Hi all

I bought a couple of 7.4 VDC batteries and I would like to run them with my Therm-ic brand insole footpads. The footpads are about 12 ohms of resistance each. The 7.3 volt batteries run the footpads too hot when hooked up directly. I am trying to figure out an easy way to lower the 7.3 volt batteries to put out somewhere between 5 and 6 volts. The insoles are a constant resistance at 12 ohms each. Is there an easy way I can accomplish this? Whatever it is, it needs to be very compact as it will hang on the back of a ski boot. A resistor, an LED? Ideas?
The batteries are Tenergy Li-ion 18650 at 7.4 volts, 2600 mAh and are rated at a max discharge current of 5.9 A and a continuous discharge current of less than 2.6 A

I dont mind if I loose a little efficiency as these batteries have plenty of power

Blair - 18650's are typ. 3.7VDC. Sounds like your pack has two in series = 7.3VDC. If you can reorient those in parallel, you'd have something maybe closer to the correct voltage & therefor wattage , but twice the capacity runtime.

Edit by moderator: The original thread is: (1) DIY ski boot heaters | All About Circuits

 

Good answer mtbcoe but almost five years too late.

Welcome to AAC.
[edit] I see that this thread has been separated into two parts. Yet I wonder "What's the question the new TS wants answered?"

 

Lithium batteries are not typically 3.7V for each cell. That is their partially charged storage and selling voltage.
They are 4.2V for one cell or 8.4V for two cells when fully charged.

You do not reduce the heat by adding a resistor that wastes battery power. Instead you turn on and off the heater with Pulse Width Modulation that wastes very little battery power. My electric stovetop uses PWM on and off.

 

The cheap trick to reduce the power without wasting any power in resistors is a variable duty cycle controller. I have done it with test system heaters and it works very well. For ski boots, where the rate of heat loss does not change very much nor very fast, a simple timer circuit will be all that you need. Consider that one second on and one second off gives an effective half power delivery and one second on with three seconds off takes it down to quarter power. You can do that with a simple 555 timer and a carefully selected FET power transistor switch. And if the transistor is located in an area needing heat then your efficiency will be close to 100%. But the controller will need to have redundant cutoff switches in case there is a failure.
The big caution is that you will need to pay attention to how warm your feet are.

 

The cheap trick to reduce the power without wasting any power in resistors is a variable duty cycle controller. I have done it with test system heaters and it works very well. For ski boots, where the rate of heat loss does not change very much nor very fast, a simple timer circuit will be all that you need. Consider that one second on and one second off gives an effective half power delivery and one second on with three seconds off takes it down to quarter power. You can do that with a simple 555 timer and a carefully selected FET power transistor switch. And if the transistor is located in an area needing heat then your efficiency will be close to 100%. But the controller will need to have redundant cutoff switches in case there is a failure.
The big caution is that you will need to pay attention to how warm your feet are.

Is that what mtbcoe wants to know? Or was he answering the question posed by the original poster in the original thread? Five years later, I'm not sure there's an issue for us to resolve.

 

Is that what mtbcoe wants to know? Or was he answering the question posed by the original poster in the original thread? Five years later, I'm not sure there's an issue for us to resolve.

My response was to the post #1 on January 2, 2023. The TS asked for a compact efficient way to control the heater temperature. That is what my suggestion was. Not sure what the 5 year reference was all about.

 

The thread this originally comes from was from five years ago. Will be 5 years I think it was March, or maybe May, 2018.

Blair - 18650's are typ. 3.7VDC. Sounds like your pack has two in series = 7.3VDC. If you can reorient those in parallel, you'd have something maybe closer to the correct voltage & therefor wattage , but twice the capacity runtime.

Doesn't sound like a question to me.

 

OK, I get the point that the portion I did not see was a 5 year old post.

The question that I responded to was in post #1, Dated Jan 2, 2023.
That question asked for an efficient way to control the heat, which my response answered. So if somebody repeats a question presented on an old post, which I did not go back and look, since this post was identified as #1, is that so very bad???
And, by the way, duty cycle modulation is a very common and effective power control used in industry.

 

Original post is from February 25, 2018. The post (post #1 in THIS thread) initially started out as a post on that old thread. Post #1 answers a 5 year old question. I saw that too.