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2 3s 16650 in parallel
#1
I need to parallel two 3s packs to get the capacity required.

The question is should the 3s packs be paralleled then connected to one 3S BMS including the cell balancing or should I use two BMS’ one for each 3s pack then connect the P+ and P- in parallel.

Thanks!!
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#2
(06-18-2020, 10:49 PM)Jkb242 Wrote: I need to parallel two 3s packs to get the capacity required.

The question is should the 3s packs be paralleled then connected to one 3S BMS including the cell balancing or should I use two BMS’ one for each 3s pack then connect the P+ and P- in parallel.

Thanks!!
You don't say how many cells you have in parallel or expected amps out - but if the functional goal is 1 physical battery (all the cells are right next to each other) and we're talking 100a or less then I'd go with 1 BMS as that's straight forward to buy.
  
Its OK to ds 2 BMSs, and makes sense if the 2 batteries are physically distinct OR you plan swap one out for a 3rd one OR each battery is delivering a current range so high that you can't find 1 BMS that can do it all - but be aware that with 2 BMSs, if 1 fails and the other continues you're pack will suddenly have 1/2 power.   Also will likely cost twice as much Smile.
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#3
connect both batteries to a y power connector and the balance leads to a y balance balance connector then everything to one bms. Once everything is connected the 2 batteries will equalize. Thats how I parallel my packs and never had problems with the packs staying in balance.
I always put quick disconnect connectors on the bms and batteries, that way I can remove a pack or even replaced the bms without having to resolder everything.

The reason to use one bms is to prevent one bms from stopping charging/discharging before the other.

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#4
If the BMS you selected can handle the Amps and balancing needs of both 3S packs, then you can (and should) parallel the connection and each balance lead before the BMS and use only one BMS.
Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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#5
(06-18-2020, 10:49 PM)Jkb242 Wrote: I need to parallel two 3s packs to get the capacity required.

The question is...

Posted in wrong section. DIY Battery Project Builds is for Project Builds only. Questions go in Battery Pack Design
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#6
Thanks to all who responded. Apologies for posting in the wrong section. I’m unable to move so if someone could make this change that would be fine.
NOW TO CLARIFY
Each three cell pack of 18650 2000ma cells are connected in series (3S) there are two such packs. The two 3S packs are to be connected in parallel for connection to the load. I am building this battery supply with either separate BMS’ for each pack then paralleling the two packs or use one BMS with the two 3S cell packs connected in parallel.

The objective is to use the 18650 cells I have on hand not purchase anything.

I fully understand the issues with cells being same capacity and initial charge potential. I am depending on the BMS to maintain the proper balancing during charge.

I believe Generic understood and answered this question, but it seems that others were confused as to the two choices in the construction of the desired battery supply. So I just wanted to restate my original question which is simply a choice of combining the two 3S packs, in an effort to gain any additional input.

Thanks again.
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#7
It's already been moved Wink

If you have 2 separate strings and connect them in parallel at the absolute Pos/Neg ends, then you should use 2 bms'.

If you have 2 separate strings and connect them in parallel at the absolute Pos/Neg ends "and" connect a balance lead across the packs to their respective groups, then 1 bms can be used.

As far as the load is concerned, if you pull a constant current of about 500mA or less per cell while they are connected in parallel, that should be fine. I would not go anymore than 1A surge/cell though

For the bms units selected, as long as the bms can handle the amps required under surge, you should be fine. If you get cheap chinesium units, they like to rate their bms units based on surge, not continuous (this also applies to their inverters/chargers/etc). So usually, for example, a 60A bms really needs to be down rated to about 30A continuous. 25A to give a margin of safety.
However, you can get around this by bypassing the bms for the over/under current over/under voltage protection and only use it for balancing. Maybe use it for charging purposes (so over voltage protection) and not use it for discharging.
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

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#8
Thanks, to all. The single BMS was my original thoughts since at this point, my understanding of the desired load is that two 3s packs will be ample. Upon a initial second look at the configuration the dual BMS looked like a way to avoid any balancing issues between the two packs but that didn’t seem like a good approach and just wanted input from the experience the forum.
Full agreement with the degradation of current rating for the cheap BMS. I’m so far below their current capacity it isn’t an issue in this application. I did note that once the BMS is removed then reinstalled, it’s necessary to connect the charge source to the pack terminals momentarily before the BMS can supply the load. Not sure why this is since the drain and source terminals of the MOSFETS are the only connection to the P+/P- terminals.

Off topic a bit, but I will ask for input on another issue I have encounter with how the BMS reacts with an inductive load like a DC motor. Typically, when powering the motor using a BMS, the motor will not start when switched on but without the BMS the same 4S cell pack there is never a start issue. Yet, 18650 cells are commonly used with BMS’ in portable EQ such as hand vac’s without issues. Any thoughts on this??

Thanks again so very much!!
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#9
Inductive loads need more current to start than to run. If you have a clamp meter that works with DC and can give you an inrush current reading, test the circuit without the BMS and find out the maximum amps the motor draws at initial startup. Make sure the reading is less than the number of Amps your BMS is rated for. If your BMS is refusing to let your motor start up, that means the BMS is either in short-circuit or over-current protection mode, and is preventing the motor from completing its circuit with the battery. Use a higher current rated BMS, or use the BMS you have as a signal to a relay or solenoid that can handle the motor's load.
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Check out my long-term capacity test of 18650s: https://secondlifestorage.com/showthread.php?tid=6868
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