Multiple pack options

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James L

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Jul 7, 2021, 8:15:24 PMJul 7
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Hello everyone.

My house (Victoria Australia) has ben off grid since 2007 running a 24v Sunny Island 3.3kw inverter charger backed up with a 9kva generator. I have around 6kw of solar running to 2 Midnite classics.  Everything has been extremely reliable.  It will soon be time to replace the 1600ah lead acids and i have prismatic cells on order.

I would like to build 2 x 2P8S 280ah cells with individual BMS's so if something was to happen to a cell the single bank will disconect and life goes on as normal. I don't ever want to have the inverter switched off automatically.  

Am i able to achieve this using relay's or contactors etc?  I work away for long periods and have wife and small kids. If one cell was to have an issue i would feel much more comfortable running on 1 pack and have the generator top it up a few times until i can get back and sort it out. I really don't want to buy cheap chinese electronics and love the idea of these products.

Thanks in advance

James




Gordon Bland

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Jul 8, 2021, 4:55:21 AMJul 8
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Hi James,  this post may help explain why you may be better off just doing one pack instead of multiple battery packs? https://groups.google.com/g/electrodacus/c/feb5T1-x2uI
Possibly other posts that explain why one pack is better. 

James L

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Jul 8, 2021, 6:51:33 PMJul 8
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Thanks.  

Dave McCampbell

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Jul 8, 2021, 9:36:53 PMJul 8
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James.  These are my comments from the post Gordonb mentioned above:  
"I also have a LiFePO4 lithium house bank on a cruising boat.  I tried two parallel batteries using 8 272ahr DIY cells configuration early on.  It is a nightmare to wire all the relays and other things you will need to properly protect and monitor even my two sets of 4 cells, 540ahrs.  And then there is the significant additional expense for multiple BMSs and other components.  Dacian can explain this to you in much more detail if needed.  We are now configured 4 sets of 2 parallel cells in series or 4S2P.  Only one BMS and much easier system wiring.  For monitoring 32 individual cells, there are many small inexpensive monitors available that can let you see each cell if you want that level of detail.  Some can be displayed on a computer/cell phone/etc via blue tooth/wifi.  We are using a very detailed display with Node Red to our remote computer via wifi to monitor our daily cell status.  If anything happens to even one cell we will be able to see it immediately.  There is more on this on Oberon's Beginners' Guide and elsewhere.  We think the Electrodacus system is the the best reasonably priced way to safely handle a lithium system  on a boat."

Others with other BMSs have done this.  See some of the other forums and Utubes.  It is not impossible, but very difficult if you want to get it right.  And there are some tradeoffs which Dacian can better explain to you.  For us, the installation difficulties out weighed the advantages.  If we have a problem with a cell it will most likely develop slowly, and we can address that when needed.  We have already had a problem with one cell self discharging excessively, so we replaced it after a month watching it.  Hopefully if it is a wiring or other component problem you will see that you have a problem by monitoring your wifi display.  

Purchasing one or two spare cells might be a good preventative measure if you are buying low end Chinese cells.  Dave 

James L

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Jul 8, 2021, 10:53:41 PMJul 8
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Will you be able to see a problem with paralelled cells?

What i really wanted to do is emulate the rack type systems that have 3.3kwh batteries all with their own BMS and breakers. When you need more capacity you just build another one, charge up and add it on. If there is a fault with one it disconects and the system keeps going.

Thanks

Dave McCampbell

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Jul 9, 2021, 11:12:43 PMJul 9
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James.  You will be able to see problem cells as the voltage of their parallel set can be seen on the SBMS0 monitor screen.  You won't know which cell is the problem until you disconnect the cells from the pack. We were able to easily see this with our single self discharging cell in a set of two parallel cells in our 4S2P configuration. 

There certainly is an advantage to using cells in a parallel battery configuration as you mention above, especially for us remote location cruisers.  However, as I mentioned above parallel batteries did not work for us due to wiring difficulties trying to disconnect individual charging sources and loads from multiple batteries.  Maybe you could disconnect individual batteries manually or with a BMS.  You should think carefully about how you would do this and if that would cause any problems with your charge sources or loads.  Also, I have read that you should not connect more that 2 batteries this way.  

Dacian has posted other reasons not to do this and I am sure will comment further.



Dacian Todea

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Jul 12, 2021, 8:07:57 PMJul 12
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James,

If you want that sort of redundancy then you will need two separate inverters that can be paralleled on the output. That way each inverter has his own battery and BMS and even if one is down the other will be able to handle half the max load.
You can not parallel LiFePO4 batteries as you can Lead Acid so is either two separate inverters and then parallel the AC output of the inverter or just use them separately for different loads.
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