cant find any dT

cornh · Jul 31, 2006

What are some chargers that use the recommended dT as a termination method? Most of the popular charger brands never mention it on the box nor website.

I looked real hard but these boast only -dV and T-max.

Varta
Sanyo
Hahnel
Minwa
GP
Duracell
Energizer
AccuPower

Didn't look any further.

What do you think?

Empath · Jul 31, 2006

Your user name has been changed to something more acceptable. We're a family forum, and expect all members to adhere to those standards.

cornh · Aug 1, 2006

thanks
but what about my question?

leduk · Aug 1, 2006

Not sure how you're searching but

Hahnel, Ansmann , Maha do them.

Cheers

cornh · Aug 1, 2006

thanks, but what I meant was why is there no mention of dT/dt on all but a very few battery chargers (lenmar, uniross)

If you open this link you'll see what I mean.
http://www.lenmar.com/Manuals/PRO66manual.pdf

Under specification page it clearly states DT (1C/1min).

Hahnel power station says dt (lowercase "t" = time) which is basically another way of saying that it has a timer.

In any case, I was wondering why none of the more reputable makers such as sanyo, GP and Varta don't have and/or mention dT/dt as their feature. All they say is something like "temperature sensor".

SilverFox · Aug 1, 2006

Hello Cornh,

Welcome to CPF.

Using DT to terminate a charge will end up with a less than fully charged cell. You need to follow the DT termination with a few hours of trickle charge to bring the cells up to a full charge state.

I can see why Lenmar uses it. A -DV of 10 mV is what you would use for NiCd cells. Charging NiMh cells to those specifications may cook the cells. By adding DT termination they did not have to redesign their charger.

When charging NiMh cells you should use 3 - 5 mV per cell for the -DV termination.

Another thing to consider is that pulse charging allows an advance charging algorithm to measure the internal resistance of the cell. High internal resistance equals heat. If you limit the charge rate based on internal resistance measurements, you avoid heating the cells up.

Tom

cornh · Aug 1, 2006

thank you SilverFox

So technically, dT/dt is not necessary for NiMH if dV termination < 5 mV, is that what you're sayin?
In other words, its possible to charge NiMH without "cooking" it (ie. shortening its life) based only on dV reading, correct?

What I'm trying to understand is the following -

Is dT/dt termination method omitted on most NiMH chargers only because its alot cheaper that way (and conversely do higher end chargers gain a lot by using it)?

wptski · Aug 1, 2006

cornh said:
thank you SilverFox

So technically, dT/dt is not necessary for NiMH if dV termination < 5 mV, is that what you're sayin?
In other words, its possible to charge NiMH without "cooking" it (ie. shortening its life) based only on dV reading, correct?

What I'm trying to understand is the following -

Is dT/dt termination method omitted on most NiMH chargers only because its alot cheaper that way (and conversely do higher end chargers gain a lot by using it)?

I don't really see how any charger can use more than one different form of termination. Like SilverFox stated, more charging is needed for a full charge. The dT occurs first then zeroDeltaV and then -DeltaV.

SilverFox · Aug 1, 2006

Hello Cornh,

I am not sure exactly why DT is not used in more chargers.

It sprung up when the industry moved from NiCd to NiMh cells. NiMh cells are more temperature sensitive to heat and this was a good way to protect against that.

It seems that as chargers moved from constant current to pulsed current and the -DV value was lowered, coupled with the customers demand for a fully charge cell as soon as the green light comes on, the use of DT as a termination signal diminished.

I believe there are also some conditions where DT may not be reliable. DT seems to work best at normal room temperatures. If you go to extremes either way, it does not seem to work as well.

Tom

eluminator · Aug 1, 2006

There is another way to do it that is simpler and might be more reliable. The slope of the voltage versus time graph changes when the cell nears full capacity. In other words the first derivative of voltage over time can be used. Saitek chargers use this method and call it delta volts square over delta time square.

When the cells are almost fully charged, the charging current is reduced and charging continues until the voltage drops by around 30 millivolts. The charging current may be important also. Saitek chargers analyze the cells before charging and use the appropriate charging current.

Apparently Saitek no longer sells chargers under their own brand, but their C Crane chargers work the same way. These Saitek/CCrane chargers are the best chargers I know of. They always seem to charge correctly no matter what the cell capacity, initial state of charge, or the health of the cell. They can charge NiCad and NiMH.

Here's the Saitek manual. It's a 1.2 MB pdf file.
http://www.hypercon.net/~blisscomm/Pictures/CPF/Saitek_CCrane_charger/Saitek_SmartCharger.pdf

MrAl · Aug 2, 2006

Hello there,

Just a note:
The squared quantity would be the second derivative... ((d^2)*v)/(dt)^2
They must be looking for the tail end of the charge curve with this criterion..possibly.

wptski · Aug 2, 2006

I think that I've read that -DeltaV is a slightly overcharged condition, so I assume that zeroDeltaV is full. A 30mv drop sounds like -DeltaV with a bigger drop. Some RC charges like the Duratrax ICE the sensitivity or voltage drop can be set. Not sure if it'll go to 30mv, 25mv might be the highest setting.

cornh · Aug 2, 2006

wptski said:
Some RC charges like the Duratrax ICE the sensitivity or voltage drop can be set. Not sure if it'll go to 30mv, 25mv might be the highest setting.

