Hello,

My name is Wan. My focus research area is on Battery chargers. I would like to know the part for Constant Current charging. As i have done a simple circuit for the cut-off battery charger system without constant current.

As I have conducted a few testing for the current control it is not constant at all. I am going to charge a 48V 3A 8Ah battery. The charging mechanism should be CC then CV during the end of the charging. I'm using a rectifier AC-DC as input and a voltage cut-off circuit with a current limiter. However, the current limiter is not working as i try to maintain at 3A the current drops while charging.

The input voltage is 56V and the OP-AMP 741 will compare the input if the battery is fully charged at 54V so the OP-AMP will cut off. It is set up by varying the trimmer pot to 54V. My consent is for constant current at 3A, can anyone share with me how to make it constant at 3A.

I am very happy if you can share with me tips or fundamentals for Constant Current Charging/battery charger.

 

The first thing to establish is the Power-Supply.
The Power-Supply is a vital part of the Circuit.

1)
What are You using for a Power-Supply ?
Can it Supply the required Voltage, AND the required Current, at the same time ?
Do You have part numbers ?

2)
What type of Battery are You charging ?
I assume that it is 4- Sealed-Lead-Acid (SLA) Batteries connected in series.
Is this correct ?
What is the Battery part-number and manufacturer ?
If this is correct, the Power-Supply must produce
more than ~60-Volts to be able to Charge the Batteries fast.

3)
The 2 Regulating Circuits that You provided have numerous problems in their design.
The Current-Regulator-Circuit is wasteful, but it could be useful.
.
.
.

 

can anyone share with me how to make it constant at 3A.

3A is likely too much charging current if it is a 8Ah lead-acid battery, and would require a large 56V supply.

Below is a variation of the two transistor current limiter you showed in your middle attachment, with PNP Q2 added to your circuit.
Q2 turns on when its base emitter voltage across R6 reaches about 0.8V to starve Q1's base current and limit its output current.

The LTspice sim shows the simulated 741 signal (green trace) slowly increasing the battery current (yellow trace) until it reaches the ≈3A limit at a little past half way of the sim time.

(Note that R5 = 10kΩ is too large to provide enough Q1 base current for 3A current. To minimize the required base current you could use a Darlington pair or a P-MOSFET for Q1).

 

You need to really revisit your circuit. You are trying to power a 741 from 48 V to 56 V and that is WAY beyond it's maximum supply voltage. On top of that, you should find a much better opamp than the oh-so-long-in-the-tooth 741.

 

Hello,

My name is Wan. My focus research area is on Battery chargers. I would like to know the part for Constant Current charging. As i have done a simple circuit for the cut-off battery charger system without constant current.

As I have conducted a few testing for the current control it is not constant at all. I am going to charge a 48V 3A 8Ah battery. The charging mechanism should be CC then CV during the end of the charging. I'm using a rectifier AC-DC as input and a voltage cut-off circuit with a current limiter. However, the current limiter is not working as i try to maintain at 3A the current drops while charging.

The input voltage is 56V and the OP-AMP 741 will compare the input if the battery is fully charged at 54V so the OP-AMP will cut off. It is set up by varying the trimmer pot to 54V. My consent is for constant current at 3A, can anyone share with me how to make it constant at 3A.

I am very happy if you can share with me tips or fundamentals for Constant Current Charging/battery charger.

Hello there Wan and welcome to the forum i see you must be a new member.

I am going to critique your design and just want you do know i am not being overly critical i just want to see if i can help you improve your design. I've worked in the power control industry and have designed various power supplies and battery chargers professionally and after i left the industry i also did so as a hobby and for other individuals looking to design stuff like this. I've always found this interesting ever since i can remember.
Again just keep in mind i am just trying to help you improve this design.

Having worked in the industry long ago and keeping an eye on how things have changed over the years, the first thing that struck me about this design is that it is what we sometimes like to call an "Old School" design, meaning that the design techniques are outdated for a circuit of this class. That doesn't necessarily mean that it is a bad idea, just that things have changed in this area that improved the designs so they better integrate into the modern era of electronics and the environment. Some may even more casually refer to this as the zeitgeist of the design. The most important part of that is the efficiency, which we try to get as high as practical.

Being a linear design at heart of it, this design can be improved greatly by moving to a switching type design where the efficiency can maybe even reach as high as 90 percent, whereas the linear design would be lucky to see 50 percent during a large part of the circuit operation period. For power levels like this that could be very significant.

