@klrman: Thank you for filling me in. Sounds like there’s a method but a bit of a squirrelly one. 
I have a MiBoxer C4-12 on the way so I’ll have two references. I got a great price for one on Amazon Prime Day (according to some our other members).
:+1: 
No problem. All my chargers annoyed me first too with their swinging IR readings, but I finally got the hang of it and always double check with my tester as a backup and it has always been consistent now.
Hello guys,
I just came across this deal on BT-C3100 on BG (no affiliate):
https://www.banggood.com/Opus-BT-C3100-V2_2-4Slots-LCD-Display-Smart-Intelligent-Universal-Battery-Charger-p-1183072.html
Do you think this deal is legit? (I’m considering the fact, the price is double at local shops).
BT-C3100v2.2 for $34
Thanks 
I like my BT-C3100, except the fan.
So far I used it only for NiMH but even at charge current 4x 300mA the fan is audible. The noise come and go, switch from noisy to whisper.
But overall it´s my favorite charger, it works well with my cells and it seems it load more to the real capacity than my Vapcell S4. The current selection is good, I use mostly 300mA for AAA (400 would be nice, but I can live with 300), 700mA for AA cells also nice (miss it for the S4 and VC4SL)
Price sure has fluctuated on this model. Original price stated back in 2017 was $38. Right now it’s about $70. But back in late Jan 2022, it had gone down to $39. Maybe a spike of purchases triggered the Amazon pricing model to a new high test ceiling price. 5 years later, I wonder if it’s exactly the same or if Opus made any minor improvements.
I’ve been using the BT-C3100 since 2015 and like you, my main peeve is the whining fan
How do you like the Vapcell S4 over the BT? I know the S4+ can do max 3A, compared to BT at 2A
I’m thinking of retiring the BT and go for the S4+, mainly to charge Li-Ion, as for Ni-Mh I will use my MiBoxer C8
Thanks
Wire up a 80mm computer fan with a 12v wall wart. pointed at it and the onboard fan will rarely turn on, if at all. Careful what you choose or that fan will drive you crazy. Or, just use an aftermarket laptop cooler.
Some personal USB/battery fans work fine and are quieter.
The stock fan is kind of lousy. There are replacements that are better. But, ALL fans of that size with any kind of decent RPM whine.
If there is no cooling, your batteries will get warm>hot. That’s physics. At least the Opus does have some attempt at cooling. Most do not.
I took a fan out of a pc that I was recycling and wired it to the psu out of that same pc. I also power the Opus and other 12v devices from the same psu. The fan is so quiet that I’m not even sure it’s on at times and as flydiver said the Opus fan never turns on.
Many pc fans also run off the 5v psu rail, although at a lower speed, which will help noisy fans run quieter.
I mostly use NiMH, and for this I prefer the C3100 because of it´s range of charge currents, especially the 700mA for AA. The S4 offers 500mA and 1000mA, but nothing between.
The S4 looks better processed and it´s quiet, except the ringing at the end of the charge. An other impression is it looks like the S4 don´t load so much in the cell than the C3100, but I will make some tests in near future.
I have the C3100 since some months and payed around 40€
Good review. I have had mine for many years. No complaints. In fact I have two of them.
I’ve had the Opus BT-3400 v. 2.2 for about 5-6 years and the newer BT-3400 v. 3.1 for the past few years and love them both.
These are almost identical to the 3100s, save for an ungraded fan and the ability to toggle off the display.
They were specifically made for ‘rdana’ who sells on EBay.
Version 2.2 has a discharge cutoff of 2.8. volts and v. 3.1 cuts off at 3.1 volts.
Chris
Thanks for your reply.
I’ve been reading about the ringing tone at the end of charge
When the 3100 is flat, cell heat is pulled downward into the body of the charger by the airflow induced by the fan. That's not natural. In addition, heat from the internal circuitry wants to rise through the slots under the cells. That tiny fan is fighting a losing battle.
I stand my 3100 on end with the fan up, to allow natural convection to assist with cooling air flow. That alone has greatly limited fan run time and, when it does run, does so at low RPM most of the time.
slmjim
Not only jack up the charger for better air flow. Operate on kitchen counter stone, or a large flat stone cooking wear, helps as a heat sink
LiIion wholesale has them for 39.99
Regarding cooling and chargers with discharge/capacity test…
It’s good that the Opus BT-C3100 has a cooling fan, although it does get noisy.
The Vapcell S4+ and the Liitokala Lii-600 (both of which do not sport any built-in fans) will get quite hot when doing discharge/capacity test (the onboard temperature sensor display will usually get into the mid to high 50 degrees Celsius) when I do a discharge test at 500mA discharge (4 batteries at the same time) — and I live in a warm temperature country whole year around (average is around 30+ degrees Celsius, maybe getting to 25 degrees Celsius room temperature during early dawn time in the cooler month (yup, just probably 1 month of “somewhat cool”). Anyway, I bought some USB-powered fans to blow at the chargers when doing discharge testing on those 2 chargers.
Many other analyzing chargers don’t have fans, eg. the Xtar VC8, VC4S also don’t have fans, but have low discharge current (300mA).
The Xtar Dragon VP4 Plus maxes at 500mA discharge current (but the physical unit is much bigger, so it can probably dissipate heat a bit better even without cooling fan).
So, aside from the Opus BT-C3100, the only other unit that has built-in fans for discharge testing might be the SkyRC MC3000 (v1 has 1 fan; v2 has 2 fans).
Although there are likely other analyzing chargers with fans that I’m not aware of.
~
some other note:
I believe the Opus BT-C3100 cooling fan may turn on depending on the ambient temperature, even though the charger is not in use (but plugged in to power).
I’ve experienced a few times that the Opus BT-C3100 built-in cooling fan automatically turning ON (the unit is plugged in to power), even when there is no battery inside any of the slots, probably due to the warm ambient temperatuer. I wonder at what temperature it does this though…
Hm, don´t have the C3100 plugged in if I don´t use it.
I´m also a bit worried about the small fan because it reminds me of some old videocards with similar fans or chipset-fans of mainboards. These ones often fail because of cheap (ball/sleeve?)bearings. And if you don´t notice it the videocard failed. The difference between the C3100 and the other fans is, the other fans work mostly on highest speed and the C3100 use lower rpm or turn it off sometimes.
Too bad that the C3100 have no USB-output, else I would combine it with an USB-fan
A question, did Opus not have a homepage?
A big difference here is you can both see and hear if the fan is functioning. For a couple years I’d just take it out, lube it, and put it back. It was good for ~ 6 months that way. There’s a thread in BLF somewhere about a superior replacement. I finally did that and it’s been fine since, over 2 years. Still whines, when it turns on, but I point an external fan at it 90% of the time.
If the fan fails, you don’t burn out anything. Your batteries will just get warmer……kind of like the ones without a fan. 
I agee, a USB output could be handy for a fan. I’m using one but it’s plugged into an external USB output.
