Is this circuit good for charging NIMH batteries?

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uraniumhexoflorite

Joined Oct 23, 2016
216
I made a constant current charger for a NIMH AA battery. When I tested it out by powering it with 9 volts, it charged the AA from about 0.65 volts to just under 1.4 volts in a matter of minutes. I'm no expert, but I have charged many batteries, and this seems too fast to charge a battery. I'm worried that the circuit may damage NIMH batteries because it's charging them so quickly. The circuit uses an LM317 to provide a constant current of about 10.4 ma and the batteries are 1.2 volt 2000 mah batteries made in 2014 (not sure if manufacture date is important, but I wanted to provide it in case it is). The battery has writing on it that says its standard charge is 200 ma over 15 hours.
 

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ian field

Joined Oct 27, 2012
6,539
I made a constant current charger for a NIMH AA battery. When I tested it out by powering it with 9 volts, it charged the AA from about 0.65 volts to just under 1.4 volts in a matter of minutes. I'm no expert, but I have charged many batteries, and this seems too fast to charge a battery. I'm worried that the circuit may damage NIMH batteries because it's charging them so quickly. The circuit uses an LM317 to provide a constant current of about 10.4 ma and the batteries are 1.2 volt 2000 mah batteries made in 2014 (not sure if manufacture date is important, but I wanted to provide it in case it is). The battery has writing on it that says its standard charge is 200 ma over 15 hours.
Its better than some of the commercial units that came from discount stores, but with no end of charge detection and cut off, its not all that difficult to ruin batteries.

Recent offerings from the better discount stores have a micro to control the charging characteristics for optimum battery life.
 

Dodgydave

Joined Jun 22, 2012
10,050
A 2000maH cell is usually charged at 10% of its capacity, so that's 200mA for 10Hours,.

Fast charging is is upto 50% of its capacity as long as you use temperature monitoring.
 
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AlbertHall

Joined Jun 4, 2014
11,506
A 2000maH cell is usually charged at 10% of its capacity, so that's 10mA for 15Hours,.
2000mAh times 10% is 20mA so it should take ≈ 30 hours to full charge.
[EDIT] WRONG! 2000mAh times 10% is 200mA so a 10mA charge should take 300 hours! If it only took a 'few' hours the battery is defunct.
 
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ian field

Joined Oct 27, 2012
6,539
A 2000maH cell is usually charged at 10% of its capacity, so that's 200mA for 10Hours,.

Fast charging is is upto 50% of its capacity as long as you use temperature monitoring.
The industry standard is detecting the small drop in terminal voltage at full charge, but its more subtle with Ni-Mh.

Temperature sensing is OK, but very hot weather can leave you with a slim margin.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
216
Its better than some of the commercial units that came from discount stores, but with no end of charge detection and cut off, its not all that difficult to ruin batteries.

Recent offerings from the better discount stores have a micro to control the charging characteristics for optimum battery life.
Would it work to add an op amp and a transistor to shut off the charger when the battery reaches a certain voltage?
 

Audioguru

Joined Dec 20, 2007
11,249
If the LM317 is working properly it has 1.25V across your 120 ohm resistor which is a current of only 10mA that will take more than 8 days to fully charge a 2000mAh battery.
Maybe the pins on the LM317 are mixed up or it is a fake one from ebay?

An Energizer AA Ni-MH cell made in 2014 by Energizer is guaranteed for 5 years so it would have 1 or two years of life remaining. A cheap Chinese cell sold on ebay might last for 1 or 2 years.
So maybe the LM317 and the battery cell are both fakes?
Did you see this:
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
216
The batteries are from amazon, and they have worked well in the past. I used them to test a solar battery charger that I made a few years ago (when I didn't know very much about how to properly charge a battery) and that probably resulted in the battery being damaged. I will try it with different batteries. The LM317 has been used before many times and has worked well, so I doubt it's a counterfeit. Assuming the new battery is ok and the LM317 isn't counterfeit, will the circuit charge NIMH batteries without damaging them? I know it will charge very slowly if the circuit works, so I will look through my power resistors for one that will produce roughly 100 ma.
 
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AlbertHall

Joined Jun 4, 2014
11,506
You could charge NiCd cells continuously at the 1/10C rate but NiMh do not take kindly to overcharging. Therefore you need some method of detecting full charge to terminate the charging. With a fast charge you can use the fact that the cell temperature will rise but this will not work at low charge rates (<400mA) so you must use the delta-V method.

Batteries are getting trickier to use. Lead acid and NiCd are reasonably immune to abuse but Li-Ion are prone to self-ignition if overcharged or over-discharged.
 

Audioguru

Joined Dec 20, 2007
11,249
Why does your 2000mAh battery fully charge "in a matter of minutes" when the charging current is only 10mA? Something must be wrong because it should take 8 days. Is the cheap battery fake and full of rice flour like in the video I posted?

Instead of guessing about using a huge power resistor, calculate the power rating needed for the current regulating resistor for the LM317. The current in it is 1.25V/12 ohms= 104mA then its heating is (104mA squared) x 12 ohms= 0.13W. A little 1/4W resistor will be fine. But measure the voltage across the 12 ohm resistor to make sure it is 1.25V and try a new battery bought locally. New AA Ni-MH cells come pre-charged and hold the charge for 1 year.
 

Thread Starter

uraniumhexoflorite

Joined Oct 23, 2016
216
I will add an op amp to turn off the charger after the battery reaches a set voltage. Here's a new circuit that will cut off the voltage when it reaches 1.475 volts (I think the charging voltage for AAs is 1.4 to 1.6 volts). I happened to be using a power resistor because it happened to be the first resistor that I could find that was close to 100 ohms. I think the battery charges quickly because it's old and I improperly charged it many many times, so it's probably dead.
 

dl324

Joined Mar 30, 2015
13,141
I will add an op amp to turn off the charger after the battery reaches a set voltage. Here's a new circuit that will cut off the voltage when it reaches 1.475 volts (I think the charging voltage for AAs is 1.4 to 1.6 volts). I happened to be using a power resistor because it happened to be the first resistor that I could find that was close to 100 ohms. I think the battery charges quickly because it's old and I improperly charged it many many times, so it's probably dead.
You've been told several times that that isn't the proper way to charge NiMH batteries.

Why ask for advice if you're just going to ignore it?
 

crutschow

Joined Mar 14, 2008
27,750
So I should use a delta-V circuit instead of the constant current one?
That would likely be best.

A simpler way, used by many cheap commercial chargers, is to charge the battery at the 1/10 Ah rate, and then have a timer terminate the charge after about 15 hours.
The limitation of this technique is that it will overcharge the battery if it is not fully discharged at the start, so it is best to have them nearly discharged before charging.
 

Audioguru

Joined Dec 20, 2007
11,249
Energizer has a Ni-MH Battery manual on their website that shows a charging voltage graph. The graph shows the "typical" voltage that I think is different for each battery and at different ambient temperatures but it clearly shows the small drop in voltage at a full charge.
 

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