[Solved] Li-Ion battery pack charger

[psysc0rpi0n]

Hello.

In the last few weeks I decided to give use to some Li-Ion batteries I had lying around here and I end up building a battery pack in a configuration of 3S4P.
Then, I bought a battery management system and also a MAX745 module to be able to charge this pack properly.

BMS:
https://pt.aliexpress.com/item/1005004635690925.html

MAX745 module
https://www.aliexpress.com/item/32851664522.html

This MAX745 module is supposed to alow us to set charging voltages and currents but there are 2 things I'm not being able to figure out so far.
The battery pack is supposed to output 16.8V. The batteries are 4.2V (when fully charged, or 3.6V for the storage recommended voltage).
1 - So, I think I need to adjust the charging voltage of the MAX745 to 16.8V by rotating the VSet rpot. But, am I supposed to do this before wiring the module to the battery pack or when?
2 - I'm using an external power supply, set to 16.8V and 500mA of current and when I connect the MAX745 to the BMS, and try to adjust the charging current with ISet potentiometer, when I get to aroun 180mA, it simply resets to like 40mA and I have to rotate the potentiometer back and then try to increase the charging current back up but I can't get over the 180mA. Any reason for this? Any help would be appreciated!

[xvr]

> So, I think I need to adjust the charging voltage of the MAX745 to 16.8V by rotating the VSet rpot. But, am I supposed to do this before wiring the module to the battery pack or when?

Before. And not only set, but a fix afterward - with a bit of glue or something like that. Voltage set should not be incidentally changed.

> I'm using an external power supply, set to 16.8V and 500mA of current

MAX745 is a Buck convertor. You should supply it with higher voltage (it accept up to 24V). It will regulate it itself. Current limiter in external poser supply also doesn't needed - MAX745 provides its own.

> when I get to aroun 180mA,

How much of current you plan to get? 3S4P configuration assumed charging with 4*C current, I think it will be much close to 4A (btw it's a maximum current, supported by MAX745. Not sure for rest of circuit on PCB from AliExpress  )

[psysc0rpi0n]

Before. And not only set, but a fix afterward - with a bit of glue or something like that. Voltage set should not be incidentally changed.

So, just to make sure I understand correctly. I connect my external power supply to In+ and In- of the Max745 module with let's say 20V and with a DMM I check the voltage on the Bat+ and Bat- side of the module and I adjust it to 16.8V with Vset, while the module is not yet wired to the battery pack, correct?

[/quote]

MAX745 is a Buck convertor. You should supply it with higher voltage (it accept up to 24V). It will regulate it itself. Current limiter in external poser supply also doesn't needed - MAX745 provides its own.

Ok, I just did that as an extra safety measure to prevent that any other unnoticed mistake of mine could end up in disaster!

How much of current you plan to get? 3S4P configuration assumed charging with 4*C current, I think it will be much close to 4A (btw it's a maximum current, supported by MAX745. Not sure for rest of circuit on PCB from AliExpress  )

I'm not sure I understand the question. You mean how much current I'm expecting the battery pack to provide when I'm actually "using" it? Or how much current I am thinking I can charge that battery pack with?
Either way, I'm not sure. The batteries are from aliexpress, therefore there is not much info about their capacity. About the carging current, I know from the MAX745 chip datasheet, it can provide up to 4A without getting hot. I think I don't need that much. If I could get 1A out of it, would be suffice.

From the valuable info you provided to me, next monday I will try to check how is the value of charging voltage as you described and if it's not correct, I will adjust it and will check again the potentiometer for carge current and see if I can get past the 180mA.

[xvr]

> So, just to make sure I understand correctly. I connect my external power supply to In+ and In- of the Max745 module with let's say 20V and with a DMM I check the voltage on the Bat+ and Bat- side of the
module and I adjust it to 16.8V with Vset, while the module is not yet wired to the battery pack, correct?


Yes

> I'm not sure I understand the question.

It is question about rated capacity of accumulator and charging current. For 1P configuration charging current should be set to accumulator rated capacity (example: 1A for 1000mA/h accumulator). For 3P it should be 3 times more. If your charging current is less then charging time will increase proportionally. If current is bigger you probably blown up your accumulator.

