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Old Apr 23, 2015, 10:20 AM
enjuhner357 is offline
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Thermoelectric/Peltier Device Discharger w/Low Voltage Cutoff Circuit

Hi all! So this is my first post as I just joined, but I have been lurking reading info here and there for a while. There's definitely some good info on here but I'm needing some extra help. My DIY electronic skills are good when it comes to reading schematics, soldering, the physical build, and multi-meter use. However, my background is mechanical engineering so my electronics design/calculation skills are lacking.

I have a 7S li-ion battery I want to discharge at >6A with thermoelectric/Peltier module(s). I'd like to build a voltage cutoff circuit that goes between the battery pack and the Peltier module(s) that will disconnect the load once the battery reaches 25.2V for storage purposes. Is this concept possible?

If possible, can a single cutoff circuit handle a single large Peltier module or X number in parallel to achieve a 10-15 amp discharge rate? Or would parallel cutoff voltage circuits need to be designed? Right now I'm using a 24V headlamp to discharge the battery at approximately 2.1 amps...not ideal as it takes a good while.

I've read that there are different ways of building such circuits but my lack of understanding of the interaction between components prevents me from designing. TL431 vs. op amp vs. MOSFET...I've read datasheets of these components on Digikey but don't really know how to apply that information to my project.

I have read this thread and helped me understand a bit more: http://www.rcgroups.com/forums/showthread.php?t=725572

Any input is much appreciated! Thank you in advance!
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Old Apr 30, 2015, 05:36 AM
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Curious why you think a Peltier would be a good idea?

Somebody suggested a 12v liquid heater in another thread. You can get a small heater element that runs on 12V and can be immersed in liquid. I really can't imagine a better device to discharge large current and the heat generated.

Google "12V Car Immersion Heater" to see what I'm talking about. WAY cheaper than a peltier junction, can't overheat, and draws the right amount of current as-is.
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Old Apr 30, 2015, 09:58 AM
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Curious why you think a Peltier would be a good idea?

Somebody suggested a 12v liquid heater in another thread. You can get a small heater element that runs on 12V and can be immersed in liquid. I really can't imagine a better device to discharge large current and the heat generated.

Google "12V Car Immersion Heater" to see what I'm talking about. WAY cheaper than a peltier junction, can't overheat, and draws the right amount of current as-is.
Good suggestion I do not want to deal with any fluid and seals. The fluid would eventually reach an equilibrium if the heat is not removed effectively i.e. cooling fan across tanks surface. I've burnt up a few water heater elements by pushing them too hard (Einstein Refrigerator project in college).

I was thinking something like this from Sparkfun or Digikey: https://www.sparkfun.com/products/10080

A couple heatsinks and fans would be easy to bolt up. I'm just not sure how to make the circuit work on a 7S li-ion battery.
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Old Apr 30, 2015, 02:16 PM
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Quote:
Originally Posted by enjuhner357 View Post
Hi all! So this is my first post as I just joined, but I have been lurking reading info here and there for a while. There's definitely some good info on here but I'm needing some extra help. My DIY electronic skills are good when it comes to reading schematics, soldering, the physical build, and multi-meter use. However, my background is mechanical engineering so my electronics design/calculation skills are lacking.

I have a 7S li-ion battery I want to discharge at >6A with thermoelectric/Peltier module(s). I'd like to build a voltage cutoff circuit that goes between the battery pack and the Peltier module(s) that will disconnect the load once the battery reaches 25.2V for storage purposes. Is this concept possible?

If possible, can a single cutoff circuit handle a single large Peltier module or X number in parallel to achieve a 10-15 amp discharge rate? Or would parallel cutoff voltage circuits need to be designed? Right now I'm using a 24V headlamp to discharge the battery at approximately 2.1 amps...not ideal as it takes a good while.

I've read that there are different ways of building such circuits but my lack of understanding of the interaction between components prevents me from designing. TL431 vs. op amp vs. MOSFET...I've read datasheets of these components on Digikey but don't really know how to apply that information to my project.

I have read this thread and helped me understand a bit more: http://www.rcgroups.com/forums/showthread.php?t=725572

Any input is much appreciated! Thank you in advance!
Peltier modules are very non-linear in discharge current versus voltage input. Those unit's I've seen are rated for 12 VDC. Putting 24VDC on one will burn it up very quickly. The Peltier unit I have pulls around 6 Amps at 14.4 Volts DC while the car is running. It drops to about an Amp with 12 Volts DC applied by my power supply.

There is another issue here. That would be the required monitoring of the individual cells in your battery pack to make certain that none of them go below safe voltage minimums. If one of the cells in your pack is slightly low in balance, it would discharge first, before the other cells, and damage can and will result.

I've made a unit that would do the job. But it was a lot of work, and was not cheap. Much easier to go to something like a CellPro PL8 or similar charger that has this function built in.

Here is a bit of info on that project:
http://www.rcgroups.com/forums/showt...0#post31493910
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Old Apr 30, 2015, 02:38 PM
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Good suggestion I do not want to deal with any fluid and seals. The fluid would eventually reach an equilibrium if the heat is not removed effectively i.e. cooling fan across tanks surface. I've burnt up a few water heater elements by pushing them too hard (Einstein Refrigerator project in college).
No troubles really. Just put the thing in a coffee cup full of liquid as it was designed for and discharge the batteries to the desired level.

As long as you're not pressurizing the liquid it can't get above 212F, which is well in the range the heater is designed for. If you've got some crazy requirement that would boil away a whole cup of water then just use a bigger cup!

If you want current control then add in some PWM switching. A PIC could pretty easily be made to do the switching and monitor the voltages.
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