Beefy 16g Cutter with a battery?

I have a quick question for you, how long can I run 16 AWG nichrome wire for continuously off of a battery? The Nichrome wire is 8" each piece run in series. 

I was using a 18v drill battery and a 12v motorcycle battery and had horrible results. Both setups melted the wire I was using that was connected to the power source. Whenever I managed to power the nichrome wire I wasn't able to get a long run time before the battery would die, maximum run time was about 1 minute for the motorcycle battery and approximately 30 seconds with the 18v battery. 

Thanks for your response in advance.

I sure have been getting a ton of battery questions lately… weird!

Thank you for your praise of the site, and enjoying its content. It has grown quite vast over the last few years, and some of the information is hard to find. I'm not sure if you found this page, but I have a WHOLE page dedicated to battery power - Link ( http://hotwirefoamcutterinfo.com/__FixedVoltageCalc.html )

There are a few problems with battery powered circuits…

Problem 1 - Fixed Voltage

First and foremost, they are a fixed voltage power supply. Obviously, meaning that they will always output their rated voltage - i.e. 12v, 18v, etc. What you've done with the 12v & 18v batteries is short circuited them both. It's probably good that the wire melted, if they'd have run for a longer duration of time, they'd probably have heated up and exploded.

16 gauge nichrome, at 8 inches, is approximately 0.173 ohms of resistance. Ohms law states that Amperage or Current is equal to Voltage divided by Resistance… A=V/R

A = 12v / 0.173o A = 18v / 0.173o

A = 69.3 amps A = 104.0 amps

That is some SERIOUS current pulled from those batteries. They'll die extremely quickly and/or heat up extremely quickly which is incredibly unsafe. 

Problem 2 - Available Amperage

Not every battery is designed to output amperages necessary to cut nichrome wire. There are internal resistances which restrict output amperage from a batter. Also, as a battery heats up, its internal resistance goes up, which also decreases output amperage. Ask any construction contractor who tries to use battery powered tools left in the sun on a HOT summer day. Their battery life is noticeably reduced. 

As a rough gauge, we say that you should not draw more than 1/10th of the Amp-hour (Ah) Rating of your battery. You'll notice the Ah or mAh listed somewhere on your battery.

So if you have a 20 amp-hour battery, it really should not output more than 2 amps for maximum efficiency.

Solution

Unfortunately there probably isn't an effective battery option for 16 gauge nichrome wire. It's liner foot resistance is far too low, and its current-to-heat demands are far too high. 

If you plan on using your 8" cutter to cut foam, I would recommend no less than 8-10 amps of current, which is about 1.4-1.8 amps. 

Jacobs-online.biz has a bunch of really effective and inexpensive AC step-down transformers that you can use to power your beefy 16g 8" cutter. Check them out, you'll be happy you did. 

I hope that answers your questions, but dive into the battery page for a lengthy explanation, and don't be afraid to ask more questions if you have them!