30/45/80/120A LiPo Discharger Build

[Greywolf74]

Originally I was going to post this under my Comparing LiPos thread but since this is more of a build unto itself I decided to post an actual build thread here for it and then Ill just post up the finished project under my other ongoing Comparison thread.

Once this build is completed I will be able to do high current discharges of LiPos for stress testing purposes. I will be able to set it to either discharge LiPos at 30, 45, 80, or 120A. 120A is actually a very significant amount of current and I'm actually expecting it to ruin most if not all of the test lipos. I think one of the big take aways from these lipo tests is that C rating are grossly exaggerated.

So firstly I had to crunch all the numbers so I know exactly how many resistors I need to use and the exact amperage Ill hit using them. So as you can see my discharger is really gonna be more like a 29.6/44.4/81.4/118.4A discharger but close enough for gubment work. You can't really get it exact with out having some sort of control board integrated in to the unit which is a little above my pay grade.

Heres all the actual numbers I needed.

Thermal adhesive to mount the resistors to the heat sink material I bought cuz these puppies are gonna get muy caliente!

Heat sink material

A 100W 1 ohm resistor

Gonna need a total of 16 of these bad boys.

Gonna use the copper wire out of this home wiring to solder all the resistors and breakers together. Undecided if I'm gonna double up and use two stands per side or if one will be sufficient.

These breakers will be my on/off switches that will allow me to control how many Amps the discharger will pull.

and last but not least I'm gonna gut this old PC power supply and use the case and the fan to house and cool everything.

Ill post more pics as I make progress.

 

[tntpoof]

https://www.mouser.com/technical-resources/conversion-calculators/ohms-law-calculator
#s are a tad off..

 

[Greywolf74]

https://www.mouser.com/technical-resources/conversion-calculators/ohms-law-calculator
#s are a tad off..

My numbers are dead on balls accurate. In fact my numbers are more accurate than the calculator you posted because I go out 10 places past the decimal point in my calculations but even the calculators numbers are very very very close to mine.
Heres an example of how close they are. My numbers state that to get a discharge of 118.4A I need a 7.4V lipo on a total resistance load of .0625 ohms for a total of 876W.

Heres what the calculator says

So tell me how thats off exactly? Is it because I decided to round 876.16W down to 876W?

 

[STONER RC]

Wow the only one crying of .16 is my wife or more HAHAHAHA.

 

[tntpoof]

starting voltage.. why discharge a 7.4 volt battery .?so wattage and current is off.

 

[Greywolf74]

starting voltage.. why discharge a 7.4 volt battery .?so wattage and current is off.

The wattage and current is going to fluctuate throughout the discharge curve so there no good reason not to use nominal voltage for the calculations. From what I've seen a fully charged lipo will average somewhere between 3.8 and 3.5v/c for the majority of the discharge so 3.7v/c seemed like as good of a starting point as any. So no my numbers are not off they are exactly what they should be.

 

[Greywolf74]

Ok, got the resistors glued on to the heat sink material. Once this has cured overnight and the resistors are permanently attached Ill be able to start wiring them up and mounting the heatsinks themselves inside my project box.

 

[Greywolf74]

Got some more work done on this build. I was hoping to get more done but I realized at some point today I needed a step up converter to convert 6-8.5VDC to 12VDC to run the two fans that will be mounted on this. I ordered the step up earlier but I dont want to go any further with the build until I get it and make sure I know where I'm going to mount it. Heres what its shaping up to look like so far.

I built some feet on the bottom of the case because I have no idea how hot these resistors are going to get.

Then the two layers of resistors are going to be stacked on top of each other like this but I put some spacers in between them for better air flow.

Then I mounted the 3 breakers to the side of the case. This will allow me to choose whether I want to do a 40, 80, or 120A discharge according to how many breakers I turn on.

And then this is what it will look like with the big 140mm fan mounted to the top. I'm also eventually going to mount an 80mm fan to the side of the case as well to make sure air gets blown in between the two layers of resistors too but I havent gotten that far yet. Mostly though at this point I just have to wire everything up and touch up a few small things like the 80mm fan and the step up converter.

 

[STONER RC]

Got some more work done on this build. I was hoping to get more done but I realized at some point today I needed a step up converter to convert 6-8.5VDC to 12VDC to run the two fans that will be mounted on this. I ordered the step up earlier but I dont want to go any further with the build until I get it and make sure I know where I'm going to mount it. Heres what its shaping up to look like so far.

I built some feet on the bottom of the case because I have no idea how hot these resistors are going to get.
View attachment 130178

Then the two layers of resistors are going to be stacked on top of each other like this but I put some spacers in between them for better air flow.
View attachment 130179View attachment 130180

Then I mounted the 3 breakers to the side of the case. This will allow me to choose whether I want to do a 40, 80, or 120A discharge according to how many breakers I turn on.
View attachment 130181View attachment 130182

And then this is what it will look like with the big 140mm fan mounted to the top. I'm also eventually going to mount an 80mm fan to the side of the case as well to make sure air gets blown in between the two layers of resistors too but I havent gotten that far yet. Mostly though at this point I just have to wire everything up and touch up a few small things like the 80mm fan and the step up converter.
View attachment 130183View attachment 130184

Dude wish I had electrical skills. Nice job. I need a better faster way to discharge my lipos other then the stupid 1amp discharge rate of charger. It takes for ever. If I don't run or use all my batteries. In a session.