 >  The batteries are from aliexpress

Ouh. AliExpress is a VERY unreliable source of accumulators. About 99% if them here is a fake.

[psysc0rpi0n]

Yes

Ok, thank you

It is question about rated capacity of accumulator and charging current. For 1P configuration charging current should be set to accumulator rated capacity (example: 1A for 1000mA/h accumulator). For 3P it should be 3 times more. If your charging current is less then charging time will increase proportionally. If current is bigger you probably blown up your accumulator.

I think I got that part. What I was not sure about was the 4*C current. If this refers to capacity units (Coloumb) or something else

Ouh. AliExpress is a VERY unreliable source of accumulators. About 99% if them here is a fake.

Yeah, I know. I am aware of that! That's why I want to go conservative on the charging current. But I purchased these batteries and I couldn't use them for the first purpose I wanted to use them so I wanted to give them so other use!

[xvr]

> If this refers to capacity units (Coloumb)

Yes, it is capacity units, but not Coulombs but Amper*Hours (for LiIon commonly used milliAmper*Hours)

[psysc0rpi0n]

> If this refers to capacity units (Coloumb)

Yes, it is capacity units, but not Coulombs but Amper*Hours (for LiIon commonly used milliAmper*Hours)

Ok, thanks.

Tomorrow I'll check the voltage I have on the MAX745 module and will go from there and will update the post here to see if I can go over the 180mA charge current or not, and why!

[psysc0rpi0n]

Ok. Voltage is correctly set to 16.83V but now to set the current, I can only do it while it is charging, right?

[xvr]

You can connect resistor instead of battery. It will be enough to check voltage and current

[psysc0rpi0n]

Ok. Voltage is correctly set to 16.83V but now to set the current, I can only do it while it is charging, right?

Edite;

Ok, somehow I can now charge them at 20V and 400mA. But if I increase current more, the MAX745 module shuts down, or at least the charging indicator led, turns off.

Actually, right now, the current is dropping and the led is already off. Current is now at 360mA. Not sure if this is normal or not! I was expecting that when the battery pack is charged, the MAX745 simply cut off the charge instantly and not slowly as it is happening right now!

And the 4 resistors on the BMS are also a bit hot!

[xvr]

This is not a 'charging indicator' This is CC/CV indicator. Charging considered done when charging current drops below 1/10 of preset of current. Not shure that module has appropriate circuit.
> Current is now at 360mA. Not sure if this is normal or not!
Check the voltage. If it 16.8 than it normal - module moved to CV mode

> And the 4 resistors on the BMS are also a bit hot!

If you supply it with 20V it also quite normal - they try to balance accumulators and turned on

[psysc0rpi0n]

This is not a 'charging indicator' This is CC/CV indicator. Charging considered done when charging current drops below 1/10 of preset of current. Not shure that module has appropriate circuit.

Hum, For sure? Because Aliexpresser seller says the LED indicates when charge is done, it turns OFF.
https://www.aliexpress.com/item/32851664522.html

Charging interface: Red LED indicator,when charing is Red,Fully charged light is out.

Check the voltage. If it 16.8 than it normal - module moved to CV mode

The charging voltage? The charging voltage is at 16.83V

If you supply it with 20V it also quite normal - they try to balance accumulators and turned on

Ok, glad to know!

[xvr]

MAX datasheet says about 'status' output -

An open-drain MOSFET sinks current when in current-regulation mode, and is high impedance when in voltage-regulation mode. Connect STATUS to VL through a 1kΩ to 100kΩ pullup resistor. STATUS can also drive an LED for visual indication of regulation mode (see MAX745 EV kit). Leave STATUS floating if not used

There is no other status indicators. PCB do not contains any other IC except MAX.
So seller just do not know what he sell (as usual)

[psysc0rpi0n]

MAX datasheet says about 'status' output -

An open-drain MOSFET sinks current when in current-regulation mode, and is high impedance when in voltage-regulation mode. Connect STATUS to VL through a 1kΩ to 100kΩ pullup resistor. STATUS can also drive an LED for visual indication of regulation mode (see MAX745 EV kit). Leave STATUS floating if not used

There is no other status indicators. PCB do not contains any other IC except MAX.
So seller just do not know what he sell (as usual)

Can't argue against datasheet...

