Li ion battery pack charging & BMS

[Adhith]

Hello everyone...
I'm making a LI ion battery pack of 4S2P configuration. Im planning to atttach a BMS to the pack but do I need two seperate BMS for each four cells in series or could I somehow use a single BMS for the project??
Also I'm planning to charge the pack through a boost module which uses a 12v,2A supply from a DC adapter as input to provide a 16.8V output for charging the pack. But upon reading some posts I came to know that the charging should be done with constant current and voltage mode. but my boost module only have constant voltage mode. So is it really necessary to maintain a constant current mode for charging??
 I'm also interested to know how the charging current requirement for the pack is calculated.

[katzohki]

It's possible to create a system that can take care of all the cells. You need to
1) Make sure that the cells charge maintain a balance with eachother
2) Protect from over-current draw (short circuit etc)
3) Protect from over-heating
4) Protect from over-voltage (while charging or whatever)
5) Prevent reverse voltage

YES, you need to control current when charging. Damage can occur (i.e. fire and explosions) otherwise. If you're considering designing a BMS yourself, you may as well plan to design a charging system for them.

On the other hand, LiIon charging systems for 4S2P should be readily available, I would think.

[KL27x]

A 4S2P is same thing as a 4S1P with larger cells. You can't put a different charge on 1 half of a "cell" than the other. So no, you can't use 2 balancing circuits. Well, I suppose you could take 2 separate 4S1P batteries and only connect them in parallel at the + and -, but the typical way to do it (which is probably more efficient) is to put each pair of cells in continuity into one larger cell.

If you allow the battery to draw too high of a current, it will increase in temperature. And this will reduce cell life and/or cause spectacular failure.

OTOH, if the cell charge rate is higher than your circuit can provide, you also have a problem. If cell can take 3A, either your boost circuit will fail via oversaturation of the inductor, and output will plummet to next to nothing. Or the 12V PSU will fail. If switchmode, it will just cut out. If transformer, same situation as with boost circuit. So if your battery is "bigger" than the PSU, which may well be the case, you need to limit current to protect the PSU, not the battery. Either way you cut it, you need to limit the max current draw to make it work. But when the cell nears full charge, you need to reduce/remove this limit, or it will take a super long time to charge the last 20% of the battery.

Rule of thumb is that max charge of an average li ion battery is probably at least 1C. Li ion batteries are made with much much higher charge/discharge to C ratios than this, maybe 20:1. They sacrifice some max capacity in order to achieve this higher charge/discharge rate. But there's decreasing gains going the other way of trying to maximize capacity at the cost of charge/discharge rate. So some batteries can be charged at 10 or 20C, but it would be rare to find a 1Ah battery which could not be charged with at least 1A. Hence the rule of thumb of 1C. But this is a max rate; if you want to extend life of the battery, you only want to charge at a small fraction of this max. I tend to use 1/5C to 1/3C as the mark for reg'lar li ion cells, unless in a hurry.

For 4S cells, I tend to charge to 16.6V in lieu of balancing. That can work ok, too. You have to examine how balanced your cells are, though.

[Adhith]

A 4S2P is same thing as a 4S1P with larger cells. You can't put a different charge on 1 half of a "cell" than the other. So no, you can't use 2 balancing circuits. Well, I suppose you could take 2 separate 4S1P batteries and only connect them in parallel at the + and -, but the typical way to do it (which is probably more efficient) is to put each pair of cells in continuity into one larger cell.

If you allow the battery to draw too high of a current, it will increase in temperature. And this will reduce cell life and/or cause spectacular failure.

OTOH, if the cell charge rate is higher than your circuit can provide, you also have a problem. If cell can take 3A, either your boost circuit will fail via oversaturation of the inductor, and output will plummet to next to nothing. Or the 12V PSU will fail. If switchmode, it will just cut out. If transformer, same situation as with boost circuit. So if your battery is "bigger" than the PSU, which may well be the case, you need to limit current to protect the PSU, not the battery. Either way you cut it, you need to limit the max current draw to make it work. But when the cell nears full charge, you need to reduce/remove this limit, or it will take a super long time to charge the last 20% of the battery.

