Well, I screwed up.
I was looking for an LFP to start a conversion and stumbled upon this Amazon listing for a 220Ah 24v LiFePO4 https://www.amazon.com/FOIIOE-24V-VESTWOODS-Bluetooth-Replacement/dp/B0BY295N5Y/. Amazon listing claims it's a LiFePo4, can handle 4500+ cycles, has bluetooth for monitoring, and it was the best from a Wh/volume, $/Wh, and lb/Wh perspective for my needs. Direct from the Vestwoods website it was $1,000 without tax, and shipping was fast. All was great in my mind. Battery arrived well packed, but I left for vacation shortly after it arrived.
Came home and hooked up a 15a power supply using the low end of their recommended charge voltage per the data sheet (29.4v) Battery was ~50% full and charged well up to 29.4v. Took the battery off the charger and a few minutes later noticed a cell overvoltage disconnect had tripped in the app (probably some cell balancing or the slow charge rate that eeked the single cell past cut-off voltage). Started a capacity test the next morning and noticed pack voltage hadn't dropped to an expected ~27.2v per some "resting" 100% lifepo4 charts, but didn't pay much attention to it.
I started to question things as my capacity test wore on... why was the spec charge voltage a little high and why didn't pack voltage drop overnight? Started to poke around Vestwoods' website more and turns out the listing says it's an "NMP" battery? As best I can tell that's just bad chinese and it's actually an NMC built pack. https://vestwoods.store/collections...f-grid-applications-and-more-built-in-20a-bms
I'm past the return window now, so I'm stuck with what I have. I know that NMC cells have a greater chance of thermal runaway and lower cycle life compared to LFP. Is there anything I can do to extend the battery's life and reduce chance of thermal issues? I'm assuming limiting charge/discharge to 10%-90% will help significantly, as well as low C rates. What voltages should I be using to hit those 10-90% levels?
With a 29.4v lower cutoff on the charge voltage, and looking at NMC cell voltages, I'm assuming there's only 7 cells in the battery (7x4.2v=29.4 spec). The BMS low discharge cut-off is specced at 19.6v which would be 2.8v with 7 cells, that seems in line with some online resources stating 2.75v per cell.
For 7 cells, and trying to limit charging to ~10-90% capacity, is a low voltage cut-off of 3.5v/cell (24.5v) and high voltage charge limit of 4.07v/cell (28.5v) reasonable?
More thoughts, see edit 2 below, is 10-90% limiting even a reasonable goal to hit? 20-90% gives 70% of total capacity and should be good for somewhere around 85-90% capacity retention after 2000 cycles.
Edit:
I've been doing some more research and have found a few scholarly articles on OCV vs. SOV estimates that all point to 3.5v being ~10% remaining SOC. 90% OCV seems to be close to ~4.02v/cell, which if you assume 0.05v relaxation after removing the charging source would put you up at 4.07v/cell bulk charging. Sources below.
www.batterydesign.net
www.mdpi.com
Edit 2:
This source talks to DoD and charge levels and their effect on longevity of Li-ion cells. I'm beginning to re-think the lower level cutoff since that seems to have the biggest effect on longevity. Perhaps a 20-90% target is a better choice?
I was looking for an LFP to start a conversion and stumbled upon this Amazon listing for a 220Ah 24v LiFePO4 https://www.amazon.com/FOIIOE-24V-VESTWOODS-Bluetooth-Replacement/dp/B0BY295N5Y/. Amazon listing claims it's a LiFePo4, can handle 4500+ cycles, has bluetooth for monitoring, and it was the best from a Wh/volume, $/Wh, and lb/Wh perspective for my needs. Direct from the Vestwoods website it was $1,000 without tax, and shipping was fast. All was great in my mind. Battery arrived well packed, but I left for vacation shortly after it arrived.
Came home and hooked up a 15a power supply using the low end of their recommended charge voltage per the data sheet (29.4v) Battery was ~50% full and charged well up to 29.4v. Took the battery off the charger and a few minutes later noticed a cell overvoltage disconnect had tripped in the app (probably some cell balancing or the slow charge rate that eeked the single cell past cut-off voltage). Started a capacity test the next morning and noticed pack voltage hadn't dropped to an expected ~27.2v per some "resting" 100% lifepo4 charts, but didn't pay much attention to it.
I started to question things as my capacity test wore on... why was the spec charge voltage a little high and why didn't pack voltage drop overnight? Started to poke around Vestwoods' website more and turns out the listing says it's an "NMP" battery? As best I can tell that's just bad chinese and it's actually an NMC built pack. https://vestwoods.store/collections...f-grid-applications-and-more-built-in-20a-bms
I'm past the return window now, so I'm stuck with what I have. I know that NMC cells have a greater chance of thermal runaway and lower cycle life compared to LFP. Is there anything I can do to extend the battery's life and reduce chance of thermal issues? I'm assuming limiting charge/discharge to 10%-90% will help significantly, as well as low C rates. What voltages should I be using to hit those 10-90% levels?
With a 29.4v lower cutoff on the charge voltage, and looking at NMC cell voltages, I'm assuming there's only 7 cells in the battery (7x4.2v=29.4 spec). The BMS low discharge cut-off is specced at 19.6v which would be 2.8v with 7 cells, that seems in line with some online resources stating 2.75v per cell.
For 7 cells, and trying to limit charging to ~10-90% capacity, is a low voltage cut-off of 3.5v/cell (24.5v) and high voltage charge limit of 4.07v/cell (28.5v) reasonable?
More thoughts, see edit 2 below, is 10-90% limiting even a reasonable goal to hit? 20-90% gives 70% of total capacity and should be good for somewhere around 85-90% capacity retention after 2000 cycles.
Edit:
I've been doing some more research and have found a few scholarly articles on OCV vs. SOV estimates that all point to 3.5v being ~10% remaining SOC. 90% OCV seems to be close to ~4.02v/cell, which if you assume 0.05v relaxation after removing the charging source would put you up at 4.07v/cell bulk charging. Sources below.
Open Circuit Voltage - Battery Design
Open Circuit Voltage is the potential difference between positive and negative terminals when no current flows and the cell is at rest.
www.batterydesign.net
A Generalized SOC-OCV Model for Lithium-Ion Batteries and the SOC Estimation for LNMCO Battery
A state-of-charge (SOC) versus open-circuit-voltage (OCV) model developed for batteries should preferably be simple, especially for real-time SOC estimation. It should also be capable of representing different types of lithium-ion batteries (LIBs), regardless of temperature change and battery...
www.mdpi.com
Edit 2:
This source talks to DoD and charge levels and their effect on longevity of Li-ion cells. I'm beginning to re-think the lower level cutoff since that seems to have the biggest effect on longevity. Perhaps a 20-90% target is a better choice?
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