Battery Vehicle battery as a power source

[Drummo1995]

If I use a vehicle battery connected to jumper cables, which is then connected to the heating element of a kettle, which is then placed in play sand, in a metal container, will the sand then heat up?

 

[James]

yes.
assuming it is a normal uk kettle it is normally 2 to 3kw at 230v
assuming it is 3kw
3000 / 230 x 12 =150watts

so it would warm the sand but not very quickly as it will only give out 1/20th of its normal power output as it is being run at 1/20th of its normal voltage.

 

[Avo Mk8]

yes.
assuming it is a normal uk kettle it is normally 2 to 3kw at 230v
assuming it is 3kw
3000 / 230 x 12 =150watts

so it would warm the sand but not very quickly as it will only give out 1/20th of its normal power output as it is being run at 1/20th of its normal voltage.

I was thinking 150W is still a lump of power
Using online calculator Watts to Heat Calculator - https://www.omnicalculator.com/physics/watts-to-heat

To heat 1Kg of sand by 10 DegC with 150W will take just under 1 minute (still sounds unlikely - mistake?)
I suspect the thermal conductivity of sand is not great, so it might ultimately get quite hot.
Whereas 1Kg (1L) of water with the same 150W would take nearly 5 minutes to rise 10 DegC!

I hope the mention of 'play sand' doesn't suggest it might be played with, as I suspect this could ultimately get dangerously hot in the vicinity of the element?

 

[James]

I was thinking 150W is still a lump of power
Using online calculator Watts to Heat Calculator - https://www.omnicalculator.com/physics/watts-to-heat

To heat 1Kg of sand by 10 DegC with 150W will take just under 1 minute (still sounds unlikely - mistake?)
I suspect the thermal conductivity of sand is not great, so it might ultimately get quite hot.
Whereas 1Kg (1L) of water with the same 150W would take nearly 5 minutes to rise 10 DegC!

I hope the mention of 'play sand' doesn't suggest it might be played with, as I suspect this could ultimately get dangerously hot in the vicinity of the element?

I don't think it is a mistake, would not be surprised if sand has a specific heat capacity of 0.2 x that of water.

 

[littlespark]

I don’t think the cup of tea is going to taste very nice….. and maybe a bit cold

 

[Engineer1969]

If I use a vehicle battery connected to jumper cables, which is then connected to the heating element of a kettle, which is then placed in play sand, in a metal container, will the sand then heat up?

Just a word of warning...

Be careful and mindful of not shorting out the battery if the jumper cables or battery are inside of a metal container

Make sure the contacts are completely isolated from the metal container even in the event of someone accidentally moving / kicking it or it could cause the battery to catch fire

yes.
assuming it is a normal uk kettle it is normally 2 to 3kw at 230v
assuming it is 3kw
3000 / 230 x 12 =150watts

so it would warm the sand but not very quickly as it will only give out 1/20th of its normal power output as it is being run at 1/20th of its normal voltage.

That calculation is incorrect
3000 / 230 is the formula for amps (I=P/V) which you are then multiplying by 12 to equal 150 amps not 150 watts

If the load was 3000 watts at 12v then the correct formula for amps would be 3000w / 12v = 250a

However it would not be 3000w at 12v. This should be calculated based on the resistance of the element not the wattage which will change with the voltage

So also assuming 3000 watts at 230v
R = V² / P
52900v / 3000w = 17.6Ω

So at 12v
P = V² / R
144v / 17.6Ω = 8.2w (or 0.68a)

I think it would take a long time to heat up at 8.2 watts

 

[Avo Mk8]

I think it would take a long time to heat up at 8.2 watts

About 17 minutes for a 10 degC rise!
I thought my sums felt wrong

 

[James]

I = P / V ......... 3000w at 230v =13.04A

R = V / I ........ 230 / 13.04 = 17.63 Ohm

I = V / R ........ 12 / 17.63 = 0.68A

P = V * A ........ 12 * 0.68 = 8.16W

As you said before, My original calculation was wrong.

I have expanded it so others can understand (particularly those new to the field)

@Engineer1969 Well done and thank you for pointing out the mistake.
its not often I make a error as fundamental as that.

 

[brianmoooore]

We've had similar mistakes on here referring to electric showers.
Shower is rated at 10.5kW at 240V = 43.75A , therefore at 230V it would draw 10500/230A = 45.65A, whereas in reality, the current is less at 230V.

