Is paralleling batteries just wrong?

frthompson

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I'm in a conversation on the Electrodacus forum in which I asked opinions on a configuration with an Electrodacus SBMS0 bms and a 2p4s battery pack versus two 4s batteries each with there own JBD 150a bms, both based on 8 280ah EVE cells. Electrodacus replied that paralleling lithium batteries is just wrong; only cells should be paralleled. Yet I see posts on this forum, particularly for RV set ups, where people seem to prefer the two batteries in parallel with there own bms compared to one 2p4s battery? I've seen tradeoffs discussed of both approaches but never a statement like this that it should not be considered at all. So is it OK or not?
 

JAS

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I've read that same recommendation by Electrodacus in their user manual. I interpreted that to be specific to the Electrodacus BMS and not the more "traditional" BMS' that sit inline of the current? EDIT: Maybe also more applicable to their solar charge controllers too? IE: How do two different Electrodacus BMS' tell the same charge controller to stop charging?
 
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Steve_S

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Clarification:

Paralleling Battery Packs is HOW we build Battery Banks, it is and always will be how it is done. This is what increases the Storage / AH of said Bank.
This configuration allows the collective number of Packs in a Bank to Share Load & Charge thereby decreasing the load any single pack has to carry, extends longevity due to lighter depth of use, and adds Fault Tolerance & Fall back in the event any Pack disconnects.

Paralleling CELLS within a Battery Packs is done successfully BUT WITH ONLY Properly Matched, Batched & Binned cells ONLY ! Fully Matched Cells cost more ! None of this Voltage & IR matching nonsense with handheld testers... DO NOT Parallel Unmatched Bulk Cells ! Stuff often happens!

EV's with Li-Ion cells often have hundreds of cells, set in Series & Parallel, these are all Highly Tested, Matched & Batched so they all behave exactly the same as their neighbour through the entire voltage & cycle ranges that apply to them.
 

time2roll

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For separate batteries, parallel is better than series. Take care in the build and connections and your batteries in parallel will be just fine.
 

frthompson

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So JAS I can see how paralleling batteries might not be appropriate in an Electrodacus system for reasons you suggested. However my question was specific to comparing a Electrodacus with a 2p4s battery versus two 4s batteries in parallel with there own JBD bms, but perhaps that detail was overlooked. Thanks Steve for reaffirming batteries are regularly paralleled in battery banks and the cautions about paralleling cells.
 
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circus

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How do two different Electrodacus BMS' tell the same charge controller to stop charging?
Question from a noob: BMS"s tell the charge controller to stop charging?
update: Found out it's unique to Electrodacus, never mind.
 
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ereams65

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I can see the anti-parallel point when using a solar charge controller, because one battery could send the message while the other could use a smidge more juice I suppose.
 

jwelter99

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This has been covered countless times.

If you are running a $12 China BMS with zero control interfacing to other equipment then having many of them is not a issue. The batteries become a simple black box that is either connected to load, or not, based on what the BMS decides is safe.

If you want a BMS that communicates with the rest of your equipment to have a single point of management and things like dynamic charge and discharge based on conditions, etc, etc, then the single BMS is the way to go, which means either parallel cells or Batrium to have strip capabilities as well.

Neither is right or wrong; just comes down to the expectations of the end user for level of integration.
 

frthompson

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In the end I went with a simpler bms (JBD) on each 4s battery because I already had a Victron Venus GX communicating with a battery monitor, inverter/charger, and solar charger and regulating the charging. The BMS in this case is kind of a fail safe or back up protection. If I was starting from scratch I think I would have gone with something like the Electrodacus that monitors things at the cell level (unlike the Venus GX) and can turn off chargers or loads. And when you have multiple options cost does factor in.
 

toms

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This is another subject that has been well discussed on this forum.

In the end it is situation specific. Both ways can work. And in both situations your total capacity is limited by your weakest cell.
 

Posplayr

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Clarification:

Paralleling CELLS within a Battery Packs is done successfully BUT WITH ONLY Properly Matched, Batched & Binned cells ONLY ! Fully Matched Cells cost more ! None of this Voltage & IR matching nonsense with handheld testers... DO NOT Parallel Unmatched Bulk Cells ! Stuff often happens!
Please elaborate on what "Stuff often happens" means. This is an extract from the ORION BMS white paper entitled
"Strings, Parallel Cells and Parallel Strings"


OrionBMS_StandardCellConfiguration_Page4.png
 

Bud Martin

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I think the key word from Steve is "DO NOT Parallel Unmatched Bulk Cells"
I expect Orion to say something about unmatched cell, I did not find anything in that document.
Also Notes from Orion page 4:
Note: While most lithium batteries can be directly paralleled together, check with the cell manufacturer to ensure that the cells can be safely paralleled and to see if there are any specific requirements for the specific cells used. In some cases (such as with some 18650 style cells), cell manufacturers may require individual fuses or fusible link wire to prevent over current through a single cell in the event of a cell failure or an internal short within a cell. Consult with the cell manufacturer to determine if such a design is necessary.
 

Posplayr

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I think the key word from Steve is "DO NOT Parallel Unmatched Bulk Cells"
I expect Orion to say something about unmatched cell, I did not find anything in that document.
Also Notes from Orion page 4:
Note: While most lithium batteries can be directly paralleled together, check with the cell manufacturer to ensure that the cells can be safely paralleled and to see if there are any specific requirements for the specific cells used. In some cases (such as with some 18650 style cells), cell manufacturers may require individual fuses or fusible link wire to prevent over current through a single cell in the event of a cell failure or an internal short within a cell. Consult with the cell manufacturer to determine if such a design is necessary.

Thanks; I have read most if not all of that a few times however I don't think this does anything to answer the question:

"Please elaborate on what "Stuff often happens""
 

wholybee

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I think that having matched cells is less important than people think. In fact, if you do the math, if you have unmatched cells, it is better to parallel them at a cell level than to have multiple 4s packs with unmatched cells.

Adding resistance in parallel;
R= 1/( 1/R1 + 1/R2 + 1/Rn)

If you have a number of cells (say 12 cells) with a wide range of internal resistance, and you group them into 4 groups of 3, then add the resistances of the cells in each group, the spread of resistances will get much closer together. It is impossible for that to not happen. If you actually know the IR of each cell, you could group them so that all 4 groups had the same IR, even though the individual cells could vary significantly. So the 4 "cells" in your 3p4s pack would be balanced.

Having the cells in parallel assures that the voltage of the cells is kept the same. So, while the SOC of each cell might vary a bit (but not much)in the flat region, as soon as the battery gets into the knee (either charging or discharging) the cells will balance to the same SOC. All the paralleled cells will reach 2.5V or 3.65V at exactly the same time. This would work even with very different cells, even different capacities. You can parallel a 200Ah cell with a 280Ah, and you will get 480Ah.

If you built multiple 4s packs with unmatched cells, each pack could have radically different IR values, and might not share the load equally.

Now, real world, not theory: It doesn't matter. You probably don't have the capacity to measure the differences of what option may or may not work better. If you are pushing the cells *very* hard, charging/discharging at their max rate, then it might make a difference on whether or not you exceed a single cells charge rate based on the IR of the cell it is paralleled with. But at any "normal" rate, it won't matter, and everything will be good again when the cells get into the knee.

One possible answer to "bad things often happen" isn't really a matched cell issue, but a state of charge issue. Before you connect 2 cells in parallel, make sure they are at (nearly) the same voltage.
 
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