regarding NiMH battery charger using LM317

[mabauti]

I had a NiMH battery pack 3.6V 1500mAH been charged with this circuit
**broken link removed**
however, after 15 hrs the voltage on the battery pack is only 2.8volts. the battery pack is new. Any advice about what could bewrong?

 

[audioguru]

You say nothing about the ratings of the mains adapter:
1) Its rated voltage.
2) Its rated current.

A 3.6V Ni-MH battery is 4.2V when fully charged. Two cells is 2.8V so maybe your battery has a shorted cell.

The output of the LM317 is 1.25V and it needs an input at least 2V higher, so its input must be at least 3.25V.

The total voltage from the mains adapter must be at least 4.2V + 3.25V= 7.45VDC. Is it?

 

[mabauti]

The adapter is voltage rated from 4.5V to 9V , 300mA max.
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The page where I found the circuit says that I can set the adapter to the lowest voltage. I have measured the voltage and it was 6.56VDC, so I have increased it to 8VCD. I'll post the result later

 

[Hero999]

The input voltage needs to be >8V in order for the battery to become fully charged.

 

[mabauti]

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update.
I found that the problem was a defective battery pack (branded as "Croton").
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So I returned it back to the electronics shop, and I bought this time a Panasonic battery pack. It's working OK now. Thanks for your for help gentlemen

 

[audioguru]

A battery cell welded in backwards??

 

[bananasiong]

Will the LED be turned off when it is fully charged? Am I right or wrong?

 

[audioguru]

The circuit is too simple to turn off the LED and to stop the charging when the battery is fully charged.
The adapter is unregulated so its voltage will keep increasing as the battery becomes over-charged and the LED will keep glowing.

It is an "overnight" charger. Put the battery in it in the evening and remove it the next morning or it will become over-charged which reduces its life.

 

[bananasiong]

Let's say the output of the adapter is 7.45VDC, and the voltage of the battery is less than 4.2V. When charging, the voltage at the input of the voltage regulator is more than 3.25V, so the output can turn the transistor on and turn the LED on. When the battery is fully charged, the voltage at the input of the voltage regulator is reduced due to the voltage drop of the battery. Then the LED will be turned off if the input voltage is less than 1.25V. Right?
But the battery is still being charged right? Then it will still be overcharged if the supply is regulated. Or I'm wrong??

Thanks

 

[audioguru]

If the adapter has an adjustable regulated voltage then its voltage could be set to 1.4V per cell plus the 1.25V at the output of the regulator plus the dropout voltage that varies with each regulator and varies with current.
Then when the battery is fully charged its charging current might stay the same and overcharge the battery.

 

[Gayan Soyza]

Hi bananasiong! If you realy need to turn a LED on or off after its charged to a certain level you must use a comparator IC.When the input voltage is higher than the reference voltage then the output goes high also it can be configured to low.This way is very accurate than the above method.

 

[audioguru]

A comparator might work with charging a lead-acid battery.
Look at the voltage for the Ni-MH battery. Its voltage DROPS when it is fully charged.

Use a Ni-MH battery charger IC that knows when the battery is fully charged and shuts off the charging so it doesn't get overcharged. The IC measures the voltage and detects when the voltage stops increasing, or when it drops. It also senses the temperature rise when a battery becomes fully charged. A timer is used as a backup. Some charger ICs can be set to discharge a battery before it begins charging it.

 

[bananasiong]

Hi bananasiong! If you realy need to turn a LED on or off after its charged to a certain level you must use a comparator IC.When the input voltage is higher than the reference voltage then the output goes high also it can be configured to low.This way is very accurate than the above method.

Yea, but how do you measure the voltage when the batteries are being charged, wouldn't the charger's voltage measured but not the batteries' voltage?

 

[Gayan Soyza]

Hi audioguru when we talk about 1.2V AA NIMH batteries my ones when it’s fully charged its reaches to 1.35V-1.4V.So its better to reference a voltage about 1.3V.when its passing 1.3V it stops the operation. If I need more can do a very small trickle charge….or whatever….what do you think.???

 

[audioguru]

A battery is charged with current, it is not charged with voltage. So you can measure the voltage of the battery as it is being charged.

Look at my graph, 1.3V is too low to stop charging because the battery cell is only just beginning to charge. Depending on how much is its charging current 1.4V is close to being fully charged.

 

[bananasiong]

From the schematic above, just a voltmeter across the batteries will do right? How if the battery is being charged by the adapter/regulated power supply directly via a diode? Then the voltage appear at that point is Vin-0.7 V.

 

[Gayan Soyza]

For energizer batteries it will be 1.4V fully charged. So better cut off charging reaches upto that level…I’m using a comparator Bcuz hard to find that MAX712 IC in my place to detect more options……