The modern way is to use a switcher for nearly everything even audio amplifiers now, and if needed use a linear back end (not needed for battery charging).
That would be the most significant point to be made in a modern design, and there are some other smaller points very worth considering also.

The first might be the constant current aspect of the design. Just to be clear, batteries do not require a so-called constant current. They just need a current that is as high as it can be without going over the limit for that type of battery, and it is perfectly fine for the current to vary even 30 percent. The only difference that makes is that with a current that is not at the limit for the whole time means the battery does not charge as fast. For an example, say you really want 3 amps as the max current to the battery. If it drops to 2 amps after a while that is not too big of a deal, except that the battery does not charge as fast. This means a drop of 20 percent isnt that big of a deal at all. If it dropped from 3 amps to 2.5 amps i would not even worry about it.
Second, that current MUST drop at some point because once the voltage regulation takes over, the current drops naturally and that is not only to be expected it is downright necessary. This is just part of the charging process for many types of cell chemistries. As the voltage rises the battery internal resistance will not allow the full current anymore and that's natural. This of course slows the charging process near the end of charge, but it's an acceptable condition. The only exception is if you employ compensation for that but that's not easy to implement making the circuit much more complicated. For your first charger i would not recommend this.

The third point is the battery does not required a smooth current. It can tolerate a pulsing current level. That means only moderate filtering if at all. Some say a current that is not smooth is actually better for the battery health.

Another point would be the voltage tolerance. Some batteries require very careful voltage level control. This may actually vary with temperature as well. Check your battery specs to see if this applies.

Another point yet is the battery longevity. Lower current levels (not too low though) can increase the battery life by a large percentage.

There is also battery discharge level. If the battery is discharged too much it could see end of life much sooner than it is rated for. This applies to regular lead acid batteries.

Another point yet is the weight to energy level. Lead acid batteries are very heavy. The current trend is to use LiFePO4 types now which weigh in at something like 1/3 of the weight of a similarly rated lead acid battery. It's amazing to pick up a LA battery in one hand and an LiFePO4 battery of the same capacity in the other hand. The weight is so different it makes you wonder why lead acid batteries are ever used anymore. There are reasons though, i wont get into too much, but the most important is drain current levels which can be higher than the LiFePO4 type.

Go over this and see what you think. I realize that not everyone wants to deal with a switcher though and prefer the simplicity of a linear. It's up to you, but if you want the best efficiency you will have to go with a switcher.

 

You are trying to power a 741 from 48 V to 56 V and that is WAY beyond it's maximum supply voltage.

He is using a 15V Zener shunt regulator to power the 741.

 

He is using a 15V Zener shunt regulator to power the 741.

Ah, I didn't notice that in the quick look I took. Thanks for pointing that out.

 

Here's my sim of the circuit with the added current limit transistor.
I modified a couple of the resistor values as needed: and changed the power source for the op amp from the output to the supply voltage.
I also re-arranged the schematic to be in the conventional left-to-right signal/current flow.

The battery is emulated by the 100 Farad capacitor CBat, and resistors RTrickle and Rbat.

Initially the battery charges at the 3A limit (yellow trace) until the battery voltage reaches 54V (green trace), at which point the op amp maintains the voltage at that value, and the current then drops to the 108mA trickle charge current due to RTrickle.

Note that starting at a discharged battery voltage of 44V, the dissipation of Q1 (red trace) is initially 33W, and will require a hefty heat-sink (possibly with a fan) when charging at 3A.
That's why a switching regulator with higher efficiency was suggested for charging.

 

The weight is so different it makes you wonder why lead acid batteries are ever used anymore. There are reasons though, i wont get into too much, but the most important is drain current levels which can be higher than the LiFePO4 type.

That, the much higher cost of Li-Ion batteries, and their needing exact voltage and current charging parameter limits or they go blooey.
That's why they are still used in virtually all internal-combustion engine vehicles, especially since the greater weight is not a significant factor in that application.

 

That, the much higher cost of Li-Ion batteries, and their needing exact voltage and current charging parameter limits or they go blooey.
That's why they are still used in virtually all internal-combustion engine vehicles, especially since the greater weight is not a significant factor in that application.

Hi,

I was talking about LiFePO4 not Li-ion. Different animal.

 

Here's my sim of the circuit with the added current limit transistor.
I modified a couple of the resistor values as needed: and changed the power source for the op amp from the output to the supply voltage.
I also re-arranged the schematic to be in the conventional left-to-right signal/current flow.

