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Cell voltage never over 3.8V? Bad idea or ok?
#11
(12-24-2019, 04:47 PM)kc8adu Wrote: at what point do we extend the cycle life so much we hit calendar life?
there has to be a sweet spot where we use up cycle life before old age takes them out.
That's a great question.   After spending booku $$ on 18650cells..  and nearly completing my home Solar,  I'm more and more focused on the long haul and wondering on the optimal DOD (and voltage range) to make my 18650 battery bank 1,000(s) of daily charge/discharge cycles that enable me to consume my PV array power Smile     

For example - I will have 70 packs (~8,000cells) and for me, if I can maintain 50% of today's capacity after 15yrs I'll have a chance to get my money back.    Its not a monolithic question since there are 8,000 individual cells.    What if I'm able to keep the battery bank functional down to an overall 50% of original and only have to replace 1,000 cells (12.5%)?  or 2,000 cells (25%)? - is that a win (for me). Will the cells age 'evenly' or will batches of them do major fails? Can I recondition packs and replace a few bad cells to help the remaining cells continue to limp along at 50% capacity? I have many questions...

This general topic will be of much interest to me as the years unfold ....
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#12
As far as I know, keeping cells permanently below 3.8V is not a problem at all. Virtually all sources state that 3.6v~3.8v is the optimal storage voltage, so those cells should last a very long time.

What you have to take care of is balancing. You'll need to have a BMS/balancer that can be configured to start working at 3.8v. Otherwise you could eventually have a very unbalanced battery that could lead to a premature failure.
Also probably a good idea to adjust the BMS cell over voltage shutoff to activate at say 4.0v, since any cell reaching that voltage means that something somewhere has gone very wrong.
Modular PowerShelf using 3D printed packs.  60kWh and growing.
https://secondlifestorage.com/showthread.php?tid=6458
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#13
(12-24-2019, 10:52 PM)ajw22 Wrote: As far as I know, keeping cells permanently below 3.8V is not a problem at all. Virtually all sources state that 3.6v~3.8v is the optimal storage voltage, so those cells should last a very long time.

What you have to take care of is balancing. You'll need to have a BMS/balancer that can be configured to start working at 3.8v. Otherwise you could eventually have a very unbalanced battery that could lead to a premature failure.
Also probably a good idea to adjust the BMS cell over voltage shutoff to activate at say 4.0v, since any cell reaching that voltage means that something somewhere has gone very wrong.

These settings should not be a problem using Batrium, am I right?
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Maniac_Powerwall: 1x 14S100P 18650 LiIo, 6x 345W LG panels, MPP Solar PIP5048GK, Batrium Watchmon4 BMS
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#14
>These settings should not be a problem using Batrium, am I right?
Correct
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#15
Batrium have No problem with that but i wouldnt run balancing below 4v personally.
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Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#16
(12-25-2019, 09:33 PM)daromer Wrote: Batrium have No problem with that but i wouldnt run balancing below 4v personally.
Why?
My Powerwall Build Thread

Maniac_Powerwall: 1x 14S100P 18650 LiIo, 6x 345W LG panels, MPP Solar PIP5048GK, Batrium Watchmon4 BMS
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#17
Because the closer to nominal voltage the flatter is the curve hence more likely to balance when not really needed. Thats also why I personally would never charge less than 3.9v and let charging start when a cell reaches 4.0V. Therefore you ensure that you only balance when actually needed.

This need to be checked on every battery bank since they differ depending on type of cells
NOTE! My links supplied in this message may be affiliated with Ebay and by clicking on them you agree on the terms.
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 100kWh LiFePo4 | 20kWh 14s 18650 |  66*260W Poly | ABB S3 and S5 Trip breakers
Upcoming: 14S 18650~30kWh
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#18
I think I will go up to 4.0V also. I will use another way to power the small inverter. It is only 260 Watts and I think I will use a stepdown converter to supply it. Or maybe a 220V to 48V power supply, as it will only get power when batteries are fully charged and panels produce excess. I hope the Panels will be installed soon and I finsh my packs. Still 8 more to complete 6 are ready. Like to bring everything up to work because it's more than one year building time now. Time flies by...
My Powerwall Build Thread

Maniac_Powerwall: 1x 14S100P 18650 LiIo, 6x 345W LG panels, MPP Solar PIP5048GK, Batrium Watchmon4 BMS
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