Lithium-iron-phosphate (LiFePO4 or LFP) is the safest of the mainstream li-ion 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 charged).
- 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 leadacid 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.
LFP batteries are easier to charge than lead-acid batteries. The charge voltage may vary from 14 V to 15 V (as long as no cell is subjected to more than 4,2 V), 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.
Battery Management System (BMS)
The BMS will:
- Disconnect or shut down the load whenever the voltage of a battery cell falls to less than 2,5V.
- Stop the charging process whenever the voltage of a battery cell increases to more than 4,2V.
- Shut down the system whenever the temperature of a cell exceeds 50°C.