2 AA ni-mh charger circuit

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Cecemel

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Hi,
I'am searching for a ni-mh 2 AA battery charger circuit. It has to run off 5v because I want to use a USB port. (Doesn't cares if the battery's charge slowly)

Regards Cecemel8-O
 

The best charger circuit is to use a battery charger IC designed for a Ni-MH battery. It can be set for as many cells as you have, its current can be set for the size of your cells and it properly detects a full charge then shuts off or goes to a very low trickle charge current.

Two Ni-MH cells in series charge to about 2.8V so the USB 5V will be fine as a power source.
 

Cecemel said:
Thanks, but I would like to build a simple circuit by myself.;-)
Click to expand...
Why do you think the "Ultrasmart" charger circuit is so complicated with four special ICs that you might not be able to buy in small quantities? Why isn't it "simple" like what you want?
Do you want to charge battery cells or do you want to destroy battery cells?

Maybe you should go to www.energizer.com and look at Technical Info on the home page. When you enter Ni-MH then it will show a list of all their datasheets and a Ni-MH Applications Manual that you should download and study. You can also learn about Ni-MH (and other batteries) at www.batteryuniversity.com .
 


This charger costs $65 without case (look at: https://www.paulallenengineering.co...ta_Logging,_External_Drain,_Without_Case.html) and I can't find any schematics
Also. Using a simple charging circuit will not destroy my battery cells. (Ex.
 

Cecemel said:
This Using a simple charging circuit will not destroy my battery cells.
Click to expand...
The red LED is about 2.0V. Then the base voltage is also 2.0V and the emitter voltage is about 1.3V. The collector current and the charging current will be 1.3V/5.6 ohms= 232mA if the transistor has a high current gain.

A modern AA Ni-MH cell has a capacity of about 2300mAh. Energizer and a Japanese battery manufacturer say that a trickle charge current should not exceed the capacity divided by 40. Then when the battery becomes fully charged the current should be reduced to 2300/40= 57.5mA or less. Your circuit does not detect a full charge so it destroys the battery cells with its high continuous current.
 

Quick Chargers Definately require Good Electronic Monitoring.

But If you trickle charge your NiMh batteries at Less than 1/10 Capacity, Within Reason, they will not overheat or be damaged.
Just Don't keep them charging for Many Days on end. One full day is OK.

So determine their Milli-Amp Hour rating and divide it by 10.

Lets Assume a 2000 mah battery in THIS EXAMPLE.
1/10 capacity is 200 mA or .2 Amps.

Assuming 5 volt supply and assuming 2.4 volt, battery voltage.
Gives a 2.6 Voltage Difference.

2.6 / .2 Amps = 13 Ohm Resistance.

So put a resistor of 13 Ohms Minimum or Somewhat Higher (I would use 15 to 18 Ohms) in series with the battery on that 5 volt supply.

CHECK THEM OCCASSIONALLY!

The Batteries Should NEVER GET HOT.
Just Warm to the touch is OK.
 

I have an AA Ni-MH battery charger from Energizer, the battery manufacturer. It charges at 360mA for about 7 hours then shuts off and the 2300mAh cells get warm. If the power fails when it is charging then the timer starts over again and it severely overcharges the battery cells and they get hot.

Sometimes I charge 600mAh Ni-Cad cells at the same time as Ni-MH cells. During charging the Ni-Cad cells get cooler and the Ni-MH cells get warmer. When the Ni-cad cells are being overcharged then they also get very hot.
 

Thanks everyone but any suggestions for a safe and cheap IC that runs of 5 volts?:bsdetector:
 

You could possibly use an LM317 as a Constant Current Source.

But Follow my Directions in my above post, as to a Recommended Current.
 

This simple voltage regulator will charge 2 cells to a maximum of 2.76 V (or 1.36 V each). Charge rate tapers to a few mA.



It is important to adjust the zener diode (or other voltage reference). It needs to be 0.6V greater than the maximum voltage you want the batteries to reach.
 

BradtheRad said:
This simple voltage regulator will charge 2 cells to a maximum of 2.76 V (or 1.36 V each). Charge rate tapers to a few mA.
Click to expand...
If the charging starts at 90mA and ends at 3mA then the average is only 47mA. It will take about 64 hours to fully charge two 2300mAh cells. That is more than 5 days.

My Energizer charger charges my two cells to more than 3V in 7 or 8 hours.
 

Audioguru said:
If the charging starts at 90mA and ends at 3mA then the average is only 47mA. It will take about 64 hours to fully charge two 2300mAh cells. That is more than 5 days.
Click to expand...

Actually it will take LONGER than that.
A Normal NiMh battery will be just below 1.2 Volts when it requires charging, so you will probably start the charging at about 50 mA.

And there is No reason for a Tapered Current Charge on NiMh Batteries.
(But Good for Lead Acid.)

Constant Current is fine on NiMh types.
 

Audioguru said:
If the charging starts at 90mA and ends at 3mA then the average is only 47mA. It will take about 64 hours to fully charge two 2300mAh cells. That is more than 5 days.
Click to expand...

You are correct of course. We can increase the charge rate, by reducing the resistors.

I have a charger which is un-smart. It was manufactured in the days when AA rechargables had a mediocre capacity. I use it for my old cells as well as new. Its charge rate is 100mA continuously. After 24 hours, my AA cells are warm, no matter what their capacity. I don't usually intend to leave them on 24 hours, except for the 2400 mA-hr type.

With that in my mind, I adjusted my simulation for a modest charge rate. It can be adjusted higher, depending on transistor gain, resistor value in the collector leg, etc.
 

My Chinese solar garden lights use old low-capacity Ni-Cad AA cells. They short or rust away in less than 1 year so I do not bother to use them for anything else.
I have purchased high capacity Ni-MH AA cells for at least 15 years. They are guaranteed for 5 years and fail at exactly 5.5 years.
 

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