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Adding 10KWh new lfp to 20KWh old lfp...

Supervstech

Supervstech

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I have the old 20KWh setup installed, and it is getting me around 10KWh... what will happen if I add in the new 10KWh?

I can isolate the SCC's to each bank, but they would be feeding the inverter paralleled...

I am not sure how they will play together.
 
I've been adding LFP to my bank over two years to get to 30kWh. Never had issues adding another pack BUT it had to be at the same voltage level or a touch above, else other packs will either Dump hugely to level it up or pull like crazy because it's heavier on charge. Now THAT can be quite an eye opener ! Once all synched up voltage wise, 5 packs are happy as clams in the perfect mud.
 
Oh, of course i would match voltages first, I dont want that mess...
I'm just worried that the weakened old packs could cause issues with the new one.
 
You can parallel packs of different capacities without issues as long as they're at the same voltage when doing so. The lowest capacity one (i.e., weakest) will just stop contributing to the power output sooner. Likewise, it will stop taking current before the others during charging. The reason is that the internal resistance increases at low and high states of charge (see image below) and the current will mostly take the path of least resistance.

Internal-resistance-variation-for-different-discharge-currents-and-different-SOC-levels.png
 
Supervstech said:
I have the old 20KWh setup installed, and it is getting me around 10KWh... what will happen if I add in the new 10KWh?

I can isolate the SCC's to each bank, but they would be feeding the inverter paralleled...

I am not sure how they will play together.
Click to expand...
holy crap i just realized you mean it’s 50% degraded, thanks to another comment

originally parsed this as a 20 -> 30 kWh upgrade. this is a misinterpretation now that i’m more awake.

this is a 10 -> 20 kWh upgrade for a system that originally provided 20 kWh
 
upnorthandpersonal said:
You can parallel packs of different capacities without issues as long as they're at the same voltage when doing so. The lowest capacity one (i.e., weakest) will just stop contributing to the power output sooner. Likewise, it will stop taking current before the others during charging. The reason is that the internal resistance increases at low and high states of charge (see image below) and the current will mostly take the path of least resistance.

Internal-resistance-variation-for-different-discharge-currents-and-different-SOC-levels.png
Click to expand...
good point.

each battery is like a straw that becomes shorter with time and usage.

the short straw is disliked by electrons, to the extent that shortness is resistance.

excessive inter-pack discharging and charging over state of charge cycle seems to be the main mode of dysfunction to be aware of?

will there be per-pack current monitoring?

per-pack current monitoring would enable real time end of life assessment if i am not mistaken.
 
upnorthandpersonal said:
You can parallel packs of different capacities without issues as long as they're at the same voltage when doing so. The lowest capacity one (i.e., weakest) will just stop contributing to the power output sooner. Likewise, it will stop taking current before the others during charging. The reason is that the internal resistance increases at low and high states of charge (see image below) and the current will mostly take the path of least resistance.

Internal-resistance-variation-for-different-discharge-currents-and-different-SOC-levels.png
Click to expand...
this graph implies internal resistance is a function of current?

which is to say, different current, different resistance?

had not considered this before, although subtle

am i misunderstanding?
 
Yes, the higher the discharge current, the higher the internal resistance. Vice versa, the higher the charge current, the lower the internal resistance.

Sources:

www.researchgate.net

Figure 5. Internal resistance variation for different discharge...

Download scientific diagram | Internal resistance variation for different discharge currents and different SOC levels. from publication: Second-Order Discrete-Time Sliding Mode Observer for State of Charge Determination Based on a Dynamic Resistance Li-Ion Battery Model | A second-order...
www.researchgate.net www.researchgate.net

and

 
Supervstech said:
Oh, of course i would match voltages first, I dont want that mess...
I'm just worried that the weakened old packs could cause issues with the new one.
Click to expand...
My Apologies, I'm sorry, I missed that the current 20kWh bank is down to 10kWh. That changes the picture a bit. If they are already that degraded (hopefully verified with tests, not just an assumption) then isolation may be a better option.

I assume you have tested the packs, verified all of the connections etc.
 
it sounds like the byd pack is “old”

is the used byd pack subject to charge or discharge in excess of 0.2C rate regularly? (regularly:greater than five seconds per week)

does it self heat noticeably during high amperage discharge or charge?
 
I agree, a completely seperate system for the 10kwh system, complete with its own inverter.
 
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