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

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,

So i recap, if i have correctly understand, A BMS board is designed for protect the cells against Overcharge, over discharge, over current, short circuit protection, right? But for protect the batteries pack life, it need to be balance charge right? Which board can you advice me for protect and balance my batteries pack? (I have Panasonic unprotected NCR18650B 18650 3.7V 3400mAh) I found a board on aliexpress, what do you think?

Or 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 original unprotected ?

Another question, Please, can you advice me a reliable BMWS for protect my batteries pack? I'am not there to save money, the safety of the pack is my priority.

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?

• You don't seem to be designing anything here, you just want to wire a pre-built BMS board to the cells, charger and load according to the wiring diagram the BMS manufacturer provided. – Dmitry Grigoryev Jul 12 '17 at 15:47
• Yes Alright, I made the wiring diagram on photoshop. I want to know if it is the good way to make a 18650 batteries pack and charge them. – BatPower Jul 12 '17 at 15:57
• Sorry, I assumed by mistake that the wiring diagram above came from your BMS supplier (the cheapest ones on Aliexpress are actually provided with drawings similar to yours as documentation). – Dmitry Grigoryev Jul 12 '17 at 16:14
• I have checked the link you provided: item not available anymore and 0 feedback look scary. Why don't you spend $4 to get something like this, which has the word "charger" in the title and 2000+ positive reviews? I know it's twice the price, but you don't want to save$2 at the risk of starting a fire, do you? – Dmitry Grigoryev Jul 13 '17 at 9:10
• Thanks for your advice, Whatever the cost, my goal is to build a safe battery pack(safe for their users and battery life). I am not there to save money on it. Please can you advise me for my case the best quality protected bms (which has demonstrated good functioning on the market) – BatPower Jul 13 '17 at 12:20

That BMS looks identical to one that I've used and it does NOT do balancing or proper charging. Just protecting from over current, over voltage and under voltage - all of which are very important.

You will need to add balance wires and get a balance charger for this battery.

Balance wires are easy - they go exactly where the bms wires go - one to each pos and neg of each cell. The balance charger will have a slot for those wires.

I recommend looking at the IMAX b6 charger because it is the cheapest balance charger I have seen and seems to work reasonably well up to batteries with 6s or less.

• This is indeed an important piece of info. Do you happen to have any documentation for that BMS you've used. I'm interested to know how exactly it can protect from over-voltage if it's not the one who charges the cells. – Dmitry Grigoryev Jul 13 '17 at 8:55
• Incidentally, I fail to see the use case for this board: if you still need a separate charger, why not just get a charger which does battery management (most of them do)? – Dmitry Grigoryev Jul 13 '17 at 9:18
• @DmitryGrigoryev - this BMS is still 100% useful because after you are finished charging the battery and the cells are balanced, you unplug the charger and use the battery and this is when the BMS is working. It prevents the battery and the individual cells from over discharging, over current, short circuit etc.,. It also acts as a back up protection from overcharge in case the charger screws up for some reason - which has been known to happen – Filek Jul 14 '17 at 7:48
• Also, the balance wires for the charger can be exactly the same as the wires for the BMS - in between each cell's connection - in might be easier when soldering to make one connection for both the bms and charger wire and this is usually how it is done, it was just easier to explain it as one wire to each neg and pls of each cell even though most put it in between even though it doesn't make a difference. – Filek Jul 14 '17 at 7:54
• You will need to show us the data sheet or link to the AliExpress BMS so we can look at the specs to see if it fits your needs – Filek Jul 14 '17 at 8:03