Hello

I am making a device which is essentially a battery powered heating pad: it consists of thin nichrome wire arranged like a miniature toaster. It has a resistance of about 30 ohms. I need it to get to about 35 degrees C when power is applied, and to get there within about 15 seconds. I only need it to stay at that temperature for about a minute, or less - this is not a long term heating application.

The question is what are the best batteries to use? With 7 AA batteries in series I can do it; also with 3 PP3 batteries. But the PP3s run down very quickly and the AA batteries are bulky. I would like to find a rechargeable battery solution which takes up less space. I don't mind if I have to recharge fairly frequently - I'm looking for something that can deliver this fairly hefty current for a fairly short period but in a small space. (I've looked at supercapacitors but my experiments with them have not been very encouraging.)

Is this something I could achieve with Li-ion batteries, or Li-poly? I want the recharging to be simple too - and I get a bit worried when I read about all the safety issues with Li-polys. And of course I want it to be cheap!

Thanks

 

Use an SLA battery like these
http://www.google.co.uk/imgres?q=sl...170&start=0&ndsp=15&ved=1t:429,r:14,s:0,i:115


but what voltage /Current are you planning on using?

 

Thanks but that is much bulkier than I can use.

It's difficult to say what voltage and current - what matters is how quickly the element heats up and I don't mind how I achieve that.

For example, experimentally, if I use 8 AA batteries (nominally 12V) the current drawn is 0.35A - and the heating is quick enough. If I could find small rechargeable batteries that could deliver that kind of current at that voltage I would be happy.

 

Thanks but that is much bulkier than I can use.

It's difficult to say what voltage and current - what matters is how quickly the element heats up and I don't mind how I achieve that.

For example, experimentally, if I use 8 AA batteries (nominally 12V) the current drawn is 0.35A - and the heating is quick enough. If I could find small rechargeable batteries that could deliver that kind of current at that voltage I would be happy.

They do smaller capacities like 0.5aH

http://www.google.co.uk/imgres?q=sl...1&start=55&ndsp=20&ved=1t:429,r:16,s:55,i:305

 

Thanks but I really doubt that lead acid is the way to go - I want something much slimmer, 15mm max. I only need a current of about 0.35A and I don't need huge capacity.

Let me ask a simpler question: is there any reason why I shouldn't connect four Li-ion 3.7V batteries in series? Would I get the 0.5A that ohm's law predicts?

 

Thanks but I really doubt that lead acid is the way to go - I want something much slimmer, 15mm max. I only need a current of about 0.35A and I don't need huge capacity.

Let me ask a simpler question: is there any reason why I shouldn't connect four Li-ion 3.7V batteries in series? Would I get the 0.5A that ohm's law predicts?

Yes the 3.7V Li-Co will do it, use the protected cells for safety.

A safer chemistry is the LifePo4. Not as much energy density as the Li-Co. You can get them in AA or 14500, or 14mm dia, 50mm long. If you are near a Wal-Mart, they may have them in the garden light section.

 

The question is what are the best batteries to use? With 7 AA batteries in series I can do it; also with 3 PP3 batteries. But the PP3s run down very quickly and the AA batteries are bulky.

You can cut open a "9V" Ni-CAD rechargeable and harvest cells, there will be either six or seven small cells. You can add cells to get 12V if that's what you want. The batteries are very cheap.

I use them in flashlights. They put out a TON of current. Body size is much smaller than AA. Much less dangerous than Li Poly and very robust for recharge.