Rule of thumb is that max charge of an average li ion battery is probably at least 1C. Li ion batteries are made with much much higher charge/discharge to C ratios than this, maybe 20:1. They sacrifice some max capacity in order to achieve this higher charge/discharge rate. But there's decreasing gains going the other way of trying to maximize capacity at the cost of charge/discharge rate. So some batteries can be charged at 10 or 20C, but it would be rare to find a 1Ah battery which could not be charged with at least 1A. Hence the rule of thumb of 1C. But this is a max rate; if you want to extend life of the battery, you only want to charge at a small fraction of this max. I tend to use 1/5C to 1/3C as the mark for reg'lar li ion cells, unless in a hurry.

For 4S cells, I tend to charge to 16.6V in lieu of balancing. That can work ok, too. You have to examine how balanced your cells are, though.

Thank you very much for your support KL27x. i didn't understand what you have said first   "but the typical way to do it (which is probably more efficient) is to put each pair of cells in continuity into one larger cell."  could you please explain a bit.

[KL27x]

Cell is one single li ion cell. Battery is more than one cell in series. Just to be super clear. Now:

When I say "put each pair of cells in continuity" I mean connect both +'s together and both -'s. Then take the four pairs of "double cells" and put them in series to make the battery. This is more efficient. It's easier to balance, and if using balancing circuit it can increase efficiency.


As opposed to making two isolated stacks of 4S1P and connecting only the output in parallel. Either way you only need one charger. But if want an active cell balancing circuit, in this latter configuration you'd need two of them.

[Adhith]

Cell is one single li ion cell. Battery is more than one cell in series. Just to be super clear. Now:

When I say "put each pair of cells in continuity" I mean connect both +'s together and both -'s. Then take the four pairs of "double cells" and put them in series to make the battery. This is more efficient. It's easier to balance, and if using balancing circuit it can increase efficiency.


As opposed to making two isolated stacks of 4S1P and connecting only the output in parallel. Either way you only need one charger. But if want an active cell balancing circuit, in this latter configuration you'd need two of them.

So are you suggestion the connection in diagram 1 over diagram 2. Does these both arrangements are equal in terms of capacity or any other??

[Adhith]

But upon reading some posts I came to know that the charging should be done with constant current and voltage mode. but my boost module only have constant voltage mode. So is it really necessary to maintain a constant current mode for charging??

Actually what I can find is a CC CV buck power supply not a boost circuit, which in my project is no useful. Does my BMS shown in the pic could limit the charging current?? I was also under the assumption that the power supply would deliver what the battery pack (4S2P) needs and it wont be a problem.

[Adhith]

OTOH, if the cell charge rate is higher than your circuit can provide, you also have a problem. If cell can take 3A, either your boost circuit will fail via oversaturation of the inductor, and output will plummet to next to nothing. Or the 12V PSU will fail. If switchmode, it will just cut out. If transformer, same situation as with boost circuit. So if your battery is "bigger" than the PSU, which may well be the case, you need to limit current to protect the PSU, not the battery. Either way you cut it, you need to limit the max current draw to make it work. But when the cell nears full charge, you need to reduce/remove this limit, or it will take a super long time to charge the last 20% of the battery.

I usually plug in an Bluetooth amplifier board which supplies 25x 2 channels and in the specification it is given that it needs a 3A supply, but i mostly power it with a 12V 2A supply and works fine.
correct me if i'm wrong, so if current draw is the case then the board will tries to pull more current from the PSU and it will get saturated right?? or does the board should have any sort of current regulation according to the supply limits??

[Adhith]

A 4S2P is same thing as a 4S1P with larger cells. You can't put a different charge on 1 half of a "cell" than the other. So no, you can't use 2 balancing circuits. Well, I suppose you could take 2 separate 4S1P batteries and only connect them in parallel at the + and -, but the typical way to do it (which is probably more efficient) is to put each pair of cells in continuity into one larger cell.

If you allow the battery to draw too high of a current, it will increase in temperature. And this will reduce cell life and/or cause spectacular failure.

