Review: Opus BT-C3100 ("dreamcharger!")

Hi, Everyone! ^^ I am really excited to post about this charger. :bigsmile:

You may be thinking, "What makes this charger so special to invoke such feeling?" Well, I would say this is a next generation multi-chemistry charger; a dreamcharger, if you will.

Finally, a truly multi-chemistry charger, combining essential features we expect from intelligent Ni-MH/Ni-CD chargers with Li-ion charging capability.

Not only can it charge several cells at once of different size and type at current the user decides, it also provides useful data on charging, has discharge and refresh capability and the ability to measure internal resistance of cells.

The BT-C3100 was supplied by OPUS Instrument Co. Ltd for review. A special thank you to kreisl for helping with it. ^^

The name Opus may not be familiar. Opus designed and manufactured the AccuPower IQ-328.

In addition to the charger unit, a 4 page (2 sheet) manual and 3A 12V adaptor was included. The adaptor is rated 100-240V and has built-in Euro plug connector, so some users will need an additional adaptor to suit their locale (fortunately I already one). For those in the UK do check the plug is rated 3A minimum and fused accordingly, as the power supply does not have a user-replaceable fuse.

Charger and PSU:

Label on PSU:

On the underside of charger there are vents, four soft feet and information about the product, including CE mark.

At back of charger a 25mm micro-fan and power input socket:

The first thing I noticed on unpacking was its dark blue livery. Casing feels like ABS and feels strong. In fact it has already survived a golf umbrella falling on it! :oops: Charger is quite light, weighing 8oz, just over 200g.

Four bays with spring sliders. Note the additional concave cutaways to bays 1 and 4, for charging 26650 cells.

The slider "feet" have two small raised bumps in addition to a single larger bump. These little bumps fit into the slightly recessed negative terminal on smaller cells with thick wrapping, such as my Ultrafire 10440s.

Each bay slider at minimum 32mm. For smaller cells such as 1/2 AAA size (10280) spacers or neodymium magnets can be used.

Slider under spring tension at maximum 73mm.

Side profile with cell inserted:

Top view with cells inserted:

With back removed:

Most notable is the 3-way slider near the edge of the PCB (top in the above picture) for an undocumented adjustable termination voltage set to 4.2V position, with option for 3.7 and 4.35V termination. More on this later. The two pin fan connection is also accessible.

On the inside of the lower casing is black tape sealing the vents. My guess is these cutaways were unnecessary:

With PCB removed, four thermal sensors are visible, attached to the upper housing near the positive terminals:

Side shots:

Plugging it in, white LEDs illuminate a familiar display. You may recognise it from the IQ-328. To be honest, I found this screen a little difficult to read at first. It takes a little getting used to (only a little!).

At power up, the display briefly shows all whilst it initializes. With backlight active, all display legends remain clear to read.

Features and Operation

There are four slots/bays, each providing up to 1A of charge or discharge. With cells placed in slot 1 and 4 only, charging current option of 1.5 and 2A become available:

Slot Charging current
1 200 300 500 700 1000 1500 2000
2 200 300 500 700 1000
3 200 300 500 700 1000
4 200 300 500 700 1000 1500 2000

Each bay is completely independent allowing for great flexibility.

The BT-C3100 has five modes of operation:

BT-C3100 Modes of Operation
Mode Display Description
Charge Mode Charge Charge to maximum capacity (for Li-ion, termination voltage of 4.2V can be changed by undocumented slider on PCB)
Discharge Mode Discharge Discharge cell to preset voltage (0.9V for Ni-MH and Ni-CD, 2.8V for Li-ion)
Refresh Mode Discharge Refresh Refresh capacity of old cells (3 complete charge-discharge cycles)
Test Mode Charge Test Check present capacity, determined by discharge (charge-discharge-charge)
Quick Test Mode Quick Test Display internal resistance of cell under load, in milliohms

Below the display are four buttons to program the charger. Pressing the Mode button cycles through the modes in the above order.

Pressing the Display button cycles through the information displayed:

Legend Description
mA Charging/discharging current
V Charging/discharging/termination voltage to two decimal places
mAh Capacity in milliampere-hours

Operation time in hours and minutes, updated every minute

Lastly, pressing the Slot button selects the slot to alter its mode, in this order: 1, 2, 3, 4, all, exit. The corresponding LCD legends begins to flash like a cursor. After ten seconds of no further input, this flashing ceases and the selected mode begins operation.

In use, I find it's a very logical, straightforward system. It takes some use to become familiar with but once familiar it is very easy to use. There are no hidden menus or special button combinations to remember.


