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Parallel vs 2 Battery Banks

WilliamFR

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May 23, 2022
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I'm building a 48v 280ah battery, and i'm going down the rabbit hole of BMS's. It's kind of draining to say the least. Right now, i'm only building one, but i'd like to plan for the future in the event i decide to build a second one. Looking at the JK BMS(for the higher amp rating), i've seen comments that you cannot parallel two together. Such as..2 Separate 16 cell banks with their own BMS. I'm not sure why on this honestly.

What are the drawbacks of not running it in parallel, but instead just feeding two battery banks into a bus bar, and then power into the AIO. I saw Will did two battery banks on his 6500ex video, but i see so much talk of putting things in parallel. What's the difference?
 
I eventually decided to go with 200a 16s Seplos BMS's. Multiple BMS's can connect to each other using CAN bus communication and read single SOC for all the entire bank of multiple packs. Each BMS has dip switches, you set one as master and the rest as slaves. I guess it can also power on/off all packs with single button press. I haven't wired it all up, so I am only reading out of the manual as of yet. I will be running three 16s battery packs, paralleled into one bus bar (16s3p), around 40KWh...
 
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What are the drawbacks of not running it in parallel, but instead just feeding two battery banks into a bus bar, and then power into the AIO. I saw Will did two battery banks on his 6500ex video, but i see so much talk of putting things in parallel. What's the difference?
There is no difference. They are both parallel battery arrangements.

If you plan to have more than one battery, each with its own BMS, select a BMS that supports being in parallel. While I'm far from a BMS expert, I can't imagine why any BMS would not support being in parallel. Supporting series is a whole other matter but parallel shouldn't be a problem.
 
Terminology Fix. !
Battery = Pack: A Pack is a set of cells assembled into a complete battery with a BMS and is a complete stand-alone entity.
Bank : A Collection of 2 or more Complete Batteries working cooperatively.
A Bank can be a combination of Batteries in Parallel _or_ Batteries set in Series to increase Voltage with Each Series Paralled into the Bank.
48V takes 16 Cells to form a Battery with the BMS to manage "that" individual Battery Pack. *Series is generally NOT Advisable, that is essentially a remnant of FLA Lead Tech which is Brute Force Tech compared to ANY Lithium based system. Imagine 16x6V Lead @ 135Lbs of lead each just to make ONE 48V "Bank"... The days of teh DoDo ! BTW: I came from an FLA Setup.

Most BMS' which are FET Based are extremely limited when it comes to Series Connections. If you have 4 12V Packs in Series to get to 48V, each BMS needs to be capable of handling the voltage changes should one or more packs fail within that series. Quite often that can melt down the FETS.

BMS' which depend on Realy's have no issues with FETS as the Relays do the switching on/off and as such isolates each BMS and the pack while the remaining relays "take the hit" (simplke terms).

I run a Larger Bank (5 packs - 1190AH/30kWh) which is all 24V/8S (no paralleled cells in packs). This has run with Relay Based BMS and zero issues as the build/wiring etc is all properly equalized. ( Same length of wires, same config across). I am now switching to all JKBMS at Great Expense and packs are IN PARALLEL and there are no issues as yet. In fact the performance of the online packs has improved quite a bit. Also within 1 hour of entering "Float Mode" all cells across everything average a Mean Delta of 0.005mv difference which is exceptional. It is a Lovely Sight to see !

Sadly there is a LOT of TWaddle coming from other tech like FLA which includes wives tales & more that get dropped all over Lithium Based system. What is worse is that not all Lithium Tech is the same, like Li-Ion (NMC, NCA etc) versus LFP (Lithium Iron Phosphate / LiFePO4) such as the "keep the charge level between 20-80%" which does NOT Apply to LFP ! but the echo chamber keeps that twaddle flying.

A SPECIAL NOTE ABOUT CELLS IN PARALLEL !
Many people are doing this and it comes from the other Lithium Bases, because they look at an EV which all run Paralell & series within their packs.. These are VERY Different animals. EV's also use Perfectly Matched Batched & Balanced cells which of course are PREMIUM ! Regardless of the particular sub-chemistry including LFP. Properly Matched & Batched LFP Cells are available but also cost more and they are suitable for having cells in Parallel. Voltage & IR (Internal Resistance) matched cells are NOT properly Matched & Batched.

Fully Matched & Batched means they have been run through test cycles of being Charged & Discharged with IR Readings AND Capacity Validation at the various set points. It is a long, tedious & costly process.

Voltage & IR Matching at a static storage voltage is of little use. This has little value because matched cells will have identical IR at 2.50, 2.75, 3.0, 3.25 etc...

