Allie33

Well:

I charged my 2 year old 1400 MA nimh pack at the recommended max rate of 1.0A on my Hobbico MK 2 field charger. Starting voltage was 4.9 V.

Voltages after meltdown are:

Burned cell 0.0v
1.21
1.26
1.27volts,

Looks like a shorted cell to me. What do you think?

I well try to connect photographs.

mr.rc-cam

If it started out with 4.9V then a cell was not internally shorted. Perhaps the "0V" cell's outer wrap was compromised and it shorted against its neighbor. Either that, or the charger went nuts and pumped out more than 1A or the peak was missed and it kept on charging.

Geistware

My Feedback: (16)

I had the same thing happen to me with my 5 cell RX pack. It melted the pack and almost burned the wood in my plane. I think I will go to a smaller fuse to prevent this from happening with RX packs.

Ryans Rebel

My Feedback: (12)

I thought you were not supposed to charge Nimh batteieas more than 25% of their rating? So if it was 1400mah then the max charge rate should have been no more than 350 mah. Correct?

Ryan

Flying Geezer

My Feedback: (14)

Do you know how long the pack was on charge? Did you stop the charge manually? Do you think the charger detected peak? Was it on long enough for the safety timer to stop the charge? Do you get a voltage reading out of the pack itself, or can you only read indvidual cell voltage?

azvmax

My opinion is the MKII failed to detect peak.
I have A MKII that I cannot trust, especialy with NiMh.
Are you sure it started at 4.9?
Its a 4 cell pack, it should of been below 4.8.
If so then it was full.
The charger still should detect peak.

http://www.rcuniverse.com/forum/m_16...tm.htm#1607487

Geistware

My Feedback: (16)

Not true.
The limit that Sanyo and Panasonic set is 1C

Silent-AV8R

No idea where you got this from. The general guideline is never exceed 1C. So for a 1,400 pack that would be 1.4 amps max. Less is better, but 1C should not be a problem if your charger is a good one.

Bill

mrfx2001

My Feedback: (2)

Gentlemen,
Someone asked "where do you get those numbers from?", Well from the Man Spec sheets of corse. Save this link: http://www.tnrtechnical.com/default.asp Go there. Click the nicad\nickel metal button, find the battery in question then look at the spec sheet. Easy. Notice that a fast charge can equal 1C for the correct cells. Amazing isn't it.

Fred

sfaust

My Feedback: (11)

A 4 cell pack is not full at 4.8v. In fact, it could be as much as 75% discharged and still read 4.8v depending on where it is in the discharge curve. A fully charged pack will read around 5.6v just off charge. At 4.7v, the pack is usually very close to the drop off in the knee of the discharge curve. So, right off charge you will get readings in the 5.x to 4.9v range for a flight or two, then it will drop into the 4.8v range for the majority of its discharge time. As it nears the knee, it will then drop to 4.7v and rapidly fall off from there.

Its a good thing it wasn't a LiPoly pack in your airplane with this problem. It would have done more than 'almost burned the wood in your airplane'!

Geistware

My Feedback: (16)

Not true, you can charge up to 1C (1400ma) and still be within the MFG specifications.

Ryans Rebel

My Feedback: (12)

Thanks guys.

Allie33

Well thanks for all the input.

First I must correct the starting voltage figure. It was 4.7 volts NOT 4.9, I sorry that I mislead you with that figure. And yes, it was charged on the NiMH setting using a 12Volt booster pac as I drove to the field, less than 30 minutes.

As I see it, here is what I think happaned.

The end cell shorted internally.

It got hot enough to melt the heat shrink and melt the solder on the tab of that cell only. That cell has zero volts and 0.4 Ohms resistance and the other 3 cells, appximatel 1.4 Volts.

I can't blame this on the MK2 charger but that shorted cell sure looks suspicious .

What do you think?