The LED sort of helps with that, taking an increasing amount of the total current as the cells charge. You may want to see what the actual battery charge current is at "full charge". Then compare that to the battery capacity and chemistry. It might be OK. If you need a better charger, you'll need a bit more circuitry to avoid overcharge.

You might consider a cheap DC-DC converter off e-bay. It'll only give you a constant voltage, but that is likely superior to the constant current strategy.

Constant voltage charging is OK for lead-acid batteries - as long as the voltage is set to the correct value.

Constant voltage charging for Ni-Mh or Ni-Cd batteries is a seriously bad idea!

 

I really don't know how to check if a battery is fully charged. I assumed that both batteries would be charged when they reach their full voltage potential. .

Ni-Cd and Ni-Mh cells exhibit a small drop in terminal voltage when they reach full charge, its much more subtle on Mh cells so using a Cd Delta-V sensing charger on Mh cells would possibly cook them.

Ni-Mh cells actually absorb heat while charging (hence the warnings not to charge them while at sub-zero temperature) they produce heat if you continue to charge them when they're full.

Its fairly simple to hook up a thermistor/resistor divider and feed the tap to a window comarator and bistable latch (you can use a 555 if you tailor Vcc and tweak the VC pin to suits the window you want) with the latch switching off the charge current when the cell(s) are warm and off when they cool, you can get away with pretty high charging current, once the battery is full the bistable cycles on and off keeping the battery in tip-top charge and ready for anything.

 

Constant voltage charging for Ni-Mh or Ni-Cd batteries is a seriously bad idea!

But shockingly common. It's not so bad as long as the set voltage is below 100% full charge, so that the battery never really gets fully topped up, and a current-limiting resistor is used in series with the battery.

I'm not saying it's ideal, but the OP wants simple and that means compromising on something.

Actually, using the proper dedicated charger ICs make for a simple build, if not a simple circuit.

 

I've been reviewing what are the advantages of using a voltage divider instead of just one base resistor, and using the additional emitter resistor that I took of from the original design. For what I found they are just meant to increase the thermal stability. So, since that wouldn't influence much the final result, and since (I think) neither the LED or the batteries will be damaged, I ended up with just one 15K base resistor.

It seems to continue regulating the current to 50mA at all times in the simulations, but the real think lowers the current by a couple of mA when I take the batteries off; which doesn't matter that much.

My only concern now is shortening the life-span of the batteries by continuously changing them with this device; so, if you can think of a reason why this will happen, please let me know.

Your circuit is dependent on the Beta of the transistor, and it is then keeping the current relatively constant, but that is the current through the battery + the current through the LED. That is not a constant current battery charger.

The purpose of the emitter reisistor in a true constant current source is to supply negative feedback. This makes it stable with changes in load, temperature, and differences in the Beta of the transistor. Your circuit does none of these.

Bob

 

Thanks Ian, that was quite informative; but I'm not sure I would be able to put all that together. I was going to give it a try anyway, but I don't think I have the right thermistor at home; just found an MF72 current suppressor and a LM35 centigrade temperature sensor.

Right now I'm using the first set of batteries charged by the circuit, and I'm noticing they are working at least for twice as long as the ones charged with the mains charger for a similar charging time. Not sure what to make of this in terms of the batteries life-span.

Also noticed that the voltage on the batteries, whether they were charged with the mains or the new USB charger, varies between 1.35V and 1.45V; instead of the 1.2V stated in the labels. Furthermore, when the batteries go down to 1.2V they behave as if they were discharged.

The label on one of the batteries (I have various brands) also states a recharge current of 220mA (that's 4 times more than what the mains charger outputs). Do you think this higher current could be used in all NiMh brands without causing any damage?

And last, imagine that I leave the circuit I posted as it is connected for longer than it should; do you think it would damage the batteries? I mean, do you think the LED will take enough current off the batteries when they are fully charged so they are not damaged?

 

Your circuit is dependent on the Beta of the transistor, and it is then keeping the current relatively constant, but that is the current through the battery + the current through the LED. That is not a constant current battery charger.

Thanks Bob. I thought that since the LED would not conduct until it reaches 3V, the batteries would get the 50mA until the voltage is high enough to break the LED's PN junction barrier; and then take the current off the batteries when they are charged.

The purpose of the emitter reisistor in a true constant current source is to supply negative feedback. This makes it stable with changes in load, temperature, and differences in the Beta of the transistor. Your circuit does none of these.

