Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Help: 4S2P pack
#1
Question 
Hello everyone

I want to design and then build a battery pack/power bank that can deliver 12V @ 3A. I will be using new 18650 cells. I want to use (as far as possible) modules I can buy from AliExpress.

My thinking was as follows:
- 4s2p pack
- 4s BMS (maybe 1 for each series strip?)
- 4s balance board (maybe 1 for each series strip?)
- Use a laptop charger to charge the pack or any other DC power source that can supply enough volt/current
- Step down DC to DC converter to go from charger/PSU to the 16.8V for charging the 18650s
- Another step-down DC to DC converter to give the desired 12V output
- Some visual indication to show the state of charge of the pack

After reading up on some BMS modules, and looking at several listing on AliExpress, I saw most specify an input voltage of 16.8V but still from specifically a lithium-ion charger. Some BMS modules claim to do balancing and others, although the same picture, claim not to be able to do balancing.

So now I am confused on what to get.  Confused  I want a BMS/charge module with a balance function as well. Or, alternatively, separate modules that I can combine to perform the desired function.

Any advice?
Reply
#2
1) Why 12V? 3s/4s is very bad for standard lithium (LiCo) cells (LiFePO4 being the chemistry to go to)
2) Why step down twice? This seems like a waste of a lot of power. Just use the proper charger voltage, more specifically you could to 24VDC (7s) and there are plenty of Lithium chargers for that voltage range that will work just fine with LiCo's
3) For being 2p, what are your loads?
4) What kinds of cells will you be using? New, Used? Laptop,Medical, PowerTool, etc?

And lastly, have you read the FAQ located on the main page?
Headrc likes this post
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#3
Thank you for your reply.

(03-13-2019, 11:02 AM)Korishan Wrote: 1) Why 12V?  3s/4s is very bad for standard lithium (LiCo) cells (LiFePO4 being the chemistry to go to)
Yes, I understand. The problem is I already have the 18650 cells  Wink

(03-13-2019, 11:02 AM)Korishan Wrote: 2) Why step down twice? This seems like a waste of a lot of power. Just use the proper charger voltage, more specifically you could to 24VDC (7s) and there are plenty of Lithium chargers for that voltage range that will work just fine with LiCo's
My understanding is that the 19V from the laptop charger must be stepped down to 16.8V to charge the 4s cells. The 4s pack then have (as per the FAQ) a range of ~12.0V when empty to 16.8V when fully charged, which must be stepped down to the required 12V. 

(03-13-2019, 11:02 AM)Korishan Wrote: 3) For being 2p, what are your loads?
No more than 2A continuous for at least 2 hours (powering a SBC)

(03-13-2019, 11:02 AM)Korishan Wrote: 4) What kinds of cells will you be using? New, Used? Laptop,Medical, PowerTool, etc?
New 18650 li-ion cells

I was hoping there is a charger module that can accept say 19V as input and then charge (and possibly balance) the pack. Also, how would the charger current be controlled?  Confused
Reply
#4
You didn't fully answer question #1. Why 12V? Why not go with 24V or something else? Do you have inverter(s) already that are rated for 12V?

2) I wouldn't step it down, I'd use a PWM charge controller for solar. It takes the higher voltage and drops it down nicely with higher amps than what most buck converters would do.

3) 2A continuous you will need more than 2P. Most laptop cells are rated at .2C or less. That equates to around 750mA max. If you plan on drawing 2A, even in short bursts (a few seconds), you need to go with at least 4P

4) Again, what kind of cell? Not chemistry or size. Are they Samsungs, Sanyos, Panasonics, etc? What's the model of the cells you're using?

Charger current on the input side is controlled by the source. You just don't want to go over the max rating of the controller. But if you go with a solar charge controller, they usually can handle 20A or more.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#5
Thank you for your reply. 

(03-13-2019, 10:25 PM)Korishan Wrote: Why 12V?  Why not go with 24V or something else? Do you have inverter(s) already that are rated for 12V?
I want to build a pack to power a LattePanda Alpha via its 12V JST 4p DC input connector. It will accept 7.4V to 15V so a fully charged pack will be too high. From what I have read, it needs about 2A when booting, but after that, it should be on 1A~1.5A.

