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[Reply #15 on:]

20,000 Watts would take 6 seconds.

...annnd a 9V battery would take 10 days. 

9V at 150ma = 1.35W
333Wh/1.35W = 246hours

Make that 15 days - you're forgetting the time taken to swap the battery every 15 minutes.

[Reply #16 on:]

Also, if you go too slow, you will be losing heat faster than the battery can put it in. Theoretically a 1.35W heater should be able to boil water. Realistically it won't even come close for any substantial amount of water.

[Reply #17 on:]

Also, if you go too slow, you will be losing heat faster than the battery can put it in. Theoretically a 1.35W heater should be able to boil water. Realistically it won't even come close for any substantial amount of water.

Oh, yes.  All those times assume perfect insulation.

33.333 Watt Hours from a 12V battery would be about 2.8 Amp Hours.  You can get that size sealed lead-acid battery for under $20.  Unfortunately that battery is rated 2.9 Ah at 20 hours.  Without perfect insulation there is no way you are going to heat the water over 20 hours and get the same temperature rise.  I think you will need a significantly larger battery, like a car battery.  You can then buy a 200W car immersion heater to turn the charge in the battery into heat: http://www.langtoninfo.com/showitem.aspx?isbn=8000692729878&gclid=CI3EqfrQ1LMCFUOK4AodA0YARA

[Reply #18 on:]

I think you will need a significantly larger battery, like a car battery.  You can then buy a 200W car immersion heater to turn the charge in the battery into heat: http://www.langtoninfo.com/showitem.aspx?isbn=8000692729878&gclid=CI3EqfrQ1LMCFUOK4AodA0YARA

Car batteries aren't great for deep cycle use (but can provide rapid heating, most will provide several kW for a short period).
Fortunately they are usually around 35 to 45Ah, which is a big margin.

Say 400A for 40s (assuming battery voltage drops to 8V or so due to internal losses) would give 120kJ and be about 20%
discharge depth or so.  Stupidly thick copper wiring would be needed (10mm or so)

A more reasonable 50A for 200s (this time about 12V) would mean more practical heater element and connecting wires (4mm diameter or so)

Alternatively a deep-cycle lead-acid battery at 20A for 8 to 10 minutes...  Capacity of 10Ah or more ought to be sufficient for a deep cycle.

[Reply #19 on:]

I'm curious now why the OP wants to heat up 2L of water as fast as possible with a battery.

[Reply #20 on:]

But not to the temperature of tea...

[Reply #21 on:]

i'd go to a camping store and buy a small gas bottle

[Reply #22 on:]

i'd go to a camping store and buy a small gas bottle

I thought of that (or a gasoline camp stove) but what if flames/combustion are not allowed? 
Perhaps an exothermic chemical reaction.

[Reply #23 on:]

Perhaps an exothermic chemical reaction.

Powdered iron, powdered magnesium, salt. Just add water to start the reaction...

[Reply #24 on:]

You could get it to 21 degrees by leaving it stood on the side.

[Reply #25 on:]

thanks all for the replays