The thing I'm trying to build is still not finished but it's something like this:

[MrAl]

Before. And not only set, but a fix afterward - with a bit of glue or something like that. Voltage set should not be incidentally changed.

So, just to make sure I understand correctly. I connect my external power supply to In+ and In- of the Max745 module with let's say 20V and with a DMM I check the voltage on the Bat+ and Bat- side of the module and I adjust it to 16.8V with Vset, while the module is not yet wired to the battery pack, correct?

MAX745 is a Buck convertor. You should supply it with higher voltage (it accept up to 24V). It will regulate it itself. Current limiter in external poser supply also doesn't needed - MAX745 provides its own.

Ok, I just did that as an extra safety measure to prevent that any other unnoticed mistake of mine could end up in disaster!

How much of current you plan to get? 3S4P configuration assumed charging with 4*C current, I think it will be much close to 4A (btw it's a maximum current, supported by MAX745. Not sure for rest of circuit on PCB from AliExpress  )

I'm not sure I understand the question. You mean how much current I'm expecting the battery pack to provide when I'm actually "using" it? Or how much current I am thinking I can charge that battery pack with?
Either way, I'm not sure. The batteries are from aliexpress, therefore there is not much info about their capacity. About the carging current, I know from the MAX745 chip datasheet, it can provide up to 4A without getting hot. I think I don't need that much. If I could get 1A out of it, would be suffice.

From the valuable info you provided to me, next monday I will try to check how is the value of charging voltage as you described and if it's not correct, I will adjust it and will check again the potentiometer for carge current and see if I can get past the 180mA.
[/quote]

Hi,

If you do not know the specs on the batteries how could you know the maximum charging current?
The max charging current varies widely for Li-ion batteries, from maybe 300 milliamps to 20 amps, so how could you know the max for the cells you have.  Even then if they are not from a reputable source the specs can even be bogus.
If they are the 18500 on the low end is usually around 500ma to 700ma, up to 20 amps or maybe higher.  I've personally delt with the 700ma and up types.  If you do not know, then 700ma is probably a good limit for the current during charging.
Charging is one of the most dangerous modes for these batteries so it has to be done with extra care.  Making a pack with cells in series/parallel also requires that each battery cell has built in BMS protection so that each cell can monitor it's own charge regimen.
For initial testing you should probably have the pack outdoors and test for overheating with a thermometer.  If anything goes too wrong it quickly turns into a disaster.

[psysc0rpi0n]

Hi,

If you do not know the specs on the batteries how could you know the maximum charging current?
The max charging current varies widely for Li-ion batteries, from maybe 300 milliamps to 20 amps, so how could you know the max for the cells you have.  Even then if they are not from a reputable source the specs can even be bogus.
If they are the 18500 on the low end is usually around 500ma to 700ma, up to 20 amps or maybe higher.  I've personally delt with the 700ma and up types.  If you do not know, then 700ma is probably a good limit for the current during charging.
Charging is one of the most dangerous modes for these batteries so it has to be done with extra care.  Making a pack with cells in series/parallel also requires that each battery cell has built in BMS protection so that each cell can monitor it's own charge regimen.
For initial testing you should probably have the pack outdoors and test for overheating with a thermometer.  If anything goes too wrong it quickly turns into a disaster.

I am aware of the dangers of charging these batteries. If you read my 1st post you see I have a BMS to help with protection for over/under voltage and current scenarios and also for shorts.
Of course that this is not 100% safe and that there is still the chance for problems, but that's inherent to anything related with electricity, generally speaking!

Ok, I'll take that value of 700mA as a reference. Also, if 400mA can charge this battery pack in like a few hours, I'm good with it. Don't need super fast charging, also because of safety concerns I rather to take longer to charge the battery pack safelly, than to charge it faster but risking fires or explosions, etc!

[Peabody]

If the batteries are mostly charged, the MAX will have shifted over to constant voltage mode, and the charge current will be falling.  You will only see the maximum charge current early in the charge process  when the batteries are mostly discharged.  You might take a look at the datasheet for the TP4056, which has a diagram of what happens to voltage and current during the entire charging process.  That's just a single cell charger, but the process will be the same for multiple cells.

[psysc0rpi0n]

Ok, to close this topic, I got this thing working and now I'm charging it with 20V@720mA, with a current limit on my power supply of 750mA just in case!

