Why lithium-iron-phosphate?
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 25,6 V battery consists of 8 cells connected in series.
Why a Battery Management System (BMS) is needed: 1. A LFP cell will be damaged if the voltage over the cell falls to less than 2,5V. 2. A LFP cell will be damaged 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 and brand). 3. The cells of a LFP battery do not auto-balance at 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, even though the overall battery voltage is within limits, some cells will be destroyed due to over- or under voltage.
A LFP battery therefore must be protected by a BMS that actively balances the individual cells and prevents under- and over-voltage.
Rugged A lead-acid battery will fail prematurely due to sulfation: • If it operates in deficit mode during long periods of time (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 winter time).
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.
LFP is therefore the chemistry of choice for very demanding applications.
Efficient 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
Expensive? 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.
Endless flexibility 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. Several batteries can be connected in parallel and no damage will occur if some batteries are less charged than others. Our 12V BMS will support up to 10 batteries in parallel (BTVs are simply daisy-chained).
A 12V BMS that protects the alternator (and wiring), and supplies up to 200A in any DC load (including inverters and inverter/chargers)
Alternator/battery charger input (Power Port AB) 1. The first function of Power Port AB is to prevent the load connected to the LFP battery from discharging the starter battery. This function is similar to that of a Cyrix Battery Combiner or Argo FET Battery Isolator. Current can flow to the LFP battery only if the input voltage (= voltage on the starter battery) exceeds 13V. 2. Current cannot flow back from the LFP battery to the starter battery, thus preventing eventual damage to the LFP battery due to excessive discharge. 3. Excessive input voltage and transients are regulated down to a safe level. 4. Charge current is reduced to a safe level in case of cell unbalance or over temperature. 5. The input current is electronically limited to approximately 80% of the AB fuse rating. A 50A fuse, for example, will therefore limit the input current to 40A. Choosing the right fuse will therefore: a. Protect the LFP battery against excessive charge current (important in case of a low capacity LFP battery). b. Protect the alternator against overload in case of a high capacity LFP battery bank (most 12V alternators will overheat and fail if running at maximum output during more than 15 minutes). c. Limit charge current in order not to exceed the current handling capability of the wiring. The maximum fuse rating is 100A (limiting charge current to approximately 80A).
Load/battery charger output/input (Power Port LB) 1. Maximum current in both directions: 200A continuous. 2. Peak discharge current electronically limited to 400A. 3. Battery discharge cut-off whenever the weakest cell falls below 3V. 4. Charge current is reduced to a safe level in case of cell unbalance or over temperature.