With no cell inserted the display reads "null". Inserting a cell into a bay, the BT-C3100 detects chemistry and the LCD flashes in Charge mode. The user can then choose the charging current by pressing the Current button. The default charge current is 500mA.

If the charger detects a full cell, it will display "Full". Pressing Display button will show the termination voltage.

To test this, I inserted freshly charged Sanyo eneloops and the charger would enter charge mode. With a multimeter connected to measure current, one could observe the cells being charged at 500mA for an additional <15 minutes before entering trickle charge.

The manual states trickle charge is "about 10mA". In my testing the BT-C3100 the trickle charge varied a little but never exceeded 40mA.

Full Li-ion cells were always detected as "Full" with no charge current applied. Occasionally the charger will detect a Li-ion cell that is near 2.8V as "Full"; more often cells were only just under 4.2V. It is a quirk and fortunately it is possible to override this by pressing the Mode button to cycle through the modes as one might to choose a different mode, and select Charge.

Is it possible to charge an already full Li-ion cell? I tried this with some unprotected Sanyo 18650 cells in each bay just to be sure:

Charging "Full" Li-ion cells (4.20V)
Slot Time to termination (mins) Termination voltage (V) Additional charge (mAh)
1 2
2 instant detection - -
3 instant detection - -
4 7 4.21 11

I would not worry about Li-ion cells being accidentally overcharged by this charger.

One criticism of this charger is not being able to tell whether the charge cycle is complete at a glance. For someone who keeps the charger close by it is perhaps less of an issue, although some might find the fan noise a little annoying (more on this later).

The LED backlight automatically turns off after ten seconds, and only turns on when power is connected, a cell inserted or a button pressed. There are no flashing lights to cause distraction.


One of the nice features in this charger is the discharge mode. For Li-ion cells, it is possible to track degradation in capacity by performing periodical discharge tests.

It is also useful for Ni-MH cells. Discharging at 1A I was able to see my Contour Energy AAA cells could not keep up with 3rd gen Sanyo Eneloop AAA. I was also able to restore some capacity to some very old Philips cells that other intelligent chargers refused to charge.

Test Mode

This mode combines charge and discharge cycles to check the capacity of a cell. The manual doesn't specify exact behaviour, however.

After inserting a cell and selecting charge current the charger enters charge mode until the cell is full. During this stage the display will read "-- --" mAh. It then enters discharge mode, discharging at the same current until the capacity is ascertained. It then enters charge cycle again to restore capacity, without updating the total discharged capacity.

So, if you do not mind leaving the charger running, you can pop cells in for a capacity test and come back to them fully charged.

Quick Test - Measuring internal resistance

The manual goes into some detail about this, so much so that I almost want to copy and paste the whole section. There are also several articles on internal resistance at Battery University.

The manual stresses that measurements will vary from slot to slot and with contact resistance. With repeated testing I observed that some bays are consistently lower than others in their resistance figures, so I would recommend noting the slot used when recording figures, and performing several tests and taking the average. Cleaning contacts of cell and charger with isopropyl alcohol can also help.

Averages of repeated internal resistance measurements



slot 1


slot 1


slot 2


slot 2


slot 3


slot 3


slot 4


slot 4

1 11 80 94 88 100 86 41 39
2 84 78 91 83 92 89 44 43
3 83 77 89 83 91 85 43 41
4 84 86 86 89 93 83 41 40
5 86 77 89 82 92 97 49 40
6 491 87 95 82 92 88 44 40
7 86 78 93 85 93 87 43 41
8 89 79 99 86 94 97 41 42
9 86 85 105 83 97 95 42 40
10 86 80 87 88 107 87 43 41
Average mΩ 118.6 80.7 92.8 84.9 95.1 89.4 43.1 40.7

In my sample, slot 4 produced the lowest internal resistance measurements, followed by slot 3, followed by slot 1 and 2 (no order).

Internal fan and thermal performance

Another thing that sets this charger apart from others is the inclusion of a small 25x25x8mm fan at the back of the charger. This fan runs at 12V and turns on automatically when temperature inside the charger exceeds 40°C, and turns off again when temperature is below 40°C.

In all my testing the cells never got hot. Even repeated charging and discharging AA eneloops at 1A, the cells only got warm.

There is also additional overheating protection with a sensor under each slot near the positive contact. If these reach over 60°C the charger will halt operations on all four slots until temperature cools to 40°C.

