1. Question: What is the difference in battery technology between Lead-Acid chemistry (PbA) and Lithium-Iron-Phosphate (LiFePO4) chemistry?
2. Question: How does a battery C rating describe its characteristics?
3. Question: What is the “Peukert exponent” and how can it be used to compare Amp-hour ratings between Lead-Acid (PbA) battery chemistry and Lithium-Iron-Phosphate (LiFePO4) battery chemistry?
4. Question: What is the difference in discharge/recharge cycle life between Lead-acid (PbA) cells and Lithium-Iron-Phosphate (LiFePO4) cells?
5. Question: What are the differences in self-discharge characteristics between Lead-acid (PbA) cells and Lithium-Iron-Phosphate (LiFePO4) cells when left unused and uncharged for a period of time?
6. Question: I’ve heard that lead-acid (PbA) battery cells are more “forgiving” than Li-family battery cells if they are overcharged. Why is a Voltage Monitoring System (VMS) required when charging and discharging Lithium-Iron-Phosphate battery cells, while not required for lead-acid battery cells?
7. Question: What is a Battery Management System (BMS)?
8. Question: What is a Voltage Monitoring System (VMS)?
9. Question: How are cell monitoring systems used to balance a Lithium-Iron-Phosphate (LiFePO4) battery pack consisting of many individual cells?
10. Question: How does the Lithium-Iron-Phosphate (LiFePO4) battery cell charger work with a Battery Monitoring System (BMS) or Voltage Monitoring System (VMS) to balance the battery pack?
11. Question: How does the LiFeBATT Voltage Monitoring System (VMS) implement Low Voltage Protection (LVP) for its companion battery cell module when there is not enough capacity from the combined cells to continue providing sufficient power to a load?
12. Question: What are the benefits of Lithium-Iron-Phosphate (LiFePO4) chemistry for temperature stability compared to other Li-family battery chemistries (LiCoO2, LiMn, etc.)?
13. Question: How effective are Lithium-Iron-Phosphate battery packs in providing power to electric vehicles?
Question: What is the difference in battery technology between Lead-Acid chemistry (PbA) and Lithium-Iron-Phosphate (LiFePO4) chemistry?
Answer: When attempting to compare LiFePO4 to PbA chemistries, the first major advantage of LiFePO4 cells are their ability to discharge power in terms of voltage (volts) and current (amps) over time (hours or seconds). LiFePO4 chemistry has far greater power discharge capability than PbA chemistry because electricity is generated more efficiently from the movement of ions rather than from a chemical reaction and transformation.
Question: How does a battery C rating describe its characteristics?
Answer: Engineers have given batteries a standardized C rating to describe discharge characteristics and safe operating ranges. This rating compares the amount of current drawn from a pack to the overall pack Amp-hour (Ah) specification. To figure the rate, multiply the C rating by total pack Amp-hours. This product determines the maximum current that can be drawn from each cell without fear of cell damage or explosion.
LiFeBATT cells have a peak 20C rating and, within a 10Ah pack, LiFeBATT cells can discharge impulses of 200 Amps for 5 seconds. LiFeBATT cells also have a 14C pulse rating, which translates to 140 Amps discharged for 18 seconds from a 10Ah pack. LiFeBATT cells also have a 12C continuous rating, which translates to 120 Amps discharged continuously from a 10Ah pack.
At 5C and less, LiFeBATT cells have such a solid voltage that for as long as the pack is in shallow discharge cycle (70% of total capacity or less) the battery pack voltage will remain unaffected by the current draw. Basically, you can draw 50A continuously for every 10Ah in the pack and the pack voltage will be sustained until current capacity is 70% depleted.
The comparative disadvantage with PbA chemistries is the dynamic internal resistance of each cell that increases as more current is discharged while the electrolyte, anode and cathode components undergo a chemical transformation that has to be reformed during the recharging period. The results of this non-linear chemical change is called the “Peukert” effect of lead-acid batteries, where cell voltage drops off exponentially as more current is drawn and internal resistance continues to increase with more rapid chemical transformation. A more rapid exponential voltage drop reduces the overall capability of PbA chemistry to supply the rated power to a load consistently. For instance, a battery pack with a 260Ah rating may provide current for 20 hours if the battery discharges current to a light load with steady current draw. However, a heavier load like an electric motor that draws current from the battery within 5 hrs, accompanied by sharp increases or decreases in current demand, may cause the pack voltage to drop off more quickly as internal resistance rises. The battery pack may only have an effective rating of 215Ah per charge cycle for this application. The more constant internal chemistry and voltage characteristics of LiFePO4 battery packs provide a more constant supply of total power to a system, as a function of the balance between voltage and current where Power = Voltage * Current. Since the voltage does not drop out as quickly in LiFePO4 chemistries as it does in PbA chemistries, the current does not need to increase proportionally in order to maintain power. This means the power delivered to the motor will be very consistent until the very end, and less power will be lost when accelerating and cruising in an electric motor vehicle.
