Or, using ΔV/Δt, when slope of voltage went negative to previously measured slope of voltage line, corresponding to about 2 degrees earlier in the temp curve cutoff.

 

GP claim that some of the new NiMH batteries can endure 0.1C continuous charging for about one year. How true this I have no idea. Just referring what I have read

 

Why doesn't Sanyo Eneloop show the voltage "hickup" at full charge that other manufacturers show for charging Ni-MH cells and that charger ICs detect?

 

GP claim that some of the new NiMH batteries can endure 0.1C continuous charging for about one year. How true this I have no idea. Just referring what I have read

Very possible. The original ones was c/40 but I have seen some newer cells that claim C/10 is OK.

 

Why doesn't Sanyo Eneloop show the voltage "hickup" at full charge that other manufacturers show for charging Ni-MH cells and that charger ICs detect?

I am skeptical of a noticable voltage change. That is the reason we had so much trouble with these NI-MH cells when they came out back in the early 90's. All the Ni-Cd fast chargers terminated an the rise/fall of the cell voltage and the NI-MH cells did not show it. Or at least, so small that it could not be reliably detected. That is actually why the dT/dt method had to be used.


Looking at the plot, you see the point where the charge should be terminated (see arrow I added) is before any significant change in voltage but cell pressure is spiking. You can usually detect a faster rate of increase in temp to get it.

 

You would think that it would be simple for a Ni-MH battery cell manufacturer to put a pressure switch inside the cell that switches off the charging.
Or a temperature switch inside.

 

You would think that it would be simple for a Ni-MH battery cell manufacturer to put a pressure switch inside the cell that switches off the charging.
Or a temperature switch inside.

I know there is a pressure vent, not sure how a switch could be done.

They definitely put temperature switches inside the battery PACKS when they build them up. Every pack I have ever torn apart had those. They open when the cell gets too hot, most do not reset so the battery is done after that.

 

The other end of charge termination we used on NI-MH was the dv/dt, where the charger terminates when the rate of voltage increase starts to get faster. What makes it tricky is the voltage is rising all through the charge process but you need to detect when it starts to rise a little faster. You can see the inflection point on the V plot. You don't want to wait until the cell voltage starts dropping because by then it is severely overcharging.

 

Here are better curves:

http://www.hardingenergy.com/pdfs/3 Nickel Metal Hydride.pdf

You can see how picking up the rising V inflection point will terminate just before the overcharge starts to force the temp to spike. It also shows the dT/dt point.

 

Have you ever charged a Ni-Cad cell at the same time as charging a Ni-MH cell (question mark).
The Ni-Cad slowly gets cooler while the Ni-MH cell slowly warms up.

The question marks on my replies here sometimes look like this: ÉÉÉ
One-half looks like this: 1é2
I have a new Windows 7 operating system that might be doing it.

 

Maybe he could use the LM317 to detect the point where the voltage rises above X volts?

Again, we are not looking for a perfect smart charger, just something better than charging it with a resistor and a prayer.

 

Maybe he could use the LM317 to detect the point where the voltage rises above X volts?

Again, we are not looking for a perfect smart charger, just something better than charging it with a resistor and a prayer.

2 LM317s, one putting out 2.75v, feeding the second LM317 set up for limiting current during charge cycle. Once max voltage is reached, regulator will go into dropout, shutting off the charge. Change output of first LM317 to determine # of cells, I wouldn't go over 2 at a time with this lashup.

I've never seen a charger like that, but that's essentially 2 regulators, 4 capacitors and a resistor.

 

Maybe he could use the LM317 to detect the point where the voltage rises above X volts?

The neg delta V detectors have to have an accuracy and sensitivity of about 2 mV.

 

Have you ever charged a Ni-Cad cell at the same time as charging a Ni-MH cell (question mark).
The Ni-Cad slowly gets cooler while the Ni-MH cell slowly warms up.

Yes. The NI-Cd charge reaction is endothermic (cools) while the NI-MH is exothermic. When I did a AA ni-cd cell at 1C rate, the temp dropped about 2C but started to warm up after that.

 

2 LM317s, one putting out 2.75v, feeding the second LM317 set up for limiting current during charge cycle. Once max voltage is reached, regulator will go into dropout, shutting off the charge. Change output of first LM317 to determine # of cells, I wouldn't go over 2 at a time with this lashup.

I've never seen a charger like that, but that's essentially 2 regulators, 4 capacitors and a resistor.

Its nice to see someone else having a go at a solution.

I'm not sure that would work though? If the NiMH has issues with constant voltage charging then it's V probbaly drops as I drops, which is why it overcharges. By using two regulators you're basically back to a constant voltage circuit, and if I drops and battery V drops it will start charging again.

What night work is some type of latch, which permanently shuts down current when it reaches >X volts.

 

Maybe with a Zener Diode and a relay ?

Sorry, I know this is a little old but I have the same problem.
I am trying to build a simple Ni-MH battery charger for a project.

My battery pack is made of 8 AAA cells in series (8x 1.8V 1800mAh)
If I didn't make any mistake so far, that makes:

Initial: 9.6V @ 1800mAh
Fully charged: 11.2V @ 1800mAh

Undercharging the batteries is not a problem here, actually any "charged" voltage from 7V or 8V is fine (I will use it to power a 7805)

I've been looking for a rather simple solution (i'm not looking for a perfect charger, just safe enough not to blow my batteries) all day long without success

What is the charging current ratio for Ni-MH ?
Would a 12V 500mA input work ? 300mA ?

To stop the charging process, how about a 10V zener diode triggering a relay ?
Of course the relay would be closed and only opens when activated by the zener diode.

Basically like this, in a way that when the battery pack reaches 10V, it stops the charging process.

The thing is, I don't know how to connect the other end of the zener circuit in a way that:
-The charger itself doesn't close the zener
-It doesn't short-circuit the battery pack
Please take a minute to help me, i'm kinda desperate of searching how to charge this battery and not be able to focus on the rest of my project

If you think the above circuit would work, can you help me with the values ?
- What plugpack to use ?
- What resistor (if needed) to step down the current fed into the battery ?
- How to connect the remaining wire to avoid the problems listed above ?
I also have a 9.5V plugpack but it's AC to AC, so I'd have to use a bridge rectifier.

(Reminder: 9.6V 1800mAh battery - Any final voltage from 7/8V is fine)

Any help would be greatly appreciated, thank you

 

Please, i'm in a dead end, I need to have this finished for a friend's birthday