# How to generate 100 watts of heat from a car battery

1. Jan 4, 2018

### John1397

I have lots of batteries 12 volt charged by the sun what I need is something that would draw 100 watts and give heat when connected to a car battery? I have ac heating pads, 2000 watt hot water elements lots of copper wire, light bulbs any other ideas?

2. Jan 4, 2018

### Staff: Mentor

Connect any device that uses 100 Watts to the car battery. The easiest I can think of is a 100 Watt light bulb (connected via an inverter, of course). But you already seem to know this, so I'm not sure what you are really asking...

3. Jan 4, 2018

### lewando

This would do the trick-- a Vishay RH1001R400HJ01.
Be very careful. That is, use this part in a manner consistent with the manufacturer's heat sink requirements.

Last edited: Jan 4, 2018
4. Jan 4, 2018

### Nik_2213

One tweak: Is the car battery being charged or providing power to anything else ? The ~ 8 amps you're drawing is based on a *nominal* 12 V battery voltage .

Hmm: Winter ? DIY cabin heater ?? You MUST have appropriate wiring, connections, insulation, line-fuse and switch. A safely mounted heat-sink, given a central power-resistor will be much hotter than the fin tips. Thermal transfer paste. A cooling fan ? Two such ??
Please consider getting a well-rated, low-voltage cut-off such as the KEMO series, as used to protect eg RV & SUV batteries against deep-discharge...
As ever, 'Due Care, Please' ??

Um, you may do better to have several power resistors, distributing the heat across the heat-sink. Just so their network appears as 1R4 ( 1.4 Ohms) load...
FWIW, if you have eg a pair of 2R8 ( 2.8 Ohm) in parallel, they may be individually switched to allow 'Full' or 'Half' power...

{ V = I x R, I = V/R, Power = I x V = V x V / R. Here, 100 = 12 x 12 / R, so R = 1R4 ( 1.4 Ohms) to first approximation per #L above ... }

5. Jan 4, 2018

### Asymptotic

Work it out.

1. Amps=Watts/Volts. 100W/12V = 8.3 amps. (you'll need to know current to determine required wire size).
2. Resistance = Volts/Amps. 12V/8.3 amps = 1.4 ohms.

You could hook up one of those 2000W water heater elements, but probably wouldn't be impressed with the results. Work it backwards from heater specifications ...
2000W/240V is a typical water heater rating. How many ohms is that?
Watts=Volts * Amps, and Ohms=Volts/Amps. A bit of algebra yields the formula Ohms = Volts2/Watts.
2402/2000W = 28.8 ohms.

How much current flows through 28.8 ohms when 12 volts is connected across it?
Amps=Volts/Ohms. 12 volts/28.8 ohms = 0.42 amps.
How much power is this? 0.42 amps * 12V = 5 watts.

Just because the heater is rated for 2000W doesn't mean that's how much power it'll provide - that's only true when supplied with 240V.If you have a bushel basket full of water heater elements you could connect 20 of them in parallel across a 12 volt source. Each one will contribute 5 watts; 5*20 = 100 watts.

@lewando spec'd a 1.3 ohm Vishay resistor (it's probably the closest standard resistance to the calculated 1.4 ohms), and that'll do the job. I'll second his inquiry, too - what is you application?

6. Jan 5, 2018

### John1397

I was wondering if one used an AC baseboard heater rated 1000 watts at 120 ac then hook it to 12 volt dc how would this work you then have ten times less voltage would you have ten times less watts? Light bulb would maybe work except I want to generate heat not light.

7. Jan 5, 2018

### lewando

Analyze the AC baseboard idea using P = V2/R.
1000 = 1202/R. R = 14.4 Ω, the resistance of the AC baseboard heater.
P12V = 122/14.4 = 10 W.

Also, incandescent light bulbs do a much better job of generating heat than light.

8. Jan 5, 2018

### Nik_2213

Snag with using 120 V-rated heaters on 12 Volts is their cold resistance may be a lot less than you expect, but more than you want. Given their non-linear thermal characteristics, they'd need some experimentation. Same applies to 120 V or 220 V incandescent lamps...

FWIW, 'rough service' lamps are less efficient than the 'regular' types, so better for turning current into heat rather than light...

9. Jan 5, 2018

### Staff: Mentor

And all of the light becomes heat once absorbed.

10. Jan 5, 2018

### John1397

I gotta try that do not know if 10 watt is going to be much heat.

11. Jan 5, 2018

### Averagesupernova

Buy some cheap sealed beam automotive headlamps. Get some that have two filaments each. You should be able to combine them up in such a way to get close to your target wattage.

12. Jan 5, 2018

### lewando

I do not know either. Even 100 W may not deliver much heat, depending on what you are trying to do. Heating up an ice fishing cabin? Probably not.

13. Jan 5, 2018

### Asymptotic

Brings back Christmas memories of my sister and her Easy-Bake oven .... it used (2) 100 watt light bulbs as heating elements.

14. Jan 5, 2018

### Asymptotic

What are you attempting to accomplish?

5 watts can seem to be far too much heat - ask anyone who had accidentally bumped up against a fully loaded 5 watt 'sand' resistor while troubleshooting a circuit board, and toasted a part of their anatomy in the process. That same 5 watts spread out over increasingly larger areas becomes decreasingly less noticeable.

15. Jan 6, 2018

### OmCheeto

This works. I just happened to have two sitting in my living room, and they each draw 5 amps @ 12.4 vdc = 62 watts/lamp

per Wikipedia; "The 9004 is rated for 65 watts (high beam) and 45 watts (low beam) at 12.8 volts."

so I'm guessing the low beam element failed on them.

16. Jan 6, 2018

### John1397

Only downside with auto headlamps is only have 300 hour life span. I have batteries and solar panels and to much power left over so I was trying to find a cheap way of using the power up.

17. Jan 6, 2018

### 256bits

There are plug in 12 v gadgets that can suit your fancy,
Thermoelectric coolers, add on windshield defrosters with a blower are just 2 examples.
Go visit the automotive section of the store and see what other creatures lurk there.

18. Jan 6, 2018

### OmCheeto

Wow. Seems like a waste. I would just disconnect the excess solar panels and put them into storage. They last longer that way. [ref]

If you're not willing to do that, just wire 4 bulbs in series and parallel. I found 9004's for $3 each, brand new. I have a feeling they would last longer than you will live. That's just too easy! 120 watt, 12 volt thingy.$9.99

ps. I was right in the middle of doing Boltzmann calculations when you posted that.
Temperature of a halogen bulb: 5300°F (filament made of Tungsten)
Melting point of tungsten: 6200°F
Temperature of 4 halogen bulbs wired in series-parallel: 3600°F​

Last edited: Jan 6, 2018
19. Jan 9, 2018

### Cameron Nicoll

Hi, I'm not an engineer but I have a train load of solar experience. Can I ask why you think you need to use this extra power?
Do you have your panels directly connected to your batteries or are you using a regulator?
If you're using a regulator, there is no need for anything else. The regulator will only deliver as much power as your load and batteries require.

20. Jan 9, 2018

### Nik_2213

I tried to get a car 'screen defroster' in UK, discovered reputable suppliers had 'pulled' all of them off their shelves.

Bunch of issues across the several brands...
Not fused. (!!)
No low-voltage cut-out, so could flatten battery when you needed it most.
No thermal cut-out, so could 'Halt & Catch Fire' if air-flow compromised.
Shabby wiring, which could over-heat.
Fake CE markings; first, 'trial' shipment was genuine, subsequent stock were low-quality, look-alike 'clones'.

So, look for 'UL' approval tag in US, and 'Due Care, Please' !!