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The charger has a fixed plug for North American outlets. The input is rated 100-240VAC, 50-60Hz. The output is rated 4.2VDC, 350mA (it's about 4.6VDC open circuit). There is an LED that lights dark yellow during charge, and green when charge is complete and when no battery is connected.

The LED actually starts to become a lighter yellow when the battery reaches 4.1V, a lighter yellow-green at 4.15V, and is green at 4.2V. If you only charge to 4.1V, you will get more cycles; but if you charge to 4.2V, you get a longer flight time - it basically averages out.

During my testing, this charger only put out its rated output on extremely discharged batteries and just for a minute. It is typically only putting out about 250mA after 5 minutes, with the battery at about 3.87V. The battery will reach 4.1V after about 40 minutes. It takes another 15 minutes to reach 4.2V, which may give you an extra minute flight time.

So you can see why I'm a little dubious about people claiming that this charges their battery in 15-20 minutes. If true, I have to assume their batteries are old and only give a couple of minutes flight time. I usually get about 7-8 minutes.

Still, this will charge the battery 20-40 minutes faster than the USB cable charger, and the LED gives a reliable indication of when the battery is filly charged. I'm only giving it 3-stars because the output is weak (which is probably good for a single 150mAh battery - I use 2 in parallel for 300mAh, with a 1.5 hour charge time, and a 15-16 minute flight time), and it would be nice if the voltage was limited to 4.2V to minimize overcharging the battery.

I also measured the USB cable charger. It puts out about 250mA max, and the open circuit voltage is about 4.6V. The battery reaches 4.1V in about 60 minutes, and 4.2V in about 85 minutes. The USB plug will start an occasional dim blink at about 3.93V and 35 minutes; a dim glow/blink at about 3.96V and 40 minutes; and a dim glow at about 3.99V and 45 minutes. The glow will get brighter as the voltage increases. After about 2 hours, the battery voltage is 4.3V and climbing.

With either the wall plug charger, or the USB cable charger, since they both have an open circuit voltage of 4.6V, I can only assume the battery will keep increasing in voltage until the safety circuit in the battery cuts off the charge. This may ultimately protect the battery from failure, but is still not good to charge LiPo batteries over 4.2V. Do not leave the battery on either charger for extended periods.

A tale of two Syma 107s.
I have 2 nearly identical looking Syma 107 helicopters, one is red and the other is yellow. I have burned out (melted) one USB charger while trying to use it to charge the yellow 'copter, after a short period the blades started turning slowly and the 'copter was warm and the USB charger was melted. :(

The yellow 'copter charges ok from its control and the red copter charges ok from its controler.

This wall charger charges the red 'copter quickly, but overheats and causes the blades to turn when "charging" the yellow 'copter.

It turns out that the two 'copters, and their respective controlers are wired backwards in polarity from one another. On the red 'copter the plug on the side is "plus" toward the tail, while the yellow has "plus" toward the front. Possibly the yellow 'copter is a "knock off", but it looks authentic and flies great!

Update: I have tried the charger on more Syma 107s (one more Yellow and one Blue) as well as a Syma 026G chinook, it charges them well, so I must conclude that the *one* Yellow 107 that does not charge (polarity reversed) is the "oddball".

Some differences:
Oddball has a red led, rather than green on the heli main board.
Oddball lights a red led, rather than green on the remote control, when it is on, and not charging.
Oddball tail blade has two "squarish" corners, rather than all rounded.
Oddball heli bottom plate does not cover all of the small components (some resistors show)

This charger is fantastic. When I charge with the USB it takes about 1hr. This charger cuts it down to about 20-25 minutes. This truely lets you use your helicopter more. Well worth it and a great addition to the S107