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Charge 18650 3s8p DIY pack
#1
First off, New user. This answer may be posted somewhere else, but at this point I have watched tons of videos, and searched that my head hurts.
I can't seem to post a jpg or png photo. New user restriction? Nevertheless..

My issue:

How do I charge a 3s8p 18650 cell pack with BMS wired into it? I assembled my pack, wired up the BMS listed below, but am confused as to the best way to charge my pack. I see people charging using laptop chargers, etc. Just supplying voltage to the BMS circuit. I have RC chargers. My most powerful being the Reaktor 300w 20A

Can I use this charger to charge my pack with it's built in BMS? The charger balances the cells as it charges. Does this "Double balancing" have any ill effects?

Also, would I need to wire balance leads to get the charger to initiate? Most chargers don't allow charging multi cell packs without said balance port in use.

I do have many laptop chargers at my disposal, but those are usually low amperage chargers. With the RC charger I could charge at any Amp up to 20a.

I intend on powering an AC plug to plug devices into. I am using a cheap Banggood 40a 3s BMS.

To whomever reads this, thank you! I hope I can get some clarity on the best way to charge 18650 packs, as this is my first. If I have left out any pertinent info, please let me know.

Mods: If I have violated any rules with links, or photo embedding, this is unintended.
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#2
You hook balance leads (as shown below) and run them to a connector.   If you have an RC balancer, you can plug the balance (leads) connector into the balance port.   You hook the main power to the power leads (12v) of the battery.     The main charging power goes in thru the main leads.    IF you hook up the balance leads...  and the charger has balance capability....   it will use the smaller/balance leads to adjust the voltage of each individual (4v) set of parallel cells during the charge process.  



If your charger only has 1 balance port - with perhaps 8 pin connector... (instead of several 8pin, 7pin, 6pin, ... 4pin) then you can typically still use it with just 4 wires. You just have to determine which end is the 'most negative' and wire your 4 balance leads to the 8pin connector so the most negative matches what the charger is expecting.
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#3
Thanks for the reply. You don't address the fact that the battery pack has a BMS soldered into the circuit. What is the best means of charging it, knowing that fact? I understand wiring the pack in order to use balance leads. If indeed they are necessary. If I wire the pack with balance leads, then charge using a RC charger, what happens with the BMS wired into the circuit?



Also, is this the correct location for this thread? I am new to this particular site.
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#4
Depends on what kind of BMS it is. If it's designed to handle overvoltage protection, then connect to the main leads on the BMS. Otherwise, you connect directly to the absolute Pos/Neg of the string.
You don't have to use the BMS during charging. It will balance regardless. If your charger is CC/CV and you can set the voltage charge limits, then go that route. The BMS only "monitors". It doesn't really do much more than that. It monitors and triggers on an event. Under normal conditions, the BMS is sorta not used at all.
Proceed with caution. Knowledge is Power! Literally! Cool 
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#5
Thanks for that reply. That makes sense.

How would you charge the pack in this video? It is what my pack is essentially.

https://youtu.be/NGp7cGBYOLc
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#6
(12-13-2019, 01:02 AM)robertjpjr Wrote: How would you charge the pack in this video? 
Voltage wise, this is a 3s lithium-ion battery.   So the max charge is 4.2v/cell * 3 = 12.6v...   and that assumes that each of the 3 cells have exactly the same voltage (i.e. are completely balanced).

For example, if cell1 is 4.0v and cell2 is 4.1v and cell3 is 4.0v...  then the battery will report an overall voltage of something like 12.1v.  If you charge up to 12.6 - then cell1 and cell3 will be fine as something a little less that 4.2 but cell2 will likely be pushed above 4.2v - which is dangerous.     

However, you have a BMS and even if it doesn't balance and you attempt to charge up to 12.6v and the one cell goes above 4.2v, the BMS should cut the power so that the charging stops and you might wind up with a 12.4v battery.

BTW - You can measure each cell with a voltmeter and find out if they are in balance or not.  You can do this at any time - at rest or while being charged etc.  It need not be a mystery.

Back to charging....   You need a charger specifically for 3s lithium-ion (i.e. 12.6v max).   A general purpose charger would be risky unless you know its 'top charge target' because left unattended, it could exceed 12.6v.    

RC chargers typically let you set 1) lithium-ion and 2) 3s ..  so that's an option.     If they don't have explicit lithium-ion (maybe 'lipo') that's OK as long as the cell level is < 4.2v and the overall is < 12.6v as long as you're OK with a < 100% charge.

Final thought..  I wouldn't obsess on trying to get 100% (4.2v/cell * 3 = 12.6v) charge.   A 4.1v/cell * 3 = 12.3v is easier on the battery and gives you a bit of wiggle room if a cell is a little unbalanced.   There isn't much power between 4.1 and 4.2v/cell - the core power of the battery is lower than 4.2 as you can see on most discharge curves.

Hope all the jibber-jaber helps, trying to poke at things because your question is such that I'm trying to guess at the info you need for it all to click Smile
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#7
for your pack a buck converter would be perfect. A buck converter can be adjusted to exactly 12.6 volts and you can power from any source as long as its above 13 volts. The bms will keep the pack from going out of balance or overcharging. As the battery gets close to 12.6 volts the buck converter drops the amps to a very low rate, it charges quickly to about 90 percent but will take hours to get a full charge. 

I recently been using the buck converters and find them to be an extremely safe way to charge lithium, and they will give you a 100 percent charge everytime. I also used balance chargers but they are very tempermental, all voltages have to be perfect, sometimes they stop charging for no reason. I been using the converters to charge all my smaller lithium packs both 4s6p lifepo4 and 3s16p li-ion, I don't even use my balance charger anymore. I'll be incorporating these converters on all my battery packs.

I use 2 different types one is 9 amps (cost 4 dollars), the other is 20 amps (cost 8 dollars) . I built cases for the converters with small cooling fans. The 10 amp volt/amp led meter I'm using cost 3 dollars. Once you set the voltage/amps on the converters its plug and play. Just connect input and output and it starts charging. 

15 amp buck converter for lifepo4 14.6 volts, I charge with a 19 volt 3 amp laptop power adapter , also has xt60 connectors on input/output.


Picture of 3 buck converters I built for 12.6 and 14.6 batteries


buck converter in use, no case needed
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#8
I appreciate all the info guys. I think I will wire up some balance leads, and charge with my rc charger. It will charge the cells to 4.2v under lipo settings. That is what I use for RC also. I feel this is the safer route?

I just don't know exactly what that BMS does as far as balancing cells. I need to explore this more.
I am familiar with lipo cells with rc, as I mentioned. But that is dealing with far less cells. At most I have 4 cells to worry about. Not 24!
I'm being cautious, as the 18650 cells came from used, older laptops.

Thanks again!

Is this what you charge your packs with? I have a few of these.
https://rover.ebay.com/rover/0/0/0?mpre=...2579111935

LM2596 Power Converter Module
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#9
Great.   One of the 'real dangers' is when an 18650 is overcharged for a long time (hours as apposed for a minute or 2).    So just don't leave your charge experiment unattended till you are comfortable and can always measure things with a voltmeter 'as you go'.   If it goes over 4.2v for any cell then you have a bit..   to just turn things off Smile
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