The only downside to this micro-fan is the additional noise, but I would rather have the cooling and peace of mind than a quiet charger that cooks my cells! Really, it is as if the designer was reading BLF last year and taking notes. :surprise:

I made two short videos to accompany this review. The first demonstrates charging operation:

In the second I test the 60°C thermal protection:

Technical data

Operating Voltage 12V DC
Power Adapter Input: 100~240V AC, 50/60Hz
Output: 12V DC, 3.0A
Charging Current Range 200,300,500,700,1000,1500,2000 mA
Discharging Current Range 200,300,500,700,1000 mA
Max. Charging Capacity 20000 mAh
Operating Temperature 0 to 40°C
Voltage Deviation <0.04V
Current Deviation <5%

Where do I get one?

Currently this charger is not available to buy in the Western market. ;_; At time of writing there are few relevant links for "BT-C3100":

It will certainly be interesting to see how much this charger will sell for, and if other manufacturers will take note. Will it replace a multimeter and hobby charger? No, nor does it strive to. What it does do is combine useful features in a very useable package. I have been using it almost every day since it arrived!

(fixed big pictures, sorry! >.<)

Noteable reviews


By UPz:;t=8246

By mjlorton:

By Mountain Prophet:

I wanted to see how the charger performed without the internal fan, so I set the charger to Charge Test at 1A on a Sanyo 18650 2600 mAh cell and noted activity and temperature of slider (the hottest part to touch). Cell was already full so BT-C3100 soon went into discharge mode.

Fanless Charge Test 1A - Sanyo 18650 2600 mAh

Time (hrs:mins)

Temperature Shutdown (mins)
Discharge 1A 00:35 41.1 5
01:00 51.5
01:40 54.5
02:00 52.2 5
02:10 45.5
02:45 12
Charge 1A 00:46 5
01:34 40.5 5
01:39 37.2 5
01:48 37.7

Charge at 1A took 3hrs 27 (an additional 15 mins for cooling). The cell was already warmer than usual from the discharge cycle prior.

Discharge Tests

Discharge Capacity Tests #1 (using Charge Test mode)
Cell Type Slot


Capacity 24hr rest? Order

Ultrafire 10440B unprotected

1 500 265 n 1
2 500 265 n 2
3 500 265 n 3
4 500 252 Y 4

Trustfire 14500A protected

1 500 816 Y 2
2 500 831 Y 1
3 500 811 Y 3
4 500 790 Y 4

Sanyo 18650A (2600mAh) unprotected

1 1000 2528 Y 4
2 1000 2521 Y 3
3 1000 2533 Y 2
4 1000 2563 Y 1

With repeated testing I observed a decrease in capacity measurements using Charge Test mode, which does not allow any rest time between discharge and charge operations. To eliminate this I will make more tests using modes Charge and Discharge, noting rest time after discharge.

Discharge Capacity Tests #2

(using separate Charge and Discharge modes, with rest time between all operations)

Cell* Charge Current (mA)

Discharge Current (mA)

Slot Capacity (mAh)
Voltage after discharge**

Discharge Time


Rest after charge


Rest after discharge (hrs)



Unprotected 2600 mAh

1000 500 1 2502 3.24 5 40 27.5 1
1000 500 1 2517 3.3 5:02 21 8.5 2
1000 500 1 2518 3.29 5:03 27 7.5 3
1000 500 1 2495 3.4 5 25 4.5 4
1000 500 1


3.2 5 39 n/a 5

Trustfire Flame



500 200 1 776 3.13 3:53 30 1.5 1
500 200 1 774 3.04 3:52 24 1.5 2
500 200 1 773 3.05 3.52 24 1.5 3
500 200 1 776 3:53 24 6 4
500 200 1 783 3.15 3:55 24 14 5

* New(ish) cells

** Cell voltage after discharge can vary depending on how long cell has rested.

Tests were performed at stable room temperature 22.1-23.9°C. There was no loss of capacity when cells are allowed to rest between charge and discharge cycles.

Just asking;-)

Cool review and fun videos :slight_smile: I wonder if the 3.7V setting is for LiFePO4 (usually 3.6V) and if it also changes the discharge level lower than 2.8 V.

I don’t know. :stuck_out_tongue:

Thanks, HBomb! I don’t have any LiFePO4 but I will test this now with some cells in case.

Update: I tried with an unprotected Ultrafire 10440 and it terminated at 2.8V as normal.

Thanks for the nice review Chloe (and kreisl, so I understand).

It looks like this charger has many sought-after features, for me especially the many charge-current options. Is this the 'dreamcharger' that Kreisler has been so mysterious about? I do not like the fan though. I do like safety so some extend but I am not fanatic about it, and if safety means an audible fan that turns on and off regularly (I charge my cells in the living room) I don't like where chargers are going.

