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Aug 04, 2015, 05:40 AM
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Reaktor 2 x 300 Watt Ext Discharge Issue


Calling External Discharge Experts? "DCHG+" on iCharger & Reaktor

Hi All, I have the Reaktor 2 x 300 Watt charger (iCharger 2 x 206B in disguise I think?) & it charges very nicely thank you!

But discharge is another story.... 13.8V Power Supply, Regen OFF.
4S LiPo at 3000mAhr.
With an external resistor of 2.35 Ohms it will discharge happily at 7 amps. If I set it to a higher current it limits at 7 amps. (Same as my iCharger 106B)

If I use a lower resistor i.e. 1.41 Ohms & set the current to 10 Amps & press START it will ramp up to the 10 Amps, then ramp back down to just 1 amp.
The manual says it should be 500 Watt capable in this mode i.e. 25 Volts / 20 Amps etc, but it looks like it limits to 7.0 Amps just like the iCharger 106B... (4S at max charge of 16.8V & 10 amps discharge is only 168 watts i.e. nowhere near the spec'd 500 watts)

Is the manual wrong or is my unit faulty or wrong Firmware or what? Any ideas much appreciated... PS - If it's limited to 7.0 Amps I can live with that :-) Just needs a Manual Update - Of course if it's me then I'm all ears :-) Thanks, Baz (UK)

NB - tried it with 2 batteries in parallel to make 6000mAh as well so as to be sure I had enough current available. Resistors are 50W metal clad in various combos of series / parallel to get the low values & high enough power ratings, mounted on a finned heat-sink AND fan cooled! Looks like the charger has an internal "power limit" that doesn't allow external discharge currents of over 7 amps?

Tried the spreadsheet info found elsewhere & didn't help. Also I'm aware that the resistors will change value as they heat up, but don't know of that's enough for the charger to throw a wobbly - could try a bigger heatsink? :-) Also as it's a dual "2 in 1 box" kind of charger I tried both "channels" separately as it were, with the same results....
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Aug 04, 2015, 09:18 AM
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Set the discharge current to maximum (20A) and try it again with the 1,41ohm resistor.
Aug 04, 2015, 09:32 AM
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Thread OP
Tried that.... ramps up to about 8.6 amps over approx 30 seconds then ramps back down...
Aug 04, 2015, 09:51 AM
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Normally it should work without any additional help.
Now we want try it with a help.
Connect another resistor, for example your 2,35ohms in series to the 1,41ohms. Then start the discharge (setting 20A) again. Wait about one minute, then take a (electrical) bridge with a crocodile wire etc. over the 2,35ohm resistor. Now it have to work.
Aug 04, 2015, 11:53 AM
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Thread OP
OK... Firstly thanks for trying to help :-) With both resistors in series & discharge at 20 amp setting, press START & it ramps up to about 4.1 amps, wait till timer says 60 seconds then short out the 2.35 ohm resistor. Current rises to 10.5 amps & holds (I actually pressed STOP so I could report back & to be in a "safe" position :-)
So, why doesn't it manage 20 amps in any case? & why does your method help it a little? I'm still thinking power dissipation in the charger? What do you think? Thanks again, Baz
Aug 04, 2015, 01:25 PM
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Race Miata's Avatar
I had the same issue when I first got my charger in January and found the cause and solution back then. The issue is that the software slowly ramp up the discharge current (bad design) assuming the external load may not like a quick ramp up to full current (e.g. motor, light bulb). Whenever it's not allowing full current, the charger internals will heat up. When it hits to 40C the charger will reduce current (another bad design which increase heat even worse) so you'll never get the full current. My fix has been to run my modded hair-dryer so that with the 2 heat settings I get 2 resistances available. I start off with the low heat setting that'll do ~4A in DCHG+ mode with 6S and let my Reaktor 300W ramp up current. After 1-2mins I switch to high heat setting that'll do 8-10A with 6S and the DCHG+ mode do the rest.
Aug 04, 2015, 02:03 PM
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I may be wrong, but to get anywhere near the 20A you are hoping for, you may have to discharge into an automotive type battery. At our flying field, we use our 4, 6-volt wet cell batteries in our solar power battery bank to discharge into. We are able to max out the discharge capability of any of our larger chargers. Our heli-flyers can discharge their 6S, 10,000 mAh lipos in under 20-minutes. Keith
Aug 04, 2015, 02:18 PM
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Great! So it will work.
You could then save discharge with 10,5amps without any problems. Only your bridge-cable (bapass), should be also thick enough to hold the current.