Bob

It's true that the circuit above has no way of stabilizing the current, but I imagined that the variations will be small enough not to matter.

I've tried to find more info on the purpose of the emitter resistor, but all I could find in my books was as a thermal stabilizer; which again, didn't think it would matter much.

 

Thanks Bob. I thought that since the LED would not conduct until it reaches 3V, the batteries would get the 50mA until the voltage is high enough to break the LED's PN junction barrier; and then take the current off the batteries when they are charged.



It's true that the circuit above has no way of stabilizing the current, but I imagined that the variations will be small enough not to matter.

I've tried to find more info on the purpose of the emitter resistor, but all I could find in my books was as a thermal stabilizer; which again, didn't think it would matter much.

LEDs do not work like that. The current is dependent in the voltage across it. If your LED conducts 50mA at 3V it will probably conduct soemthing like 10mA at 2.8V, which is before the batteries are fully charged. Note that there is nothing wrong with this. It does not hurt to reduce the current as you approach fully charged.

Bob

 

But shockingly common. It's not so bad as long as the set voltage is below 100% full charge, .

Incorrect - Both Ni-Mh & Ni-Cd exhibit a small drop in terminal voltage at full charge, if that's less than the fixed voltage you're charging at, the battery will be ruined. If your fixed voltage is low enough to not damage the cell - it will never reach full charge.

 

I should have said "well below" or "below, say, 90%", which is what I meant. My wording of "below 100%" implies that 99% would be OK, and we both agree that's not the case.

 

LEDs do not work like that. The current is dependent in the voltage across it. If your LED conducts 50mA at 3V it will probably conduct soemthing like 10mA at 2.8V, which is before the batteries are fully charged. Note that there is nothing wrong with this. It does not hurt to reduce the current as you approach fully charged.

Bob

Just measured the I/V curve for this particular type of LED, and you're right, it's not as narrow I as thought. I begins conducting at 2.9V, but it doesn't reach 10mA until the 3.5V; then 20mA at 4V and the 50mA just passing 4.5V.

But I think this actually works in my favor, since once the LED begins to light up at around 3V, it means that the batteries are ready to be taken off.

 

Both Ni-Mh & Ni-Cd exhibit a small drop in terminal voltage at full charge

About this small voltage drop... do you have any idea around how much it would be for a AAA 1.2V NiMh cell? I guess it depends on the load, so let's say the load is the 3V LED...

 

I should have said "well below" or "below, say, 90%", which is what I meant. My wording of "below 100%" implies that 99% would be OK, and we both agree that's not the case.

Then the battery would get so little charge as to be useless.

All the *REALLY* cheap chargers I've had apart contained current limiting resistors - they are *NOT* constant current chargers.

 

About this small voltage drop... do you have any idea around how much it would be for a AAA 1.2V NiMh cell? I guess it depends on the load, so let's say the load is the 3V LED...

You're failing to understand what I said!

The volt drop has nothing to do with being under load - it is a product of the internal chemistry as it approaches full charge.

That is why you should use constant current (or at least current limited) charging - as the terminal voltage drops, the cell will draw more charge current from a constant voltage source - at that point it is already full and will then get very hot and self destruct.

 

You're failing to understand what I said!

The volt drop has nothing to do with being under load - it is a product of the internal chemistry as it approaches full charge.

That is why you should use constant current (or at least current limited) charging - as the terminal voltage drops, the cell will draw more charge current from a constant voltage source - at that point it is already full and will then get very hot and self destruct.

To be honest, it was the first time I heard the term "terminal voltage"; so I did a google search. What I found was a formula to calculate the terminal voltage, which needed a known load; so that confused me, as I supposed you needed it to know the terminal voltage.

While looking on the internet I also found a page that describes various methods for charging NiMh cells from www.talkingelectronics.com; which are in essence what I did with the transistor. Unfortunately, it doesn't explain how to know when the batteries are charged; other than keeping track of the charging times.

What I need to find out now is how much approximately would this terminal voltage drop in my circuit. I guess I could risk leaving a set of batteries charging for long enough until I see the voltage go down instead of going up.

 

Hello,

Have a look at the following picture about the voltage drop when charging a NI-MH battery:

This comes from this maxim page:
http://www.maximintegrated.com/app-notes/index.mvp/id/4496

Bertus

 

Thanks a lot Bertus; that's exactly what I was looking for...

