Nimh overheat

Nimh overheat
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All Forums >> Radios, Batteries, Clubhouse and more >> Batteries & Chargers >> Nimh overheat

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Nimh overheat - 7/4/2004 3:30:38 PM

Allie33

Posts: 158
Joined: 12/24/2001
From: Palm Bay, FL, USA
Status: offline

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.

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Post #: 1

RE: Nimh overheat - 7/4/2004 5:34:17 PM

mr.rc-cam

Posts: 536
Joined: 6/6/2002
From: West Coast, CA, USA
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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.

(in reply to Allie33)

Post #: 2

RE: Nimh overheat - 7/4/2004 9:58:51 PM

Geistware

Posts: 12946
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From: Locust Grove, GA, USA
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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.

< Message edited by Geistware -- 7/5/2004 12:19:08 PM >

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Post #: 3

RE: Nimh overheat - 7/5/2004 3:41:18 PM

Ryans Rebel

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From: St. Martinville, LA, USA
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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

(in reply to Geistware)

Post #: 4

RE: Nimh overheat - 7/5/2004 3:48:05 PM

Flying Geezer

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From: Bowling Green, KY, USA
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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?

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Post #: 5

RE: Nimh overheat - 7/5/2004 5:18:17 PM

azvmax

Posts: 54
Joined: 10/24/2003
From: Snottsdale, AZ,
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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_1607487/anchors_1607487/mpage_1/key_mkii/anchor/tm.htm#1607487

< Message edited by azvmax -- 7/5/2004 10:23:15 AM >

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Post #: 6

RE: Nimh overheat - 7/5/2004 5:20:27 PM

Geistware

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

quote:

ORIGINAL: Ryans Rebel

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

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http://www.rcaircraft.net/
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Post #: 7

RE: Nimh overheat - 7/5/2004 8:52:13 PM

Silent-AV8R

Posts: 1824
Joined: 3/16/2004
From: SOCAL, CA, USA
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quote:

ORIGINAL: Ryans Rebel

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

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

(in reply to Ryans Rebel)

Post #: 8

RE: Nimh overheat - 7/9/2004 4:13:29 PM

mrfx2001

Posts: 48
Joined: 11/12/2003
From: Mechanicsville, VA, USA
Status: offline

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

(in reply to Silent-AV8R)

Post #: 9

RE: Nimh overheat - 7/9/2004 8:01:07 PM

sfaust

Posts: 1804
Joined: 9/6/2002
From: Boston, MA, USA
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quote:

ORIGINAL: azvmax
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_1607487/anchors_1607487/mpage_1/key_mkii/anchor/tm.htm#1607487

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'!

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(in reply to azvmax)

Post #: 10

RE: Nimh overheat - 7/9/2004 10:46:06 PM

Geistware

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From: Locust Grove, GA, USA
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Not true, you can charge up to 1C (1400ma) and still be within the MFG specifications.

quote:

ORIGINAL: Ryans Rebel

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

_____________________________

Aircraft Proving Grounds
http://www.rcaircraft.net/
Information on the building and flying of Radio Control Aircraft.
Site for hobbyist in the R.C. Aircraft modeling world.

(in reply to Ryans Rebel)

Post #: 11

RE: Nimh overheat - 7/10/2004 11:08:08 PM

Ryans Rebel

Posts: 1171
Joined: 7/6/2002
From: St. Martinville, LA, USA
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Thanks guys.

(in reply to Geistware)

Post #: 12

RE: Nimh overheat - 7/11/2004 2:26:18 PM

Allie33

Posts: 158
Joined: 12/24/2001
From: Palm Bay, FL, USA
Status: offline

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?

quote:

ORIGINAL: 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.

(in reply to Allie33)

Post #: 13

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