Cheap high power discharge load - Page 4 - RC Groups
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Feb 06, 2013, 08:52 PM
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Ultimate Discharger

Many thanks to all that have contributed to this project. Again thanks to redbird300 at Heli freak for getting me started on this awesome project.
This is what I came up with combining all our knowledge, The perfect discharger.
The Cell-log 8 operates the trip circuit through the power of the discharge batteries and shuts off.
Of course you can set your own specific cut off voltage as you please.
This unit uses amp settings instead of battery cell count for selecting the discharge rate. Totally your choice of settings, mine is set
at 6 - 14 amps discharge for 4s, and 4 - 9 amps for 6s packs. Amps selectable with switches, and led's show switch position, up to 250 watts for me.
My original schematic shows the trip circuit with load resisters.
Cost is around $50 , most parts in my garage from computers and years of saving.
Last edited by Honest Abe; Jun 20, 2014 at 12:03 AM. Reason: Correct info
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Feb 07, 2013, 03:34 AM
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That's a very nice discharger Honest Abe. Well done.
Mar 30, 2013, 12:57 AM
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I finally got around to upgrading my discharger from 15 to 20 amps. This is my finished ghetto cheapo compact 250 watt 3S discharger, completely built with parts I had. The current is minimum 7.5 amps, up to 20 amps in 2.5 amp increments. Every two pins of the connector adds another lm317.

Yes, I'm too cheap to buy a box and a switch. Note that the positive lead is well insulated up to the fuse. Do this at your own risk!
Mar 30, 2013, 01:01 AM
ancora imparo
jj604's Avatar
I like it! But are you sure the clothes peg has passed the electrical safety requirements tests?
Mar 30, 2013, 08:50 AM
Registered User
Originally Posted by jj604
I like it! But are you sure the clothes peg has passed the electrical safety requirements tests?
If you zoom in and look closely at the clothes pin you can see the UL and CSA logos.
Mar 31, 2013, 09:46 PM
Registered User
Now you just need to invent a plane that charges the batteries and you'll have the complete system!
Mar 31, 2013, 10:14 PM
Registered User
I didn't have a switch so I made one. That DIY switch has survived two years now and will probably last for many more.

By the way, the reason I can get 250 watts out of that fairly small heatsink is due to the high rpm and high static pressure fan. It's about all it can take though, my best guess is the heatsink reaches about 80 degrees C which is still not hot enough for the LM317's to cut back current on overtemperature. It takes a fully charged 3S 2200mah pack to storage voltage in about 3.5 minutes.
Last edited by spog; Mar 31, 2013 at 10:23 PM.
Jun 06, 2013, 08:30 PM
Registered User
Sprog, Is it fair to say that 8 LM317T's is the max for a CPU cooler such as the one you're using?
Jun 07, 2013, 04:48 AM
Registered User
I may have improperly estimated the temperature of my heatsink in my previous post. I can hold my hand to it indefinitely which means it's probably not at 80 deg C. I'd say my heatsink could accept another two LM317's. It's not a big heatsink but a very powerful fan. The Pentium 4 heatsink others have used is bigger than mine, I'd say they could probably take 10 LM317's with the stock fan. Maybe one of those users could comment on how hot their heatsink gets.I also noticed that most have been getting about 2.2 amps per LM317 at 3S voltage, while I get 2.5 amps. I think this is due to who manufactured the LM317, mine are made by Fairchild.
Last edited by spog; Jun 07, 2013 at 05:11 AM.
Jun 07, 2013, 04:09 PM
Registered User
Thanks Sprog. I'll go for Fairchild's LM317T then.
Jun 07, 2013, 05:55 PM
Registered User
The Pentium 4 heatsink may be able to take 12 or even 14 devices, so you may want to space them tightly on the heatsink when you drill and tap. I am impressed by how much power these cpu heatsinks can dissipate.
Last edited by spog; Dec 20, 2013 at 08:44 PM.
Jun 07, 2013, 06:45 PM
Registered User
Maybe I'll try a few more then. Not like it will be expensive to retrofit or even start over.

I picked up the fan below.

From the ad:
The copper base and high-density aluminum fins optimize heat dissipation
Plus a 92mm, 2200RPM cooling fan delivers an air flow of up to 48.4 CFM for exceptional cooling performance

$8.49 delivered to my door.
Jun 07, 2013, 08:13 PM
Registered User
Wow, super cheap heatsink with a nice thick base for tapping. That should work well. A bonus of not needing an isolation kit on the TO220 cases is that you can use 6-32 bolts instead of 4-40, since there is no shoulder washer. This makes it much more difficult to strip the threads in the aluminum. Please post some pics of your discharger.
Jun 07, 2013, 08:56 PM
Registered User
The thick base is exactly why I choose this model. Some of the other units I looked at had a really thin base.
Jun 24, 2013, 08:42 PM
Registered User


Wrong stuff sorry
Last edited by Honest Abe; Jun 20, 2014 at 12:05 AM. Reason: Bad diagram

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