Thanks to all that somehow helped|

[MrAl]

Hi,

If you do not know the specs on the batteries how could you know the maximum charging current?
The max charging current varies widely for Li-ion batteries, from maybe 300 milliamps to 20 amps, so how could you know the max for the cells you have.  Even then if they are not from a reputable source the specs can even be bogus.
If they are the 18500 on the low end is usually around 500ma to 700ma, up to 20 amps or maybe higher.  I've personally delt with the 700ma and up types.  If you do not know, then 700ma is probably a good limit for the current during charging.
Charging is one of the most dangerous modes for these batteries so it has to be done with extra care.  Making a pack with cells in series/parallel also requires that each battery cell has built in BMS protection so that each cell can monitor it's own charge regimen.
For initial testing you should probably have the pack outdoors and test for overheating with a thermometer.  If anything goes too wrong it quickly turns into a disaster.

I am aware of the dangers of charging these batteries. If you read my 1st post you see I have a BMS to help with protection for over/under voltage and current scenarios and also for shorts.
Of course that this is not 100% safe and that there is still the chance for problems, but that's inherent to anything related with electricity, generally speaking!

Ok, I'll take that value of 700mA as a reference. Also, if 400mA can charge this battery pack in like a few hours, I'm good with it. Don't need super fast charging, also because of safety concerns I rather to take longer to charge the battery pack safelly, than to charge it faster but risking fires or explosions, etc!

Hi again,

That's sounds like it is not too bad.  However, these batteries do not pose an ordinary threat like "anything related with electricity" they pose a fire and/or explosion hazard which only some electronic gadgets can create.  The kinds of fires with these things are unimaginably bad and the only way to get a good idea what can happen is to check out some web sites that do worst case testing of these things and also read up on some of the really bad problems that had come up from what seemed like just an ordinary battery.  In New York they made special laws for this now.
One case I remember is the whole store burnt down just because of one battery.  The fire department got there but found there was no way to put out that kind of fire so they had to just wait it out.
Another case the guy's pants pocket started on fire.
Another the car burnt up because the cell phone battery started on fire while charging in the front seat.
The FAA made special rules about these batteries, and now when I go to the post office I have to sign something that states there are no Li-ion (or similar) batteries in the package.  There are special rules about shipping them.
The list goes on and on.  This is completely different than any regular electrical problem that causes a fuse to blow or a person gets a shock or even a person dies when they get a shock.  It could be a complete disaster.
I myself take precautions even when not charging them and have backup plans worked out in case something does go wrong.

That's why these batteries are so different.  They are normally safe, but there are times when they just are not, and when they are not, they are REALLY *NOT*.

Good luck with your charger.

[psysc0rpi0n]

Yeah, I know, I'm aware of the dangers. I've seens quite a few really bad situations regarding Li-Ion Batteries catching on fire an even exploding. And how hard is to deal with the fire.

But a warn is never too much and I'm not more concerned about using this pack of batteries where I wanted to use them. I'll have to think a bit more about if I really want to use it where I wanted to use it, which was in the trunk of my car to feed a survaillance system I was building to my car!

[MrAl]

Yeah, I know, I'm aware of the dangers. I've seens quite a few really bad situations regarding Li-Ion Batteries catching on fire an even exploding. And how hard is to deal with the fire.

But a warn is never too much and I'm not more concerned about using this pack of batteries where I wanted to use them. I'll have to think a bit more about if I really want to use it where I wanted to use it, which was in the trunk of my car to feed a survaillance system I was building to my car!

Hi,

Yeah and some of the cells are protected internally.
I just tested a few yesterday because I had a problem trying to recycle the old Li-ion cells.  They only accept them once about every three months, and when I went up there the other day the only access road was blocked by 100 cars or so, so I could not even get to the place that accepts them.
I decided to discharge them all and keep them in storage for whatever time it takes me to get back there again, if I even decide to go back to that place.  What I found during the discharge was that two of the cells would discharge to around 2.4 volts and then the current would turn off all by itself.  That was the internal protection circuit kicking in.  Another two cells actually discharged all the way to zero volts, which says they did not have internal discharge protection.
I did not check them of overcurrent protection though, I would have had to set that experiment up.

Good luck with the battery pack.