Why large parallel connection in battery packs ?

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qwerkus

10 kW
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Jul 22, 2017
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Something I've not quite understood yet: what's to point of large parallel connection in ebike battery packs ? From what I understand, series is where the juice flows. The only current flowing in parallel (besides both terminals) would be balancing current, which is very small.

=CELL1==CELL2==...==CELLN= high current flow
I low current flow
=CELL1==CELL2==...==CELLN= high current flow

.. and so on

The scenario in which large parallel connections seem to make sense, is in the case one series connection breaks - for one reason or another. The flow would than be redirected over the parallel connection to the remaining series. But in that case, wouldn't it make sense to use the parallel connection as a fuse, and disconnect the faulty series to save the other ones from overcurrent ?
 
Most buses are laser-cut from a sheet of pure nickel. For "some types" of battery pack builds, the individual cells are not supported enough, so the buses actually provide some mechanical support (which I do not recommend).

You are right, the series connections carry the bulk of the current, and the parallel connections could be MUCH thinner and narrower.
 
As long as there are series connections from every cell in a group to the next series cell, then there's no need for anything but "token" parallel connections. That's probably the better way to do it. "series first, then parallel" is one way to say it.

But many packs I've seen are built with only one or two series connections between each parallel group, regardless of the number of cells in each parallel group. In that kind of pack, you have to have thick parallel connections in order to pass current from all group cells to the series connection(s) to the next series group. These are usually built by paralleling all the cells in each group first, to make 1sXp modules, that are then put in series, probably because it is very easy to build packs this way, and it's less easy to make a big mistake wiring up balance leads (as there's only one balance lead for each "p" group).


In the "series-first, then parallel" type of pack, you build all teh series modules first (Xs1p), then parallel those with thick connections at the main +/-, and thin ones between each cell that would be parallel neighbors, or individual balance leads from the BMS (or balance connectors for RC chargers, etc.) running to each cell in each series group (lots of wires, very easy to make a mistake with one or more, and start a fire or blow something up in the electronics).
 
amberwolf said:
As long as there are series connections from every cell in a group to the next series cell, then there's no need for anything but "token" parallel connections. That's probably the better way to do it. "series first, then parallel" is one way to say it.

But many packs I've seen are built with only one or two series connections between each parallel group, regardless of the number of cells in each parallel group. In that kind of pack, you have to have thick parallel connections in order to pass current from all group cells to the series connection(s) to the next series group. These are usually built by paralleling all the cells in each group first, to make 1sXp modules, that are then put in series, probably because it is very easy to build packs this way, and it's less easy to make a big mistake wiring up balance leads (as there's only one balance lead for each "p" group).


In the "series-first, then parallel" type of pack, you build all teh series modules first (Xs1p), then parallel those with thick connections at the main +/-, and thin ones between each cell that would be parallel neighbors, or individual balance leads from the BMS (or balance connectors for RC chargers, etc.) running to each cell in each series group (lots of wires, very easy to make a mistake with one or more, and start a fire or blow something up in the electronics).
Click to expand...

Not sure I get your point: in "parallel first, than series" config, you still need to connect the bms wires. No more, no less bms wires, than in the "series first, than parallel" config.
 
If you series first, then parallel the strings, you need a bms for each string. So #P x more bmss = lots more wires. Then there's all the problems you'll have when one bms shuts down. Discussion on this currently on another thread.
 
1st - "Battery" is a collection of cells working together. Single cells are not batteries!

Basic parallel connectors with good matched cells will effectively carry 0A current.
My battery builds, as are a considerable percentage, are a combination of several series batteries, paralleled together.
Parallel leads are essential for BMS monitor and function, but, unless afflicted with bad cells, will only carry minimal current.

However, each series carries the full current of the cells, and the end parallel connectors (what I call Rails) must carry the full current x the number of series batteries.

For example:
With a 14s4p build with 10A rated cells, built as 4x 14s1p would require 40A rated rails.
Normal parallels, with good cells, require only sensor capability!
But! To survive and function with a complete cell failure ... 10A capable parallels would be recommended. This, in combination with BMS, PCB etc, will prevent immediate catastrophic failure. Battery build will need prompt repair!

However 2, if parallels are sufficiently light, a failed cell will burn the parallels (wire) as current is shunted to the failed series! While this will pinpoint the bad cell, it can be dangerous!

7s 12p battery with horizontal 18ga parallels and 8ga+ end rails. (also connected to balance leads for connection to 1 BMS)

file.php
 
qwerkus said:
Not sure I get your point: in "parallel first, than series" config, you still need to connect the bms wires. No more, no less bms wires, than in the "series first, than parallel" config.
Click to expand...
If a pack is built with a bunch of Xs1p groups, with no built-in parallel interconnects, then you need a complete set of balance wires for each series string (perhaps excluding the main + and - wires, if those go to the BMS already). So if you have a 5p pack, that's five times as many balance wires as the other way of building the pack (or if using built-in parallel interconnects). A 10p pack would take ten times as many balance wires.
 
Parallel connections balance the cells in each group at the same voltage potential. Series connections don't.

Parallel groups of cells act like a single larger cell. Series strings don't.
 
I like to monitor my pack. And parallel then series is the ez way. I like large format cells like 20ah so not many connections. Not much luck. So looking for a 20s 9p or 7p triangle pack of Samsung 30q.
 
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