OTOH, if the cell charge rate is higher than your circuit can provide, you also have a problem. If cell can take 3A, either your boost circuit will fail via oversaturation of the inductor, and output will plummet to next to nothing. Or the 12V PSU will fail. If switchmode, it will just cut out. If transformer, same situation as with boost circuit. So if your battery is "bigger" than the PSU, which may well be the case, you need to limit current to protect the PSU, not the battery. Either way you cut it, you need to limit the max current draw to make it work. But when the cell nears full charge, you need to reduce/remove this limit, or it will take a super long time to charge the last 20% of the battery.

Rule of thumb is that max charge of an average li ion battery is probably at least 1C. Li ion batteries are made with much much higher charge/discharge to C ratios than this, maybe 20:1. They sacrifice some max capacity in order to achieve this higher charge/discharge rate. But there's decreasing gains going the other way of trying to maximize capacity at the cost of charge/discharge rate. So some batteries can be charged at 10 or 20C, but it would be rare to find a 1Ah battery which could not be charged with at least 1A. Hence the rule of thumb of 1C. But this is a max rate; if you want to extend life of the battery, you only want to charge at a small fraction of this max. I tend to use 1/5C to 1/3C as the mark for reg'lar li ion cells, unless in a hurry.

For 4S cells, I tend to charge to 16.6V in lieu of balancing. That can work ok, too. You have to examine how balanced your cells are, though.

so for a 4S2P battery pack where each cell is of 5800mah (I'm using ordinary low cost batteries and thus I know that the actual capacity will be low) what should be the current ?? 1/5c is for each cell right so for 8 cells (8*1/5c) = 1.6A right??

[adras]

1 cell with 5800mAh? Those are big ones. For a cell with 5800mAh, 1C means that you charge at 5800mA which means 5.8 Amps. 2C would be twice as much so 11.6Amps, or 0.5C means 2.9Amps . These numbers are for cells connected in serial. If they are parallel you can multiply that by the number of cells.

Another example, having 4S2P, so 4 in serial, and these 4 another time in parallel. So 2 blocks, where each block consists of 4 cells in serial. These two blocks are connected parallel. Resulting in 8 cells total would result in Capacity * number of cells, which is 5800mA * 2 => 11.600mA => 11.600 Amperes

Please check the datasheet of your cells regarding the C rating, that's usually specified there. 1C may be a good rule of thumb but I saw cells which take more or even less.

Charging the cells with too much current can result in a steam/fire/explosion.

I sometimes use twice the specified current, but I also check the temperature every 5-10 minutes since it's really dangerous. I think fast charging also reduces the livetime so better stick to the recommended values.

Edit: Also very important: If you connect cells parallel  you need to ensure they are evenly charged. If one cell is full and the other one is empty, the full one will charge the empty one uncontrolled. It might be possible that the current is way too high and the empty cell starts steam/fire/explosion.

And as katzohki said, use balance charging. That means if you have cells connected in serial you have an additional wire between each cell.

So for a Block of 3 cells it looks like this: -C x1 C x2 C x3 +

C is a cell, - is minus, + is plus, x are balancing connectors. Where minus and plus give you the total voltage of the block. x1 is the first balancing connector providing 4.2 volts between - and x1, x2 is the second balancing connector at 8.4 colts between - and x2, and x3 is the same as + giving you the total of 12.6 Volts. A balancing charger can use these different voltages to determine the voltage of each cell, and these points are also used to charge cells independently. The balance charger first charges the empty batteries until all batteries have equal charge, then they are charged together.

In whatever way cells are connected, serial or parallel it's getting dangerous if they are not charged equally.

Edit2 You should also think about buying a battery charger from the car/airplane/quadrocopter area like the Imax B6. I'm really happy with mine, I can charge EVERYTHING with it. https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR12.TRC2.A0.H0.Ximax+b6.TRS0&_nkw=imax+b6&_sacat=0
But for your case you might want to look for a stronger one which can provide more Amps.

[Adhith]

1 cell with 5800mAh? Those are big ones. For a cell with 5800mAh, 1C means that you charge at 5800mA which means 5.8 Amps. 2C would be twice as much so 11.6Amps, or 0.5C means 2.9Amps . These numbers are for cells connected in serial. If they are parallel you can multiply that by the number of cells.

