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Apr 08, 2016, 06:45 AM
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Gemini AC/DC dual channel charger

Big thanks to Jim from Banggood who sent me an Eachine Gemini 2100 battery charger for review.

This charger is also sold under the Venom brand with slightly different cosmetics.

See picture 2

I normally only review stuff I have bought but this was a potentially attractive charger and I was interested to see how good it was. Most of my charging is done with higher powered units with separate power supplies but an AC charger is handy for those occasions when you don’t want all the hassle and size of the larger unit. A while back, I bought a Hitec X1 (made by SkyRC) just for the convenience and use it a lot, particularly for smaller packs.

There are plenty of dual and even quad chargers around that can charge two or 4 totally different packs at one time, and there are plenty of AC chargers around that do a single pack but there are not many that combine AC input and dual charging and decent power. The obvious one that comes to mind is the Graupner Polaron AC/DC Sports 240W made by SJ Electronics. The Polaron is a very nicely thought out quality charger with a unique vertical form factor, excellent color touch screen and menu system and dual 120W chargers when used with a separate DC supply. It drops to a total of 120Watt on AC however with that power shared over the two channels. The most powerful dual AC/DC charger I have found is the Ultrapower 400AC which has 400 Watts charge/40 Watts discharge on one channel with a 600mA balancing circuit and 200W/25W/300mA on the other. It is a conventionally shaped flat format charger.

UPDATE: I have since found this Ultrapower charger which claims to be a dual 120W (ie full 120W each side) on AC. It can do 4.3V not 4.35V LiPo by adjusting the TVC and has a touch screen and PC link but does not look like it is firmware upgradable.

My simple assessment of the Gemini 2100

It is a poor alternative to the Graupner in every respect except two important ones:

1) It provides a full independent 80 Watts per channel on both DC and AC input.
2) It’s a lot cheaper – well below half the price of the Graupner.

Those two factors alone will make it attractive and it is a good buy at Banggood’s $83 price.

There is an RCGroups thread on the Gemini 2100 charger here.

In detail

There are plenty of reviews around on Youtube and RCGroups from others who were sent a sample by Banggood with lots of pictures showing what you get and descriptions of it working and the functionality so I thought I would concentrate on a more technical opinion.

1) It does the job as described. In my tests it will do a constant twin 77-78 Watt charge of two 4S 5000mAh packs running off AC. Not quite the rated 80 Watts per side but very close.
2) Functionally it seems to work. Balance voltages are reasonably accurate and all the various charge modes do what are expected.
3) It is convenient. The vertical form factor originated by Graupner is a neat and compact benchtop solution. The Gemini is wider and shallower than the Polaron but it is similar in size.

From a design point of view it is pretty basic. What they have done is taken two separate charger boards and two separate off-the-shelf AC/DC 15V switched mode power supplies and packed them into a “mirror image” clamshell case with a control board across the front that simply switches between the two charger units. The controls are the usual basic 4 buttons you find on cheap chargers plus a fifth one that toggles between the two sides. Red LEDs show which side is currently being controlled. Simple but effective.

The actual construction is cheap. The two clamshells are accurately molded but are of quite brittle thin black plastic. The two power supplies (which are the heavy components) are held in by only two small screws. I doubt this charger will survive a drop from a bench without some damage. Alignment of the control panel relies on two small notches in the plastic and the skill of the assembler. A number of users have reported sticky buttons due to a slightly misaligned panel, and this charger is almost impossible to take apart and do a fix and reassemble without damaging the control panel.

On the other hand the quality of the PC boards and soldering is good. The OEM 15V supplies seem reasonably well made with good heat sinking, thick PC board and added insulation.

See pictures 3 and 4

The charger boards themselves consist of a daughter board with the Freescale (Now NXP) MC9S08P A60 8 bit microprocessor in a 64 pin package soldered to a main board. Both boards are good quality and the control FET is on a substantial heat sink. I am not sure if this is another standard OEM part used in other “50 Watt” size 4 button chargers or an Eachine original. It is possible Eachine make only the case and control board, or the whole thing is made by someone else and sold under different finishes to Eachine and Venom. Hard to tell these days with this stuff.

See pictures 5 and 6

The control board is less impressive. Electrically it is fine but in my view the display is a low-cost let down and the 5 push buttons are a weakness. See also Disassembly and Assembly section below.

See picture 7

Functionally it does the basic Li, Nixx, Pb charging fine but leaves a bit to be desired when you come to the detail. It has nothing of the sophistication, functionality or quality design of the Polaron.

