How to build your own automatic and programmable 6S discharger

[redbird300]

Originally Posted by EatMorePizza

amazing raf. I cant wait to get back home and try one. As an enthusiastic electronics kit builder this will be great to try.

I just hope hobbyking wont start selling your creation.

Thx EMP. I'm working on a final version right now. As long as you follow the electric diagram exactly, you should be OK. If you have any ideas for changes or improvements, let us know ! And we want pictures of course !

HobbyKing is planning on making a 500 Watt / 19.99$ version as I heard, flames and explosions included. But no doubt that there will be a V2 soon after that, where the explosions will be much smaller.

[chansen1953]

ok Raf, here goes ( sorry for the bad photos !)

i used 2 x 6.8Ohm 100W resitors, , but put them in parallel, so the circuit should run about 7amps thru the load. seems to be ok heat wise, and I also have a 24V computer fan to add, which is why I setup the heat-sinks like I did...to form a "funnel" to enclose the air-flow better. (plus it was the only heat-sinks I could find )

I originally had the diode wrong way round, but changed it.. hopefully I did not bust the ALM output trannie ?
Is there any simple test I can do with a multi-meter to check the ALM circuit .. ?
I do have a few batts with a low cell voltage - but this does not seem to set the ALM working !!
what did you mean by "activate it " ??.. btw, I find the manual completely unintelligable.. in that it only has diagrams on what the menu displays.. no real list of default settings or how to change them.. ??
maybe you can tell me the key steps to "activate " ?
Thanks in advance !!

[redbird300]

Originally Posted by chansen1953

ok Raf, here goes ( sorry for the bad photos !)

i used 2 x 6.8Ohm 100W resitors, , but put them in parallel, so the circuit should run about 7amps thru the load. seems to be ok heat wise, and I also have a 24V computer fan to add, which is why I setup the heat-sinks like I did...to form a "funnel" to enclose the air-flow better. (plus it was the only heat-sinks I could find )

I originally had the diode wrong way round, but changed it.. hopefully I did not bust the ALM output trannie ?
Is there any simple test I can do with a multi-meter to check the ALM circuit .. ?
I do have a few batts with a low cell voltage - but this does not seem to set the ALM working !!
what did you mean by "activate it " ??.. btw, I find the manual completely unintelligable.. in that it only has diagrams on what the menu displays.. no real list of default settings or how to change them.. ??
maybe you can tell me the key steps to "activate " ?
Thanks in advance !!

Great job Chris ! Nice to see that somebody is actually building this !

The ALM circuit should give an output of around 23-25 Volt (6S voltage) when activated, you can simply measure this with a voltmeter over the ALM wires. A fully charged pack will activate the ALM output IF the alarm is set as "NC" in the menus. An empty pack will not, unless the alarm setting is "NO", which we don't want. It is easily possible to bust the alarm output, yes. The diode suppresses the large, inverse peaks caused by the coil when it is deactivating.

There is plenty of info in the manual on how to change the values, look at page 10, 11 and 13, I posted the manual on page 1 of this thread. All the steps are decribed there.

Do you use "Skype" ? If not, install it and I will talk you through it in no time, let me know your Skype username if you would have that. Mine is redbird300, lol. You'll need a microphone and sound of course.

www.skype.com (it's free)

[redbird300]

Meanwhile, om my workbench….

Four larger 0.6 K/W heatsinks, and four 0.82 ohm resistors in series (3.28 ohm in total) for more discharging power. That should bring the discharge capacity to about 170-180 Watt.
Already mounted them on a support plate, and wired the resistors.

[redbird300]

Have to wait a few more weeks before the new Cell-Logs will be here, but that didn’t stop me from already testing the new resistors and heatsinks.

I simply hooked up the pack to the resistors with a loose cable, while closely monitoring the voltage with another lipo monitor, and measuring currents and temperatures. I did this two times in a row, without a cooling down period in between.

Like this:


Here are the data and results:

Resistors: 4 x 0.82 omh in series / 50 Watt max dissipation each
Heatsinks: 4 x 0.6 K/W
Packs: 6S / 3000 mAh fully charged
Discharging from 4.2 Volt/cell to 3.85 Volt/cell (23.1 Volt for the pack)


1) First pack:
Start current = 7.12 A
End current = 6.70 A
Start temperature = 25 °C (ambient temperature)
End temperature: heatsinks = 50 °C / resistors = 60 °C
Ellapsed time = 8 min 0 seconds, give or take a second, lol.

2) Second pack:

Currents: identical to above
Start temperature = heatsinks = 50 °C / resistors = 60 °C
End temperature: heatsinks = 64 °C / resistors = 74 °C
Ellapsed time = identical to above

BTW, the packs needed 950 – 1000 mAh to become fully charged again afterwards.

Conclusion:

discharging a 6S pack in only 8 minutes is great, but you need a big heatsink if working without any active cooling like I did here. Temperatures are still a little bit high to my liking, guess I overreacted a bit on the new resistor values aka discharge power, you know me, lol. Four 1 ohm resistors would be perfect, and still make for 150 Watt discharge power.

I will leave mine like it is, I like the speed, it will allow me to discharge 7 packs/hour and I don’t wanna get even bigger heatsinks. The facts that the circuit will need no external power and that it makes no noise at all, are great, IMHO.

