Is paralleling batteries safe? 50/50 maybe yes maybe no..

[batteryGOLD]

Hi there people, hope Ur having a nice Sunday

Soo do you think paralleling batteries is safe? at 50% cases yes, but other 50% cases may be unknown behavior/safety, it's a matter of probability/lucky that you get two batteries containing BMS with common charge/discharge port
50% of BMS worldwide are same charge/discharge port, and the other 50% are SEPARATE charge/discharge port.
So it's a matter of lucky when U combine parallel two or more batteries..

if BMS has separate charge/discharge ports, paralleling power lines will lead to non protected share transfer between batteries..
This happens to all parallels, even if batteries are same or different serial configuration.
Imagine you have one 16S battery parallel to other 16S, if BMS has separate ports this is a non protected parallel case.
If one battery debalances then will receive charge from the other battery. this may overcharge unbalanced cells at debalanced pack

Allways play SAFE and check BMS type before any kind of parallel !!
and MAKE SURE at the moment of parallel connection, batteries are at same voltage level or near. If those at different voltages level, it could lead to 100 or more Amperes current flow from one battery to other.. Also a matter of lucky depends how much is the voltage difference between batteries delta Volts at the connection moment

Fun thing: Some chinease batteries have BMS separate charge/discharge port and China people connect output to charge port
strange thing.. but It works! I've seen many times that invention at CityCoco scooters. charge port is able to supply 30A peak dont know how.. Those Chinese people are masters at quick solutions!

Is also possible to paralell Lithium Ion with LiFePO4 and LiPo and Pb acid and vice versa, as long apply the math rules common interval working zone & BMS have same charge/discharge port

 

[batteryGOLD]

take this math :thumb:

Imagine U parallel x2 16S batteries containing SEPARATE charge/discharge port BMS's (the case is also valid even at combination one battery w/ BMS different C/D port and the other battery w/ same Chrg/Dischrg port BMS --> this changes the probabilities of success vs possible failure, and also take into account the DUMB BMS's at market,ex Daly and more.. just sayin.. )

Imagine all battery cells at around 4,2V (100% SOC) , soo 4,2Vx16S = 67,2V
I've seen some times at Chinese batteries, one Block dies because sudden internal short circuit cells, this happen sometimes at low cost cells & many cycles cells.similar to a sudden death (play with low cost cells probabilities/statistics)
so at this sudden death moment, one battery goes from 16S to 15S (one S block died! RIP)
math 15Sx4,2V = 63V

sooo tha battery w 67,2V 16S will drain to tha 63V "15S" battery. Imagine batteries with IR 60mOhm and 80mOhm, this is total 140mohm . now math using ohm law I=U/R 4,2/0,14 equal 30Amperes charging to non healthy 100% charged battery. this will over charge all pack cells and generate a lot of heat at dead cells

more math.. the healthy battery with 67,2V(100% SOC 16S) will charge the 63V(now a 15S battery at 100% SOC)
67,2V will met at mid way 63V. this means 2,1V mid way (65,1V)
soo unhealthy battery will take more 2,1V/15S = 0,14V more per cell This leads to all 15S cells go up to 4,2V+0,14V = 4,34V
maybe safe :thumb:
but those 30A initial current could cause a fire because overheat the dead block and cells surround could go thermalrunway and start a fire! (but we don't need to be pessimists)

and about to do SERIAL batteries configuration? never/don't think about that. Is danger and only 1% is on tha safe side.
NEVER serial batteries, and don't full trust at PARALLEL , 50% parallel cases could have unstable behavior. (is a matter of technical wisdom or lucky..)
SERIAL solution is to rebuild packs or hard connection
* edited