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Simple NIMH Charger Modifications

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Gayan Soyza

Active Member
Hi all this is one version of my ideas for NIMH charger.The R (sense) is calculated for 0.1C from the battery capacity.

I have small questions

1) What’s the minimum supply voltage this needs? Can it work from 3V?
2) What’s the best place to add a charging indicator?

Thanks
 

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ericgibbs

Well-Known Member
Most Helpful Member
Hi all this is one version of my ideas for NIMH charger.The R (sense) is calculated for 0.1C from the battery capacity.

I have small questions

1) What’s the minimum supply voltage this needs? Can it work from 3V?
2) What’s the best place to add a charging indicator?

Thanks
hi Gayan,
If you said the two 1.2Vbty were at a nominal 2.4V, also you have ~0.7V across Rsense and allowing say 1Vce on the TIP.
I make that about 4.1V, so allowing for a above 2.4V across the batteries when charging, I would say 4.5V is the minimum supply.

Adding an LED in the charge loop will add to the required supply voltage.
By adding a small transistor/led across the Rsense will give some indication of 'charging'.
 

Gayan Soyza

Active Member
hi Gayan,
If you said the two 1.2Vbty were at a nominal 2.4V, also you have ~0.7V across Rsense and allowing say 1Vce on the TIP.
I make that about 4.1V, so allowing for a above 2.4V across the batteries when charging, I would say 4.5V is the minimum supply.
That makes sense,so the minimum will be 4.5V & above.

Adding an LED in the charge loop will add to the required supply voltage.
By adding a small transistor/led across the Rsense will give some indication of 'charging'.
I can't configure this eric.Do you have a schematic to connect a charging LED for the above?

Thanks
 

ericgibbs

Well-Known Member
Most Helpful Member
hi Gayan,
The charger will overcharge the batteries if left connected, the current tail off, for voltage of the 'charge circuit' is about 1.9V/cell !!!

Do you use LTSpice.?
 
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Gayan Soyza

Active Member
hi Gayan,
The charger will overcharge the batteries if left connected, the current tail off, for voltage of the 'charge circuit' is about 1.9V/cell !!!
Yes eric thats too much but I'm using 0.1C rate,

The battery companies recomend to charge their batteries on constant current sources.When charge with constant current if their is no voltage limits on the charger it will exceed its rated battery voltage.They don't speak this incident.They speak only constant current..............

But many people charging their batteries with higher voltages using constant current configuration.Still don't know when to terminate :D

Do you use LTSpice.?
Oh no this is the hardest simulator software I have found on earth :( I use only Proteus
 
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audioguru

Well-Known Member
Most Helpful Member
Energizer recommends using a charger IC that detects when the battery is fully charged and either turns off or switches to a 1/40th C trickle charging current.
Their old Ni-cads could be over-charged continuously at 1/10th C but their capacity was much less than Ni-MH so the current is 1/40th of a Ni-MH cell.

Your simple circuit will over-charge a battery that is already charged and will over-charge when the battery becomes charged.
An over-charged battery does not last long.

I have a cheap Energizer Ni-MH battery charger. It is designed to charge 2500mAh AA Ni-MH cells for 6 hours so its charging current is 500mA.
It is stupid with just a simple timer so it over-charges cells that are already charged.
It almost fries old Ni-Cad cells by over-charging them.

Did you know that Ni-Cad cells get cooler when they are charging at a fairly low current? Ni-MH cells get a little warmer as they charge at a fairly low current.
Both types get hot when they are over-charging.
 

Gayan Soyza

Active Member
Energizer recommends using a charger IC that detects when the battery is fully charged and either turns off or switches to a 1/40th C trickle charging current.
Their old Ni-cads could be over-charged continuously at 1/10th C but their capacity was much less than Ni-MH so the current is 1/40th of a Ni-MH cell.

Your simple circuit will over-charge a battery that is already charged and will over-charge when the battery becomes charged.
An over-charged battery does not last long.

I have a cheap Energizer Ni-MH battery charger. It is designed to charge 2500mAh AA Ni-MH cells for 6 hours so its charging current is 500mA.
It is stupid with just a simple timer so it over-charges cells that are already charged.
It almost fries old Ni-Cad cells by over-charging them.

