3 parallel battery with one external bms

chronosteam

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Jul 27, 2020
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This is my first post in here. Thank you for all the membersinformation sharing that help me to start my electric vehicle journey.
I'm building 20S9P,20S12P from recycled 18650 and 20S2P from recycled 26650. The BMS i'll be using is 20S 72V 100A BMS

I'm thinking about connecting3 battery in parallel and using one BMS that is connected to the 2 or 3 battery with the balance lead that go to the 2 or 3 batteries.Is this possible? What do you think about this setup? I'm trying to save cost by using only 1 high amp bms, or is it better to use 3 separate bms?

Thank you for your help and information.
 

OffGridInTheCity

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Dec 15, 2018
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chronosteam said:
This is my first post in here. Thank you for all the membersinformation sharing that help me to start my electric vehicle journey.
I'm building 20S9P,20S12P from recycled 18650 and 20S2P from recycled 26650. The BMS i'll be using is 20S 72V 100A BMS

I'm thinking about connecting3 battery in parallel and using one BMS that is connected to the 2 or 3 battery with the balance lead that go to the 2 or 3 batteries.Is this possible? What do you think about this setup? I'm trying to save cost by using only 1 high amp bms, or is it better to use 3 separate bms?

Thank you for your help and information.
Yes you can. @DIY Tech & Repairs even did ayoutube -
Note: This bothered me as I'm imagined the sense leads burning up under load... but other discussions on this over the last couple of years indicate that the 'main power' will flow thru the bus/terminals of the batteries rather than the sense wires - so it works great.

Of course you can't have the different battery/cells with wildly different voltage levelwhen you 1st join the sense wires as that might burn them up as they do the initial balance.
 

gauss163

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OffGridInTheCity said:
Yes you can. @DIY Tech & Repairs even did ayoutube -
Note: This bothered me as I'm imagined the sense leads burning up under load...[...]

Another remark: at 11:51 they show a 30?load of 3 series'ed small aluminum-housed 10?100W resistors - see below.


image_ycaosz.jpg


Many beginners are not aware that the power ratings on such small power resistors presume thatthey are mountedto much bigger heatsinks (here typically a 12 x 12 x 1/8 inch aluminum heatsink). Without any heatsink they need to be derated ("free air rating"), i.e. in free air with no heatsink typically they can only handle 40-50% of their rated power. So in the video they can only handle 40W each, so 120W total (only a bit above what he's running them at, so they will get extremely hot, see below).

Further, the standards used for suchpower ratingspermitthem to rise to about 350C (662F), andthey mayactually get near that hot when pushed to their limits, so even a moments touch can lead to severe burns (but it will take a little timeto reach full temperature). If you exceed the power ratings for long periods then they can violently explode, ejecting shrapnel - something you don't want to be near (esp. if you are not wearing safety goggles - like the video author). So be careful with such (high) power resistors - be aware oftheir design limits.

Below is an excerpt from a Dale Vishay datasheet showing typical free air deratings (RH100 is their 100W models). A good introductionto these matters is Ohmite's Resistor Selection Application Notes.


image_lpedmk.jpg
 

Wolf

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Super now that we have the Resistor lesson out of the way let's get back to assisting the OP with his question about
"connecting3 batteries in parallel and using one BMS"

Yes The 1 BMS for the 2 -3 batterypacks should work just fine as OffGridInTheCity mentioned.

Do be aware of the chemistry differences between batteries as in ICR (low drain) and INR/IMR(high drain).
Additionally IR can have a significant effect.
In small p packs like this it can make a big difference in longevity especially if pulling more than 1A per cell and also discharging below 3.5V.
See my threadon behaviour of cells in parallel.

Wolf
 

ajw22

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I absolutely and strongly recommend AGAINST wiring like that, particularly with recycled cells.

Suppose you're drawing (for simplicity) equal 20A from each battery, 60A in total. What happens if the CID of one of the 26650 pops? The voltage of the other (now lone) 26650 cell will sag much faster than all the other cells, and then the thin sensing/balancing wire is eventually forced to carry nearly 10A to equalize the voltages between the cells. The thin wire then might overheat and potentially start a fire.

My explanation might be difficult to understand, and the exact dynamic will be difficult to predict... but I'm 100% convinced that this way of wiring is potentially very dangerous.
 

daromer

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The balance leads is not what to carry the current. Thats done by the main connection lead of course. The balance wire should only be connected to 1 cell and rest is by the interconnect circuit.

I hope thats the intention? If not what Ajw said may be an issue with pack like that.
 

ajw22

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Previous post is based on my understanding from OP that the 3 batteries will be connected with just 2 thick current carrying wires (neg and +72V), and all the other interconnects between the 3 batteries will be extended thin balancing leads.

Another concern is a physical failure of those interconnect cables, perhaps by vibration, tugging, or crushing of wires. The BMS would continue to see the proper voltages from the other 2 batteries, and fail to sense a dangerous situation developing in the 3rd battery. And of course no more balancing for those cells. Stuff move in cars - need to make really sure that those interconnects cannot possibly fail.
 

chronosteam

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daromer said:
The balance leads is not what to carry the current. Thats done by the main connection lead of course. The balance wire should only be connected to 1 cell and rest is by the interconnect circuit.

I hope thats the intention? If not what Ajw said may be an issue with pack like that.
yes, that is my intention


What do you think about his setup
20S8P battery connected to single 20S 45A BMS
20S12P and 20S2P is connected to a single 20S 45A BMS

The 2 BMS is only used to balance the battery during charging and discharging, and drive a 500A contactor that have separate direct lead to the 3 batteries that is connected in parallel.

I appreciate any input on this.
 
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