Charging NIMH Batteries - Getting HOT in Charger

[koala]

Re: Charging NIMH Batteries - Getting HOT in Charg

Regarding the thermistors, has anyone here seen one that works? How does a thermistor mounted underneath a layer of plastic(charger body) able to correctly measure the temperature of a cell? My point of view being these thermistors 'trip' only when the cells temperature are way over the limit.

I recently posted a picture of a NiMH cell cooked in a Sony charger. The heat not only destroyed the cell's plastic sleeve but also melted the charger's charging bay. The termistor did not work or perhaps the intelligent charger did not even have one.

I guess the way manufacturers can sell more rechargeables is to build 'hot' chargers that cook cells so normal consumers would have to buy new cells frequently. Looks like new technology creating more landfill. Well, at least each rechargeable cell is reused a couple more times than primaries.

-vince.

 

[Lynx_Arc]

Re: Charging NIMH Batteries - Getting HOT in Charg

I would *guess* the thermistors are connected thermally to one of the metal charging tabs as this would be the best way to measure temperature of a cell.

 

[MrAl]

Re: Charging NIMH Batteries - Getting HOT in Charg

Hi again,


Vidpro:
My charger has four slots, but each pair of cells are individually charged.
Yes, i'd have to monitor 4 cells i guess.
And yes, they dont charge evenly in this stupid charger i have
The temp rise is usually chosen to actually be a 'change' in temp, like
say 40 deg C or something, above ambient.
The reason Li-ion can be handled differently is because the charge regimen
is very different. Using a current limit, the voltage detemines when the
charge is to be dropped, and this is very easy to measure. The delta Vf
is much harder to measure because it involves only a couple tens of mv
of change.
I have to agree that with all the problems they still work for some things
very well. I've said that about the Rayovac rechargeable alkies too.

Sub_Umbra:
Dont worry, be happy, and be prepared to buy more cells when needed /ubbthreads/images/graemlins/smile.gif

koala:
Yes, there are lots of thermistors that work.
Heat travels across short distances very well, so it doesnt take much
to get through thin plastic. I'd prefer to mount the thermistor on
top however, with some moderate but firm pressure, right in the middle
of the cell. I envision a small, wide arm with a thermistor installed
in a cutout indentation in the bottom of the arm so it can contact
the cell directly when the arm comes down, or perhaps through a tiny,
thin piece of metal for protection. The cell placement has to be just
right so it has to be constructed carefully to allow for variations
in cell diameter or whatever.
As Lynx had mentioned, it might be a good idea to mount it at one end
of the cell instead.

Any method that involves NiMH cells is *supposed* to be backed up by
a timer that shuts off after the longest possible required charge time.
This shows that 'none' of the methods are 100 percent reliable.


Russ (aka Lynx):
Yes, that's probably a good idea.


Take care,
Al

 

[VidPro]

Re: Charging NIMH Batteries - Getting HOT in Charg

MR AL
""The delta Vf
is much harder to measure because it involves only a couple tens of mv
of change.
""

i never realized it was such a SMALL drop that they were trying to read, untill i read about those Controllable Computerized chargers.
i am really tempted to buy the one that does the 4 seperate channels with the LCD display in it, as it sounds totally cool.

and trying to stick with the temp high idea, there is also temp "breakers" and temp "fuses" i have seen them pasted into packs of both types.
a temp fuse blows if the cells were getting to hot, and the pack becomes useless, but then again, chances are the cells in it were useless anyways
i assume that uses a tiny spring type thing, in there that gets releaced when the solder melts that holds it in. various melting temperatures of solders make various temperatures of thermal fuses?

and
a temp breaker which disconnects when the temps get to high, then reconnects when it goes back down.
i assume they use dissimilar metals that flex the contacts away when the temp gets to high.

of course both of those are for catastrofic failures.
BUT
what if we can get hold of a specific temperature thermal breaker , and use that, and a locking logic curcuit, or locking relay. so when it goes open the first time, the curcuit doesnt reconnect.
Always trying to find the EASY way out of the situation /ubbthreads/images/graemlins/smile.gif

course the prefered
a microcontroller, that watches temp rise on a minute basis, and when temp rise is to a certian level it starts to watch for a fast increase in temps.

still wont work for cold cells, that are charged, but stuck back in the charger anyways. or weird thermal changes in ambient temps.

do you have any ideas how long the temp has gone very high, before the normal cutoffs (besides timer) kick in?

and how MUCH higher it gets when it has gone from charging fast, to overcharging.

