# Battery required for a portable immersion boiler

Immersion heaters like the one pictured are typically plugged in. I'm interested in potentially developing one that is battery operated so it can be used on backcountry camping trips. A water heating calculator (https://www.easycalculation.com/physics/thermodynamics/water-boiling-time-calculator.php) informed me that 500W will bring half a liter of 20°C water to 70°C in 4 minutes assuming 80% efficiency.

My question is: what size battery would be required to deliver 500W for 4 minutes say 20 times before requiring recharge? Would it be small/light enough to be practical to carry on an extended hike?

• 500W * 80 minutes = 2.4 MJ of energy. A battery is probably the least effective way to carry that quantity of energy around. It's equivalent to about 1/4 pound of wood, or 2.5 fl.oz. of gasoline. – Dave Tweed Apr 16 at 13:46
• True, but I am often in environments where I can't build a fire. I could carry a Jetboil but I'd rather use something that doesn't require fuel. A Jetboil and fuel cannister together weigh about a pound. If I can get a rechargeable device like this to come in at less than 2 pounds, it is worth carrying (to me). – Tony Ahn Apr 16 at 14:03
• 1 kg of LiIon batteries is about 1 MJ, so you are short by a factor of 2.4 here. – winny Apr 16 at 14:06
• Gotcha. Thanks. – Tony Ahn Apr 16 at 15:37
• efficiency would be much closer to 100% than 80%, not sure where 80% comes from... – dandavis Apr 16 at 19:24

You want to get 500W for 10 minutes, plus whatever losses from the water during the heating. Call it 600W for 10 minutes or 60Wh (assuming the water is well insulated and heat loss due to evaporation is very limited). So roughly the size and weight of a big power tool battery pack, and close to the limit for what you are allowed to carry on an airliner.

Might be useful for situations where open fires are prohibited but otherwise a bit silly compared to a small camp stove, since you'd only get one fairly weakly heated cup of water out of a charge cycle. Kettles draw around 1500 to 3000W (240V land) and still take a few minutes to boil a small amount of water so you may find that estimate on the low side when you actually try to build it. And if you're melting snow, forget it, the phase change eats up way more energy.

A light (395g) camp stove such as Firefly produces about 2,500W on white gas (8700 BTU/h) according to the specs, and it doesn't take many ml of fuel to do a lot of cooking.

• Okay, so too big for thru hiking. That explains why nobody else has developed one. Thanks! – Tony Ahn Apr 16 at 14:06
• With lithium battery technology, it's do-able (a Milwaukee M18 battery weighs 1.5kg and is >200Wh), just not competitive if you have other options. – Spehro Pefhany Apr 16 at 14:55
• Oh, no, I'm about to disagree with @SpehroPefhany! This is dangerous. Anyway -- there are no batteries that are as volume- and weight- efficient as common fuels. Just ain't. If your goal is to head out for a week and have a cuppa every morning and night, then if you do the math I think you'll find that a small camp stove is the way to go. The only exception would be if it's an area that allows absolutely no open flame. – TimWescott Apr 16 at 16:11
• @TimWescott Whatever happened to those micro turbines that were supposed to power our notebook computers? There are chemical one-time heating packs sold for this application. – Spehro Pefhany Apr 16 at 17:08
• @SpehroPefhany I missed out on those. I had so much hope for the booze-powered fuel cells for laptops, simply for the extra options it would give you for whiling away those long flights. – TimWescott Apr 16 at 17:27

A quick google search and it looks like a few immersion heaters run on 12V

500W at 80% efficiency is 625W/12V = 52A, which you want for 4*20 minutes = 80 Minutes = 1.3 hours

1.3 * 52 = 68Ah

That size battery is.....kinda large

car battery

• Batteries are not all that efficient, especially at high discharge rates. Better go with a 100 Ah version -- it's only 64 pounds! – Dave Tweed Apr 16 at 13:56
• Good point! would have to make a backpack for that one - call it wearable tech! – Andy West Apr 16 at 13:57

Cheaper, easier and more efficient in terms of what you have to carry to hang a can over the fire...

But work out the energy use and compare that to the energy in the battery you are prepared to carry...

Matches and kindling are lighter...

• I was looking for something I could use in open flame prohibited areas. – Tony Ahn Apr 16 at 14:09
• @TonyAhn leaving out critical info does not help... "open flames prohibited"... – Solar Mike Apr 16 at 14:37
• My question was clear: what size battery would be required to deliver 500W for 4 minutes say 20 times before requiring recharge? Would it be small/light enough to be practical to carry on an extended hike? – Tony Ahn Apr 16 at 15:39
• There was no more information required to answer the question as presented. – Tony Ahn Apr 16 at 15:40
• But I got the answers I needed from other commenters, so it is all good. – Tony Ahn Apr 16 at 15:41