Steve_S
Offgrid Cabineer, N.E. Ontario, Canada
A clarification as there is a touch of confusion.
When looking at AH ratings and trying to do the MATH. With LFP (LiFePO4) we ALWAYS use 3.200V per Cell (Nominal Voltage) X #S of STD Cells for the Range. 3.2 volts X 4 cells = 12.8 Volts. 12.8 Volts X 170AH = 2176Wh or 2.17kWh.
STD Cells meaning 12V=4, 24V=8, 48V=16.
The amount of AH from (2.500-3.000) & (3.450-3.650) typically represents about 8% of total Gross Capacity of the cells (WHEN NEW) and is generally not used as most will use (limit) their battery packs to use between 80-90% of cap[acity for maximized life and reduced "stress".
Please Download this resource (PDF) and refer to pages 12 & 13 on voltage charts & charging matrix. It WILL simplify many things and answer some questions coming out here. Luyuan Tech Basic Lifepo4 Assembly Guide
Some say Float is not needed, well I use float on a larger complex bank of LFP Battery Packs and it most certainly helps get the batteries & their cells topped up. Float will also be keeping the Constant Voltage - VARIABLE AMPS going so the batteries can actually saturate (deep charge to low amps taken which is best) and should the System Demand increase, if there is enough incoming solar, then that increase to cover the demand... IE during the day once batteries are in float, I run my coffee maker and NONE of that power comes from BATT it comes direct from the panels. Float increases relative to demand and can provide all the solar needed, if not capable then the balance is pulled from the batteries.
This is how Float works and always has and to NOT utilize it is Handicapping your systems capabilities, which isn't smart at all... tossing $ in the form of power away.
If you Bulk Absorb @ 13.80V (3.450Vpc) and have an EndAmps / Tailcurrent set to 8.5A (170AH X 0.05) Then float @ 13.60 (3.400Vpc) that will allow to top off at the top of the "Working Voltage Range" which is 3.000-3.400V per cell, Nominal being 3.200 for LFP. Rebulk @13.4V.
A CRITICAL WORD OF CAUTION !
As you see in the voltage charts in my PDF resource above, LFP has a fairly flat voltage curve. This means that your system MUST BE CALIBRATED! Even a 1/2V offset can mess things up ! BMS' are Millivolt & Milliamp, sensitive folks ! The Voltage @ Battery Terminals will not be the same at the SCC or Inverter/Charger, due to wire voltage drop, resistance from connections, fuses & breakers and whatever else is "on the wires".
! The SCC or other "Charger" devices MUST be set to correct for any difference DURING CHARGE so that when the battery pack reaches 13.80V at the terminals and you want to stop Absorb the SCC needs to see that too, but if there is a 0.3V difference (loss) the SCC will overshoot to 14.1V at the battery terminals... So that MUST be corrected / adjusted / compensated. Different gear handles it differently, some have actual offset params, others you have to add the difference to the settings themselves.
! The Inverter MUST ALSO BE CORRECTED for DISCHARGING ! Again if there is a 0.5V Difference (loss) and you want to cut off the battery at 11.0V (2.75Vpc) the inverter will actually cutoff at 10.5V (2.625) remember absolute 0%SOC is 2.50), you will likely hit an LVD (Low Volt Disconnect) by the BMS which could "drop" the Inverter hard (not a good thing). Cutoff points are REALLY IMPORTANT and even a 1/2 Volt over/under can cause the BMS' to Trip and Fault or worse get damaged if a dumb BMS fails and you don't know.
In my signature there is a resource on Calibration and more, much of this would be handy to review and check over your gear.
Hope it helps, Good Luck.
Steve
PS: It is my understanding from reviews / teardowns etc that the BMS' used in those BigBattery are just fairly basic Dumb BMS' without any type of balancer or interface to interact with & set params. Apparently, some of the params as devined by other techies are pretty "loose" and not very optimal. Sorry but to me, this is a RED FLAG especially as they use Used Cells & Packs, and being able to tweak & adjust such is "critical" for healthy long-term operation. It also seriously handicaps troubleshooting capabilities because you cannot "see" what is going on internally with the cells.