1) Balancing and discharge currents are small. Balance is 300mA which is reasonably typical for low cost chargers, but discharge is limited to a pathetic maximum power of 5 Watts per side. That is only about 200mA for a 6S. Will take an eternity to discharge a large full pack to storage.
2) It can only do 3.3V, 3.6V and 3.7V chemistry Lithiums. No HV pack capability.
3) It cannot connect to a PC and be updated or use a PC software for logging and display. The 3 pin connectors are only for a (non-supplied) temperature probe. Importantly this means you cannot update the charger with new firmware if they ever do add High Voltage LiPo chemistry.
4) The two red LEDs which clearly show which side is active are good and the menu system looks logical enough on paper but it is actually rather confusing in practice for Lithium chemistry charging which is what I think most people will use it for. The “Select” button toggles through LiXX, NiMH, NiCd, Pd packs then Save Data, Load Data, User Setting and Li cell monitoring. However to change from LiIon to LiPo or LiFe you have to enter the User Settings and change the actual nominal voltage there. For people moving regularly between say LiFe and LiPo packs this is neither simple nor very obvious. Why there is a separate main item for NiMH and NiCd but not for LiPo, LiIon and LiFe is a bad decision in my opinion. The save and load data functions let you store 5 frequently used configurations however which overcomes some of the inconvenience.
5) There are a couple of questionable safety issues. Although it comes with a full 3 wire grounded power cord the 3 wire connection in the charger simply has no ground connected. This is not a true Cat II double insulated device, but I think the risk is fairly small. The OEM power supplies appear to be well made with adequate strain relief. The soldering to the power connection is a bit rough but the risk of a mains wire coming off and connecting with anything internal and conducting that is a potential danger is low.

See picture 8

6) More of concern is that they have simply connected the output of the two switched mode power supplies together (bad practice) and also connected these outputs direct to the DC input socket. This is common in other AC/DC chargers but means that if you use a LiPo as a DC source and leave it connected by mistake when you plug into the mains then an uncontrolled constant 15V is applied to the LiPo. Adequate warning is given in the manual but a couple of Shottky isolating power diodes would have been very low cost insurance.
7) Cooling looked like it might be an issue. This charger is powerful for a dual AC charger but the cooling design is very basic. There is no attempt to design a proper cooling flow with inlet venting as you might expect. A 12V case fan is simply mounted in a cutout in the top empty part of the case. The air flow just blows across the edge of the upper power supply with no attempt made to guide it around other components. Because the case halves are a mirror image, one of the fans should have been mounted backwards, otherwise they simply oppose one another and there is no airflow. This was apparently too hard, so Eachine just cut off one fan plug and soldered the leads with reversed polarity.

See picture 9

It means one fan is running the wrong way for the blade design but it seems to work OK. The fans go through some initiation ritual when you turn it on, no matter what cell capacity or charge rate is chosen, to tell everyone they are there - then they sulk and just go quiet. When the internal temperature gets up to about 70˚C they turn on and quickly ramp up to full speed. Not particularly noisy and they seem quite effective at keeping the temperature around a constant 70˚C. I measured the internal temperature by installing two thin film 100k thermistors on one of the power supply heatsinks and one of the charger FET heatsinks before I reassembled mine.

See picture 10 and 11

8) The next picture shows internal temperatures during a full power (80 watt per side) test at an ambient temperature of 28˚C. Given the fairly primitive design I was surprised how well they work.

See picture 12

The bottom line?

This is a basic but powerful charger for its type. It gives you a full 80 Watts charge capacity on both sides because of its use of completely separate charger boards and power supplies. This is simply two chargers in the one box with a common control board. It does the basics well but it is limited and fairly primitive by modern standards.
1) The display is hard to read and very agricultural. No color or touch screen here. Not even brightness and contrast adjustments.
2) Mechanically it is of only reasonable quality. The case is not very robust, internal components are held only by small screws, sticking buttons have been reported and it is difficult to disassemble and reassemble for repair.
3) It lacks common modern features like High Voltage LiPo charging and Internal Resistance measurement
4) It cannot be upgraded to include these as there is no PC connection.

Overall it is good value for the money provided you accept these limitations and there is nothing much else available with this power in an AC/DC twin output charger for the price in such a compact form factor.


Disassembly and assembly.

Best of luck if you need to take the charger apart to fix something like a sticky button. My advice – don’t. Here’s why.

The problem is that the two case halves have to be slid together while very well aligned as there are three components that have to slot into retaining sections in both halves: the mains power connector, the DC power connector and the internal control board with the push buttons and display. The last of these is tricky, as it has to fit on two small retaining notches, one in each case half and there have been no guide slots molded into the case to hold it in alignment during assembly.

Because these three components are on opposite faces of the case, the two halves have to slide together exactly and getting the control panel to click into place is not easy.

BUT four of the buttons protrude beyond the case halves and yet have to fit through holes in it so the whole job is impossible.

The centre button is no problem as it is on the case split line.

See picture 13

What they do is to assemble it without the button covers in place, then pop them on through the holes in the case after it is all screwed together. If done properly it all aligns and works fine but it is very easy to get the control board "not quite" seated which accounts for the sticky button issue I think.

It's fine for assembly but disassembly is almost impossible as the button covers are a tight friction fit and if they are too tight pulling them off in order to disassemble the case will pull the switch apart.

If you don't lose the tiny disk from inside the switch when it comes apart it can be repaired with some heat shrink. Looks a bit precarious but it is fine once assembled as the only force on the switch is then downwards.

See picture 14

Alternatively you can desolder the switches and replace them. They are a standard extended shaft miniature PC board push button. The 4 moulded covers just push on - AFTER you have assembled the charger.

Don't be surprised if you manage to break at least one of the replacements while struggling to get it all back together though!

Best advice I can give is - do not attempt to disassemble this charger. If it arrives with sticky buttons or any other fault return it immediately.
Last edited by jj604; Apr 15, 2016 at 03:42 AM.
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