Ohhh Silverfox /ubbthreads/images/graemlins/smile.gif


a thought on the tab temperature reading.
that would read the "center" of the battery better, and be GREAT designed into the charger itself, no thermal probe to deal with, just stick the battery in.
BUT :-(
there can be a GAP between the electrolyte roll thing , and the tabs. so it would still read off the metal, but it might have less thermal contact to the part were trying to measure.
on the other hand, the anode wire is right behind the top tab (often) and it comes from the center of the cell ???

oh hey, how about a FOAM at the bottom of the battery tray, with the themister attached, the foam will keep the thermister tight against the battery, and no action from the user required. as sort of spring loaded thermister holder.

 

[lasercrazy]

Re: Charging NIMH Batteries - Getting HOT in Charg

2 of my 2400mah titanium batteries get really hot and can sometimes take 7-10 hours @500-700mah to show charged. Seems kind of odd and dangerous, not to mention sever overcharge and reduced life. This happens in both my vanson and ansmann chargers.

 

[VidPro]

Re: Charging NIMH Batteries - Getting HOT in Charg

[ QUOTE ]
lasercrazy said:
2 of my 2400mah titanium batteries get really hot and can sometimes take 7-10 hours @500-700mah to show charged. Seems kind of odd and dangerous, not to mention sever overcharge and reduced life. This happens in both my vanson and ansmann chargers.

[/ QUOTE ]

when they have been on the charge for 7+ hours, and you measure the current (WITH a SEPERATE meter) , is it still 500+ ma?

there is some waste, meaning you do have to put in more than you get out.
and a primo charger might be slowing down, or stopping temporarily if it saw heat.

do you have a way to put a sepearte meter in there to read the actual current the battery is getting?

 

[MrAl]

Re: Charging NIMH Batteries - Getting HOT in Charg

HI again,

VidPro:
The 'time' it takes to get hot isnt quite as important
as the fact that it does get hot...because this time
depends on the cell capacity, cell size, and charge rate.
For a 2000mAh AA cell at 1 amp it probably takes about 5 mins
to reach a temp where it's getting hot (after full charge
has been reached) but at 2 amps it gets hot much faster.

A tiny thermistor should work the best because it doesnt
take much heat energy to raise it's temperature.
I thought if it was on top it would work best because
heat rises. This doesnt mean it cant be on the 'bottom'
when the cell is installed in the charger, then turn the
charger upside down.

Foam has elastic properties that cause it to deform
permanently over time so i'd use a spring or something.


Take care,
Al

 

[VidPro]

Re: Charging NIMH Batteries - Getting HOT in Charg

but if the charger is upside down, then the heat rising wont get off of the batteries as quickly. it might be reduced slightly. unless the batteries are suspended in air (hmm another idea)

heat rises in air, which could negativly effect the thermal probe with turbulent convection currents, heat isnt going to move upward in the battery as well as it does in the air.
the top shouldnt be hotter than the bottom, even if the heat from the bottom is going past (via convection) the top to get out.
usually the top of the cells are cooler, because the air gets to them most (i sometimes roll them over)

Reverse that, and the bottom or the lower part of the batteries wouldnt want to be even hotter because some thermal probe was blocking convection and normal airflow.

(just tell me to shut-up anytime

so its a toss up, IF you were going to use the themal probe to not only discover the cutoff, but somehow also use it for thermal managment, then it wouldnt matter how much you blocked heat flow, you could even manage the temps for ANYTHING the user did with it, putting the cover on, putting it in a drawer, the sun blasting it from a window, or high ambient temperatures.
course i wouldnt want to be the one who had to do the programming

if you wanted to see more heat, it seems to occur where the most blockage of the air is, on some of my stuff the cells lay FLAT (horizontal) and the bottom is plastic already touching the cells, the top is more exposed and cooler.
(no big heat loss in sticking a probe there)

do we want to read the hottest place, or the coolest place on the battery? umm probably the coolest, because when it does the overcharge heating , that place will get just as hot as the other, and have the largest thermal change.
(so the probe should be on the top, when the bottom is a heat trap)

isnt there foam on the thermal probes for batteries already, to insure good contact with the battery, and reduced exposure to the ambient air?
the only thermal probes i have seen, have 2 sides to them, not that it made that much difference that the heat was only on one side.
to me, the probe needs to make excelent contact to the battery only, and insulated to anything else.

