LiFePO4 Cycle Life Effect

e185640

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I am hoping someone can spread some knowledge, for may seem like a dumb question for this newbie. My question involved cycle life of the batteries, and sizing the bank to maximize that cycle life.

I am installing a 24V LiFePO4 bank of QTY=6 Victron 12V/200Ah batteries (2 three batteries series strings, run in parallel). If desired, I have the option to add another pair....$$$$.

The biggest load in this application draws 450A for a max 15-25sec duration, potentially 10 times in a 15min period. This would happen once every 10 days...maybe. The batteries would be recharged by solar and/or a 110/230 supplied Victron charger/Inverter.

With this type of intermittent current draw I presume there is high heat generation and that heat would effect the cycle life of the batteries. Does increasing the battery stack help in limiting the heat damage by adding capacity to draw upon? Is it worth the money?

Any shared wisdom would be greatly appreciated. Thank you in advance!
 

mrzed001

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I am hoping someone can spread some knowledge, for may seem like a dumb question for this newbie. My question involved cycle life of the batteries, and sizing the bank to maximize that cycle life.

I am installing a 24V LiFePO4 bank of QTY=6 Victron 12V/200Ah batteries (2 three batteries series strings, run in parallel). If desired, I have the option to add another pair....$$$$.

So you build a 2s3p set of 12V batteries. (2 series, and this 3 times parallel)

The biggest load in this application draws 450A for a max 15-25sec duration, potentially 10 times in a 15min period. This would happen once every 10 days...maybe. The batteries would be recharged by solar and/or a 110/230 supplied Victron charger/Inverter.

For a 450A x 24V = 10,8 kW load I would suggest 48V system. 450A ... is a LOT.
For battery cable you need at least 3/0 AWG (95mm2)

With this type of intermittent current draw I presume there is high heat generation and that heat would effect the cycle life of the batteries. Does increasing the battery stack help in limiting the heat damage by adding capacity to draw upon? Is it worth the money?

Any shared wisdom would be greatly appreciated. Thank you in advance!
Yes, more battery, less stress.
Also LFP is best used under 0.5C so (24V) 3x200Ah = 600Ah ... 0,5C is 300A.
Not continuous 450A (0,75C) the LFP can handle. But more battery is better.

If you have a plan for the installation with load, can you show it? Not sure 24V is the right solution for you.
 
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e185640

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mrzed001, thank you very much.

The system is already built, was built in 1998. The system is a power supply for sailboat systems. The original system has 8 lead acid Group 31 (or Group 29) batteries. The bank is wired to provide 400Ah of capacity at 24V. The major current draw is the bow thrust on the boat that delivers somewhere in the range of 10hp of power at 24VDC (manufacturer states max intermittent 450A draw). The system has been proven reliable since the 1980s and is still used today in new production.

I have about a 1kW of solar, a 7.5kW genset and 100A engine alternator all capable of feeding the battery stack at some point of time.

I just don't have the knowledge base to know the effect on LiFePO4 chemistry. I recognize the design as-is cooks the LA batteries (why we replace them every 3 years). But, the LiFePO4 batteries are much much much much more expensive. Does the 6-battery stack give me capacity design margin to for this use case? Would the 8-battery stack be better, or just overkill?

Thank you again.
 

mrzed001

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The system is already built, was built in 1998. The system is a power supply for sailboat systems. The original system has 8 lead acid Group 31 (or Group 29) batteries. The bank is wired to provide 400Ah of capacity at 24V. The major current draw is the bow thrust on the boat that delivers somewhere in the range of 10hp of power at 24VDC (manufacturer states max intermittent 450A draw). The system has been proven reliable since the 1980s and is still used today in new production.

I have about a 1kW of solar, a 7.5kW genset and 100A engine alternator all capable of feeding the battery stack at some point of time.

I just don't have the knowledge base to know the effect on LiFePO4 chemistry. I recognize the design as-is cooks the LA batteries (why we replace them every 3 years). But, the LiFePO4 batteries are much much much much more expensive. Does the 6-battery stack give me capacity design margin to for this use case? Would the 8-battery stack be better, or just overkill?

OK, so I understand it better :)

The main differences from lead to LiFePO4 you will have (for the same amount of lets say 400Ah battery):

Lead-AcidLiFePO4
Charge0,1C : 40A0,5C: 200A
DischargeDepends on batterycontinuous 0,5C: 200A
peak 1C: 400A
Usable capacity50% : 200Ah90%: 360Ah
Operating temperatures-20-50 Celsius0-50 Celsius
Cycle life6003500+


To longer LiFePO4 life usually: (for exact numbers need the battery data sheet)
  • Temperature control.
    Under 0 Celsius not to charge (but can be discharged).
    Over 45 Celsius cooling is needed .
  • Not to exceed 0,5C charging
  • Not to exceed 0,5C continuous discharge, temporally 1C is OK
  • Not to discharge under 5-10%
  • Not to charge over 90-95%
  • Needs a precise CC-CV charger

So from LiFePO4 a 600Ah battery is enough (max short 0,75C discharge).
With 800Ah the max discharge is 0,5C ... so total no stress for the battery.
Your choice :)
What else do you use it ? Fridge, AC, TV, Micro ?
Calculate them into the Wh you need. No need to start the generator often.


The ca 3500 cycle life (then it looses 20% storage capacity) for LiFePO4 is when you
- charge with 1C (from 0% to 100% charge in a hour),
- discharge with 1C (from 100% to 0% discharge in a hour).

If you use lower charge and discharge rate values ... really noone knows how much the cycle life goes up, but a lot (x2 - x10 ?)

So if you charge and discharge the full (600Ah x 24V = 14,4 kWh battery) every day then it should work for at least 10 years (and even then with 80% capacity).
 
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e185640

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So that may be the most data rich thorough answer to a question I have asked online. I can't thank you enough. Just Fantastic!
 

mrzed001

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So that may be the most data rich thorough answer to a question I have asked online. I can't thank you enough. Just Fantastic!

That is why this forum exists
Feel free to ask anytime.
Also to share, like experience and pics from your setup :)
 
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