Did you know that Ni-Cad cells get cooler when they are charging at a fairly low current? Ni-MH cells get a little warmer as they charge at a fairly low current.
Both types get hot when they are over-charging.
Hi audioguru thanks for your reply.

I want to build with discrete components without using max ICs.I have built many designs & checked one by one.

When charging with constant current (using LM317) it will charge very nicely.But while charging when I measure the voltage its going upto 1.65 -1.75V range per cell.Whats your idea on this while charging the voltage is rising?If I leave the charger after the battery is fully charged whats your idea.

I don't think its a problem because I'm charging with a C/10 rate.
 

audioguru

Well-Known Member
Most Helpful Member
I charge AAA Ni-MH cells with an LM317 at a current of 0.3C and I set its max output voltage to 1.5V for each cell. Then the cells are becoming overcharged because they are getting hot.

I have two graphs that show a Ni-MH cell reaches 1.5V then becomes overcharged and hot.

Since the voltage of your cells is so high then aren't they extremely hot?
I think their life is shortened.
 

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kchriste

New Member
Forum Supporter
If you use a 0.5C - 1.0C charging rate you could terminate the charge when the battery temperature rises apx 10-15C above the ambient temperature. Two thermistors, a comparator and a flip-flop would be all you'd need for a simple charger.
The other, more popular but difficult, method of charge termination involves monitoring the battery voltage. The battery voltage will slowly rise until the battery is fully charged. When fully charged, the battery voltage will dip by apx 5-20mv. By sensing this dip, which is difficult to do, you can determine when to stop charging.
 
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Gayan Soyza

Active Member
I charge AAA Ni-MH cells with an LM317 at a current of 0.3C and I set its max output voltage to 1.5V for each cell.
There is the hidden point. Now only you told this :D are you using two LM317? One for current & other for voltage?

Or you configure the LM317 as constant voltage current limited?

Since the voltage of your cells is so high then aren't they extremely hot?I think their life is shortened.
You are correct my ones are getting hot.
 

audioguru

Well-Known Member
Most Helpful Member
My single LM317 is a current regulator with one sensing resistor from the output to the ADJ pin.
The wall-wart's voltage limits the max charging voltage.
My mains voltage is regulated very well.

I made it to replace a Chinese charger that was just a wall-wart. It and the battery got extremely hot when the battery was over-charged.
 

Gayan Soyza

Active Member
My single LM317 is a current regulator with one sensing resistor from the output to the ADJ pin.
The wall-wart's voltage limits the max charging voltage.
My mains voltage is regulated very well.

I made it to replace a Chinese charger that was just a wall-wart. It and the battery got extremely hot when the battery was over-charged.
Hi is your wall-wart is supplying 6v into the current regulator?
3V for the dropout in the regulator (including 1.25 drop) + 3V for the two batteries.
 

audioguru

Well-Known Member
Most Helpful Member
The LM317 uses about 1.5V and the current regulating resistor uses an additional 1.25V.
 

Gayan Soyza

Active Member
Gentleman!! I have been asked from the battery manufacture. I asked from the “GP Batteries Technical Support Team”

My question
I have 2500mAh NIMH 1.2V cells. I'm going to charge them on a constant current configuration with a 0.1C rate that is 250mA. My problem is when charging with constant current, the voltage is rising on cells like 1.75, 1.8V per cell. But the current is constant (250mA). Do you recommend charging this way?
Reply
Dear Sir,
Charging at 0.1C no problem for long term.
regards.....Raymond.
Now see they don't care about the voltage while charging :D
 

audioguru

Well-Known Member
Most Helpful Member
Energizer shows a typical charging voltage graph on their datasheet. When the current is only 0.1C then the cell is fully charged when the voltage is about 1.4V.

For the longest life Energizer recommends stopping the charging when the cell is fully charged or reducing the over-charging current to 0.025C.

My Energizer charger has a timer that is set for charging 2500mAh cells. The cells become warm when they are fully charged then it stops charging.
But the charger is stupid. If the cells were not completely discharged then it over-charges them and they get hot.
 
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