12v water heating element selection help

I have one of those DIY ABS water tanks on my truck and I want to add a 12v heating element but not sure what is safe and sufficient. The tank is 4” ABS and ~8 gal capacity. I will thread on a metal pipe section where the element will install so as to not melt the plastic. Brass, aluminum, etc., not sure yet.

I have a single battery atm, an x2 power group 27M @ 90ah.

Looking at something like this:

12 Volt 300 Watt 9 3/8 Inch DC Submersible Water Heating Element

12 Volt 300 Watt 9 3/8" DC Submersible Water Heating Element

https://www.amazon.com/dp/B01BT9JEV6/ref=cm_sw_r_cp_api_i_zNRDCb3CEG7BV

If I calculate a 25amp draw (did I do my math right?: 300w / 12v = 25amps), I can safely run this while the truck is running, and for about an hour or so with the engine off.

Amirite?

 

Your amp draw math is right. Since this is a constant power device when you're running your truck and it's charging at 14 V, you'll use ~21 Amps and you'll use ~25 when it's off. You'll heat your 8 gallons of water ~15 Deg F an hour given no heat loss from the tank.

Note that your battery has a 20hr capacity rating of 90AH. This means that over 20 hours, it can discharge 90 amps, or 4.5 amps/hour. If you discharge at a faster rate, you'll get less capacity out of the battery. Peukert's law predicts that you'll get around a 63 AH capacity discharging at the 25 amp/h rate.

If you're going to build this, make sure you incorporate a temperature switch for the heating element and a T & P relief valve, and your materials are rated for at least the temperature and pressure (simultaneously, remember PVC's strength decreases sharply with elevated temperatures) the relief valve are set for. A water heater left too long could eventually burst your tank, and if it's gotten hot enough, you'd be looking at a steam explosion.

 

Impressive.
I was like.........