Why lithium iron phosphate?
Lithium iron phosphate (LiFePO4 or LFP) is the safest of the mainstream liion battery types. The nominal voltage of a LFP cell is 3,2V (lead-acid: 2V/cell). A 12,8V LFP battery therefore consists of 4 cells connected in series; and a 25,6V battery consists of 8 cells connected in series.
A lead acid battery will fail prematurely due to sulfation:
•If it operates in deficit mode during long periods of time (i.e. if the battery is rarely, or never at all, fully
•If it is left partially charged or worse, fully discharged (yacht or mobile home during wintertime).
A LFP battery does not need to be fully charged. Service life even slightly improves in case of partial charge instead of a full charge. This is a major advantage of LFP compared to lead acid.
Other advantages are the wide operating temperature range, excellent cycling performance, low internal resistance and high efficiency (see below).
LFP is therefore the chemistry of choice for very demanding applications.
In several applications (especially off-grid solar and/or wind), energy efficiency can be of crucial importance.
The round trip energy efficiency (discharge from 100% to 0% and back to 100% charged) of the average lead-acid battery is 80%.
The round trip energy efficiency of a LFP battery is 92%.
The charge process of lead-acid batteries becomes particularly inefficient when the 80% state of charge has been reached, resulting in efficiencies of 50% or even less in solar systems where several days of reserve energy is required (battery operating in 70% to 100% charged state).
In contrast, a LFP battery will still achieve 90% efficiency under shallow discharge conditions.
Size and weight
Saves up to 70% in space
Saves up to 70% in weight
LFP batteries are expensive when compared to lead-acid. But in demanding applications, the high initial cost will be more than compensated by longer service life, superior reliability and excellent efficiency.
LFP batteries are easier to charge than lead-acid batteries. The charge voltage may vary from 14V to 16V (aslong as no cell is subjected to more than 4,2V), and they do not need to be fully charged. Therefore several batteries can be connected in parallel and no damage will occur if some batteries are less charged than others.
With or without Battery Management System (BMS)?
1. A LFP cell will fail if the voltage over the cell falls to less than 2,5V(note: recovery by charging with a low current, less than 0,1C,is sometimes possible).
2. A LFP cell will fail if the voltage over the cell increases to more than 4,2V. Lead-acid batteries will eventually also be damaged when discharged too deeply or overcharged, but not immediately. A lead acid battery will recover from total discharge even after it has been left in discharged state during days or weeks (depending on battery type a nd brand).
3. The cells of a LFP battery do not auto-balanceat the end of the charge cycle. The cells in a battery are not 100% identical. Therefore, when cycled, some cells will be fully charged or discharged earlier than others. The differences will increase if the cells are not balanced/equalized from time to time.
In a lead-acid battery a small current will continue to flow even after one or more cells are fully charged (the main effect of this current is decomposition of water into hydrogen and oxygen). This current helps to fully charge other cells that are lagging behind, thus equalizing the charge state of all cells.
The current which flows through a fully-charged LFP cell however, is nearly zero, and lagging cells will therefore not be fully charged. Over time the differences between cells may become so extreme that, eventhough the overall battery voltage is within limits, some cells will fail due to over-or under voltage. Cell balancing is therefore highly recommended.
In addition to cell balancing, a BMS will:-
Prevent cell under voltage by timely disconnecting the load.
Prevent cell overvoltage by reducing charge current or stopping the charge process.
Shut down the system in case of over temperature.
A BMS is therefore indispensable to prevent damage to large Li-ion battery banks.
With cell balancing , but without BMS:12,8V LFP batteries forlight duty applications
In applications were excessive discharge (to less than 11V), overcharge (to more than 15V) or excessive charge current will never occur, 12,8V batteries with cell balancing only may be used.
Please note that these batteries are not suitable for series or parallel connection.
1. A Battery Protect module (see www.victronenergy.com) may be used to prevent excessive discharge.
2. The current draw of inverters and inverter/chargers is often still significant (0,1A or more) after low voltage shutdown. The remaining standby current will therefore damage the battery if the inverters or inverter/chargers are left connected to the battery after low voltage shutdown during a long period of time.
With cell balancing and interface to connect to a Victron BMS: 12,8V LFP batteries for heavy duty applicationsand parallel/series connection
These batteries have integrated Cell Balancing, Temperature and Voltage control (BTV).Up to ten batteries can be paralleled and up to
four batteries can be series connected (BTVs are simply daisy-chained) so that a 48V battery bank of up to 2000Ah can be assembled.
The daisy-chained BTVs must be connected to a Battery Management System (BMS).
Battery Management System (BMS)
The BMS connects to the BTVs and its essential functions are:
1. Disconnect or shut down the load whenever the voltage of a battery cell falls to less than 2,5V.
2. Stop the charging process whenever the voltage of a battery cell increases to more than 4,2V.
3. Shut down the system whenever the temperature of a cell exceeds 50°C.