The battery is emulated by the 100 Farad capacitor CBat, and resistors RTrickle and Rbat.

Initially the battery charges at the 3A limit (yellow trace) until the battery voltage reaches 54V (green trace), at which point the op amp maintains the voltage at that value, and the current then drops to the 108mA trickle charge current due to RTrickle.

Note that starting at a discharged battery voltage of 44V, the dissipation of Q1 (red trace) is initially 33W, and will require a hefty heat-sink (possibly with a fan) when charging at 3A.
That's why a switching regulator with higher efficiency was suggested for charging.

View attachment 291046

Hello again,

You are kidding right? I hope so
As soon as i saw the letters "FTZ" i knew something was up. How long before the transistor blows off the board?
Check the power dissipation for one thing. For another thing, who would run a 60v, 3 amp transistor at 56v VCE and 3 amps IC?
Also, what's with all the zeners or are they voltage reference diodes? Only need one anyway.
It i missed something i apologize but something does not look right.
Go with a switching design anyway this is no place for a linear unless you've got energy to burn

 

No Thread-Starter ?
Oh well, here's my thoughts .............
.
.
.

.

 

You are kidding right?

Never.
Well, maybe a little.

As soon as i saw the letters "FTZ" i knew something was up. How long before the transistor blows off the board?

I picked a transistor I had a model for.
The one in the TS's circuit appears more appropriate but I didn't have a model for that.
I was interested in showing the circuit operation, not whether that particular transistor is completely appropriate.

what's with all the zeners or are they voltage reference diodes? Only need one anyway.

How many is "all"?
The 15V Zener (D1) is used as a shunt regulator for the op amp power.
The 5.1V Zener is the reference.
The 15V Zener has a much poorer temperature coefficient than the 5.1V Zener so does not make a good reference.
But it likely would be better to use a stable programmable reference, such as the TL431 for the 15V supply, and then divide that down to 5V for the reference.

 

I was talking about LiFePO4 not Li-ion. Different animal.

My understanding is that the LIFePO4 chemistry is still considered a Li-ion type battery, but I see that it is more appropriate for vehicle batteries than other Li-ion types, however still not compelling over lead-acid for use in a vehicle with an internal combustion engine.

 

what's with all the zeners or are they voltage reference diodes? Only need one anyway.

Okay, so here's the circuit with one TL431 reference for both the op amp supply, and the reference voltages:
Better?

 

Never.
Well, maybe a little.
I picked a transistor I had a model for.
The one in the TS's circuit appears more appropriate but I didn't have a model for that.
I was interested in showing the circuit operation, not whether that particular transistor is completely appropriate.
How many is "all"?
The 15V Zener (D1) is used as a shunt regulator for the op amp power.
The 5.1V Zener is the reference.
The 15V Zener has a much poorer temperature coefficient than the 5.1V Zener so does not make a good reference.
But it likely would be better to use a stable programmable reference, such as the TL431 for the 15V supply, and then divide that down to 5V for the reference.

Hello again,

Oh that makes sense about the transistor, but if you care enough to go through the trouble of coming up with a good circuit why not just get a model for a transistor that would really fit the bill, and how would you know the thread starter would know not to use that very transistor.

I talked a little about the history of power electronics since i had worked in the power control industry but i have seen more recent improvements in the methodology too when i started designing chargers for Li-ion cells, mostly the 4.2 volt type like the 18650 cell. It started with a pure linear with no real attention to power consumption, then moved to a pure linear with limited input like mandatory 5 volt maximum. Then finally to a switcher. I went to a switcher early on because i worked with mostly high power switchers in the past so i was familiar with the advantages in power savings. I went from a design that got pretty warm using a 9v DC wall wart to a simple switcher that gets barely warm to the touch using a wall wart with voltage from about 6v up to 15v or above with no real restraints on the input voltage (maybe 30v max though for that design). The interesting part is, the higher the input voltage the lower the current draw from that input source. At 6v it might draw 1 amp, at 12v just 1/2 amp, at 24v just 1/4 amp. No heating at all, very little loss of energy, and that had a National Semiconductor "Simple Switcher" at the heart of it, which was not even the top of the line efficient converter. Id's say maybe 80 to 85 percent. Imagine going to 90 percent. Yeah, a little extra effort, but why not in today's environmental climate.