HERE is one company in US and this is what they want for Matching & Batching (and not a great description either)
www.batteryspace.com
 
Terminology Fix. !
Battery = Pack: A Pack is a set of cells assembled into a complete battery with a BMS and is a complete stand-alone entity.
Bank : A Collection of 2 or more Complete Batteries working cooperatively.
A Bank can be a combination of Batteries in Parallel _or_ Batteries set in Series to increase Voltage with Each Series Paralled into the Bank.
48V takes 16 Cells to form a Battery with the BMS to manage "that" individual Battery Pack. *Series is generally NOT Advisable, that is essentially a remnant of FLA Lead Tech which is Brute Force Tech compared to ANY Lithium based system. Imagine 16x6V Lead @ 135Lbs of lead each just to make ONE 48V "Bank"... The days of teh DoDo ! BTW: I came from an FLA Setup.

Most BMS' which are FET Based are extremely limited when it comes to Series Connections. If you have 4 12V Packs in Series to get to 48V, each BMS needs to be capable of handling the voltage changes should one or more packs fail within that series. Quite often that can melt down the FETS.

BMS' which depend on Realy's have no issues with FETS as the Relays do the switching on/off and as such isolates each BMS and the pack while the remaining relays "take the hit" (simplke terms).

I run a Larger Bank (5 packs - 1190AH/30kWh) which is all 24V/8S (no paralleled cells in packs). This has run with Relay Based BMS and zero issues as the build/wiring etc is all properly equalized. ( Same length of wires, same config across). I am now switching to all JKBMS at Great Expense and packs are IN PARALLEL and there are no issues as yet. In fact the performance of the online packs has improved quite a bit. Also within 1 hour of entering "Float Mode" all cells across everything average a Mean Delta of 0.005mv difference which is exceptional. It is a Lovely Sight to see !

Sadly there is a LOT of TWaddle coming from other tech like FLA which includes wives tales & more that get dropped all over Lithium Based system. What is worse is that not all Lithium Tech is the same, like Li-Ion (NMC, NCA etc) versus LFP (Lithium Iron Phosphate / LiFePO4) such as the "keep the charge level between 20-80%" which does NOT Apply to LFP ! but the echo chamber keeps that twaddle flying.

A SPECIAL NOTE ABOUT CELLS IN PARALLEL !
Many people are doing this and it comes from the other Lithium Bases, because they look at an EV which all run Paralell & series within their packs.. These are VERY Different animals. EV's also use Perfectly Matched Batched & Balanced cells which of course are PREMIUM ! Regardless of the particular sub-chemistry including LFP. Properly Matched & Batched LFP Cells are available but also cost more and they are suitable for having cells in Parallel. Voltage & IR (Internal Resistance) matched cells are NOT properly Matched & Batched.

Fully Matched & Batched means they have been run through test cycles of being Charged & Discharged with IR Readings AND Capacity Validation at the various set points. It is a long, tedious & costly process.

Voltage & IR Matching at a static storage voltage is of little use. This has little value because matched cells will have identical IR at 2.50, 2.75, 3.0, 3.25 etc...

HERE is one company in US and this is what they want for Matching & Batching (and not a great description either)
www.batteryspace.com

ah thanks for the correction. oddly after i posted this i scrolled down on the jk bms product page on ali, and it states that it can be in parallel with new info from this forum and they posted a diagram lol.

if you don't mind me asking, i'm wondering how the 2 BMS wires are to be utilized. I wanted to have my positive and negative wires going to an anderson connector, that will lead out of one box and back into another for the AIO input. would the idea be to run both P- wires to a bus, and then out of the bus to wire into the anderson connector? or is there a simpler way?
 
2 or more packs are Attached to (+) & (-) Bus Bars. All + goto the common + Busbar with Equal Length Wires and all - goto common - Busbar ad again all with Equal Wire legnths. This is Very Important to have matching lengths of cables. Each Battery Pack MUST be fused as well !

Please Review pages 18-19 of this Victron Wiring doc.

I have to wonder WHICH diagram they posted... We had a bit of a Go Around over the whole Series / Parallel thing because of translation issues that resulted in some muddled information.

PS: I am no longer Fully Active here, I more or less "lurk" and answer selectively only.
 
2 or more packs are Attached to (+) & (-) Bus Bars. All + goto the common + Busbar with Equal Length Wires and all - goto common - Busbar ad again all with Equal Wire legnths. This is Very Important to have matching lengths of cables. Each Battery Pack MUST be fused as well !

Please Review pages 18-19 of this Victron Wiring doc.

I have to wonder WHICH diagram they posted... We had a bit of a Go Around over the whole Series / Parallel thing because of translation issues that resulted in some muddled information.

PS: I am no longer Fully Active here, I more or less "lurk" and answer selectively only.
 

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