 

[olds97_lss]

Dude wish I had electrical skills. Nice job. I need a better faster way to discharge my lipos other then the stupid 1amp discharge rate of charger. It takes for ever. If I don't run or use all my batteries. In a session.

https://www.amazon.com/ISDT-Discharger-Wireless-Capacity-Discharging/dp/B07VS2JSN9
https://www.amazon.com/ISDT-Discharger-Discharing-Capacity-Discharging/dp/B0775PDWZM
https://www.amazon.com/Discharger-Intelligent-Discharge-Efficient-Dissipation/dp/B07ZD2D8YW
https://www.amazon.com/SKYRC-BD250-Battery-Discharger-Analyzer/dp/B087NFJN67

Or wait for the slow boat and get a cheap one like this:
https://www.aliexpress.com/item/1005002701340137.html

I use one of those, works well enough if you set it to discharge to about 3.6 and when the lights shut off/start flickering, unplug it, then plug the pack into your charger and storage charge.

 

[Greywolf74]

https://www.amazon.com/ISDT-Discharger-Wireless-Capacity-Discharging/dp/B07VS2JSN9
https://www.amazon.com/ISDT-Discharger-Discharing-Capacity-Discharging/dp/B0775PDWZM
https://www.amazon.com/Discharger-Intelligent-Discharge-Efficient-Dissipation/dp/B07ZD2D8YW
https://www.amazon.com/SKYRC-BD250-Battery-Discharger-Analyzer/dp/B087NFJN67

I recommend the SKYRC BD250 35A discharger. I have one and it works great. You can set the Amperage you want it to discharge at and the voltage you want it to stop at and it will show you how much mAh was drained too. I'm only building this high current discharger to torture test lipos for my tests.

 

[STONER RC]

https://www.amazon.com/ISDT-Discharger-Wireless-Capacity-Discharging/dp/B07VS2JSN9
https://www.amazon.com/ISDT-Discharger-Discharing-Capacity-Discharging/dp/B0775PDWZM
https://www.amazon.com/Discharger-Intelligent-Discharge-Efficient-Dissipation/dp/B07ZD2D8YW
https://www.amazon.com/SKYRC-BD250-Battery-Discharger-Analyzer/dp/B087NFJN67

Or wait for the slow boat and get a cheap one like this:
https://www.aliexpress.com/item/1005002701340137.html

I use one of those, works well enough if you set it to discharge to about 3.6 and when the lights shut off/start flickering, unplug it, then plug the pack into your charger and storage charge.

Thanks man appreciate the info and links

 

[Doom!]

@Greywolf74 Lookin' good, and good thinking on the feet!

 

[Greywolf74]

Thanks guys. Once I actually get it all wired up its prolly gonna look like something from madmax with all the wires the will be running in and out of the box lol

 

[Greywolf74]

My voltage step up converter is finally here!

I just realized though that this isnt going to work. If I used this and wired it up the way I was going to then once the lipo is done discharging the fan will stop running and I dont want that so I'm just going to wire up a switch and some connectors that can be attached to a separate battery. I may still use this if I decide to use a 2S lipo to power the fans but if I'm using a separate battery then I could just go for a 3S and skip the converter. I'll have to see what extra batteries I have that I could use. At any rate, I can get back to work on this now! I want to get in to battery testing already!

 

[STONER RC]

Man everytime you think up something the idea to run the tests. You get thrown a curve ball.. got some good electric skills and thinking.. that's awesome.. I'm dieing to see these test..and read read report... my info the better and I have no problem throwing some $$ towards the cause... and towards R&D of GREYWOLF BATTERY TESTING INDUSTRY.. LLC

 

[Greywolf74]

Man everytime you think up something the idea to run the tests. You get thrown a curve ball.. got some good electric skills and thinking.. that's awesome.. I'm dieing to see these test..and read read report... my info the better and I have no problem throwing some $$ towards the cause... and towards R&D of GREYWOLF BATTERY TESTING INDUSTRY.. LLC

I'm hoping to get the discharger finished up this weekend and then its just a matter of getting the new datalogger set up and working which shouldnt be to hard. That reminds me. I need to get some pics of the new datalogger I bought to show everyone.

 

[STONER RC]

I'm hoping to get the discharger finished up this weekend and then its just a matter of getting the new datalogger set up and working which shouldnt be to hard. That reminds me. I need to get some pics of the new datalogger I bought to show everyone.

That's awesome.. oh nice post up some pics sir

 

[Greywolf74]

Ok made a lot of progress today. Working with 8AWG wire is a PITA and it might not look like it but just getting 85% of the resistors soldered up today and getting the first coat of electric tape on was about 4.5 hours worth of work. Went through 1/2 a roll of solder too. I should be able to get this thing finished up tomorrow or if not Ill be reaaaaaaaaaaaaally close.

 

[STONER RC]

Ok made a lot of progress today. Working with 8AWG wire is a PITA and it might not look like it but just getting 85% of the resistors soldered up today and getting the first coat of electric tape on was about 4.5 hours worth of work. Went through 1/2 a roll of solder too. I should be able to get this thing finished up tomorrow or if not Ill be reaaaaaaaaaaaaally close.

View attachment 131437View attachment 131438View attachment 131439

Sweet good job man... guessing that's the liquid tape you talked about