compressor (what most fridge makers use) will normally consume 4.5 to 4.9 amps per hour at 12 volts, so that's roughly 55watts per hour. Therefore 4 hours of fridge running would be 220 watt/hours. (you know a fridge will probably cycle for 10 hours plus per day depending on the ambient temp, insulation effectiveness, cold temp setting. could be 16 hrs). If you take into account the charging inefficiency of the batteries as being around 80% (could be as good as 85% or whatever...) then you will have to put 275watt hours back in. I don't know where you are BUT if at your location you are looking at 5 peak sun hours (PSH) per day, then a 15 watt panel would need 18.3 PSH or in real terms , 3.7 days to recover from 4 hours fridge running (providing there were no other loads on the battery at the same time). Phew, my brain hurts.
A lot of "if"s thrown around there, change the detail around if you need to but that's the basic math.
If you aren't sure about your PSH, google a chart off the net and it will give you an approximate figure of available solar energy for your area based on average rainfall, cloudy days, latitude, etc. but don't expect more than 7 hours in the Sahara on a summer's day when you are doing your calcs.
I tell guys here to work on needing 2 x 64 watt panels if you want to keep stuff frozen in a reasonable size fridge when camped for any length of time. I know what you're trying to get at. If you are on the go most of the time then car will provide a lot of the charge anyway. Seriously, I'd just go with a smart solenoid (so that means VSR controlled) or at the very least a solenoid triggered of your ignition or at the very very least, a manual battery switch.
BTW some fridges here have an optional low volt disconnect (LVD's) unit built in too.
Regards John
Last edited:
lostworldexpedition
Explorer
I hope it is OK to repost someone else's comment... here is what Mario of Adventure Trailers said about solar panels on their rigs:
"Measuring consumption on an Engel 45 at our shop using a "Doc Watson" from www.powerwerx.com, we have seen daily consumption of 20 to 28 A/h per day at 12 VDC on the "Ref" setting (around 38-40F) with ambient temps is the 70-80F range. Our experience with solar panels is that unless they are mounted on a tracking system, perfectly aligned for latitude and the season, the weather is cool and the the sky is un-obstructed by clouds, we can only expect 70% performance. This means for example that a 100 watt panel will only produce 70 watts per hour. This means 5.8 A/h rather that the rated 8.33 A/h @ 12 VDC.
So...if you were to back into the consumption of an Engel 45 you would need 60 to 80 watts of PV based upon 20 to 28 A/h of consumption per 24 hr period for a PV to keep up with your needs on the "Ref" setting on the MT-45.
The 22.5 W panel per our calculations will produce as follows: 22.5W/12VDC = 1.875 A/h x .7 = 1.312 A/h x 6 hrs of average usable sunlight = 7.85 A/day ( 6 hours is based upon information from sources such as http://www.energyatlas.org/contents/default.asp). Considering that your draw is 20 to 28 A/h per day, you will be coming up short 12 to 20 A/h short with 22.5 watts.
I know this is a lengthy explanation but is is based upon Ohms law:
Watts / Volts = Amps. As in all equations, you can flip this around such as Amps x Volts = Watts etc. Throw in the factoring of .7 to the PV for performance and you will not be disappointed.
Based upon these formulas and numbers, we supply 80 watt panels to our trailer customers who express a desire to be "self contained" with their fridges and laptops for indefinite periods. The calculations have been successful to date."
Hope that helps. I am having battery issues as we speak and have received invaluable help from wrcsixeight. All I can say is design it well and spend the money ONCE and you won't have the problems I am having...
Cheers.
"Measuring consumption on an Engel 45 at our shop using a "Doc Watson" from www.powerwerx.com, we have seen daily consumption of 20 to 28 A/h per day at 12 VDC on the "Ref" setting (around 38-40F) with ambient temps is the 70-80F range. Our experience with solar panels is that unless they are mounted on a tracking system, perfectly aligned for latitude and the season, the weather is cool and the the sky is un-obstructed by clouds, we can only expect 70% performance. This means for example that a 100 watt panel will only produce 70 watts per hour. This means 5.8 A/h rather that the rated 8.33 A/h @ 12 VDC.
So...if you were to back into the consumption of an Engel 45 you would need 60 to 80 watts of PV based upon 20 to 28 A/h of consumption per 24 hr period for a PV to keep up with your needs on the "Ref" setting on the MT-45.
The 22.5 W panel per our calculations will produce as follows: 22.5W/12VDC = 1.875 A/h x .7 = 1.312 A/h x 6 hrs of average usable sunlight = 7.85 A/day ( 6 hours is based upon information from sources such as http://www.energyatlas.org/contents/default.asp). Considering that your draw is 20 to 28 A/h per day, you will be coming up short 12 to 20 A/h short with 22.5 watts.
I know this is a lengthy explanation but is is based upon Ohms law:
Watts / Volts = Amps. As in all equations, you can flip this around such as Amps x Volts = Watts etc. Throw in the factoring of .7 to the PV for performance and you will not be disappointed.