It's up to you though the circuit does look like it will do the job but it will need a pretty decent heat sink i think for when the battery is very low and is charging. Remembering that the input is 56 volts and the output may only be 46 volts (or even less) and at 3 amps that's 30 watts already. That's a lot to dissipate.
For a decent switcher should be a lot less.
Also, for a switcher i dont know if there is any off the shelf simple device you may have to use a controller chip with external MOSFET, but still not the end of the world as we know it

The zeners...
Yes i see what you mean about the two zeners. I guess i tend to use voltage reference diodes now since they are widely available and cheap enough. I always hated zeners because of their variable nature when the whole point of using them in the first place was to get a decent regulated reference source.


The battery chemistries...
The LiFePO4 types have less tendency for thermal runaway because their rate of temperature increase is much lower. There could still be a problem though of course, and mostly with all these batteries the worst time is when charging.

 

Hello again,

Oh that makes sense about the transistor, but if you care enough to go through the trouble of coming up with a good circuit why not just get a model for a transistor that would really fit the bill, and how would you know the thread starter would know not to use that very transistor.

I talked a little about the history of power electronics since i had worked in the power control industry but i have seen more recent improvements in the methodology too when i started designing chargers for Li-ion cells, mostly the 4.2 volt type like the 18650 cell. It started with a pure linear with no real attention to power consumption, then moved to a pure linear with limited input like mandatory 5 volt maximum. Then finally to a switcher. I went to a switcher early on because i worked with mostly high power switchers in the past so i was familiar with the advantages in power savings. I went from a design that got pretty warm using a 9v DC wall wart to a simple switcher that gets barely warm to the touch using a wall wart with voltage from about 6v up to 15v or above with no real restraints on the input voltage (maybe 30v max though for that design). The interesting part is, the higher the input voltage the lower the current draw from that input source. At 6v it might draw 1 amp, at 12v just 1/2 amp, at 24v just 1/4 amp. No heating at all, very little loss of energy, and that had a National Semiconductor "Simple Switcher" at the heart of it, which was not even the top of the line efficient converter. Id's say maybe 80 to 85 percent. Imagine going to 90 percent. Yeah, a little extra effort, but why not in today's environmental climate.

It's up to you though the circuit does look like it will do the job but it will need a pretty decent heat sink i think for when the battery is very low and is charging. Remembering that the input is 56 volts and the output may only be 46 volts (or even less) and at 3 amps that's 30 watts already. That's a lot to dissipate.
For a decent switcher should be a lot less.
Also, for a switcher i dont know if there is any off the shelf simple device you may have to use a controller chip with external MOSFET, but still not the end of the world as we know it

The zeners...
Yes i see what you mean about the two zeners. I guess i tend to use voltage reference diodes now since they are widely available and cheap enough. I always hated zeners because of their variable nature when the whole point of using them in the first place was to get a decent regulated reference source.


The battery chemistries...
The LiFePO4 types have less tendency for thermal runaway because their rate of temperature increase is much lower. There could still be a problem though of course, and mostly with all these batteries the worst time is when charging.

Hello again,

Oh that makes sense about the transistor, but if you care enough to go through the trouble of coming up with a good circuit why not just get a model for a transistor that would really fit the bill, and how would you know the thread starter would know not to use that very transistor.

I talked a little about the history of power electronics since i had worked in the power control industry but i have seen more recent improvements in the methodology too when i started designing chargers for Li-ion cells, mostly the 4.2 volt type like the 18650 cell. It started with a pure linear with no real attention to power consumption, then moved to a pure linear with limited input like mandatory 5 volt maximum. Then finally to a switcher. I went to a switcher early on because i worked with mostly high power switchers in the past so i was familiar with the advantages in power savings. I went from a design that got pretty warm using a 9v DC wall wart to a simple switcher that gets barely warm to the touch using a wall wart with voltage from about 6v up to 15v or above with no real restraints on the input voltage (maybe 30v max though for that design). The interesting part is, the higher the input voltage the lower the current draw from that input source. At 6v it might draw 1 amp, at 12v just 1/2 amp, at 24v just 1/4 amp. No heating at all, very little loss of energy, and that had a National Semiconductor "Simple Switcher" at the heart of it, which was not even the top of the line efficient converter. Id's say maybe 80 to 85 percent. Imagine going to 90 percent. Yeah, a little extra effort, but why not in today's environmental climate.

