I did search and think I maybe found a formula but have only a slight memory about formula equality. Maybe that isn’t the right term but I remember in 9th grade in the very early 70’s someone telling me swapping up variables could be useful. I remember thinking “How could that be useful”!
I have an Artic pack that my Hughes Bulldog tells me uses 50 watts. How do I predict how many AH that draw would pull? Until I get a number that I can feel confident about I can only stop at plug in spots this weekend in OH and Western PA.
Is there anyone here who payed attention in 9th grade?
1) The pads are not "on" all the time. They cycle based on the temperature of the water in the tank and elbow(s). How long they are on determines how much of your battery is consumed over time.
2) The efficiency of the pads (actual heat produced) decreases with time as the battery is consumed (Voltage decreases) and thus produce less heat (Watt's Law). (P=V^2/R)
3) The battery will be heavily tasked to provide 50 amps for less than an hour till dead. Operating the heating pads in winter on battery is a BAD idea.
Additionally, you will need a source of house current to survive the cold since the furnace will be adding to the current demand on the battery.
Why:
The resistance in the pads is fixed based on the number of pads and length of resistance wire in each pad.
If we use the OP's numbers for amp draw in his camper at 50 amps DC for a full charged battery (12.6 volts), we can calculate the resistance of the heaters 50W/(12.6*12.6) = 50W/158.76 V^2 =~ 0.32 Ohms.
When the battery voltage drops to about 50% capacity, 12.0 Volts nominal, since the resistance is fixed at 2 ohms, the wattage produced to heat the tanks is reduced to 12^2*0.32 =~ 46 Watts.
That just means the pads will be "on" longer to try and maintain the tank temperature as your battery gets used up until the battery no longer produces amps.
Further complicating the problem is the battery's ability to deliver amps (Capacity) DECREASES as the battery's temperature goes down and the current demand goes up.
Batteries are "rated" in AH and that is based on a 5-amp load (not 50). So, a 100 AH rated battery will last 20 hours at 5 amps.
That is not to say a 100AH rated battery will last 10 hours at 50 amps. The more demand you place on a battery the less capable it is to deliver those amps without decreasing the voltage (capacity of the battery).
The attached graph is for a typical 100AH battery with capacity available plotted vertically and the current demanded on the horizontal axis. You can use this graph as a "percentage" to determine current draw effect on YOUR battery (typically 75AH in a Marine/RV OEM battery in a trailer).
With a single 75AH battery and a constant 50-amp draw, we have to estimate the life of this battery due to the excessive amperage. Using the graph, we will estimate the capacity at 43% of the 75AH for a fully charged battery. (32AH)
32AH/50 amps = 0.64 hours =~ 38 minutes
Cycling that load to maintain tank temperature would increase that since the battery would be allowed to recover some between cycles.
The furnace is also a huge battery killer, so then there is that.
__________________
Lou & Freya the wonder dog
2008 GMC Sierra 3000HD Allison Duramax
2019 Flagstaff 8529FL
The resistance in the pads is fixed based on the number of pads and length of resistance wire in each pad.
This might be true with old school heating pads but today's pads are made using PTC (Positive Temperature Coefficient) elements. Simply put, resistance increases as pad temp rises and current drops.
Self regulating, cutting off current demand as they reach their upper design temp, and no "hot spots" as with resistance wire based units.
Just an FYI
__________________
"A wise man can change his mind. A fool never will." (Japanese Proverb)
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2023 f-150 SCREW XLT 3.5 Ecoboost (The result of a $68,000 oil change)
This might be true with old school heating pads but today's pads are made using PTC (Positive Temperature Coefficient) elements. Simply put, resistance increases as pad temp rises and current drops.
Self regulating, cutting off current demand as they reach their upper design temp, and no "hot spots" as with resistance wire based units.
Just an FYI
Thanks for the update.
I made note of that in my library.
__________________
Lou & Freya the wonder dog
2008 GMC Sierra 3000HD Allison Duramax
2019 Flagstaff 8529FL
Much of your great answer leads me to think you are thinking I wanted to draw 50 amps.
Thanks for the in-depth answer.
When first turned on, all pads are drawing current.
Actual amperage drawn going forward will depend on the quantity of liquid in each heated tank/and elbow and the temperature of the liquid when you turn the pads on and ambient temperature.
With relatively full tanks, the pads will run a constantly in order to bring the contents up to temperature, then cycle less frequently but for a longer duration to maintain it.
With relatively low tanks, the pads will cycle on and off more frequently but for a lesser duration because the lower volume of liquid inside the tank will cool quicker.
In addition, tanks will not have equal contents so pads will be turning on and off at different times (though low hanging elbows will always be "full"). Remember that the freshwater tank is also heated.
Thus, the actual duty cycle of the system would be hard to generalize. All pads on at the same time would be worst case and if those tanks are relatively full, your battery could be depleted quickly.
It is best to utilize electric hookup sites in winter for the reasons stated.
__________________
Lou & Freya the wonder dog
2008 GMC Sierra 3000HD Allison Duramax
2019 Flagstaff 8529FL
Thanks for the update.
I made note of that in my library.
Here's a fairly simple explanation of how PTC heaters work. They are showing up in all kinds of applications from the small cube style space heaters to electric heat elements in EV's, and of course RV tank heater pads.
One of the nice characteristics is that they can be designed for a max temp without any external controls which allows for a heater that will warm, but not get hot enough to melt or ignite combustibles.
__________________
"A wise man can change his mind. A fool never will." (Japanese Proverb)
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2023 f-150 SCREW XLT 3.5 Ecoboost (The result of a $68,000 oil change)
Here's a fairly simple explanation of how PTC heaters work. They are showing up in all kinds of applications from the small cube style space heaters to electric heat elements in EV's, and of course RV tank heater pads.
One of the nice characteristics is that they can be designed for a max temp without any external controls which allows for a heater that will warm, but not get hot enough to melt or ignite combustibles.
It also explains why they should not be used on empty tanks.
Thanks again.
Herk
__________________
Lou & Freya the wonder dog
2008 GMC Sierra 3000HD Allison Duramax
2019 Flagstaff 8529FL
It also explains why they should not be used on empty tanks.
Thanks again.
Herk
Tank heaters usually only get to ~60 F +/- Polypropylene has a melting point ~340F, Polyethylene ~230 F, and ABS ~390 F.
Most tank heater pads I've researched lately have a temp cutoff temp ~60F-70F.
I think the admonition to turn off tank heaters on empty tanks is a legacy from the "wire heating element" days.
Other than wasting energy, especially when boondocking, I don't see tank heaters being left on in this day and age as a real issue. As with lots of "manuals", many are not always up to date and a lot of "This is how we've always done it" information left over.
Remember, the high limit temps in PTC devices can be determined in their composition.
__________________
"A wise man can change his mind. A fool never will." (Japanese Proverb)
"You only grow old when you run out of new things to do"
2018 Flagstaff Micro Lite 25BDS
2023 f-150 SCREW XLT 3.5 Ecoboost (The result of a $68,000 oil change)
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