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Old Sep 23, 2012, 01:59 PM
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NiMH Batteries Overcharge Wrecked A Radio Transmitter

Yesterday (Sept. 22nd 2012) I accidentally wrecked a Turnigy 9X transmitter by overcharging its NiMH batteries.

Usually I leave the 8 NiMH batteries in the transmitter and charge them from time to time by plugging a 12V supply in the charging connector of the transmitter.
This charges the batteries over several hours.

Yesterday I plugged by mistake my netbook supply which supplies 2A at 20V instead of 12V.
The NiMH batteries overcharged and started to boil inside the transmitter.
I was next to them but unfortunately I had my headphones on so I only heard them "boiling" when it was too late. They make small "pop" sounds and other sounds similar to boiling water (but at lower intensity).

When I realized what happened, I disconnected the transmitter from the supply and put it in the bath tub. Then, I removed the back "hatch" which was very soft because of the heat and took out the hot boiling batteries using a pair of scissors to pry them out (I couldn't touch them).

I left the transmitter in the bath tub for a while face up so that the whatever boiled would flow downward and not towards the transmitter electronics.

Today I took some pictures of the damage (see attached pictures).

Even though at least one of the batteries melted its way through the plastic, nothing seemed to have contaminated the PCB.
I tested the transmitter using a 3s LiPo battery and everything works well.

The worst part is that the same thing happened to me before. I wrecked another transmitter which came with a RTF Art-Tech SU-27 exactly the same way about 2 years ago.
In that case I noticed the problem earlier and the result was that the battery holding compartment got melted and deformed. No battery melted its way though the plastics to the PCB.

Since I managed to "get stricken by the lightning" twice, I decided that it is too dangerous to continue as before. It's only a matter of time until I plug in the wrong supply again.
I ordered some round LiPo cells which have the same shape as AA batteries. Only two in series are needed for the Turnigy 9x. They are protected by an electronic circuit against overcharging, over discharging, etc..
For more details about them see:

http://www.rcmodelreviews.com/tendollarbattery.shtml

Meanwhile I could use some 3s LiPo batteries I have around, but they are too big and don't fit inside the transmitter. Maybe some improvisation is in order.
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