Excellent review! Very thorough.
Forgot to expect a bear. Caught me. :smiley: Was that a Rilakkuma? Looked different.

Thanks for a review :slight_smile:
that fan at the back side really looks like a patchwork, but, I would pay $30 for the charger with psu, not more thou…

I just picked up a BT-C2000 charger to replace my dearly departed La Crosse BC-900. I like the BT-C2000 even better than the old La Crosse- it is easier to operate and shows more data about the cells. It was on sale for under $40 here
I wonder if they are clearing out the old BT-C2000 model to make way for this new BT-C3100?
I hope my 26650 cells can fit in the new BT-C3100.
Edit- I just noticed a 26650 being charged in the photos at the link provided in the first post.

Chloe Thanks for the Very Good Review.

Nicely written, thorough review.

Thanks Chloe. Looks like a nice charger.

Nice review Chloe. :)

(Kreisl, looks like a dleam. A dleam come tlue.. :party: )

Now lets hope price is good..

Thanks for review, Chloe and all the fine folks around here who helped to make it possible, even those, who are away(Kreisdinoboyler) :D!

For someone, who doesnt have neither proper Li charger nor Ni charger, I mean, no fancy features, fast charging etc, this seems like it could become one charger to rule them all!

My subjective observations based on this review:

What sets this apart in positive way is internal resistance measurement - I think its good to test really old or abused cells, for cells that work fine and arent ooold, I think one can live without this feature(I mean, I own the king of NiMh's - C9000, I had to say something in its defence :D! )!

Second thing - ability to charge lithiums and test their capacities just the way its been possible with NiMh batteries(by discharging and not charging) and it also looks like it will support ALL major lithium chemistries, nice! No other charger can do this with both NiMh and Li batteries!

Now, IMO comes the part where something could have been improved even more:

Charge current is little too low for such a versatile device, 1A x 4 and 1.5-2A with 2 batteries, not an issue with everything sized up to AA, but everything of 2A+ capacity will take a while. I expected 2A x 4 this time :D!

Fan - noise might not be an issue if used during daytime, but longevity of one could be. Then again, I hope its standard size fan so replacing one shouldn't be a problem.

As I understood - Li chemistry switch is hidden inside and is not available without taking charger apart?

Price factor might play a big role - while its not yet available, it would be no-questions-asked purchase for 35$ shipped world wide for many, with at least 1 year warranty, but given how pricey BT C2000 is, which is above 40$+shipping around 20$(perhaps some have it cheaper), I might as well just get myself higher quality hobby charger and get busy with it(read on why) :D!

Other than that, for 35$ I could buy it, however, given that I have C9000, Sunopux(which I trashed in my review, but its still usable, its just not as impressive as Shan's appears to be for the same price), i6 clone(still learning it) and some TP4056 boards with tiny voltmeters to play with, so, given all this, even 35$ is a stretch for a device that isnt yet perfect for someone, who owns lots of other chargers that all have more charging power and ports and features when combined :D!

YET, if this keeps on receiving good reviews and long term feedback turns out to be great(remember NiteCore i4 V2 - everyone hailed it when it was released, but it turned out to overcharge-fry AAA NiMh's and over/under charge lithium batteries in many cases), then it is definitely going to be on the top of my charger-addiction addition list. Ok, lets be honest, ADDICTION list :P!

I now have more chargers than I need for every day tasks, so owning one that does them all would be nice addition to the list :D!

But only when it turns out to be great! Oh and, I hope they start selling this thing worldwide fast, so other companies will start to move to the same direction and release their versions of smart Ni-Li charger!

Once again, thanks for everyone who made this happen!

Now, HKJ's review should tell us whether it has proper charging algorithms and behavior! I hope he has one on his table :D!

impressive Chloe,mostly impressive :smiley:

Thanks, everyone!

It is possible to disconnect the fan (I may add a little switch). If it gets too hot the thermal protection will halt any charging/discharging if the cells or inside reaches 60°C. I will do some tests to see how it fares without it. An IR thermal camera would be nice!

That was Control Bear (wearing a Hello Kitty hat)! :bigsmile:

Looking for some discharge figures!! Thanks for the review Chloe!

Oh, and the 6th number in your resistance test is quite unique.

My thoughts as well; 2A charge is more than double my current (hah) chargers. That and the selectable charge rate, capacity testing, and a few other features are quite nice. Hopefully the price is reasonable.

Thanks for the review! :)

I have been testing on this charger the last two weeks (That means around 40 charts).

Most of the time it is ok, but I have recorded some examples of bad behaviour.