Quote:
Originally Posted by bigbaz
...So, why doesn't it manage 20 amps in any case? & why does your method help it a little? ..
The start up, is the bottle neck of this method.
I will explain:
All the discharge current, has to flow through the charger. At the beginning, the discharge current is zero, the voltage is the full battery voltage on the positive charger output.
Then, during a time of about 1,5minutes, the charger is reducing the resistance of it`s output to such a low value, that the voltage on it, is about between 0,3 and 1,0volt. The charger is switching through its fets. The discharge current is rising to the max. value. Limited mainly only by the external resistor.
When the charger reaches this stadium, the further discharge is no problem.
Example: The discharge current should be 15amps, the residual voltage (on the output) should be 0,7volts, then the internal discharge power is: 0,7V x 15A = 10,5watts.
The problem is on the half way there.
Example: Your 4s battery, we will discharge at 15A. On the half way of the start up process, the voltage on the chargers output should be the half of the battery: 8,4volts and the half of the discharge current: 7,5amps. This means, the charger was loaded at this time at 63watts!
The junsis are so programmed in this mode, to hold the multiple of the normal internal discharge power (up to 100watts) for a very short time. However, not without limits. So on higher cell counts and higher discharge currents, the charger will stop the start up process.

Now, the trick is, to bring the charger with a lower load easier through this start up stadium. After this, switch on the full load, when the chargers fets are fully switched, without to interrupt it.
Aug 05, 2015, 06:57 AM
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Reaktor 2 x 300 Watt External Discharge Testing


Ok. Thanks to everyone who took the time to reply - I think I "have it" now :-)

As I suspected the charger, battery & external resistor are in series & the charger has an internal limit. It monitors it's own power dissipation during the External Discharge. (REGEN on OR off) (Power dissipation coming from V * A, therefore high volts drop at low current can mean high Watts OR low volts drop but high current - e.g. 1.0 Volt drop across the charger at 20 amps just hits the 20 watt limit.

As RNFLY says, if we can keep the volts to 0.7V or less, even better, BUT over 1.0V at anything over 20amps makes MORE than 20 Watts :-)

Unfortunately due to the ramp up from zero towards maximum amps as set, the internal MosFet(s) are working in their linear region (not completely on OR off) & cannot help but dissipate power (i.e. get warm due to "RDS ON" in milli ohms) In order to self protect, when the charger sees the dissipation within itself as greater than 20 watts it ramps back down.

If we fool it & set it running into a bigger resistor & let it settle, the MosFet(s) will be hard ON at that time & experience minimum voltage drop such that when we reduce the value of the External resistors (place the crocodile short across the bigger value) even though the current increases, because the voltage drop in the MosFet(s) is minimum the power limit is NOT exceeded & we can increase the actual current that we are discharging at - Phew!

For the record, I tried External Discharge with "REGEN ON" & it also ramped up then down.....

Time to either accept it "as is" (shame on the spec!) OR come up with a simple AUTO circuit that will reduce the resistance after, say 60 secs discharge.

I feel an additional MosFet circuit coming on :-)

I'll let you know if I manage it!

Once again, Thanks to all!
Aug 05, 2015, 09:26 AM
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Quote:
Originally Posted by bigbaz
.... OR come up with a simple AUTO circuit that will reduce the resistance after, say 60 secs discharge.
I feel an additional MosFet circuit coming on :-)...
I think, it is not absolutely necessary, to have a automatic solution for this. You could replace the crocodile cable, by a banana plug cable. Or you may install a switch. So only one movement of the hand after a minute, is enough.

Instead of the additional series resistor, a simple halogen light bulb, could also do the job.

If you want to observe the voltage drop, you could switch in a multimeter, to the chargers output terminals.
Aug 05, 2015, 01:23 PM
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Race Miata's Avatar
Quote:
Originally Posted by RNFLY
I think, it is not absolutely necessary, to have a automatic solution for this. You could replace the crocodile cable, by a banana plug cable. Or you may install a switch. So only one movement of the hand after a minute, is enough.
Or did what I did, mod a $10 hair-dryer by shorting out part of the heater elements with shirt pins to decrease resistance. Then just flip the low/high heat setting switch built-in the hair-dryer.

OTOH, I can understand why OP wants an automated solution so he doesn't have to wait for the 1st minute to go by before flipping a switch (or working an alligator clip).


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