Help: 4S2P pack

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Kenjutsu

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Mar 5, 2019
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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. :s 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?​
 
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?
 
Thank you for your reply.

Korishan said:
1) Why 12V? 3s/4s is very bad for standard lithium (LiCo) cells (LiFePO4 being the chemistry to go to)
Click to expand...
Yes, I understand. The problem is I already have the 18650 cells ;)

Korishan said:
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
Click to expand...
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.

Korishan said:
3) For being 2p, what are your loads?
Click to expand...
No more than 2A continuous for at least 2 hours (powering a SBC)

Korishan said:
4) What kinds of cells will you be using? New, Used? Laptop,Medical, PowerTool, etc?
Click to expand...
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? :s
 
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.
 
Thank you for your reply.

Korishan said:
Why 12V? Why not go with 24V or something else? Do you have inverter(s) already that are rated for 12V?
Click to expand...
I want to build a pack to power a LattePanda Alpha via its 12V JST 4p DC input connector. It will accept7.4V 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.

Korishan said:
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?
Click to expand...
LG HG2
 
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
 
Thank you for your reply.

Korishan said:
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
Click to expand...

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 anXL4015 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?
 
Yes, that's a common buck converter.

And correct, the BMS will take that 19.5V and greet you with some magic smoke.
 
Kenjutsu said:
Thank you for your reply.

Korishan said:
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
Click to expand...

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 anXL4015 5A DC to DC CC CV moduleto set the charge voltage and current to the required values.

So far, am I on the right track?
Click to expand...

Hi,

Looks like were 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.

Ive 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 :)

Regards
Adam
 
MonkeyThink said:
Ive 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 :)
Click to expand...

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