 

[lasercrazy]

Re: Charging NIMH Batteries - Getting HOT in Charg

[ QUOTE ]
VidPro said:
[ QUOTE ]
lasercrazy said:
2 of my 2400mah titanium batteries get really hot and can sometimes take 7-10 hours @500-700mah to show charged. Seems kind of odd and dangerous, not to mention sever overcharge and reduced life. This happens in both my vanson and ansmann chargers.

[/ QUOTE ]

when they have been on the charge for 7+ hours, and you measure the current (WITH a SEPERATE meter) , is it still 500+ ma?

there is some waste, meaning you do have to put in more than you get out.
and a primo charger might be slowing down, or stopping temporarily if it saw heat.

do you have a way to put a sepearte meter in there to read the actual current the battery is getting?

[/ QUOTE ]

I have no way to put a meter between the cell and charger since I don't have a meter anymore. It's only those 2 that do it, no matter which bay on either charger I put them in.

 

[VidPro]

Re: Charging NIMH Batteries - Getting HOT in Charg

do you know the "time" cutoff for the charger? is it ending only after time, and missing the Vdrop thing?

do the still have all thier capacity?

it sounds like they are overcharging, and might be total toast by now. BUT only somebody with one of those chargers would know for sure.

the vanson is that 10x single charger? is its rate that hign? my knockoff version of a 10x is slow, takes 10-12 hours to FULLY charge 2300-2500s.

 

[SilverFox]

Re: Charging NIMH Batteries - Getting HOT in Charg

Hello Reviewum,

Keep in mind that it usually takes a few "conditioning" cycles to bring NiMh cells up to the point where they will charge properly.

The first charge on a NiMh cell should be a timed charge (ideally at 0.1C). You need to determine the charge rate of your charger and the capacity of the cells you are charging and keep track of the charge time. When the time is up, pull the cells from the charger. Don't wait for the green light.

By the third cycle, the cells should be "conditioned" and the charger should be able to be "trusted" to properly charge the cell.

Tom

 

[SilverFox]

Re: Charging NIMH Batteries - Getting HOT in Charg

Hello Sub Umbra,

I noticed your comments when you first posted about the problems you were having with bulged cells. It slipped my mind to comment on your problem.

Recently I have re-discovered that the end of charge signal is difficult to determine at low charge rates.

The best charge rate to determine the end of charge signal is 0.5C - 1.0C. That means you would need a charger capable of about 5 amps per cell...

Your 700 mA charge rate works out to about 0.08C. At that rate, you may want to consider it a "timed" charge and terminate the charge after a certain time has passed.

By the way... It was really quiet yesterday and I was wondering what the sucking noise was... /ubbthreads/images/graemlins/evilgrin07.gif /ubbthreads/images/graemlins/smile.gif

Tom

 

[SilverFox]

Re: Charging NIMH Batteries - Getting HOT in Charg

Hello Al,

The information I have on charging NiMh cells suggests terminating the charge when the cell reaches a temperature that is 20F to 40F above ambient. This is for a charging rate of 1C.

This comes from the RC people.

Tom

 

[STEVENT6]

Re: Charging NIMH Batteries - Getting HOT in Charg

Months ago I could have went insane trying to figure out what would be a decent charger. After a lot of reading I concluded that the Accupower20 would be pretty good all around charger. Right?? So may I ask what is a good charger that doesn't damage cells? In other words one that charges them properly to get a good life span?

 

[vcal]

Re: Charging NIMH Batteries - Getting HOT in Charg

I've used a charger from CCRane ($35.) for many years. It has never made batteries so hot as what's been described in the above posts. There are quite afew chargers (above $20.) that don't mistreat batteries.

My rule: if any charger is making batteries so hot that I can't hold them in a closed fist comfortably, I get rid of it! /ubbthreads/images/graemlins/icon15.gif

 

[MrAl]

Re: Charging NIMH Batteries - Getting HOT in Charg

Hello again,

VidPro:
Yes turning the charger upside down might cause other
problems which would have to be dealt with no doubt.
I was going to try a flip up arm with a thermistor
mounted underside.

Silverfox:
Ok thanks. I might end up trying the thermistor method
at some point because i really dont like the charger i
have now.


Take care,
Al