Because the PbA cell internal resistance increases and causes the pack voltage to drop, more current must be drawn from the battery to produce the same power. Current draw continues to increase over time as more internal resistance is formed with further chemical transformation of the battery components, resulting in an accelerating discharge of the total power available within the PbA battery pack.
Question: What is the “Peukert exponent” and how can it be used to compare Amp-hour ratings between Lead-Acid (PbA) battery chemistry and Lithium-Iron-Phosphate (LiFePO4) battery chemistry?
Answer: Comparing Amp-hour ratings between PbA and LiFePO4 is tough. Peukert’s exponent explains the capacity expected from a PbA cell at given discharge currents. The equation is:
I^x * t = Ah
I = current discharged
x = exponent
t = time
Ah = total Ah's
Most PbA cells have a Peukert’s exponent rating between 1.3 and 1.6. Good PbA cells have an exponent the gets closer to 1.15. We can’t assign this exponent to LiFePO4 because of chemistry differences, but an approximation of the Peukert’s exponent for comparison would be 1.05 for LiFePO4. As an example, a 20Ah LiFePO4 cell will produce approximately as much or more power as a 50Ah PbA cell at the same rated voltage.
Question: What is the difference in discharge/recharge cycle life between Lead-acid (PbA) cells and Lithium-Iron-Phosphate (LiFePO4) cells?
Answer: Lead-Acid (PbA) deep-cycle battery packs can usually be discharged and recharged for 300-700 cycles, depending on the quality of manufacture. However, discharge rates are also subject to temperature and weather conditions throughout the life of the pack. This means potential replacement for deep-cycle battery packs is about every one to two years. LiFeBATT guarantees its cells for 1,500 deep discharge/charge cycles before a 15% degradation of overall capacity may occur. Past the 2,000 cycle threshold, the cells still don't need replacement, but the user will notice a 15% reduction in battery capacity after a full charge. For an electric vehicle, a driver might see a 15% reduction in distance traveled as the battery pack ages beyond 2,000 cycles.
The definition of 2,000 deep discharge cycles for LiFePO4 chemistry is 72% - 98% depletion before recharge. Shallow discharge/recharge cycles will prolong battery pack life, extending it beyond 5,000 cycles with little loss in overall capacity, up to 10,000 cycles before degradation of overall battery capacity starts exceeding 15%. Shallow cycles are defined as less than 72% discharge and can be maintained by always allowing more than 28% of the battery pack capacity to remain undischarged when reconnecting the battery pack charger. At 2,000 deep discharge/charge cycles, averaging 1 cycle per day would allow for 5.5 years of use before a noticeable loss of capacity. 10,000 shallow discharge/charge cycles would last around 13+ years.
Question: What are the differences in self-discharge characteristics between Lead-acid (PbA) cells and Lithium-Iron-Phosphate (LiFePO4) cells when left unused and uncharged for a period of time?
Answer: If left unused and uncharged for a period of time, PbA battery cells can self-discharge their current capacity by up to 20% due to their internal resistance characteristics. LiFePO4 battery cells will only self-discharge about 2% of current capacity over the same period of time.
Question: I’ve heard that lead-acid (PbA) battery cells are more “forgiving” than Li-family battery cells if they are overcharged. Why is a Voltage Monitoring System (VMS) required when charging and discharging Lithium-Iron-Phosphate battery cells, while not required for lead-acid battery cells?
Answer: Unlike PbA battery cells that can tolerate a certain amount of overcharging when exceeding normal voltage and current thresholds, Li-family battery cells do have strict requirements to not exceed their maximum and minimum thresholds. The chemistry advantages gained in performance by Li-family battery cells do come at a cost that requires strict compliance to narrower charge/discharge constraints during each battery cell cycle. LiFePO4 charging systems must never exceed thresholds where voltage and current applied to a Lithium-Iron-Phosphate cell could overcharge it. When discharging Li-family cells, the applied load should also not discharge the cell completely but leave some residual current available to maintain cell chemistry. These differences are due to how the electrolyte is used in PbA and LiFePO4 chemistries.
