Diy 18650 4s battery pack [How to charge, how to build?]

CyPF · Jul 12, 2017

Hello everyone,
I'm building a bluetooth portable speaker. However, I have some questions about building my first 18650 battery pack. I have 4 pcs of Panasonic unprotected NCR18650B 18650 3.7V 3400mAh.

My goal is to build a 4s 18650 pack with these batteries, and the battery pack must:
- be inside the portable speaker
- Fully protected
- Safe
My question is, how do I design this battery pack? I have a idea, but want to know if it is a good way to build it?:

I also have some others questions: During my searches on internet some people use balanced charge, is it necessary to it use in my case? If yes how?
Do I need to use a power adapttor or lithium li-ion battery charger adaptor?
Thanks for your future help Cordially,

SilverFox · Jul 12, 2017

Hello CyPF,

Welcome to CPF.

It looks like you understand how to hook things up for the discharge side of the pack. With multiple cells in series you do need to use a balancing charging system.

During charging the balancing system monitors the voltage of each cell and when it gets too high there is a circuit that bleeds some charge off of the cell. In the end you end up with each cell balanced in voltage to each other cell.

You should be able to find a circuit to do this. It requires access to each cell to measure individual voltage but you basically have that in your protection system.

Tom

CyPF · Jul 12, 2017

SilverFox said:
Hello CyPF,

Welcome to CPF.

It looks like you understand how to hook things up for the discharge side of the pack. With multiple cells in series you do need to use a balancing charging system.

During charging the balancing system monitors the voltage of each cell and when it gets too high there is a circuit that bleeds some charge off of the cell. In the end you end up with each cell balanced in voltage to each other cell.

You should be able to find a circuit to do this. It requires access to each cell to measure individual voltage but you basically have that in your protection system.

Tom

Hello,
Thanks for you reply,
If i have correctly understand, the Bms board is designed for protect the cells against Overcharge, over discharge, over current, short circuit protection.
The link of the bms: http://goo.gl/s8g2bQ
But for protect the batteries pack life, it need to be balance charge right?
Which board can you advice me for protect and charge my batteries pack? (I have Panasonic unprotected NCR18650B 18650 3.7V 3400mAh)
I found a board on aliexpress, what do you think ?
https://goo.gl/SKVdhn

I have an other idea,
what do you think if i put a balanced charger in the speaker,
does it possible the wires stay always connected to the charger and the speaker circuits?
Does it is necessary to have a bms for protect my batteries because they are unprotected?
Link of the balanced charger: https://goo.gl/8JRHTZ

SilverFox · Jul 12, 2017

Hello CyPF,

It looks like you have presented two options.

One is to use the s8g2bQ linked protection circuit along with the balancing charger.

The other is to use the SKVdhn linked board that both protects and balances during charging.

You will have to study both and pick the one that best fits your application.

Tom

CyPF · Jul 12, 2017

SilverFox said:
Hello CyPF,

It looks like you have presented two options.

One is to use the s8g2bQ linked protection circuit along with the balancing charger.

The other is to use the SKVdhn linked board that both protects and balances during charging.

You will have to study both and pick the one that best fits your application.

Tom

Which solution is the most safe for the battery life ?
Cordially,

SilverFox · Jul 13, 2017

Hello CyPF,

I believe they are equal.

Tom

CyPF · Jul 14, 2017

The bms specifications is compatible with my batteries?
link batterries: Panasonic unprotected NCR18650B 18650
https://www.fasttech.com/p/1345500

SilverFox · Jul 15, 2017

Hello CyPF,

The balancing protection circuit will work with your cells. You still need to supply a charging voltage and make sure your application draws less than 30 amps.

Go through the specifications for the protection circuit board line by line and see if they match up to your cells and application.

Tom

CyPF · Jul 18, 2017

I checked the existing BMS on the market, but i noticed that Over-discharge Protection is too low compared to my batteries specs. The common bms on internet cut the current at approximately 2.55 this is too low for my batteries. Indeed minimum voltage of my batteries is 2.75 V. So i found a new BMS with balance charge function: link
I have several questions, i made a wiring diagram and i want to know if don't have made mistakes:

However, several questions remain vague in my mind: The Positive cable and the negative cable are the same cables to charge the battery pack and use it? On the diagram (Output / Input Positive + and Output / Input Negative -) Can i use the battery pack at the same time as it charging? For me it seems odd to me because I wonder what the intensity and the voltage sent to electric circuit of the bluetooth speaker. On the diagram (Intensity: A? Voltage Rating: V?) Those of the adapter or the battery?

SilverFox · Jul 18, 2017

Hello CyPF,

At this point you need to explore isolation circuits to separate the charging side from the application.

Unfortunately I understand the basics but don't have any experience with the details. I can look at something and say that doesn't see correct, but don't have the experience to go on to indicate how to correct the problem.

With what you have, you can use the battery pack but need to disconnect it for charging. There should be a way to isolate things so you can charge and use at the same time.

Tom

Diy 18650 4s battery pack [How to charge, how to build?]

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