Cells in parallel vs whole packs

[curtis.perrin]

Hi hopefully this is a quick question though preliminary searching didn't reveal an existing answer.

Say you have 8x 3.6V identical cells. To make a 12V battery I understand the best approach would be to do sets of two cells in parallel then 4 sets in series. 2P4S. Then this could be managed by a single BMS.

I'm trying to understand what the reasons against instead putting two packs of 4 in parallel. (4S2P?)
Are the following statements correct? Are there other reasons?
Each pack would need its own BMS, contactor and fuse.
Over time the two packs could become dissimilar in capacity which would mean:
-Charging them with a single charger runs the hazard of over charging the lower capacity one
-If you're load is set based on the combined C rating then you'd be running too much current through the lower capacity pack

I appreciate the knowledge of this community in helping me understand this quesiton.
Cheers,
Curtis

 

[SafeDiscDancing]

Ultimately all packs are defined by the weakest cell.

So if you have two series strings which you then combine as parallel "afterwards" then the weakest cell will exist "somewhere" in one of those two series strings.

Let's take an example:

Series One:

10.1 ah
10.2 ah
10.3 ah

Series Two:

10.1 ah
10.2 ah
8.3 ah

The overall battery will drain down about 8.3 ah and then Series Two is in serious trouble. (16.6 ah in our parallel example)

Since the 10.1 ah and 10.2 ah cells are still rather full while 8.3 ah is empty they cheerfully trample all over this weak cell causing it to go towards zero voltage.

You see the "problem" is anytime there is a weak cell anywhere that goes to zero and you cannot cut off the power it will eventually go past zero and go NEGATIVE.

That's total death and possibly fire.

But this tragedy won't happen at exactly 8.3 ah... no... while Series One has plenty of energy left it is willing and able to supply a little extra to compensate for the weak cell of Series Two.

Eventually the Series One has expended enough to "force" the Series Two with the weak cell to reverse.

And that's why.

BMS stands for "Battery Murdering System" and they sell them by saying "Trust the Science".

Well what "science" does the BMS do?

Ideally it at minimum can correctly tell the pack to shut down the moment the weakest cell goes too low.

So really the BMS helps to stop you from doing damage, but it alone will not increase capacity.

What if the BMS has the ability to balance series cells?

Now when you are going up in charge the weakest cell is getting excess energy REMOVED but on discharge as the weak cell goes low the balancer must ADD energy.

This means the weakest cell is working extra hard on BOTH ends of the energy usage cycle.

The extra abuse does more damage to the weak cell increasing it's aging and lowering it's capacity.

And a last comment on balancing... the RATE at which the weakest cell either needs more energy or wants less of it will increase as the cell gets worse and worse.

Most balancers do one amp which if your ebike is pulling 30 amps is not nearly enough to save the weakest cell.

So what happens is the balance function falls behind and eventually... fire...

 

[DogDipstick]

Each pack would need its own BMS, contactor and fuse.
Over time the two packs could become dissimilar in capacity which would mean:
-Charging them with a single charger runs the hazard of over charging the lower capacity one
-If you're load is set based on the combined C rating then you'd be running too much current through the lower capacity pack

Yes yes yes and yes. If not its own BMS, you would need a harness that serves every packs cells, and yes this is called a " pigtail"....

Pigtail parallel charging is frowned upon int he hobby RC community just because of the truth in your statement number two...
Really comes down to this: Certainly. People have done it, but yeah you are right there. Just for that reason. You cannot assure a balance with pigtail charging.

-Charging them with a single charger runs the hazard of over charging the lower capacity one

Load should not be set on the combined C rating. C rating does not add up. Will be as strong as the weakest group, or weakest cells in the pack in the series... So yeah they wont add up and may very well be only as strong as the weaker one.

So if you have two series strings which you then combine as parallel "afterwards" then the weakest cell will exist "somewhere" in one of those two series strings.

... the RATE at which the weakest cell either needs more energy or wants less of it will increase as the cell gets worse and worse.

Most ...

A friend of mine once layed it out for me on paper like this: Look at the diagram and you can see how the line resistance changes with each setu... Now, a healthy battery wont have problems, but a battery with IR resistance and connection resistance all over the place will suffer from wasted energy, short life, and detriment.

Now this example is exaggerated ( you dont lose amps per connection, but milliamps maybe, 1/1000 of an amp, in practice) for showing the point. Imagine: The numbers getting worse, here or there, over time, connection building resistance, not taking charge well, never filling and never balancing because of tiny variation int eh design of the current path. Causing detriment over time. Eventually you have a " heater" weak cell that takes and wastes heat on discharge and recharge, goes out of balance, and takes more energy to charge cause its collumbic efficiency is getting worse and worse... How this bad cell affects the pack, either type of battery, is shown below. This is jsut diagramming the line loss and you can imagine the difference if one number here or there goes fluke.

