Kelpie
Well-known member
The main thing we like about electric cooking is the lack of extra heat given off. Perhaps we have a particularly badly ventilated galley.
12v Kettle
- Thread starter Laysula
- Start date
Commercial Dave
Member
The laws of physics dictate that product simply cannot be good. It takes an immense amount of energy to heat water from 10°C to even 80°C (the perfect temperature for a cup of tea). Most home kettles nowadays are in the region of 3000 watts. A standard USB power supply can probably deliver about 15 watts maximum. Even with a 280ml capacity it's going to take a very very long time to get that amount of water to any significant temperature. Unless it is vacuum sealed I'm pretty sure it will never get anywhere near boiling. I'd be surprised if it could even get water hot enough to make tea.
12V kettles are crap, 24V kettles are still...meh. Realistically if you want to boil water on a boat you either need to have mains power (from generator or shore) or use gas. You can get mains electric kettles in the range of 500W-1000W which can actually boil a useful amount of water in <5 minutes but you'll still need a chunky inverter to run them.
Boiling 1 litre of water in a decent kettle uses about 0.15kWh, once you add in efficiency loss through inverter etc that could cost up to 15Ah from your 12V battery system.
William_H
Well-known member
As said that would be useless on USB supply. However if you were willing to risk it (ie destroy it) you might cut the USB plug and connect it directly to 12v supply. Should be like 9x wattage so actually heat water if element does not melt. ol'will (known source of madness)
Snuffly
New member
ok guys, you have made your point, i dropped the thing off at a charity shop, and ordered 1 of those kettles that you put on the hob
AntarcticPilot
Well-known member
No; the wattage is controlled by the resistance of the element. If it is connected to a 12v supply, the resistance of the element controls the amps drawn. If E = volts, R = resistance, power = P and I = amps, you can write the following equations:
1) E = R * I (Ohms Law)
2) P = E * I
rearranging 1) we get
3) I = E / R
substituting 3 into 2 we get
4) P = E * E / R
So, for a constant voltage such as that supplied by a battery (and neglecting the internal resistance of the battery, which is very low), the power drawn by an appliance depends solely on the resistance of the appliance.
Pete7
Well-known member
True, but we don't boil a litre very often, more like 0.5L for two mugs of tea. Boiling it in a cheap kettle means its plastic rather than a huge great stainless steel thing, so less heat required. I think we use about 5Ah each time, or if you prefer a draw of about 62A if its sunny and 75A at night.
The inverter is a Pure Sine Wave 1600w from Sterling. Agreed they are getting expensive, but deals do happen mid winter when the boating and caravan world is dormant.
William_H
Well-known member
Quite right this suggestion was one to try before committing to rubbish bin. The resistance of the element remains the same except perhaps some reduction in resistance as temp of the element rises. So my statement remains about 9 times that power (so heat) of the device on 12v battery compared to USB 5v.But probably still not enough to generate much heat. ol'will
Commercial Dave
Member
Assuming 15W heating power (5V 3A USB power supply with no efficiency loss)
R=V/I so 5/3=1.66 ohms
The resistance of the heating element is constant (in reality it probably changes a little as it heats up). If you connect it to 12V:
I=V/R so 12/1.66=7.2 amps
P=V*I so 12*7.2=86.4 watts
That's quite an increase over the standard 15W but not quite 9x. But I guarantee a USB cable will not handle 7.2 amps for more than a couple of seconds and it'll simply melt. So personally I wouldn't risk it.
Moral of the story: do not connect electrical appliances to a supply that is higher than their rated voltage.
R=V/I so 5/3=1.66 ohms
The resistance of the heating element is constant (in reality it probably changes a little as it heats up). If you connect it to 12V:
I=V/R so 12/1.66=7.2 amps
P=V*I so 12*7.2=86.4 watts
That's quite an increase over the standard 15W but not quite 9x. But I guarantee a USB cable will not handle 7.2 amps for more than a couple of seconds and it'll simply melt. So personally I wouldn't risk it.
Moral of the story: do not connect electrical appliances to a supply that is higher than their rated voltage.
William_H
Well-known member
Yes your maths are quite correct. I estimated 9 times as the square root of 5 volts versus 15v ie 3 times voltage. Which might only occur with smart charger on alternator or charger. Perhaps realistically 14v supply would result in 118 watts (assuming 15w at 5v) a bit less than 8 times power. Say 7 times for less than 14v supply. Yes probably the USB cable with plug cut off would not like 8 plus amps. Not to mention actual heating element. I did say "before throwing it away" ol'will