 

[Drummo1995]

I was thinking 150W is still a lump of power
Using online calculator Watts to Heat Calculator - https://www.omnicalculator.com/physics/watts-to-heat

To heat 1Kg of sand by 10 DegC with 150W will take just under 1 minute (still sounds unlikely - mistake?)
I suspect the thermal conductivity of sand is not great, so it might ultimately get quite hot.
Whereas 1Kg (1L) of water with the same 150W would take nearly 5 minutes to rise 10 DegC!

I hope the mention of 'play sand' doesn't suggest it might be played with, as I suspect this could ultimately get dangerously hot in the vicinity of the element?

 

[Drummo1995]

Hello mate,

Thankyou for your advice and no! For a bit of context I’m in the army and have been tasked to create something, that is portable, quick to set up and can be hot enough to be seen on a thermal sight.

 

[mainline]

Hello mate,

Thankyou for your advice and no! For a bit of context I’m in the army and have been tasked to create something, that is portable, quick to set up and can be hot enough to be seen on a thermal sight.

You could use a battery and a lamp, wrap the lamp in foil if you don't want the light to be seen.

 

[James]

https://www.amazon.co.uk/DERNORD-Submersible-Element-Stainless-Heating/dp/B091DJ7C6Y/ref=sr_1_7?dib=eyJ2IjoiMSJ9.bpkaPfEP7o2S-OI913KaKISKW1Pl3WKr2162O0DntIgQ513BFyFR9O9sc0WTSXdbvA4iyoib_rchENjnQr8c6ENc1CN9Tm3jGT19IHbrYILMUD6589s3uzx1YrqmtNzKr_jwgZPu4oKWejUjjEhUFh2P0nFq_2WYQNA72HpuHNCDKfhRcQBipExHR0CnWIgzGkRYOHgjBjYkj0sK--iAZuiUSB_rvgQT4wVZP-njK1_x964Jq7Jh4iOqTwDB2UDX8nvnhNSt8F0H3MNu-L0tR9kNHwcFsZ3cOJ3Ta5bJCd6T6O3gNT9c4O9ZObbdhTJLKpAbv_eVQvM_tSKLRiaSTmHPcgSsl0obUJAodv1FYfnKpGjZ4pkULjgBzhuiWNBk0gr7aPVxXwuQRYB3F_jeuJGIO7YqexvdDIrFG5CbskojL74XMZzQ8akP8HpIGPmD.G3xcPXC0HKxe1cVDgzlrg_5Gwg_1mhgk84cg4VmCYe0&dib_tag=se&keywords=12v%2Bwater%2Bheater&nsdOptOutParam=true&qid=1737227329&sr=8-7&th=1

 

[mainline]

https://www.amazon.co.uk/DERNORD-Submersible-Element-Stainless-Heating

Warning: Don’t run the fuel tank dry when the heating element is turned on!​

 

[Simon47]

Pop into the vehicle workshop and blag a headlight bulb - they'll regularly be replacing them because dipped beam has blown, while main beam is fine. Or just blag a new one.
While you are there, blag some cable, Croc clips, and a fuse.

Then pop down to the kitchen and ask for a largish tin wih a lid - post scran removal. Make a hole, fix the bulb into the hole with light inside the tin.
You now have a 55W (maybe more) heater inside a tin. The tin will get warm if uncovered, hot if covered.

Years ago, dad made something like that as a bed warmer, using a 10lb trade tin of Horlicks - I think it only needed something like a 15W bulb.

 

[James]

Warning: Don’t run the fuel tank dry when the heating element is turned on!​

Fixed the link now.
It’s 300w 12v
Instead of sand, fill the box with water.
Much lighter to carry home, although it is still heavy to carry it there.

 

[mainline]

I think it's going to depend on how large and hot the heat source needs to be for detection.

 

[James]

I think it's going to depend on how large and hot the heat source needs to be for detection.

Absolutely, if your trying to imitate a head sticking out of a trench, 10w in a 5 litre tin is fine, maybe a little bit too much.

If your trying to imitate a tank, 5000w is probably about right, maybe even a bit too small.

 

[brianmoooore]

Don't underestimate 15W of heat. My precision soldering iron is 15W, and it definitely gets hot.

 

[UKMeterman]

yes.
assuming it is a normal uk kettle it is normally 2 to 3kw at 230v
assuming it is 3kw
3000 / 230 x 12 =150watts

so it would warm the sand but not very quickly as it will only give out 1/20th of its normal power output as it is being run at 1/20th of its normal voltage.

