4S2P Li ion charging module and BMS

[Adhith]

hello everyone...
I'm making a 4s2p Li ion battery pack for my led project, but I'm totally new to making battery packs. i thought to connect 2 pairs (each with 4 cells in series) in parallel. but I found from the web that making 4 pairs ( each with 2 cells in parallel) in series is the better option but don't know why, also I would like to know how to choose the BMS and its connection to the pack.
I would like to charge the pack by micro usb so is it possible to charge this pack with USB power supply??
The battery pack are used to light a of 5m RGB led strip, Since the pack could provide around 14.8v then should i use any buck converter to reduce the voltage to 12v for the strips??

[Audioguru]

Don't you know the voltage of a Lithium cell when its voltage is low enough for the BMS to disconnect it and know its voltage when a cell is fully charged? Your guess of 14.8V is when the voltage of the 4s battery is halfway between almost dead and fully charged.
Will a buck converter work when its input voltage is lower than its output voltage setting (when the battery is almost dead)?

[sleemanj]

Parallel pairs is better (that is a single series string of cell-pairs)

Consider what happens when a cell dies with high internal resistance (or has a protection circuit which switches off).  In with 2 parallel strings of 4 cells, you lost half of your capacity because the other three cells in that string of 4 with the bad cell can't work either because their current can't flow through the dead/off cell, where with parallel pairs you really only lose that one cell because the other in the parallel pair can carry on working and carry the current, not quite that simple since it's current delivery capacity of that cell pair is now half, but it's not as bad as totally losing half your pack and if you're not pushing the cells to their limit in current delivery terms anyway it won't matter.

Similarly consider what happens if the cell develops a shorting fault (dendrite formation), in 2 parallel of 4 series cells you basically have the current of 4 cells able to dump through into that short, where in a 4 series of 2 parallel cells you have only the single paired cell which can feed the short. Either way the result probably won't be great but the lesser of two evils is better!

Similarly if you will monitor the cell health there is much less to monitor if you use parallel pairs, if you use 2 parallel of 4 series cells then you have 7 points to measure to get the cell health, while in 4 series of 2 parallel cells you have only 4 points to measure to get the cell-pair health.

Laptop packs invariably use parallel pairs in series.

[Geerant101]

Hi arithmetic. A BMS is required to keep the each cell within safe limits during charge and discharge, ~2.5v - 4.2v for standard li-ion cells. ~3v  can be used for the lower threshold to increase cell life. so if you have a 4s1p pack you will need a selwrate BMS node to monitor each cell individually, generally done with a BMS capable of multi cell monitoring having multiple channels. If you then connect 2 of these packs in parallel ( 2x (4s 1p)) then you will need 8 BMS nodes (2 x 4 Channel BMS units. However if you use a 1s2p then you only need 1 BMS node to monitor BOTH of the cells in parallel. So when you connect 4 of these in series (1 x(1s2p) You only need a total of 4 BMS nodes or a BMS with 4 channels. Both approaches are fine but one needs twice as many BMS nodes.

To charge it from USB you will need a boost converter charger to get the 16.8v minimum for the cells (you will need more due to losses..)

To run the lights you will need a 12v buck converter.

I hope this makes sense as I am rushing a bit..

Sent from my SM-G920I using Tapatalk

[Adhith]

Don't you know the voltage of a Lithium cell when its voltage is low enough for the BMS to disconnect it and know its voltage when a cell is fully charged? Your guess of 14.8V is when the voltage of the 4s battery is halfway between almost dead and fully charged.
Will a buck converter work when its input voltage is lower than its output voltage setting (when the battery is almost dead)?

Yes I'm aware of the fact that the voltage of the LI ion cells would decrease toward the end offull discharge. Actually I'm getting 4.1V from the cell when fully charged, So does the 3.7V written on the battery indicates the voltage towards the end of full discharge??

[sleemanj]

3.7v is the nominal voltage.  Simply speaking, it is the voltage where the cell is for the majority of it's capacity. 

Lithium-ion cells have a pretty flat discharge curve, they drop rapidly from 4.2 down to around 3.7v within the first 25-30% or so of discharge.  Then they are somewhere around the 3.7v for the next 50-60% of dicharge, and the last 10-25% of discharge they will rapidly drop down to about 2.7v, at which point there's really nothing left in them to speak of.

Look at some Lithium-Ion Discharge Curves.

[Adhith]

Actually I'm planning to charge the the pack with usb power from my phone charger  but I found that inorder to use it in that way each cell should have a seperate charging module(TP4056 module), but then there is no BMS. On the other hand if I use a BMS then I should provide a min of 14.8 v as its input which is impossible for my 12v adapter. So is there any way such that I could charge with usb along with BMS??

[sleemanj]

You can not use TP4056 or similar in series arrangement (unless each TP4056 has an isolated power supply of it's own, 5 TP4056, 5 isolated power supplies).

Each cell should have a protection circuit (use protected cells) which will prevent over charge (high volt), over discharge (low volt) and over current / short circuit (note that the protection circuits only usually take into account discharge current, not charge current).

If you use protected 18650 cells, the simplest way to charge is just use a spring-terminal 18650 battery holder and take the cells out and recharge in a proper 18650 charger when they are flat.

If you really want to use USB to charge then for starters you obviously are going to need a boost converter and preferably a 4s charge controller or at least a boost converter with constant voltage and constant current (current limit), as long as the cells are well balanced (same charge) and matched (same capacity) when you connect them in your parallel/series string AND PROTECTED then the protection circuit on each cell will kick in to protect them well enough without a balancing charge setup.