It uses DC 5.5x2.1mm plug, isn’t it? Maybe something like this will work:
(it indicates up to 1.5A only though, which may not be powerful enough if charging at full speed)
I noticed the BT-C3400 comes with a car charger plug and was hoping to get that one as it looks a little more sturdy.
That is the one I purchased , OPUS BT-C3100 V2.2 Charger. I use it on all rechargeable batteries. So far so good.
I purchased 10 EBL 18650 Li-ion 3.7V 3000mAh Rechargeable Batteries on Ebay for $16.00 and tested them at 2600 mah so i kept them. Then i purchased 4 LG HG2 18650 3000mAh 20A Batteries | Authentic Original Battery LGDBHG21865 on Ebay and checked them at 1800 mah 
so i’m returning them for a refund.
This charger is a big help for me, bad for Ebay, Never buy from them again.
yes it is a great charger i’ve had it for about 2 years now and it is still running strong
and for the battery problem, the place I buy all of my batteries is: https://www.imrbatteries.com/ (link is external) every cell I have ever purchased from this site was real and I trust them, this is another site in case the first one was out of stock or vise versa https://liionwholesale.com
I just bought this charger and it behaves the same way for the IR test.
HKJ says to take the lowest reading when jiggling, rotating, putting the cells a few times in and out as chargers do that. I got the SM8124A tester and confirmed a few times that the lowest IR reading from the chargers is accurate. The chargers use a different method to determine cell IR than my tester, so I add 60% more from what my tester shows and so far it always ends up at the lowest IR reading from any charger.
@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
