RC LiP0 combined packs: parallel balance wiring

[pickworthi]

I want to combine two RC LiPo packs in parallel, in such a way as they form a single battery, rather than two batteries connected in parallel.

I've had a good read of this thread: https://www.endless-sphere.com/forums/viewtopic.php?f=14&t=52240
Based on the excellent description, I created a sample wiring diagram for the creation of two parallel cables for two 3 cell RC LiPo packs:
- one parallel power output (and charging input) cable
- one parallel balance cable

I've attached the diagram below.
Since these are LiPo I thought it would be a good idea to check that I have understood things correctly, mainly for the balance cable. My understanding is:
- The parallel balance cable connects each end of the two pack cells together, in this order: + to +, +/- to +/-, +/- to +/-, and - to -.
- This is sufficient to create a two cell parallel grouping for each pair of cells
- This will equalise the voltage of the pairs of cells while discharging

I would really appreciate someone validating my understanding, and telling me if I'm wrong! Not having an runaway fire trumps my ego every time

 

[flippy]

you can do it that way but if the packs or even 1 cell is even slighty off to the rest you will see some serious currents flowing between them and burn out those balance wires.

dont connect the balance wires together. keep them separate. if there is a issue with a cell you can see wich pack it is.
you could charge them in bulk and just connect one of those little balancers to each pack during the charge.

ofcourse if you were to use a bms for each pack this would not even be an issue...

 

[pickworthi]

you can do it that way but if the packs or even 1 cell is even slighty off to the rest you will see some serious currents flowing between them and burn out those balance wires.

Given the cells are matched when the packs are connected, my understanding is that voltage is equalised continuously as long as they remain connected, thus keeping balance current low.
How would the situation you describe arise?

 

[flippy]

Given the cells are matched when the packs are connected, my understanding is that voltage is equalised continuously as long as they remain connected, thus keeping balance current low.
How would the situation you describe arise?

simple, a simple temperature difference, wear, tolerances or whatever can/will cause differences between cells. in high power applications that effect is much more visible, especially when the pack ages. at thait point the better cells will give more power then the worse one. and if your interconnect between these cells is too small you can have the main current flow go from pack A to pack B and back to A again. the path that electricity seeks is always towards the lowest resistance. and it does not care if that lower resistance is a balance wire.

 

[eMark]

Given the cells are matched when the packs are connected, my understanding is that voltage is equalised continuously as long as they remain connected, thus keeping balance current low.

paralleled two older 2200mah 3S ParkZone LiPos (like new condition with little use) that had been in cool/dry storage for five years at 3.85V ... would you believe both were still at 3.85V after five years.

The paralleled pack doesn't get much use ... just for my etrike 12V rear red blinking light. Could have used just one, but just for the heck decided to parallel the two to see if I could do it without causing a short. It does require a certian degree of surgical skill as i uncovered the top wrapping so paralleling the hidden balance leads wouldn't be visible and then carefully rewrapped with sufficient protection.

How would the situation you describe arise?

Too many variables so better to be safe than sorry, also to err is human. Thus flippy's caution. Anyway, for some it could be too risky leading to a fire depending on their knowledge and skill. I'm retired and a hands on guy. Even so have learned over the years via more than a couple mistakes that "haste makes waste" so took my time.