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 four cells connected in series; and a 25.6V battery consists of eight cells connected in series.
A lead-acid battery will fail prematurely due to sulfation, that is, if the battery is rarely, or never at all, fully charged, or if it is left partially charged or worse, fully discharged.
An LFP battery does not need to be fully charged. 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. LFP is therefore the chemistry of choice for very demanding applications.
In several applications – especially boats 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 an 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, an 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 14V to 16V (as long as no cell is subjected to more than 4.2V), and they do not need to be fully charged.
Huge Battery Life
What a lot of people don’t know is that a Lithium Phosphate battery is not finished/dead after 2,000 cycles, unlike lead-acid batteries, Instead, their long-term performance is as follows:
• 2,000-2,500 cycles – the battery is reduced to 80% of its original capacity (up to 14 years based on one cycle every two days)
• 3,000 cycles – the battery is reduced to 70% of its original capacity
• 4,000-5,000 cycles – the battery is reduced to 60% of its original capacity (up to 28 years based on one cycle every two days)
• 6,000-10,000 cycles – the battery is reduced to 50% of its original capacity (has not been tested by the manufacturer.)
Also at any point you can add new batteries to increase capacity (no need to trough out the old ones)
Two more key advantages are that size and weight LFP batteries weigh just 25% of what standard lead-acid batteries weigh, and most existing charging systems can be used without any major changes.
1. An LFP cell will fail if the voltage over the cell falls to less than 2.0V
2. An LFP cell will fail if the voltage over the cell increases to more than 4.2V.
3. The cells of an LFP battery do not auto-balance at the end of the charge cycle.
4. The cells in an LFP 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/equalised 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 equalising the charge state of all cells.
The current through an LFP cell, however, when fully charged, is nearly zero, and lagging cells will therefore not be fully charged. Over time the differences between cells may become so extreme that, even though the overall battery voltage is within limits, some cells will fail due to over or under voltage. Cell balancing is therefore highly recommended.
One last important warning to remember is that LFP batteries can be damaged due to over discharge or over charge.
– Octopus Electrical Services
This article is part of the Wired on Water series provided by Octopus Electrical Services, based at Phuket Boat Lagoon, with more than 40 years combined experience working in the marine industry on the latest in marine technology. For more information, contact Octopus Electrical Services at Phuket Boat Lagoon. Tel/ Fax: +66 (0) 76 600143. Email: firstname.lastname@example.org Visit www.octopusasia.com