[chansen1953]

Raf,
tested the ALM output with it disconnected form circuit... looks like I busted it by either diode wrong way or when tried without any diode !!
no voltage reading when set to "NC" , with a full pack connected....
btw.. what does it mean when you hold down the top button and it says "starting ".. (still no voltage however !) is that to start logging ( a feature mine does not have ?)

I will see if I can find a new cell-log, that dosen't take too long to get here !!

[redbird300]

Originally Posted by chansen1953

btw.. what does it mean when you hold down the top button and it says "starting ".. (still no voltage however !) is that to start logging ( a feature mine does not have ?)

Strange, with my Cell-Log the top button is the "type" button", and brings me to the "select type" menu. It doesn't say "starting..."
The middle button ("hold") does, it says "start holding", and freezes the values on screen, until you push and hold the middle button again.
Logging is a feature that's only included in the more expensive version, I don't have it either Chris. Hope this clears things out ?

[chansen1953]

hi raf, yes, mine is the same.. just got it wrong.... have confirmed that the ALM output is DEAD.. so have ordered 2 more cell-log ( this is starting to get expensive now !!)

so.. now waiting for the new units.. meanwhile, I might add the 24v fan unit, and then think about how to convert it to version 2, without changing the relay ..

[redbird300]

The final version all wired up, no more supporting circuit board here, I simply ran all the wires directly from one component to another. The relay is a heavy duty type now (25A/30VDC - 30A/230VAC), which can easily be fixed with two screws.




And the housing closed. Too bad this is still the broken Cell-Log, I took the pic just to get an idea how it will look like.

[redbird300]

Completely finished now, resistors connected and battery included, lol.
Just need to hook up a Cell-Log with a working alarm port.
Well, that was it, up to another project, I guess.

[sutty]

Really good looking job Raf. Looks professional, I guess we shouldn't expect anything less.

I have to say though, you are the very last person in the world I would have expected to design and build this project. By the very design function of your charging station, you have a plan to charge only when necessary, and quickly, then go fly, and then leave them like that, discharged. For the odd time this doesn't work out, you could use your charger discharge function, no?

Not detracting from the project, it is a very carefully designed device, and I could certainly have used it until now, when you convinced me to operate in a different manner, like you, lol.

I know why you did it really. It is in the design, and solving a problem, and enjoying your hobby, and it will of course be useful, on the occasion that you forget to fly.

Just pulling your leg really Raf.


Cheers

Sut

[redbird300]

Originally Posted by sutty

I know why you did it really. It is in the design, and solving a problem, and enjoying your hobby...

You nailed it there, Andrew.

[gbrockway]

Does this only work on 6S?

[redbird300]

Originally Posted by gbrockway

Does this only work on 6S?

A quote from the first post in this thread at page 1:

- Neither did we try to make the device usable for any packs other than 6S lipos, the most popular kind nowadays, right after the 3S packs. As the 3S packs have less capacity, the problem seemed less urgent to me. You could still discharge these with any normal charger in a reasonable amount of time, or even build a second discharger suitable for 3S packs, by using a 12Volt relay, etc…

So yes, this specific version that I build is only for 6S, but in fact it takes only a few changes in the components (not in the diagram) to work on 3S, so it's kind of universal. Hope this helps.

[jperkosk]

My stubborn $0.02: 6Ss store a scary amount of energy, so I fully agree with the concept of discharging them in sudden flight cancellation situations strictly for safety reasons, even if it it costs you an additional charge cycle out of battery's life. 3Ss are not as scary, so just store them carefully, and that will do it IMO.

[redbird300]

It costs you an additional charge cycle indeed, but on the other hand the pack is discharged at a slower and more steady rate compared to a real flight. Furthermore, it is not discharged as deep, only to storage level, which corresponds to only 1/3 discharge in terms of capacity. It won't suffer that much, in other words. So IMHO, this does not count as a "real discharge" cycle, as the average pack would last much more cycles if it were always discharged this way. Let's say it counts for half a cycle or so, lol.

[jperkosk]

Good point, Raf. Can we settle on 40% of the charge cycle as a approximate total cost (discharge and then recharge) of discharging a Lipo to a storage level?

[redbird300]

Originally Posted by jperkosk

Good point, Raf. Can we settle on 40% of the charge cycle as a approximate total cost (discharge and then recharge) of discharging a Lipo to a storage level?

40% and a beer, and we have a deal.

[jperkosk]

No problem, I'll throw in a six-pack

[redbird300]

My new Cell-Log battery monitors arrived today, and I quickly hooked one up to the discharger. A fully charged, but badly balanced 6S pack was used for the test, I pushed the red button and waited.



The alarm output of the monitor was set at 3.85 Volt/cell or 23.1 Volt for the pack, whichever would come first. It is clear that in this case one cell would have to trigger the alarm long before the pack would reach 23.1 Volt. And it did.

Here's the screen right before the device stopped discharging, watch the weakest cell, that's cell 1, it is at 3.85 Volt:





And a few seconds later, the device de-activated and the voltages jumped right up, with no load to the pack connected anymore. But the discharger did not engage again, like we wanted it to behave. It's finished and approved. If I would want to end with some lower voltages, I could simply set that in the Cell-Log, it's a flexible setup.