Thursday, April 11, 2013

Battery Power

This message was submitted through the online contacts form. Great project and questions regarding using batteries in heating elements or even just foam cutters...

User in Blue, my response in Orange


I am creating a heating element with 22gage nichrome wire. Using 3.7 volt 3800 mAh Battery, trying to reach a stable temp of 500F degrees.
Using the Jacobs Calculator it states with 3.3amps and 3.7volts a 12.75 inch straight wire would reach the 500F temp. As i understand when coiling the wire you cut the amps needed in half to reach the desired temperature. 

Does this mean that the same 12.75 inch length of wire coiled will still reach the 500F temp and only draw from the battery 1.65 amps and make the battery last longer?


Does the battery still push 3.8 amps through the wire and damage the power supply?

Do I need a longer or shorter piece of nichrome?

It looks like your estimates are correct. 3.7volts applied to a 12.75" length of 22 gauge nichrome wire will yield 3.3 amps, a theoretical temperature of 500°F

Here is a small section of the website dedicated to fixed voltage / batteries - Link - There might be some information there worth reading.  But I have a few things to say.

I've tested NiChrome wire in the coil arrangement on a heater project I was working on for a customer. And I've never found that "half amperage" fact to be true. It still seems like it requires the fully calculated value for amperage to achieve the desired temperature. I suppose I understand where that statement is coming from. In a coil arrangement, you are trapping and saving more heat into the empty space of the coil and less is lost to the environment. But honestly, I've only ever seen the calculated amperage to truly being effective.

The other thing is, you may not actually be able to get 3.3 amps out of that battery and expect to achieve longevity from the battery.  Theoretically if you pulled 3.3 amps from a 3.8Ah (3800mAh) battery, it would run for 69 minutes. But as a rule of thumb you should never pull more current (amps) than 1/4 the rated Ah (or mAh).  Since it's a 3800mAh battery or 3.8Ah, 1/4 would put you around 0.95 amps, or damn near 1 amp. 

The problem is two fold...

One, not every battery can physically establish a current that high. The internal salt bridge and internal resistance of the battery actually limits the total output amperage. 

Two, if you force a battery to output more than it can comfortably handle, the battery will heat up. Now, it might not explode, but with increased battery heat, comes increased battery resistance. The higher battery resistance, the lower the amperage output, and eventually the battery will die quicker. So rather than 69 minutes, you may only get 20 minutes per battery charge (just a guess). 

So the 1/4 rule is really more so that you can MAXIMIZE both the output current as well as the longevity. If you go above the 1/4 rule, in most cases you will be able to achieve the desired current, but you may not achieve the desired battery life / time. 

I kinda rambled a bit, but does all that make sense?

In any event, when people are asking about these custom projects I always tell them that calculations are one thing.. but in-field application is another. Try to do as much on paper as you can, but eventually you're just going to have to build your heater and try it out. Some times the real life applications works better than it does on paper, which is a win-win.  

So don't be too afraid of going forward with your current project as-is, but if it doesn't work out like you plan, my explanation may be the reason why. 

Just a great question for those interested in batteries, hope it helps some of you out!

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