Although frequent overcharging of PbA battery cells will shorten their total life cycle performance over time, PbA batteries can tolerate some overcharging and total discharging under load. Because a PbA cell’s anode, electrolyte and cathode materials regularly undergo chemical reactions and changes every charge/discharge cycle, PbA battery packs can tolerate more abusive charging and discharging conditions. A PbA battery pack can be simply charged by just applying voltage and current across the positive and negative ends of the entire pack from a simple AC or DC source to charge up all the battery cells at the same time. Some individual cells may charge more quickly than others, causing the electrolyte to “boil over” and evaporate. PbA cells can tolerate this more volatile chemical change after discharge if the electrolyte can be rebalanced and regenerated from the anode and cathode materials to restore some current capacity when recharged.
The same is true when discharging a PbA battery under a heavy load such as an electric motor. The PbA cells may stop performing when most of the current has been drained from the battery pack while driving an electric vehicle (EV). EV drivers have found however, that unlike gasoline, residual current can be coaxed out of the remaining cell capacity after resting the battery cell for several minutes and allowing the PbA electrolyte to slightly regenerate from the anode and cathode materials. Although this abuse can shorten the overall life cycle of individual cells, it does provide an extra margin of safety to EV drivers if they accidentally run out of “fuel”. The residual current capacity that can be regenerated in the electrolyte from the anode and cathode by just resting the PbA cell can be enough to drive the electric vehicle a little further to a safe location.
In contrast, overcharging LiFePO4 battery cells just one time will disable them by damaging the ion-transference capability of their electrolyte chemistry. During normal operation, the LiFePO4 electrolyte material remains relatively unchanged as cations and anions move through the electrolyte between the anode and cathode materials of the cell during charge/discharge cycles. The electrolyte material can also be damaged when totally discharging the LiFePO4 cell under load. These two conditions must never occur and strict thresholds must be established when charging and discharging each LiFePO4 cell within the entire battery pack to prevent this.
Fortunately, modern day electronics technology has enabled the creation of inexpensive “smart” charging and monitoring systems that can effectively control how voltage and current are applied to or discharged from each individual cell inside the LiFePO4 battery pack.
Question: What is a Battery Management System (BMS)?
Answer: Battery Monitoring Systems (BMS) use specialized sensors to monitor the battery characteristics of each LiFePO4 cell during actual battery pack operation. BMS manufacturers can configure their systems to sense cell voltage, current, temperature, and other characteristics within the anode, cathode and electrolyte materials of each battery cell. The overall goal of a BMS design engineer is to optimize individual cell usage in parallel with many other cells to compose a much larger battery pack that discharges and recharges the total battery pack current as one unified system. If just one LiFePO4 cell exceeds its maximum or minimum threshold and becomes disabled, it can lower the performance of the entire battery pack.
Question: What is a Voltage Monitoring System (VMS)?
Answer: The most important indicator and easiest characteristic to monitor in any battery cell is how the cell voltage changes over time. Voltage Monitoring Systems (VMS) are inexpensive subsets of a larger, more comprehensive Battery Monitoring System (BMS) that can be attached to each individual LiFePO4 battery cell or multi-cell module.
Question: How are cell monitoring systems used to balance a Lithium-Iron-Phosphate (LiFePO4) battery pack consisting of many individual cells?
Answer: Lithium-Iron-Phosphate battery cells from LiFeBATT are constructed from proprietary materials developed by Phostech in Canada, a subsidiary of Sud-Chemie. Phostech developed this technology from patented research by Dr. John Goodenough, a physicist at (the) University of Texas- Austin. LiFeBATT refines the raw Phostech materials with its own proprietary processing technologies to create finished battery cells of high quality and consistent performance.
Though the high reliability of automated processing systems can produce consistent battery cell yields, each cell will exhibit slight variations in chemistry after production that are still within an acceptable range of performance. When combined into a high voltage battery pack with many cells, however, these small variances may affect overall battery performance during charge and discharge cycles that can include self-leakage, self-discharge, rush discharge (also known as pulse discharge), and temperature sensitivity.
Dynamic balancing of battery pack cells during charge/discharge operation by using a Battery Monitoring System (BMS) can help all the cells in the pack perform together in harmony. Because of the speed of microelectronic switching circuits, the voltage levels and other information generated by the sensors can be interpreted by analog comparator circuits or by digital microprocessors according to a set of rules or constraints. The analog comparator circuits or digital microprocessors then activate control circuits that limit the amount of voltage and current flowing in and out of each LiFePO4 cell within microseconds (millionths of a second)
Question: How does the Lithium-Iron-Phosphate (LiFePO4) battery cell charger work with a Battery Monitoring System (BMS) or Voltage Monitoring System (VMS) to balance the battery pack?