Based upon these formulas and numbers, we supply 80 watt panels to our trailer customers who express a desire to be "self contained" with their fridges and laptops for indefinite periods. The calculations have been successful to date."
Hope that helps. I am having battery issues as we speak and have received invaluable help from wrcsixeight. All I can say is design it well and spend the money ONCE and you won't have the problems I am having...
Cheers.
FurthurOnTheFly
Glamping Society
I'm going the battery brain route as well. I was planning on doing a dual battery set-up, but after several extended trips with the fridge, leaving it on overnight, and having no problems starting up in the morning I've decided to forgo that route. Instead I have the largest battery I can fit in the space, and the battery brain will keep me from ever being stranded. Granted it might cut off the fridge and I risk spoiling food but thats better than being out in the middle of nowhere with a dead truck!
I was thinking of doing a trickle charge via a solar panel just in case, and may still do that down the road, but I just haven't seen any need for it so far.
I was thinking of doing a trickle charge via a solar panel just in case, and may still do that down the road, but I just haven't seen any need for it so far.
TreeTopFlyer
Adventurer
Datalogs for 12v fridge
I posed this info a year ago. This is a Norcold 630 that i was logging using Hobo components. I did several tests, half full, full, empty, two beer cans, etc. I measured current draw and temp so you could see the where to optimize temperature and keep cycling of the compressor to a minimum. The attached xls file has about an 8hr observation. You can see the compressor kick in, then off. Throughout the day I lowered the temp setting of the thermostat, i forget which temp I started at, but I lowered the temps about every few hours. The pink is the temp. Blue is the current.
I did these test to get an idea of what kind of consumption be looking at. One thing I never did test is the effect of the blanket around the fridge. I tried to get engel to send me one to demo but they never followed thru. The blanket or whatever they are called would be an interested test to see run because you could see how well the insulated blanket smooths out the temperature curve consequently stretching out the cycle times of the compressor.
I posed this info a year ago. This is a Norcold 630 that i was logging using Hobo components. I did several tests, half full, full, empty, two beer cans, etc. I measured current draw and temp so you could see the where to optimize temperature and keep cycling of the compressor to a minimum. The attached xls file has about an 8hr observation. You can see the compressor kick in, then off. Throughout the day I lowered the temp setting of the thermostat, i forget which temp I started at, but I lowered the temps about every few hours. The pink is the temp. Blue is the current.
I did these test to get an idea of what kind of consumption be looking at. One thing I never did test is the effect of the blanket around the fridge. I tried to get engel to send me one to demo but they never followed thru. The blanket or whatever they are called would be an interested test to see run because you could see how well the insulated blanket smooths out the temperature curve consequently stretching out the cycle times of the compressor.
whatcharterboat
Supporting Sponsor, Overland Certified OC0018
Hi Luis.
Lots, and lots of factors here.
Solar panels need to derated for cell temperature. The hotter a panel is the less power it puts out and that figure is different depending on the type of PV panel. Mono -crystalline, poly- crystalline or amorphous. So as an example polys and especially monos really need to have an air gap to allow air flow. Amorphous don't.
Also sun angle or the amount of air mass the sun's rays have to pass through to reach the panel. Example...So roughly on the equator when the sun is straight above there is much less air than at the poles.
And the type of panel. Most amorphous panels will be less effected by low sun angle than poly or mono crystalline.
BTW when they say that an amorphous panel is less efficient it means they are bigger for the same power. In an RV situation when the panels are normally fixed flat on the roof, sure deduct more than 20% off for mono and poly crystalline but you'll find that amorphous don't need this derating well certainly not anywhere near the same extent.
Don't just work on 6 hours of sun. Find the PSH for you region.
Well you wallet may be dissappointed more than it needs to be but your beer will be cold....haha. Actually we call the ".7" bit the "fudge factor" and it would be a good general safety margin if you didn't want to do all the math. Just a bit of an overkill.
Regards John.
Lots, and lots of factors here.
Solar panels need to derated for cell temperature. The hotter a panel is the less power it puts out and that figure is different depending on the type of PV panel. Mono -crystalline, poly- crystalline or amorphous. So as an example polys and especially monos really need to have an air gap to allow air flow. Amorphous don't.
Also sun angle or the amount of air mass the sun's rays have to pass through to reach the panel. Example...So roughly on the equator when the sun is straight above there is much less air than at the poles.
And the type of panel. Most amorphous panels will be less effected by low sun angle than poly or mono crystalline.
BTW when they say that an amorphous panel is less efficient it means they are bigger for the same power. In an RV situation when the panels are normally fixed flat on the roof, sure deduct more than 20% off for mono and poly crystalline but you'll find that amorphous don't need this derating well certainly not anywhere near the same extent.
Don't just work on 6 hours of sun. Find the PSH for you region.
Well you wallet may be dissappointed more than it needs to be but your beer will be cold....haha. Actually we call the ".7" bit the "fudge factor" and it would be a good general safety margin if you didn't want to do all the math. Just a bit of an overkill.