I personally think the top example is better, with the 5 (5s) packs in parallel each with a 1p, then paralleled), that the bottom pic, showing the typical way an nickel ebike pack is built ( with many p in groups). But this ( the way I like) is not the way many build the packs. It is generally accepted that it is better to have all the pack make up or many p for a single set in the total string. Have them all leveled and balancing themselves.. ie, parallel first, series second. The ideal behind this is larger application, larger cell, and you need less ( I use one cell for 25Ah, not (10) 2.5Ah cells) ( little cells are for flashlights in y opinion). The cells stay in balance in the set, you dont need a physical balance wire to service every cell ( and a bunch of wires that need protection, and physical design constraint). Like SDD said above, the weakest cell IS gonna exist somewhere...

Now this works great if you have a lot of money that you can replace packs, but, even the Chevy Volt HV battery parallels its two cells first then strings them along... I guess it is alot easier to do 96s2p, in one battery, rather than two 96s1p packs in parallel. Alot of modern EV are like this... and managed by very * fancy * BMS.... one 290v module.

Top would be like if you get five HK lipos and string them along in parallel.. Bottom is is you parallel 5p first then string the five parallel groups along for the 15v of the 5s ( easily battery murdere.. er... managing system installed) like most ebike packs. Losses are exaggerated.

Some of it is packaging convenience, some is electrical constraints. Whats more important to you. Design constraint is always a tradeoff. For your intended application.

 

[calab]

Its better to do what the professionals do with tool pack batteries.

Parallel first then series. I copied the pic for the battery itself not the added graphics from https://endless-sphere.com/forums/viewtopic.php?f=14&t=28285&hilit=makita#p407768
Doctorbass







Then to Series first then parallel the + and -

 

[SafeDiscDancing]

POUCH

In one word you solve all the problems relating to parallel.

Use pouch cells as big in amp hour as your design requires and just do a simple series string of them.

But then you devote your time to pouch compression and making sure those tabs are tightly connected... so it's always a tradeoiff.

----------------------

One trick I discovered with pouch cells is if you spend the time to wrap each pouch first with tape to give it some initial tension then with fiberglass cloth set up with resin you can get satisfactory compression without long bolts and heavy plates.

That's something a DIY builder can do because it's slow and tedious, but might be difficult for mass production.

---------------------

And don't forget the idea of just buying a used EV module.

https://batteryhookup.com/products/samsung-ev-battery-44-4v-10ah-444wh

 

[DogDipstick]

POUCH


And don't forget the idea of just buying a used EV module.

I hit my first EV pouch cell battery for 2000A. More. 2000A is the most I could read at the time.

It is still kickin.

Yes. I agree.

 

[calab]

I once did a series first and had the ends + and - connected together and it lasted a long time.

Then I once did the exact same layout but had each row paralleled and had one 18650 go low and took out a half to a dozen.
I figured that was a bad way of doing it and never had a problem with the first way series 1st, and parallel the + and -

Another time I had the Makita tool pack battery put 2 in series to make 10s 36v and paralleled each row, that did not work out either taking out a half to a dozen or so.

Maybe just to much heat, but on those Makita's with the tabs already in place, it did not take much heat to solder 12-14awg solid wire (for parallel, 10awg used for series) from 3/ house wire from home depot.

Currently the tool pack method on my 36v is just connected at + and - with 10awg solid copper wire at the series connection between the 2 series bricks. Its lasting a long time.

 

[ColinB]

Thanks for this discussion. Is there a way to check how the cells are doing to make sure they are performing normally? I'm still trying to understand the parallel first vs series first issues.

If you put cells in parallel first, we know they'll stay in balance with each other. What happens when a single cell goes bad? Can we know we have a bad cell without taking the pack apart?

In use, would we notice the parallel group with the bad cell dropping voltage faster than the others? (That would require a BMS or some way to show the voltages.) I assume it would be most obvious when the bad cell is empty.

For someone like myself, who isn't up to building my own battery right now, what should I do? Most off the shelf batteries I see don't even show the cells are full and balanced, let alone voltages.

Colin

 

[amberwolf]

I once did a series first and had the ends + and - connected together and it lasted a long time.

Then I once did the exact same layout but had each row paralleled and had one 18650 go low and took out a half to a dozen.
I figured that was a bad way of doing it and never had a problem with the first way series 1st, and parallel the + and -

Assuming no BMS on each series string, then if the series-first pack had had the cell go low (as in fail to 0v, internally shorted), then the entire series string would then be damaged by the overcharge it would get every time the pack is charged; if this is a severe enough overcharge it could damage the cells in a way that causes a fire.

Additionally, if the cell failed low (self-discharged/shorted it's entire capacity) and the pack is being discharged, it would then drive it's voltage negative, which can be much worse for fire risk than just being "dead" (0v), once it is then recharged.

If you have something attached to each string (BMS, etc) that shuts off charging / discharging at cell-hvc, or cell-lower-than-lvc, at least to that series string, then it just reduces the pack capacity/capability...but if there's no protection like that, then fire risk is much greater than for a pack built parallel-first. (assuming a standard bms on it, or some form of cell-level monitoring that you are paying attention to either during charge or discharge).