Another example, having 4S2P, so 4 in serial, and these 4 another time in parallel. So 2 blocks, where each block consists of 4 cells in serial. These two blocks are connected parallel. Resulting in 8 cells total would result in Capacity * number of cells, which is 5800mA * 2 => 11.600mA => 11.600 Amperes

Please check the datasheet of your cells regarding the C rating, that's usually specified there. 1C may be a good rule of thumb but I saw cells which take more or even less.

Charging the cells with too much current can result in a steam/fire/explosion.

I sometimes use twice the specified current, but I also check the temperature every 5-10 minutes since it's really dangerous. I think fast charging also reduces the livetime so better stick to the recommended values.

Edit: Also very important: If you connect cells parallel  you need to ensure they are evenly charged. If one cell is full and the other one is empty, the full one will charge the empty one uncontrolled. It might be possible that the current is way too high and the empty cell starts steam/fire/explosion.

And as katzohki said, use balance charging. That means if you have cells connected in serial you have an additional wire between each cell.

So for a Block of 3 cells it looks like this: -C x1 C x2 C x3 +

C is a cell, - is minus, + is plus, x are balancing connectors. Where minus and plus give you the total voltage of the block. x1 is the first balancing connector providing 4.2 volts between - and x1, x2 is the second balancing connector at 8.4 colts between - and x2, and x3 is the same as + giving you the total of 12.6 Volts. A balancing charger can use these different voltages to determine the voltage of each cell, and these points are also used to charge cells independently. The balance charger first charges the empty batteries until all batteries have equal charge, then they are charged together.

In whatever way cells are connected, serial or parallel it's getting dangerous if they are not charged equally.

Edit2 You should also think about buying a battery charger from the car/airplane/quadrocopter area like the Imax B6. I'm really happy with mine, I can charge EVERYTHING with it. https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2380057.m570.l1313.TR12.TRC2.A0.H0.Ximax+b6.TRS0&_nkw=imax+b6&_sacat=0
But for your case you might want to look for a stronger one which can provide more Amps.

Thankyou very much adras for the detailed explanation for my questions, but i still got somethings to be cleared before going further.
1)So for a 5800mah battery 1C means 5.8amps, i got it and I'm planning to go for 1/3 C as the charging current. but I have seen on internet that the real capacity of batteries are quiet less than specified so do I really want to take into account the specified value of 5800 or better stick around 3000 or less to be practical??
2)Regarding the 4S2P battery pack would it be good to make 4 groups of 2 cells in parallel first and then connecting the 4 groups in series?? thus making it easy for balancing (I'm using a BMS with balance option).
3So If I'm going with 1/3C as the charging current then for a 5800mah cell it would be
1/3 X 5800 = 1933.33  since I'm using 4 groups(each with 2 cells in parallel) how to calculate the charging current ??
would it be 2X 1933.33  or 4 X 1933.33 ??

[Nusa]

Assuming you're talking about 18650 cells, 5800mAh is clearly in the total fiction marketing hype range, especially since you combined it with the words "low cost" and "ordinary". The only thing you can do with those is actually measure their capacity with a charge-discharge cycle, see what you ended up with, and go from there. Don't be shocked if it's in the 600-1000mAh range.

Name-brand batteries from trusted vendors will typically be in the 2200-2500 area for those that look for a good price/performance ratio. If performance trumps price, then you can find real capacities up to around 3500. Plus you can find valid datasheets for genuine batteries that tell you what C is, and the recommended charge cycles!

[adras]

• You can go for 5.0A or 5.5A. That should be enough safe margin. But you're going to use 1/3C already to improve safety and lifetime, so I say you can use the full capacity as base for your calculation. 5.8A / 3 = 1.7A
• You should do "4 groups of 2 cells in parallel first". This will balance the batteries a little bit. Just watch them while you're building the first groups. Check temperature with your hands. Getting warmer is ok, getting too hot to touch is not. And before you make the final connection in series wait half an hour.
• Current increases in parallel, because at the same voltage, double capacity you need to deliver twice the energy. In case of serial you have the higher voltage, but the same capacity, so the current stays the same. This means 1.9A * 2 because you've got the double capacity. The charging voltage increases as well, one time for the obvious reason in the next sentence, and the other time because a higher current needs to flow, and therefore U=R*I where R is the batteries internal resistance. But the voltage is managed by the BMS. Obviously you can't charge 3*4.6V batteries, connected in series with 5V.
  
