De-ice the stock tank with 12 V heater

molniya

Folks are selling 12 volt stock tank heaters.  Chopping ice for the ponies every day is getting...old.

I now have a 600W 12 volt heating element.  The application charts always show the heating element wired
directly to a 12 volt battery.  Only one battery, not a bank of batteries, and there are no low voltage drop out circuits
involved.  The only device between the heating element and the battery is a "freeze switch", a sealed [probably bimetallic]
switch that is supposed to close and connect the heating element when the ambient temp is around 32F and open at
about 40F.  If there is NO provision for a low voltage disconnect to save the battery from the anticipated 50 Amp load,
[600W / 12 Volt] how is this NOT a "battery self destruct circuit?    I have built several very small scale solar powered
systems, and the battery always had a low voltage disconnect circuit [usually the charge controller].

The battery I intend to use is a marine type with a 180 min reserve rating. [25 Amp discharge at 80F]
It would appear that at best I can run the water tank heater for about an hour or so [50 A load]
and then have a depleted battery with essentially a dead short across the terminals. [the heating element].

The wiring chart that appears all over the internet for this heater seems to be dangerously over simplified.
How can this work [safely] without a disconnect circuit?

? is there some subtle bit of "water tank heating" mojo that is escaping me?  

dennis461

Most 'freeze' switches I've encountered are set to 45 degrees F, not 32.  Do you have product names, model numbers you can share?
I just looked at some farm forums, and it looks like they deliberately let the battery go dead, then recharge it or swap it out the next day.
You would need a fairly large solar array to heat a water trough.
How many gallons?

Estragon

If my arithmetic is right, that element would raise the temp of about 30 gallons of water from 32f to 40f in one hour.  Heating anything with solar electric gets expensive pretty fast, especially if batteries are involved.

If you have sun near the tank, maybe a DIY solar thermal setup with some black poly waterline and a small pump?

westbranch

some of the PWM charge controllers have a circuit to turn on lights and other small loads..., you could use the built in timer to turn the heater switch on, via solenoid,  and off after X hours of melting based on the batteries capacity. I assume you want it there when you feed in the AM,..?

dennis461

Estragon said:

If my arithmetic is right, that element would raise the temp of about 30 gallons of water from 32f to 40f in one hour.  

Good starting point, now we need to know if the tank/trough is insulated, thickness of insulation, size and shape of tank, ambient temperature and desired water temperature to calculate losses.   

mvas

molniya,

All of your concerns are justified.

I would definitely add a Low Voltage Drop-Out device.
I would never, ever, ever, ever want the heater to discharge the battery to 0 volts = quick death.
I would not discharge more than 50% because you have a hybrid "marine" battery.

What if you used a heavy duty $20 PWM like ...
https://www.amazon.com/dp/B01HEXJUX8?psc=1
and dialed back the power, to say 10% PWM, just enough to keep the water from freezing?

I think, approx 25 watts can raise 10 Gallons of water 1° F
Insulate the water container - all sides and bottom.
If you minimize the heat lost then you will significantly reduce the load on the battery.
Maybe, now the battery could last 180 minutes /  60 min/hour / 10% = 30 hours ?

If you purchase two batteries you can swap daily.
Use bolted-on Power Pole Connectors to make swapping, less of chore.

Or skip the battery and buy a 12 V DC 50 Amp MegaWatt power supply for $110.

mike95490

My sister just uses a hatchet to chop the ice out.  Easier to swing than changing a battery that froze soild