NiCD <=> NiMH setting on some chargers is probably nothing more than a sensitivity regulator i.e. more sensitivity for NiMH and less for NiCD

eluminator · Aug 2, 2006

wptski said:
I think that I've read that -DeltaV is a slightly overcharged condition, so I assume that zeroDeltaV is full. A 30mv drop sounds like -DeltaV with a bigger drop. Some RC charges like the Duratrax ICE the sensitivity or voltage drop can be set. Not sure if it'll go to 30mv, 25mv might be the highest setting.

If you are refering to my post, realize that -DeltaV is only used after the cell has been nearly charged as seen by dV²/dt² and the charge current reduced. It's used when the charger is in "top off" mode. I think just reducing the charge current causes some voltage drop.

Actually I'm not sure if the criteria for ending top-off mode is -30 mv. Saitek doesn't reveal all it's secrets. It seems that the charger switches from top-off to trickle when the voltage drops by 30 mv from what it was when switching from fast to top-off. That's based on sitting there and staring at the voltage display. It's boring but it's easier than building a data logger

I guess I should mention that using this method instead of the delta temperature seems to work in cases where there is virtually no temperature rise. For instance if I charge a fully charged AA cell, the charger will go through all three charge phases and switch to trickle in about 5 or ten minutes. The cell stays almost at room temperature I think. (gotta build that data logger

)

Anyway I've been charging NiCad and NiMH this way for over 4 years and I see no ill effects. How warm the cells get depends on what I'm charging. I think the highest temperature I measured was 130°F (54°C). Maybe some day I will try to see if the highest temperature occurs during fast or topoff mode.

eluminator · Aug 2, 2006

MrAl said:
Hello there,

Just a note:
The squared quantity would be the second derivative... ((d^2)*v)/(dt)^2
They must be looking for the tail end of the charge curve with this criterion..possibly.

I forgot much of what I ever learned about calculus. I was taking a chance that nobody else remembered their calculus either. I should have known I couldn't get anything past MrAl

wptski · Aug 2, 2006

cornh said:
NiCD <=> NiMH setting on some chargers is probably nothing more than a sensitivity regulator i.e. more sensitivity for NiMH and less for NiCD

The Triton refers to it as peak detection 5-20mv for Ni-CD and 3-15mv for Ni-MH. The ICE states that it determines accuracy of the peak detection circuit with 5-25mv for Ni-CD and 3-15mv for Ni-MH. Additional on the ICE is a zeroDeltaV setting which I've never used.

wptski · Aug 2, 2006

eluminator said:
If you are refering to my post, realize that -DeltaV is only used after the cell has been nearly charged as seen by dV²/dt² and the charge current reduced. It's used when the charger is in "top off" mode. I think just reducing the charge current causes some voltage drop.

Actually I'm not sure if the criteria for ending top-off mode is -30 mv. Saitek doesn't reveal all it's secrets. It seems that the charger switches from top-off to trickle when the voltage drops by 30 mv from what it was when switching from fast to top-off. That's based on sitting there and staring at the voltage display. It's boring but it's easier than building a data logger

I guess I should mention that using this method instead of the delta temperature seems to work in cases where there is virtually no temperature rise. For instance if I charge a fully charged AA cell, the charger will go through all three charge phases and switch to trickle in about 5 or ten minutes. The cell stays almost at room temperature I think. (gotta build that data logger )

Anyway I've been charging NiCad and NiMH this way for over 4 years and I see no ill effects. How warm the cells get depends on what I'm charging. I think the highest temperature I measured was 130°F (54°C). Maybe some day I will try to see if the highest temperature occurs during fast or topoff mode.

I've read a few different papers but never heard of dV²/dt². I read that a cells voltage levels off when fully charged(zeroDeltaV) and starts to drop when it's being overcharged(-DeltaV).

eluminator · Aug 2, 2006

Here are a couple of voltage and temperature graphs. You can see that the slope of the voltage changes at about the same time as the temperature increases.

It seems to me it's a lot easier to measure the voltage, and it is instantaneous. Measuring temperature requires a sensor and unless it's an IR sensor, it requires some kind of reliable thermal contact with the cell. At best there would be a time lag involved.

This graph is from this pdf file:
http://www.panasonic.com/industrial/battery/oem/images/pdf/Panasonic_NiMH_ChargeMethods.pdf

This graph is from this pdf file:
http://www.hypercon.net/~blisscomm/Pictures/CPF/Saitek_CCrane_charger/linearTechnology.pdf

cant find any dT

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