EEVblog Electronics Community Forum

Electronics => Beginners => Topic started by: Ed.Kloonk on April 26, 2011, 11:35:32 am

Title: Running a 24v water heater on 12v.
Post by: Ed.Kloonk on April 26, 2011, 11:35:32 am
Hi.
I'm about to purchase a small portable water heater and I have two uses for it. I'm looking at a few different types of various wattages. I'd like to connect it to our big truck's supply which runs on 24v. But I'd also like to take the heater with me down into the snowfields and use it in an ordinary car which is of course 12v.

I only need to warm up enough water for a cup or two of coffee. I realize it would take longer to boil on 12v. However, I'm more concerned there may be other consequences.

I'd really prefer to avoid fitting a 24v - 12v converter as I will only occasionally use it with 12volts. The rest of the time it will be connected to 24v.

Does running a 24V heating element at 12v affect the lifetime of the element? Will lowering the voltage shorten or even increase it's life?

Title: Re: Running a 24v water heater on 12v.
Post by: scrat on April 26, 2011, 11:50:47 am
If it's only a heater element (power resistor, in practice), the power will be 1/4, passing from 24 to 12 V. The element will be less thermally stressed, so lifetime should increase rather than decrease.
Title: Re: Running a 24v water heater on 12v.
Post by: tnt on April 26, 2011, 11:54:07 am
First, I'm assuming that there is only a resistance heater (and no advanced eletronics or such).

At DC you have U = R * I  and P = U * I  so P = U^2 * R

For a constant R, dividing the voltage by 2 will result in reduction of 4 in power (what's gonna eat the water).

Now that's not gonna result in you water taking 4 times more time ... it's probably be worse than that because the power transmitted to the water it self is :
P_to_water = P_heater - P_loss(temp)

P_loss(temp) are the losses in the air ... and they don't change so it's probably gonna be more than 4 times slower.
That also mean if the P_heater if now smaller than some P_loss, the water will actuall not heat anymore. (P_loss depends on temperature of the water so the water might start to heat then "top-off" at some point).

Should be safe to try however ....
Title: Re: Running a 24v water heater on 12v.
Post by: Ed.Kloonk on April 26, 2011, 11:56:05 am
Thanks for answering. That's great. Yeah, just an element and cig lighter plug. Sound ok then. I'll get the biggest wattage one for 24v so I wont have to wait too long at 12v.

 :)
Title: Re: Running a 24v water heater on 12v.
Post by: saturation on April 26, 2011, 12:07:51 pm
I use a single cup AC based immersion heater and a DC-AC converter, rated for load.  You can thus use them in vehicles or from line voltage.  Other models are multivoltage, 110-240V.

You need to use it on a ceramic or glass mug for safety.  Not sure why they are $10 now, I bought mine at a dollar store for $1; its fairly old technology.

(http://ecx.images-amazon.com/images/I/51q91NjzgeL.jpg)


Title: Re: Running a 24v water heater on 12v.
Post by: Psi on April 26, 2011, 12:12:11 pm
Another idea is to get two heaters and either connect them in parallel or series depending on if your using 12 or 24v.
Then again, if you got two, you might as well get a 12v and a 24v and use whichever is needed :P

Might be worth considering though if you could mount two different rated heater elements inside one unit and have a switch.

But a 24v unit on 12v is definitly the easiest soluton, assuming you can get the unit to heat enough from 12v.
Due to thermal losses the maximum temperature the water is ever going to reach will be lower with 12v than with a 24v. Exactly how low is the real question.
Title: Re: Running a 24v water heater on 12v.
Post by: Tony R on April 26, 2011, 01:42:32 pm
I would use it what it is rated for...