P= (V*V)/R so the element resistance is 3000=(230*230)/R R=17.6 ohms, at 12V this will dissiapte 8W... as the current is 0.68A at 12V

 

[UKMeterman]

If you want something that is safe and less hassle than the kettle then try someihing like Arcol, 27Ω 25W Aluminium Chassis Mount Resistor HS25F 27R F ±1% | RS - https://uk.rs-online.com/web/p/chassis-mount-resistors/1860145?gb=s this will dissupate 5W, the same kind of heat as a sidelamp on a car and comes with wires ready attached. get it bolted to a small bit of metal say a couple of inches across after a few minutres I would expect a decent themal night site to see it from miles! To make 12V batteries safe you must have an inline fuse such as H7335 | iMaXX Fuse Holder | RS - https://uk.rs-online.com/web/p/fuse-holders/2884498 (holder) and a fuse such as 0287003.PXCN | Littelfuse 3A Purple Blade Car Fuse, 32V dc | RS - https://uk.rs-online.com/web/p/car-fuses/7874142 If you short out a car battery it will melt the offending item, probably cause a fire and the car battery may explode. RS have trade counters across the country and also do mail order.

 

[Drummo1995]

If you want something that is safe and less hassle than the kettle then try someihing like Arcol, 27Ω 25W Aluminium Chassis Mount Resistor HS25F 27R F ±1% | RS - https://uk.rs-online.com/web/p/chassis-mount-resistors/1860145?gb=s this will dissupate 5W, the same kind of heat as a sidelamp on a car and comes with wires ready attached. get it bolted to a small bit of metal say a couple of inches across after a few minutres I would expect a decent themal night site to see it from miles! To make 12V batteries safe you must have an inline fuse such as H7335 | iMaXX Fuse Holder | RS - https://uk.rs-online.com/web/p/fuse-holders/2884498 (holder) and a fuse such as 0287003.PXCN | Littelfuse 3A Purple Blade Car Fuse, 32V dc | RS - https://uk.rs-online.com/web/p/car-fuses/7874142 If you short out a car battery it will melt the offending item, probably cause a fire and the car battery may explode. RS have trade counters across the country and also do mail order.

Thankyou, will I have to attach anything to the chassis mount resistor? And how long can I run it on a 12 v battery before the battery dies

 

[mainline]

Thankyou, will I have to attach anything to the chassis mount resistor? And how long can I run it on a 12 v battery before the battery dies

View: https://www.youtube.com/watch?v=9mAmg0D5ArY

 

[Avo Mk8]

Thankyou, will I have to attach anything to the chassis mount resistor? And how long can I run it on a 12 v battery before the battery dies

It would be a good idea to attach the resistor to a small piece of metal, as advised in the post you responded to, but from the spec. the resistor is capable of dissipating 9W without a heatsink, so it shouldn't come to any harm if you don't.

How long it will run depends on the battery you use, and how healthy and charged it is. Your battery will likely be labelled with its capacity in Ampere hours (Ah). So as an example, if it was a 100Ah battery, it could supply 100Amps for 1 hour, or 1 Amp for 100 hours, or 2 Amps for 100/2= 50 hours etc. The suggested resistor would take (roughly) 0.5A, so the 100Ah battery would last 100/0.5=200 hours.

 

[UKMeterman]

The big question is how big do you want your target and how hot do you want your target?

 

[Simon47]

Your battery will likely be labelled with its capacity in Ampere hours (Ah). So as an example, if it was a 100Ah battery, it could supply 100Amps for 1 hour, or 1 Amp for 100 hours, or 2 Amps for 100/2= 50 hours etc.

It doesn't actually work like that.
For a vehicle SLI (starter, lights, ignition) battery, as well as a nominal AHr rating, it has a CCA, cold cranking amps, rating which indicates it's ability to turn the engine over.

For the AHr rating, this will be specified at a particular rate - the faster you discharge, the lower the apparent AHr capacity. Back when I worked in IT, it could be really obvious how much adding batteries would disproportionately increase runtime. E.g. adding two battery packs (trebling capacity) could result in a 5-fold increase in runtime.
Pulling a few 10s of W from a large vehicle battery will result in a very long run time.

But, of course, unless you've a dual battery setup (as on FFR, fitted for radio, vehicles) then you'll be wanting to keep enough charge to start the vehicle.