  

One final thing, as Nusa mentioned. Which types of batteries are you using? I know there are packs at 5800mAh, but if you've got 18600 batteries you got ripped off and those batteries will be dangerous!

[Adhith]

Assuming you're talking about 18650 cells, 5800mAh is clearly in the total fiction marketing hype range, especially since you combined it with the words "low cost" and "ordinary". The only thing you can do with those is actually measure their capacity with a charge-discharge cycle, see what you ended up with, and go from there. Don't be shocked if it's in the 600-1000mAh range.

Name-brand batteries from trusted vendors will typically be in the 2200-2500 area for those that look for a good price/performance ratio. If performance trumps price, then you can find real capacities up to around 3500. Plus you can find valid datasheets for genuine batteries that tell you what C is, and the recommended charge cycles!

Thank you very much for your suggestion and yes I'm using 18650 cells. Let me check how much it could last and then I'll make a decision to upgrade the batteries or not. I heard that Panasonic batteries are good and have a closer range to the specified capacity. 

[Adhith]

• You can go for 5.0A or 5.5A. That should be enough safe margin. But you're going to use 1/3C already to improve safety and lifetime, so I say you can use the full capacity as base for your calculation. 5.8A / 3 = 1.7A
• You should do "4 groups of 2 cells in parallel first". This will balance the batteries a little bit. Just watch them while you're building the first groups. Check temperature with your hands. Getting warmer is ok, getting too hot to touch is not. And before you make the final connection in series wait half an hour.
• Current increases in parallel, because at the same voltage, double capacity you need to deliver twice the energy. In case of serial you have the higher voltage, but the same capacity, so the current stays the same. This means 1.9A * 2 because you've got the double capacity. The charging voltage increases as well, one time for the obvious reason in the next sentence, and the other time because a higher current needs to flow, and therefore U=R*I where R is the batteries internal resistance. But the voltage is managed by the BMS. Obviously you can't charge 3*4.6V batteries, connected in series with 5V.
  
  

One final thing, as Nusa mentioned. Which types of batteries are you using? I know there are packs at 5800mAh, but if you've got 18600 batteries you got ripped off and those batteries will be dangerous!

Thank you again adras for your valuable suggestion and I'm cleared with my doubts. As i mentioned just above I'm using 18650 batteries for the purpose. These are the common type of Li ion batteries usually used right ?? is there any other type of Li ion for the same purpose??

[Nusa]

18650 is by far the most popular cell size, but there are quite a few cylindrical sizes available, both smaller and larger. https://en.wikipedia.org/wiki/List_of_battery_sizes#Cylindrical_lithium-ion_rechargeable_battery

There are also non-standard sizes, such as prismatic cells (cell phone batteries) and pouch cells.

Regardless of physical form, the theory is pretty much the same from the charging/discharging/balancing point of view, once you have the capacity and basic specs of the battery.

[adras]

Hmmmm, 18640 batteries with 5800mAh?!. Those don't exist! Yet! Or, well, in China they exist. But Chinese use different measurement methods like looking over your thumb gazing at the stars.

Throw them away! Either they only have a capacity of couple of hundret mAh, or maybe 1300 if you're lucky. But even if they are 1300 ones, you would charge them with way to much current. Throw them away! Even if you charge them right they can be dangerous! Throw them away!

Please!

Here, have a look at these. They were at 10-12€ each, but now they are at 7€. https://www.amazon.com/2500mah-discharge-Samsung-INR18650-25R-li-ion/dp/B00QKD4V4C/ref=sr_1_3?ie=UTF8&qid=1508442076&sr=8-3&keywords=samsung+inr18650-25r

I use those in my flashlights. You can charge them with 0.5C in 180 minutes or with 1C in 60 minutes(quickcharge). You get 20A out of them. They are LiNiMnCo batteries. They are safer than LiPo, not as safe as LiFePo. So they should forgive you a small mistake, but you should still be careful.