Extending battery

ArranP

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LiFePo4 3.2v cells

I currently have 32x 100Ah cells in 2p 16s configuration like this
bf4a04061edfb7420f226fa585734541.png
If 300Ah cells are added to the existing 100Ah cells, like this does any foresee a problem ?​
439d9d9390a008597c0c4d94e25a36ed.png
 

sunshine_eggo

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You'll get pretty substantial flow imbalance within each "cell."

I would build a separate 304Ah battery with its own BMS and parallel it to the 200Ah at the main terminals.

Neither option is ideal, but having two separate batteries with more similar capacity is less objectionable than "cells" constructed from significantly dissimilar capacity cells.
 

DThames

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What some people see as "undesired" or "less than ideal", others see as a problem. So be aware that different people will have different points of view.
Largely related to internal resistance and capacity, different cells will give up or take current more easily than its parallel partner(s). So if you pull a heavy load for a short period then reduce that load to zero, you can observe a current between parallel cells. I like to think if this as a sloshing type affect. Some call it micro-cycling. I think this affect would be more of a concern if there was a lot of changes in the load and charger output all of the time, where a steady load would less of a concern.

Sunshine suggested two batteries. I have 2 old BYD 24v in parallel (paralleled at cell level) as one battery and that battery is parallel to 24v 280ahr Eve cell battery. You can see the same sloshing on a battery the same as at the cell level. The BYD will give up more to drive a load and then the Eve will continue to discharge to get balanced with the BYD after the load is removed.
 

ArranP

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What some people see as "undesired" or "less than ideal", others see as a problem. So be aware that different people will have different points of view.
Largely related to internal resistance and capacity, different cells will give up or take current more easily than its parallel partner(s). So if you pull a heavy load for a short period then reduce that load to zero, you can observe a current between parallel cells. I like to think if this as a sloshing type affect. Some call it micro-cycling. I think this affect would be more of a concern if there was a lot of changes in the load and charger output all of the time, where a steady load would less of a concern.

Sunshine suggested two batteries. I have 2 old BYD 24v in parallel (paralleled at cell level) as one battery and that battery is parallel to 24v 280ahr Eve cell battery. You can see the same sloshing on a battery the same as at the cell level. The BYD will give up more to drive a load and then the Eve will continue to discharge to get balanced with the BYD after the load is removed.

Then the smaller cell(s) give up more to drive a load than the larger cell...

Application is my home, I have 2x 5kw inverters working in parallel... Discharge: I have 6kw shower, several 2kw appliances. Charge: I have 6kw solar panels which I may extend to 12kw.
 

time2roll

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I am not sure if there will more balance issues as separate batteries in parallel or with the cells paralleled in the same battery.

I would tend to leave the existing alone and build the second battery separate.
 
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DThames

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Then the smaller cell(s) give up more to drive a load than the larger cell...

Application is my home, I have 2x 5kw inverters working in parallel... Discharge: I have 6kw shower, several 2kw appliances. Charge: I have 6kw solar panels which I may extend to 12kw.
My observation (and electrical training) is that the battery with the lower internal resistance will flow more current. The internal resistance is in series with the power source and the load. Think of the battery cell as a power source in series with its own internal resistance. Connect to batteries in parallel with different internal resistance, different ability to deliver amps, and you will get different amps from each battery. So draw the circuit out and do the math (use random numbers as examples) and you will see what is happening.

When the load is removed the state of charge of each battery is a bit different, so they balance again, at what ever level they are at. Edited to add, this is not ideal but it is not really that bad either. The currents are low and just a cost of doing business with batteries that are not the same in parallel.
 
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DThames

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I am not sure if there will more balance issues as separate batteries in parallel or with the cells paralleled in the same battery.

I would tend to leave the existing along and build the second battery separate.
I would agree because there are several reasons this might be a benefit someday. For example, you could remove either battery from the battery bank to do service work on it and the other would keep your system running. It would also be easier to spot a weak cell someday.
 

RCinFLA

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For thick electrode cells, the greater the charge and discharge current percentage of C rate the faster the degradation of cell. Exceeding 0.5 CA cell current regularly significantly decreases cell longevity.

If mismatching of current sharing causes a cell to hog much of charge or discharge current and that cell current is a high percentage of its C rate it will be degraded at a faster rate.

I would put the 300 AH cells on their own independent string pack with their own BMS.
 

time2roll

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Only way to know would be to build the cells in parallel and measure the cross current to see if it is high enough to cause concern.
I assume there will be parallel connections at each group of cells.
 

DThames

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Only way to know would be to build the cells in parallel and measure the cross current to see if it is high enough to cause concern.
I assume there will be parallel connections at each group of cells.
If the total available charge current was way less than 0.5C for any cell circuit path, this specific concern would pretty much go to zero.
 
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