Never reached the 1.55V with my circuit; since I always took the batteries off when they passed the 3V (1.5 each).

 

The delta V termination method really only applies when you are charging the batteries at the maximum rate (about 1C or 2A for typical NiMH cells). What you are doing is more like a trickle charge, and you will not see the delta V, and termination is not necessary because you can trickle charge at C/20 (i.e. 100mA) or less continuously.

This site has great info on battery charging:

http://batteryuniversity.com/


Bob

 

To be honest, it was the first time I heard the term "terminal voltage"; so I did a google search. What I found was a formula to calculate the terminal voltage, which needed a known load; so that confused me, as I supposed you needed it to know the terminal voltage.

While looking on the internet I also found a page that describes various methods for charging NiMh cells from www.talkingelectronics.com; which are in essence what I did with the transistor. Unfortunately, it doesn't explain how to know when the batteries are charged; other than keeping track of the charging times.

.

The early cheapo chargers just used a transformer with slightly higher voltage than needed and a current limiting resistor, but it was too easy to cook the cells if you left them in too long. For a while they tried including a shut-off timer - the charger usually had a label on the bottom indicating the time to set for various size/type of cell.

Nowadays most chargers have Delta-V full charge detection - the cell's temperature coefficient means that the terminal voltage drops slightly as the cell starts to warm up at full charge.

If you have limited means, you can just detect temperature rise - nickel chemistry batteries actually absorb heat while charging, and cross over to producing heat if you continue charging when they're full - aim for a shut off trigger point no more than 45c, and a resume trip point just slightly above the highest room temperature will reach.

 

The delta V termination method really only applies when you are charging the batteries at the maximum rate (about 1C or 2A for typical NiMH cells). What you are doing is more like a trickle charge, and you will not see the delta V, and termination is not necessary because you can trickle charge at C/20 (i.e. 100mA) or less continuously.

This site has great info on battery charging:

http://batteryuniversity.com/


Bob

Yes, that seems to be the case. I left a pair of cells charging all day... they went up to 3.14V, and just an hour ago came down to 3.13V and stayed there without further changes.

The early cheapo chargers just used a transformer with slightly higher voltage than needed and a current limiting resistor, but it was too easy to cook the cells if you left them in too long. For a while they tried including a shut-off timer - the charger usually had a label on the bottom indicating the time to set for various size/type of cell.

Nowadays most chargers have Delta-V full charge detection - the cell's temperature coefficient means that the terminal voltage drops slightly as the cell starts to warm up at full charge.

If you have limited means, you can just detect temperature rise - nickel chemistry batteries actually absorb heat while charging, and cross over to producing heat if you continue charging when they're full - aim for a shut off trigger point no more than 45c, and a resume trip point just slightly above the highest room temperature will reach.

I just opened one of the mains chargers (the oldest one, which seems to be specific for NiCd batteries): it's only a step down transformer, a couple of rectifying diodes, and then the resistors to limit the current for each size of batteries; there is no transistor to keep the current constant. It also states that AAA batteries should be charged at 50mA for 5 hours; as the newer mains charger which is for NiCd and NiMh (both are the same actually, slow or trickle charge).

Just a moment ago, when I took off the batteries that I had charging all day, I think I noticed a slight increase in temperature; but barely noticeable.

Do you think I could use the LM35 precision temperature sensor that I have at home for this purpose? If so, how do you do it; do you just place it near to the batteries? I'm not sure how it's meant to detect their temperature unless it's physically attached to the batteries.

 

Yes, that seems to be the case. I left a pair of cells charging all day... they went up to 3.14V, and just an hour ago came down to 3.13V and stayed there without further changes.



I just opened one of the mains chargers (the oldest one, which seems to be specific for NiCd batteries): it's only a step down transformer, a couple of rectifying diodes, and then the resistors to limit the current for each size of batteries; there is no transistor to keep the current constant. It also states that AAA batteries should be charged at 50mA for 5 hours; as the newer mains charger which is for NiCd and NiMh (both are the same actually, slow or trickle charge).

.

I have several "regular rate" chargers that are for Ni-Mh only - they give up long before they reach full charge if you put Ni-Cd in them. The fast chargers have Ni-Mh/Ni-Cd switches - they also have cooling fans and charge/float the cells so they're just barely noticeably warm.