Regards John.
wrcsixeight
Adventurer
Slightly off topic from the OP's topic, but since he is mostly concerned about safely powering the Fridge and still having enough juice to start the vehicle here's some possibly relevant info from my experience.
#1. We should all be happy that compressor fridges have become efficient enough to be run off batteries for a day or two or more.
Without more solar than you need, and a large bank of isolated auxiliary batteries here are a couple tips to keep the duty cycle down, and the beers frosty. Some are obvious, others might not be.
-Keep it out of the sun.
-If you are keeping it in the vehicle, keep it low, and use sun shades in all the windows to keep the interior of the vehicle as cool as possible.
- Make sure the intake and exhaust vents are not obstructed, make sure the cooling fins are not obstructed with dust, lint and hair.
-The more it is opened, the more air cold air spills out. So opening in a windier area is even more detremental to keeping duty cycle low.
-Know where everything is inside the fridge so you don't have to waste time looking.
-When adding warm beer to the cold fridge, if possible, do so first thing in the morning when the ambient temps are the coolest, and the fridge has to work less hard to suck the heat from them.
- An empty fridge runs more than one full of already cold things. If you are nearly out of food, keep the empty, or full water bottles inside so less air can spill out.
-If you do have solar and the batteries are full, or are driving the vehicle, turn it way up effectively storing the extra energy by having the food/ beer colder. Just don't freeze the beer. And turn it back down once the sun is down or you stop driving.
I have a front loading 'dorm' style Norcold. It's compressor drew 2.7 amps when new, and ~33 months later draws about 2.4 amps. I have added 1/2 inch of insulation to the sides, with another 1/2 inch of dead air space before the sides of the insulated cabinet it resides within. I removed the compressor from the back and rolled on 3 layers of aluminum backed 1/8 inch thick self adhesive rubber sheeting, and reinstalled the compressor over that, more for vibration suppression than extra insulation. I have added a computer muffin fan that draws air from the floor underneath the fridge and blows it over the compressor and cooling fins. I have added a 12 volt 25 mm muffin fan in the interior. Directly behind and above the cooling fins, I have a sliding window actuated by my former sofa bed motor. I covered the window with louvered vents. I can direct the forced air to exhaust through the window, or inside the vehicle.
I find that in my normal use, my duty cycle is well under 30%, meaning the maximum my fridge is likely to use in 90 deg f ambient daytime temps in 24 hours is under 20 amp hours. From 10 pm to 9 am, It uses about 6 a/h including my fans and TV, and LED reading lights with ~60 f degree over night temps.
I also have 130 watts of solar, So I don't even stress the electricity unless I get 5 or 6 seriously overcast days.
#1. We should all be happy that compressor fridges have become efficient enough to be run off batteries for a day or two or more.
Without more solar than you need, and a large bank of isolated auxiliary batteries here are a couple tips to keep the duty cycle down, and the beers frosty. Some are obvious, others might not be.
-Keep it out of the sun.
-If you are keeping it in the vehicle, keep it low, and use sun shades in all the windows to keep the interior of the vehicle as cool as possible.
- Make sure the intake and exhaust vents are not obstructed, make sure the cooling fins are not obstructed with dust, lint and hair.
-The more it is opened, the more air cold air spills out. So opening in a windier area is even more detremental to keeping duty cycle low.
-Know where everything is inside the fridge so you don't have to waste time looking.
-When adding warm beer to the cold fridge, if possible, do so first thing in the morning when the ambient temps are the coolest, and the fridge has to work less hard to suck the heat from them.
- An empty fridge runs more than one full of already cold things. If you are nearly out of food, keep the empty, or full water bottles inside so less air can spill out.
-If you do have solar and the batteries are full, or are driving the vehicle, turn it way up effectively storing the extra energy by having the food/ beer colder. Just don't freeze the beer. And turn it back down once the sun is down or you stop driving.
I have a front loading 'dorm' style Norcold. It's compressor drew 2.7 amps when new, and ~33 months later draws about 2.4 amps. I have added 1/2 inch of insulation to the sides, with another 1/2 inch of dead air space before the sides of the insulated cabinet it resides within. I removed the compressor from the back and rolled on 3 layers of aluminum backed 1/8 inch thick self adhesive rubber sheeting, and reinstalled the compressor over that, more for vibration suppression than extra insulation. I have added a computer muffin fan that draws air from the floor underneath the fridge and blows it over the compressor and cooling fins. I have added a 12 volt 25 mm muffin fan in the interior. Directly behind and above the cooling fins, I have a sliding window actuated by my former sofa bed motor. I covered the window with louvered vents. I can direct the forced air to exhaust through the window, or inside the vehicle.
I find that in my normal use, my duty cycle is well under 30%, meaning the maximum my fridge is likely to use in 90 deg f ambient daytime temps in 24 hours is under 20 amp hours. From 10 pm to 9 am, It uses about 6 a/h including my fans and TV, and LED reading lights with ~60 f degree over night temps.
I also have 130 watts of solar, So I don't even stress the electricity unless I get 5 or 6 seriously overcast days.