So the voltage dropping before the battery reaches full charge is the reason why having an op amp shut off the charging circuit wouldn't work? Since the battery has 200 ma for 5 hours written on the side, does that mean that if I have the constant current power supply set to 200 ma, I could have a timer cut it off after 5 hours?

 

So the voltage dropping before the battery reaches full charge is the reason why having an op amp shut off the charging circuit wouldn't work?

From the article I referenced at batteryuniversity:

It is difficult, if not impossible, to slow charge a NiMH battery. At a C rate of 0.1C to 0.3C, the voltage and temperature profiles do not exhibit defined characteristics to trigger full-charge detection, and the charger must depend on a timer. Harmful overcharge can occur when charging partially or fully charged batteries, even if the battery remains cold.

 

So it could work to discharge the batteries and then use a constant current circuit to charge the batteries and then have the charging circuit stopped by a timing circuit?

 

So it could work to discharge the batteries and then use a constant current circuit to charge the batteries and then have the charging circuit stopped by a timing circuit?

Until one of the cells gets a bit low on capacity then it will get overcharged and go faster and faster downhill.

 

Are there microchips that I can buy to detect when the battery is charged?

 

So it could work to discharge the batteries and then use a constant current circuit to charge the batteries and then have the charging circuit stopped by a timing circuit?

You're being very stubborn about this.

From the second paragraph in the batteryuniversity article I referenced in Post #2:

NDV in a NiMH charger should respond to a voltage drop of 5mV per cell or less. This requires electronic filtering to compensate for noise and voltage fluctuations induced by the battery and the charger. Well-designed NiMH chargers include NDV, voltage plateau, delta temperature (dT/dt), temperature threshold and time-out timers into the full-charge detection algorithm. These “or-gates” utilize whatever comes first. Many chargers include a 30-minute topping charge of 0.1C to boost the capacity by a few percentage points.

NDV = Negative Delta Voltage

 

Are there microchips that I can buy to detect when the battery is charged?

Loads. Google "nimh ic"

 

So the voltage dropping before the battery reaches full charge is the reason why having an op amp shut off the charging circuit wouldn't work? Since the battery has 200 ma for 5 hours written on the side, does that mean that if I have the constant current power supply set to 200 ma, I could have a timer cut it off after 5 hours?

Some of the not so bad discount store chargers had timers. The timer means nothing if the cell isn't fully discharged when you set it.

Continual development means that higher cell capacities get marketed as time goes by - at some point you have to increase either charging current or charging time.

 

I think that I found an IC that I can use to shut off the charger at the right time. I will most likely use this or something like it instead of timing the charger. I will do some more reserch to see if there are any other controllers that would do better.
http://www.ti.com/product/BQ2002D

 

What about this one?
http://www.linear.com/product/LTC4060
It says it stops charging by negative delta voltage, maximum voltage, or maximum time.

 

So the voltage dropping before the battery reaches full charge is the reason why having an op amp shut off the charging circuit wouldn't work? Since the battery has 200 ma for 5 hours written on the side, does that mean that if I have the constant current power supply set to 200 ma, I could have a timer cut it off after 5 hours?

Before the battery was 2000mAh. 200mA for 5 hours is only 1000mAh. Which is it?
A timer can be used to charge a battery that has truthful ratings and if the battery is completely discharged.
It heats all the time it is charging so some extra charging time is needed to replace the heating current.
Did you know that a Ni-Cad battery gets cooler as it charges and if you charge a Ni-Cad and a Ni-MH you can feel the difference?

 

Before the battery was 2000mAh. 200mA for 5 hours is only 1000mAh. Which is it?
A timer can be used to charge a battery that has truthful ratings and if the battery is completely discharged.
It heats all the time it is charging so some extra charging time is needed to replace the heating current.
Did you know that a Ni-Cad battery gets cooler as it charges and if you charge a Ni-Cad and a Ni-MH you can feel the difference?

I just realized the battery said 200 ma for 10 hours.

 

I just realized the battery said 200 ma for 10 hours.

Then it is 2000mAh.

 

What about this one?
http://www.linear.com/product/LTC4060
It says it stops charging by negative delta voltage, maximum voltage, or maximum time.

That's what the industry uses mostly - you need to make sure the chip you select can handle the more subtle voltage change on Ni-Mh.

 

The chip looks like just what I need. Thanks a ton for the help.
Edit: Does anyone know if this chip comes in a DIP casing?

 

Does anyone know if this chip comes in a DIP casing?

Your next question will be, 'Where can I buy one?".
The answers are given by your electronic parts distributor. Digikey says: DIP case is not available anymore.
Maybe you can find a distributor that has a real old one but it might be fake.

 

Hello,

You might want to have a look at the BQ2002 battery charger controller.
It is available in 8 pin dip.

Bertus

 

What about this one?
http://m.onsemi.com/product?part=MC33342
It uses negative delta voltage. The BQ2002 doesn't appear to do this.

 

Hello,

From the attached PDF of BQ2002:



Bertus

 

just ordered 2 BQ2002 ICs from TI. Now the waiting game begins. Thanks a ton for all the advise. It was very helpful and without it, I would most likely have accidentally destroyed many many more batteries.