Parallel and series packs

[Harani]

Watching various youtuber building 18650 powerwalls there seems to be two very different approaches. Those who build multi paralleled 1S "Super cell" packs, then series them together to get the desired voltage for their system,
and those who build a pack (or packs ) direct to their Voltage spec. For example 7S-xP (and then parallel more packs together to get the desired system capacity if using multiple packs)


Say i wanted a "24v" system

my understanding is as follows

7S packs ( of whatever parallel configuration)
Pros:
I could add new packs of any capacity in parallel at any time to increase the system capacity ( couldn't i ? Though i can see it would be desirable to keep them all roughly the same capacity and also to ensure that any additional pack was brought up to the existing system voltage level before introducing it
I could remove a pack for maintenance without bringing the system down, just reducing its capacity.
Bus bars and connecting leads for individual packs need not be so hefty. Only the main bus they all plug into.

Cons
Voltage is fixed and cannot be altered without an entire pack rebuild
positive and negative cell ends are mixed on each face of the pack meaning more care has to be taken over accidentally shorting them while being worked on
Monitoring/balancing needs 7S cables run to every pack


1S packs
pros:
The loss of a few cells in a pack won't unbalance the system voltage
monitoring/Balancing is easier with only 7 wires plus common to be run to the packs
You can reconfigure the system voltage easily by adding or removing more 1S packs (though I'm not sure why you would want to)
Each pack is positive on side and negative the other providing good separation and minimiszing danger of accidental shorting

Cons:
(there can be) a massive amount of current in each pack. If it did short out things could get out of hand quickly
Busbars for each pack need to be substantial
If you pull a pack for maintenance your entire system goes down as you are breaking the series. unless you have another to replace it your system will be out of action for the duration of the maintenance
Harder to add extra current capacity. You would have to add another seven 1S packs, instead of just one in a parallel system


Are the any other factors to take into account I may have missed or anything I've misunderstood?
Apologies if these questions are addressed elsewhere i wasn't sure of if the two different approaches have a naming convention i could do a search on.

 

[OffGridInTheCity]

Interesting and a credible effort at beginning a conversation on this. So in the spirit of conversation, I've added comments

Under 7s Pros....
- "...add new packs of any capacity..." Its better that each battery (in parallel) have the same capacity as the other batteries else the underpowered ones will cause unnecessary current flow to compensate when they discharge faster than their neighbors causing unnecessary strain on things. It it was the end-of-world and that's all you had, then OK but if you're designing a system I would do it on purpose

- "...could remove a pack for maintenance..." Yes.

- "...leads for individual packs need not be so hefty. Only the main bus.." Yes.


Under 7s Cons...
- "Voltage is fixed ..." In my experience, the voltage is fixed because of the equipment you buy (charge controllers, inverters) to interact with the battery rather than the battery itself.

- "positive and negative cell ends are...." To me, this is just a physical battery design issue rather than 7s issue. Some design the 'p' part to have + and - at the same side and some (as in my own design) do it with + and - at opposite ends to avoid shorts.

- "..needs 7S cables run to every pack..." Yes..


Under 1s Pros...
- "... loss of a few cells in a pack won't unbalance the system voltage..." - True in the short run, but depending on the %, loss of cells will begin to unbalance the pack relative to the others and will require maintenance to maintain healthy packs after a while. There's no free lunch here.

- "... monitoring/Balancing is easier with only 7 wires plus common to be run to the packs..." - Yes.

- "...reconfigure the system voltage ..." - Same comment as above, its the equipment that interacts with a battery that plays the largest role in the voltage. If cells are physically designed for mix and match then is entirely possible to reconfigure the battery itself.

- "...Each pack is positive on side and negative the other providing good separation and minimizing danger of accidental shorting" - a matter of physical design rather than inherent issue with 1s vs 7s. Eventually you'll hook these 1s into series - and you're back to a potential for shorts.


Under 1s Cons...
- "(there can be) a massive amount of current in each pack. If it did short out things could get out of hand quickly" - This is mitigated buy cell fusing (in 18650 type cases) which works very well from personal experience. If you're doing 100ah or 200ah prismatics then it is different.

- "Busbars for each pack need to be substantial" - I use 6awg wire as my busbars for 260ah 1s packs... not really that difficult. The physical weight of a 1s as it gets 400 cells, 800 cells etc.. begins to be the issue more than finding suitable bus bar wire. That's why you don't find a lot of 18650 1s that are larger than 200 or so cells

- "...If you pull a pack for maintenance your entire system goes down as you are breaking the series. unless you have another to replace it your system will be out of action for the duration of the maintenance..." - True.

- "...Harder to add extra current capacity. You would have to add another seven 1S packs, instead of just one in a parallel system..". - Agreed but harder is defined by how much you want to add. In my personal case, I started with 1 x 14s100p and have added 5 more so far, battery by battery. Adding 1s in a 14s is not significant capacity to be something I wanted to do.

Overall recap:
Each person's situation is different - e.g. how much physical space do they have, what kwh do they need (affects voltage choices), is it a small system or larger system, is the goal to grow year by year or a 1 time shot, what kind of cells, do they want Batrium BMS level monitoring or just something to forget about, is it a high-end equipment where battery monitoring is built into the all-in-one, and x and y and z.

Many many factors all go into a battery design suited for the individual situation - so I don't see it as 1s/7s/14s pros and cons so much. All these options are viable and it get's down to a combination of power requirements, equipment requirements, and DIY skills that work for that person

 

[Harani]

many thanks for your input.

my use case is a follows. I have a holiday home (work in progress) on a small island it is quite rural and power cuts although not terribly frequent are also not uncommon.
I'd like some basic solar for the following scenarios:
As backup for a chest freezer
As backup for a water pump. ( mains water is not pressurized 24/7 so you take water into your tank when it is available and use a an electric pump to serve the house from it. When electrics go off you are without water. So a backup DC pump would be handy otherwise it's open up the cistern and chuck a bucket down there.)
Keep the internet router running.
Even when mains power is on i would utilize a DC solar system for charging other devices laptop, phone, power tools etc. and to run DC lighting in rooms that don't yet have mains run to them, as well as outdoor courtyard lighting. ( and beer fridge through a small inverter. !)
It would also be good to be able switch mains off completely when i am not there and leave solar running a few basics. ( with an internet connection kept live i can keep an eye on the place remotely and if anything ever fails get a neighbor to come in and restore the mains)

So my needs are not great. I'm lookin to set up a system that i can start small and add to as needed.

And maybe it's a pie in the sky idea at the moment but it would be awesome one day in the future to pull a removable pack from a larger battery system in the house and use it to run an electric outboard on a small boat for day trips. Then recharge it from the system and plug it back in when it's equalised and the next time use a different pack to keep them rotated and the "wear on them equal.