When looking at AH ratings and trying to do the MATH. With LFP (LiFePO4) we ALWAYS use 3.200V per Cell (Nominal Voltage) X #S of STD Cells for the Range. 3.2 volts X 4 cells = 12.8 Volts. 12.8 Volts X 170AH = 2176Wh or 2.17kWh.
STD Cells meaning 12V=4, 24V=8, 48V=16.
The amount of AH from (2.500-3.000) & (3.450-3.650) typically represents about 8% of total Gross Capacity of the cells (WHEN NEW) and is generally not used as most will use (limit) their battery packs to use between 80-90% of cap[acity for maximized life and reduced "stress".
Please Download this resource (PDF) and refer to pages 12 & 13 on voltage charts & charging matrix. It WILL simplify many things and answer some questions coming out here. Luyuan Tech Basic Lifepo4 Assembly Guide
Some say Float is not needed, well I use float on a larger complex bank of LFP Battery Packs and it most certainly helps get the batteries & their cells topped up. Float will also be keeping the Constant Voltage - VARIABLE AMPS going so the batteries can actually saturate (deep charge to low amps taken which is best) and should the System Demand increase, if there is enough incoming solar, then that increase to cover the demand... IE during the day once batteries are in float, I run my coffee maker and NONE of that power comes from BATT it comes direct from the panels. Float increases relative to demand and can provide all the solar needed, if not capable then the balance is pulled from the batteries.
This is how Float works and always has and to NOT utilize it is Handicapping your systems capabilities, which isn't smart at all... tossing $ in the form of power away.
If you Bulk Absorb @ 13.80V (3.450Vpc) and have an EndAmps / Tailcurrent set to 8.5A (170AH X 0.05) Then float @ 13.60 (3.400Vpc) that will allow to top off at the top of the "Working Voltage Range" which is 3.000-3.400V per cell, Nominal being 3.200 for LFP. Rebulk @13.4V.
A CRITICAL WORD OF CAUTION !
As you see in the voltage charts in my PDF resource above, LFP has a fairly flat voltage curve. This means that your system MUST BE CALIBRATED! Even a 1/2V offset can mess things up ! BMS' are Millivolt & Milliamp, sensitive folks ! The Voltage @ Battery Terminals will not be the same at the SCC or Inverter/Charger, due to wire voltage drop, resistance from connections, fuses & breakers and whatever else is "on the wires".
! The SCC or other "Charger" devices MUST be set to correct for any difference DURING CHARGE so that when the battery pack reaches 13.80V at the terminals and you want to stop Absorb the SCC needs to see that too, but if there is a 0.3V difference (loss) the SCC will overshoot to 14.1V at the battery terminals... So that MUST be corrected / adjusted / compensated. Different gear handles it differently, some have actual offset params, others you have to add the difference to the settings themselves.
! The Inverter MUST ALSO BE CORRECTED for DISCHARGING ! Again if there is a 0.5V Difference (loss) and you want to cut off the battery at 11.0V (2.75Vpc) the inverter will actually cutoff at 10.5V (2.625) remember absolute 0%SOC is 2.50), you will likely hit an LVD (Low Volt Disconnect) by the BMS which could "drop" the Inverter hard (not a good thing). Cutoff points are REALLY IMPORTANT and even a 1/2 Volt over/under can cause the BMS' to Trip and Fault or worse get damaged if a dumb BMS fails and you don't know.
In my signature there is a resource on Calibration and more, much of this would be handy to review and check over your gear.
Hope it helps, Good Luck.
Steve
PS: It is my understanding from reviews / teardowns etc that the BMS' used in those BigBattery are just fairly basic Dumb BMS' without any type of balancer or interface to interact with & set params. Apparently, some of the params as devined by other techies are pretty "loose" and not very optimal. Sorry but to me, this is a RED FLAG especially as they use Used Cells & Packs, and being able to tweak & adjust such is "critical" for healthy long-term operation. It also seriously handicaps troubleshooting capabilities because you cannot "see" what is going on internally with the cells.