(03-13-2019, 10:25 PM)Korishan Wrote: Again, what kind of cell? Not chemistry or size.  Are they Samsungs, Sanyos, Panasonics, etc? What's the model of the cells you're using?
LG HG2
Reply
#6
With that wide voltage range and specifically for the Panda, I'd go with 3s, then. Forget the double buck converters and use a controller that can charge up to 12.3V (4.1V/cell).

This will make it so you don't have to worry about bucking the pack voltage down to the Panda, or bucking a power brick down to pack voltage. Then use an inexpensive bms that will disconnect charge input when the pack gets full
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#7
Thank you for your reply.

(03-15-2019, 05:32 PM)Korishan Wrote: With that wide voltage range and specifically for the Panda, I'd go with 3s, then. Forget the double buck converters and use a controller that can charge up to 12.3V (4.1V/cell).

This will make it so you don't have to worry about bucking the pack voltage down to the Panda, or bucking a power brick down to pack voltage. Then use an inexpensive bms that will disconnect charge input when the pack gets full

I will order the following components: My laptop power brick is rated 19.5v @ 3.34A and it is not clear from the BMS specifications if it will be able to handle the 19.5v, and also I still need to set the charge current. After some Googling, it seems as if I can use an XL4015 5A DC to DC CC CV module to set the charge voltage and current to the required values.

So far, am I on the right track?
Reply
#8
Yes, that's a common buck converter.

And correct, the BMS will take that 19.5V and greet you with some magic smoke.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#9
(03-21-2019, 01:03 PM)Kenjutsu Wrote: Thank you for your reply.

(03-15-2019, 05:32 PM)Korishan Wrote: With that wide voltage range and specifically for the Panda, I'd go with 3s, then. Forget the double buck converters and use a controller that can charge up to 12.3V (4.1V/cell).

This will make it so you don't have to worry about bucking the pack voltage down to the Panda, or bucking a power brick down to pack voltage. Then use an inexpensive bms that will disconnect charge input when the pack gets full

I will order the following components: My laptop power brick is rated 19.5v @ 3.34A and it is not clear from the BMS specifications if it will be able to handle the 19.5v, and also I still need to set the charge current. After some Googling, it seems as if I can use an XL4015 5A DC to DC CC CV module to set the charge voltage and current to the required values.

So far, am I on the right track?

Hi,

Looks like we’re both looking to do almost exactly the same thing. I want to build a 4s pack and charge it with my 20v 3.25A rated laptop power supply and had the same question about what voltage to feed to the BMS and controlling the current draw.

I’ve come the the same conclusion as you I think, need to use both a BMS and a XL4015 CC CV module. I guess one of us just needs to try it now Smile

Regards
Adam
Reply
#10
(03-23-2019, 09:06 AM)MonkeyThink Wrote: I’ve come the the same conclusion as you I think, need to use both a BMS and a XL4015 CC CV module.  I guess one of us just needs to try it now Smile

I have ordered all the required modules from AliExpress and now just the long wait for it to arrive...  Wink
Reply


Who read this thread?
58 User(s) read this thread:
Sean, mike, Majorphill, Korishan, PAF, wim, The-J-Man, watts-on, Dan Lim, BlueSwordM, Geek, Oz18650, djuro, gpn, Chablis_m, Headrc, Crimp Daddy, jonyjoe505, Redpacket, Ibiza, Cherry67, Riplash, 100fuegos, bukvaahmed@gmail.com, Gummiadler, chuckp, Truth, Leipi35, Shawndoe, Wolf, Generic, Speicher, kevinjaye, davidj, Crash_3174, Richard, Legolfeur95, Overmind, Church1182, beserker786, VOLTRON78, winny, Lucan, Omalik, 100kwh-hunter, Bilabob, JimU, Kenjutsu, MonkeyThink, loyd, DudeBro, friedpenguin, varro, ruolanace, BitcoinBandit, sunkmail, centrecore, cx530559

Forum Jump:


Users browsing this thread: 1 Guest(s)