It's up to you though the circuit does look like it will do the job but it will need a pretty decent heat sink i think for when the battery is very low and is charging. Remembering that the input is 56 volts and the output may only be 46 volts (or even less) and at 3 amps that's 30 watts already. That's a lot to dissipate.
For a decent switcher should be a lot less.
Also, for a switcher i dont know if there is any off the shelf simple device you may have to use a controller chip with external MOSFET, but still not the end of the world as we know it

The zeners...
Yes i see what you mean about the two zeners. I guess i tend to use voltage reference diodes now since they are widely available and cheap enough. I always hated zeners because of their variable nature when the whole point of using them in the first place was to get a decent regulated reference source.


The battery chemistries...
The LiFePO4 types have less tendency for thermal runaway because their rate of temperature increase is much lower. There could still be a problem though of course, and mostly with all these batteries the worst time is when charging.

Hello again,

Oh that makes sense about the transistor, but if you care enough to go through the trouble of coming up with a good circuit why not just get a model for a transistor that would really fit the bill, and how would you know the thread starter would know not to use that very transistor.

I talked a little about the history of power electronics since i had worked in the power control industry but i have seen more recent improvements in the methodology too when i started designing chargers for Li-ion cells, mostly the 4.2 volt type like the 18650 cell. It started with a pure linear with no real attention to power consumption, then moved to a pure linear with limited input like mandatory 5 volt maximum. Then finally to a switcher. I went to a switcher early on because i worked with mostly high power switchers in the past so i was familiar with the advantages in power savings. I went from a design that got pretty warm using a 9v DC wall wart to a simple switcher that gets barely warm to the touch using a wall wart with voltage from about 6v up to 15v or above with no real restraints on the input voltage (maybe 30v max though for that design). The interesting part is, the higher the input voltage the lower the current draw from that input source. At 6v it might draw 1 amp, at 12v just 1/2 amp, at 24v just 1/4 amp. No heating at all, very little loss of energy, and that had a National Semiconductor "Simple Switcher" at the heart of it, which was not even the top of the line efficient converter. Id's say maybe 80 to 85 percent. Imagine going to 90 percent. Yeah, a little extra effort, but why not in today's environmental climate.

It's up to you though the circuit does look like it will do the job but it will need a pretty decent heat sink i think for when the battery is very low and is charging. Remembering that the input is 56 volts and the output may only be 46 volts (or even less) and at 3 amps that's 30 watts already. That's a lot to dissipate.
For a decent switcher should be a lot less.
Also, for a switcher i dont know if there is any off the shelf simple device you may have to use a controller chip with external MOSFET, but still not the end of the world as we know it

The zeners...
Yes i see what you mean about the two zeners. I guess i tend to use voltage reference diodes now since they are widely available and cheap enough. I always hated zeners because of their variable nature when the whole point of using them in the first place was to get a decent regulated reference source.


The battery chemistries...
The LiFePO4 types have less tendency for thermal runaway because their rate of temperature increase is much lower. There could still be a problem though of course, and mostly with all these batteries the worst time is when charging.

What method do you suggest that using switch?

 

What method do you suggest that using switch?

You mean using a switching regulator?

If so and if you are interested in some decent efficiency when it is running, you could look up some switching regulator control chips. These are chips that regulate the output by turning a transistor on and off as the output requires. They are used in conjunction with a transistor, and these days a MOSFET is probably the best choice as they are fast and relatively low voltage drop.
For a regular voltage regulator application, the controller chip measures the output voltage and then makes changes to the pulse width in some way. Pulse width modulation is fairly common. It turns the transistor on when the output is too low, and off when it is too high, but this can happen very fast, like 20000 times a second or in more modern designs 100000 times a second or even faster.
For a current regulator application, the controller chip measures the output current and does the same thing only it tries to keep the output current constant.
For a battery charger it may be both. It measures both the voltage and the current and allows the battery to charge with some maximum current and once the voltage reaches a certain point it starts to regulate the voltage instead of the current. As the voltage reaches the set point, it will start to regulate it to try to keep it at that set point.

I can see you are familiar with CC and CV charging, so you probably have a good idea what to do just cant get something going yet.
If you would like to stick with a linear circuit for now, we can look at some designs that would do what you need, then if you like later you can move to a switching circuit for better efficiency, if you want to do that. The only difference is in the controller and some filter components.

To start, do you understand how a basic linear voltage regulator works?

 

That is a 5 volt Zener on the power supply. It is unlikely that the LM741 can do much with that low of a power supply. To make it worse, the non-inverting input is connected to another 5V Zener.

 

but if you care enough to go through the trouble of coming up with a good circuit why not just get a model for a transistor that would really fit the bill,

Well, if you really must know, I'm a lazy old fart.
And I'm not changing at this point in my life.