2P4S vs 4S2P, and paralleling question

[lateral]

No right/wrong imo. Whatever your constraints justify. Budget was a biggy for me and being comfortable with the logic.

[fxykty]

Originally Posted by hjohnson

…

By the same token, I was also building a fully integrated system, with the BMS managing the voltage/current using Victron’s DVCC system. The victron setup only can track one battery, so unless you’re running something like pylontech batteries which can combine their BMSs and present as a single battery to the system, you’re kinda SOL for that functionality if you have two independent BMSs.

It depends on the BMS. Our two BMS communicate with the Cerbo and each other via CAN, with one BMS the master and the other slave. The slave BMS tells the master BMS everything that it is measuring and/or doing and the master BMS integrates that data with its own battery measurements and presents to the Cerbo a combined view. Works perfectly well, not SOL at all.

[Leakie Marine]

I was going to go the 4S2P with 2 BMS's and bought all the gear. I ended up going 2P4S as I was a bit worried about what would happen if the batteries ended up way out of balance and you dropped them back in together.
At least I have a spare BMS now.

[s/v Jedi]

Originally Posted by Leakie Marine

I was going to go the 4S2P with 2 BMS's and bought all the gear. I ended up going 2P4S as I was a bit worried about what would happen if the batteries ended up way out of balance and you dropped them back in together.
At least I have a spare BMS now.

Yet you never asked about that

[Leakie Marine]

Originally Posted by s/v Jedi

Yet you never asked about that

I found this group after, and good point, it's not too late! I do like the idea of redundancy and the reduced C.

so... If the OP goes 4S2P, and the batteries get separated for whatever reason and end up at 5% and 100%. What happens when you parallel them again?
How could you control the balancing current?

[lateral]

Originally Posted by Leakie Marine

I found this group after, and good point, it's not too late! I do like the idea of redundancy and the reduced C.

so... If the OP goes 4S2P, and the batteries get separated for whatever reason and end up at 5% and 100%. What happens when you parallel them again?
How could you control the balancing current?

In practice this will never happen with common bus unless you have some bad connections or bad cell. Your two 4S banks should be isolatable with switches see pic previous.
In any case, the bms for "5%" battery will stop it discharging past a set %, usually 10% but user configurable. Then you just turn that low SOC 4S discharge off (not charge) in the app, and let it catch up. You can set a device to audibly warn you of this or done automatically. ($$) Set your warning higher than BMS auto discharge off.
My 4s2p has never had more than 5% delta. If you want to start a fire, couple two largely different SOC Li banks in parallel. You would have to be grossly negligent in monitoring to let SOC delta get any where near that. IMO.
C/cell is not changed.

[CaptainRivet]

I run 4P4S with 272AH Lishen cell and i would only recommend to parallel cells when
A) you have a lot experience with Lifepo4 batteries
B) you have a 100% secure source where to purchase real grade A factory matched 280AH EVE cells as with cells in parallel if they are significantly different the bank will go out of balance over time
C) your existing system design is in a way that one source is spider in the web and steer all.

Otherwise i highly recommend to build 2 batteries with 2 BMS and parallel them. BMS sees every cell and can even balance a junk set of grade C cells very well. Also 2 BMS share the load and if eg 200A JK BMS is 130Euro, 2 are 260 which is very reasonable.

[s/v Jedi]

Originally Posted by Leakie Marine

I found this group after, and good point, it's not too late! I do like the idea of redundancy and the reduced C.

so... If the OP goes 4S2P, and the batteries get separated for whatever reason and end up at 5% and 100%. What happens when you parallel them again?
How could you control the balancing current?

Find the video about doing exactly that on the Offgrid Garage YouTube channel
(It’s a non event)

[Fuss]

Originally Posted by s/v Jedi

Find the video about doing exactly that on the Offgrid Garage YouTube channel
(It’s a non event)

Absolutely, it’s so much a non event that when I parallel my electric outboard battery to the house bank, it doesn’t event get charged up properly, yet about a year ago when I asked for opinions it was all doom and gloom and pictures of blackened fingers and warnings that I was an extremely dangerous incompetent forum member….so…only partially correct responses then!!
But seriously, it was a good example of members posting when they had no real world experience, which happens a lot!

[s/v Jedi]

Originally Posted by Fuss

Absolutely, it’s so much a non event that when I parallel my electric outboard battery to the house bank, it doesn’t event get charged up properly, yet about a year ago when I asked for opinions it was all doom and gloom and pictures of blackened fingers and warnings that I was an extremely dangerous incompetent forum member….so…only partially correct responses then!!
But seriously, it was a good example of members posting when they had no real world experience, which happens a lot!

True. It’s simple laws of nature and many look at the low internal resistance only for calculating current while the external components like BMS, wiring, crimps, fuses, switches etc. form the bulk of resistance and voltage difference is low as well.

The problem is that those without the knowledge are often very loud voicing misinformation

[CaptainRivet]

Originally Posted by Ryban

Partly it comes down to how little space a 560Ah bank will take up, and how cheap the cells are these days. At least for the 2P4S option, where the BMS costs are low still. Upgrading our 460Ah LA bank to a 560 Ah bank will still be freeing up 2 GC2 cells worth of space.

I've also experienced plenty of days where tropical waves come through and we have clouds for days, and had to cut back our usage. I want to ensure that we're able to continue cook/charge our outboard battery (if we decide to go electric there) on days where we're not getting sun.



Aren't the individual cells in a 2P4S pack discharging the same current as if they were in a 4S2P arrangement though (assuming cells are balanced/matched and the 4S2P arrangement is discharging equally too)? There's more current going through the FET BMS, and battery cables, but I'm planning to use a BMS that controls a contactor.

Unfortunately I haven't found a high Ah cell that I can accommodate in that location, because most cells are not built tall, and some manufacturers specifically advise you not to put cells on their side (e.g. Wisnton).

No they don't in reality.
For parallel to work you need very high quality cells eg the winston will do or EVE directly from factory from a supplier that only gets top notch cells and that won't be resellers.

I have 20 years experience with Lifepo4 and needed 3 iterations till i had a stable 4P4S bank which eg needed 10x60mm busbars that put on an engine lapping machine to be absolutely even so contact to all 4 cells are equally perfect, if one is not perfect you won't realize and the other parallel cell takes over and gets stressed/wears and the bank gets out of balance quickly.
My whole install was around a electrodacus BMS which cannot be paralleled and work like spider in the web and controlls all. So it would be a major change and lot of effort go to 4 paralleled bayteries with 4 BMS, thats why i went the 4p4S route and i have the experience to deal with it, you clearly don't reading all the statements here and will mess that up in the end.
For your small ship i highly recommend to use the 200A JK BMS with 2A active balancer, each is 130Euro with a 1p4S battery and build 2. The 200A one is enough that you can emergency start the engine on your little vessel. Makes things a lot easier for a beginner then a contactor BMS. Use a Victron BMV 712 bat monitor as last resort cut off and the FET BMS is as good as a contactor BMS. You can mess up a lot stuff with the contactors they are not that simple then you think,devil is in detail.