When I go to discharge 3 to 4 full batteries, after around 10 mins, it just stops discharging because the mA shows zero. Fan’s still blowing and display still on. If I do a charge, then all’s good.
I’m discharging the following…
26650
21700
18650
18650
Can someone solve this enigma??
It discharges by converting the power to heat, and it’s doing that at 1000ma x 4, so it overheats. I upgraded my fan to avoid the issue and save me lots of time.
PS - yours will kick back on and continue after it cools.
Ohhhh she’s just too hot. Whew I thought the charger was going out the window. Hey I got lots of different PC fans so is that what you used or is it a special fan? Thanks for help man. 
Oh on a side note, if the Opus has a fan and Xstar or Littlecola doesn’t and they both have discharge, what happens when those units overheat? Do they shut down too?
I tried different fan configurations, but the case has so few air holes it only helped. I finally removed the factory fan and adapted a 40x40x11 fan I had in my parts bin. It plugged right into the same old connector and actually blew air. You should hang a strip of Toilet Paper in front of the factory fan. It’s fascinating that it spins but moves NO air. What a joke. Anyway, here’s the adapter I am running right now: Opus BT-C3100 v2.2 40mm Fan Mod - Super Silent - by 3d_droide - Thingiverse
I put a case fan from an old pc under my Opus 3100 and I’ve never had issues with it getting too hot when discharging 4 18650’s at 1A. Since you have lots of pc fans you might try doing that.
Yeah I actually did the toilet paper test too when I first got it. I when so far as tearing little strips off till I saw movement.
I thought hmmm not much movement but hey it’s got a fan so good to go.
I’m gonna replace the fan now that I seen your mod. Hmmm maybe stick on an RGB fan! Give the Opus some razzle dazzle! 
Hey that’s an idea too. Kinda like on top of a laptop cooler or something. Hmmm it’s winter here and so maybe stick the Opus in the garage while discharging!
But I was just thinking. What if I did the fan mod then put the Opus on top of pc fans and put everything in the cold garage! Damm, charger probably turn into solid block of ice!
I remember reading in one of the long Opus BT-C3100 message threads here about this issue - the built-in fan will not be enough to cool the charger when it is discharging several batteries at higher than about 700mA (eg. discharging 4x 1A max discharge). The charger will stop discharging until it cools down a bit and then should resume discharging, although when this happens, (according to what I read), the discharged capacity rating will then become a bit too high (ie. becomes more incorrect capacity reading)… Try discharging at 4x 0.5A (check if the unit will also stop when discharging at 4x 0.7A).
The remedy as mentioned above (based on what I read) is to replace the fan with a stronger one so it doesn’t overheat. (the Opus fan is so small…)
Thanks d_t_a and I was about to ask about what happens to the readings when the unit cools down. I figure it’ll mess up the discharge count somewhat. Will try 700mA next time. 
Yeah I read that this unit uses large resistors to do discharge and those suckers get hot quite a lot.
Thanks all for the great response. Was a little worried that the unit was defective or something.
I stand mine on end, fan up, to let convection help with airflow. Also when charging. Never had an overheat problem. Admittedly, it's in a cool-ish basement. It's always in a milsurp steel ammo can with the lid open.
slmjim
I actually prop mine up on one end. I think mine overheats cuz I have a heater in the computer room so room temps are actually quite warm already. Gonna try different scenarios next few days. Main thing is unit not defective.
Are you using the standard wall plug power supply ?
I put in a better fan, upped the power supply to 5A, prop up the charger on it’s side and point a small (personal cooling type) USB fan that moves a lot of air at it.
Before that I used an 80mm PC cooling fan wired to a 12v wall wart and just set the charger on it. I used the metal screws that would normally fix it to a PC case to make legs on both sides so there was plenty of air movement. That worked fine also.
The reason the Opus even works at this discharge level is because it has fans. Pretty much all the chargers create heat on discharge, so are often limited to 0.5A. You need to get more dedicated devices if you intend to discharge at higher levels.
I’m using the one it came with which is 12v 3A I believe.
I have couple big pc fan and will try out different combos. Yeah heat is the thing to watch on these things. Just like Turbo mode in our flashlights. Should be fun to used summer time!
As flydiver mentioned. The Solution is a better fan AND Power supply[SMPS].
I have a .12A/1.44W fan and a 5.5A/66 Watt power supply.
The results are No more stoppages during discharge, Heat is a Non issue and the capacity[mAh] numbers are more accurate.
There’s nothing wrong with my power supply. And surely it has nothing to do with the discharge function, since the charger’s not drawing hardly any power at all while running the discharge cycle.
As far as accuracy improvements from the fan, it’s hardly worth speaking of. What can happen is a stoppage that occurs right before cut-off can give the battery a rest it doesn’t deserve, meaning it’s slightly more capable of meeting demands when the process starts again. In that one special case.
Mostly the stoppages just waste time.
I threw my Opus BT-C3100 in the trash after I had an issue with overcharging my 18650 battery.
Well that was wasteful as hell. There’s a switch on the bottom of the PCB to change the top charge voltage from 4.20 to 4.35 to 3.70. You should have asked for help.
At the CRITICAL transition between charging and discharging mine would stop/reset and lose ALL information. I think that certainly has something to do with the SMPS.That is certainly a waste of time.
Sometimes four plus hours depending on what batteries were discharged.
EDIT:Several years later and with the new SMPS[and fan] that has not happened once!
It was defective as I was charging 2 18650 batteries, and it charged 1 battery over 5.04 volts and the other battery was charged only to 3.6 volts.