A BMS is a Battery Manager. It manages & monitors the cells within a battery pack. It's primary purpose is to ensure that cells neer go below or above the "Allowable Voltage Range" which is from 2.500 to 3.650 Volts per cell. If the cells go out of that range they will suffer damage. Now LFP has a "Working Voltage Range" (just like every other chemistry) which is from 3.000-3.400 Volts per cell. This is because the chemistry has a Very FLAT Voltage Curve and this is the Range that delivers the actual specified AH from the cells.
The BMS will also monitor the battery pack Temperature and if Below 0C or above 55C Temp it will cutoff charge/discharge (Some BMS will allow a lower temp discharge which is OK to -20C Temp. LFP CANNOT BE CHARGED Below Freeing Temp!
SmartBMS can do more things like Bluetooth Interface & Device Interface to Inverters (Not all BMS' can interact with Inverters or only selected models). They also usually incorporate some form of Balancing. Passive Balancing only burns off excess voltage from cells that are higher than the others. This is pretty much useless on cells above 100AH - it is intended for smaller cell sizes. Active Balancing actually moves voltage from High Cells to Low Cells and this is far more effective. A rule of thumb for Active Balancers is to have 1A per 100AH of Capacity, so a 280AH Battery can comfortably use a 2A to 3A Active Balancer. This will keep cells within 0.010 - 0.020 volts between them (most are programmable).
LFP doe NOT EVER use Equalize !!!
It is Strictly CC (Constant Current)/CV (Constant Voltage)Charging
Where Float is CV which is when the battery cells get topped off & balanced internally.
* Active Balancing SHOULD start at 3.300Vpc so as to stay ahead of any imbalances and to prevent a large "job" at the end of charging cycle... Remember that Active MOVES power and does ot waste it. It will only run when cells reach a certain (programmable) voltage delta difference.
Please provide Make/Model of Battery Pack(s) you have,
Make / Model of BMS if known,
Specifics on your SCC and the Inverter/Charger
LFP is the closest Lithium Based chemistry to Lead Acid and essentially mimicks FLA with regards to charge profile.
Using a conservative profile and staying within the working voltage range + the little extra that is safe to use.
* Note that BMS Low Voltage Cutoff must be set, so that even if cutoff for low volts under load, to allow them to recover to a point where they can take charge without manual intervention. The operable voltage range is 2.800-3.450
12V: 4 cells, 11.2V - 13.8V
24V: 8 cells, 22.4V - 27.6V
48V: 16 cells, 44.8V - 55.2V
Bulk/Absorb up to the High Voltage Side shown above.
Float to same Hi Voltage -0.1V so 13.7, 27.5, 55.1
EndAmps/TailCurrent (when to switch to Float) is calculated as follows:
100AH X 0.05 = 5A or 280AH X 0.05 = 14A
If you have multiple packs in parallel (same sized) the EndAmps stays the same... If you have big/small packs then there is another formula for averaging it.
YES you CAN have different AH Packs in Parallel without issue.
YES you can add Battery Packs into your bank over time without issues (quite unlike FLA).
* BEST is t stay "Native" 12V Battery for 12V system, 24V for 24V system and 48V battery for 48V System... So NOT do the series batteries with LFP, it's a nasty path to trod.
Hope it helps & Good Luck,
Grab the resources from my Signature which will help you get more up to speed on LFP - building them, voltage charts & more.
Please Note: I was on FLA seems like forever ago... There is a LOT of stuff that you will have to forget & relearn. Many of the things that affect FLA do not even play with Li. Forget Peukerts Laws and all that... There is a bit of a curve but not bad. BTW: Beware of common Wives Tales that get pushed about... also look at people signatures, if they have no links to their systems etc, sprinkle their advice with lot's of salt.