Answer: Control of voltage and current flow to Lithium-Iron-Phosphate (LiFePO4) cells during a charge cycle can be achieved through passive dissipation (using passive electronic components like resistors, capacitors and fly-back transformers) or through active dissipation (using active electronic components such as analog operational amplifier comparator circuits or digital microprocessor circuits). Rather than one large charger providing voltage and current to the entire pack as with lead-acid (PbA) battery cell chemistry, a LiFeBATT LiFePO4 battery pack is divided into sub-modules that each have their own charger and Voltage Monitoring System (VMS). Each sub-module is charged from a constant voltage and constant current source until the Voltage Monitoring System (VMS) senses that the upper threshold voltage of a cell is being reached. At that point, the VMS signals the charger to start dissipating the amount of current applied to each cell, eventually stopping the applied current completely when the cell is fully charged. When all cells in the sub-module have reached their upper thresholds, the charger is automatically turned off without the need for the user to actively disconnect it.
When all chargers have been shut off and all cells in the pack are fully charged, the supervising Battery Monitoring System (BMS) will indicate the status of the overall pack to its user.
As the LiFePO4 battery pack is discharged, each cell is monitored as it approaches its lower threshold. If the cell is deeply discharging its current to the point that it is approaching its lower threshold, the VMS activates control circuits that will gradually dissipate current flow out of the cell. Like a valve on a water hose, current flow is automatically shut off from each cell at a point where there is just enough residual current capacity to preserve the LiFePO4 chemistry for the next charge cycle. The Battery Monitoring System (BMS) must monitor all modules, as well as the current draw at the battery pack load, in order to shut down the entire battery pack current flow when there is not enough capacity from the combined cells to continue providing sufficient power to the load
Question: How does the LiFeBATT Voltage Monitoring System (VMS) implement Low Voltage Protection (LVP) for its companion battery cell module when there is not enough capacity from the combined cells to continue providing sufficient power to a load?
Answer: Each LiFeBATT battery pack module is equipped with a Voltage Monitoring System (VMS) that sends out control signals to trigger user-defined shut down circuits when there is not enough capacity from the combined cells to continue providing sufficient power to the load. These signals are routed through an RS-232 connector found on each LiFeBATT battery module. The signals can drive an audible buzzer, light a warning LED and send a sustained digital logic level (voltage and current) to trigger peripheral control circuits such as relays, contactors, electronic controllers, etc. During systems integration, a design engineer must account for “worst case” scenarios for load requirements when integrating peripheral shut down circuits to protect the battery module from dropping below its maximum discharge threshold.
Question: What are the benefits of Lithium-Iron-Phosphate (LiFePO4) chemistry for temperature stability compared to other Li-family battery chemistries (LiCoO2, LiMn, etc.)?
Answer: Unlike the other Li-family cells, LiFePO4 does not have problems with thermal runaway under normal operating conditions that other Li-family batteries have exhibited in the past. The maximum measured temperature radiated from the cells during normal operation is 55'C. LiFeBATT cells can handle over 160'C without any loss of total capacity. At 180'C or ~450'F the cells release Lithium gases that are non-toxic in small doses. However, this worst case scenario would only occur under extreme conditions, say a major fire in an electrical system. As a safety design, pressure valves inside the individual cells would keep the cells from exploding. The internal Voltage Monitoring System (VMS) and casing construction would protect the cells from electrical and physical damage.
Question: How effective are Lithium-Iron-Phosphate battery packs in providing power to electric vehicles?
Answer: To understand a vehicle’s reaction to power, we must understand the vehicle characteristics. Weight, dimensions, motor/controller, wheel (rolling) resistance, wind resistance, etc… The most economical pack is one designed to handle all currents, levels of discharge (load demand) and regeneration, but also a design that stays within voltage tolerance, and exceeds power requirements for range by just enough to be cost-effective. Keeping the pack operating in shallow discharge/charge cycles not only allows for peak performance of the entire system, but it more than doubles the life expectancy of the battery pack. Selection of battery pack size depends on how much performance power you are expecting from an EV, and how far the EV is expected to travel each day.
As a benchmark, the LiFeBATT 120V / 30 Ah pack provides 3.6kWh of power. A moderate-sized Electric Vehicle (EV) configured with a 15kWh battery pack, operating at 300Wh per mile, could provide a range of about 50 miles before the need to recharge. As a very basic configuration for an EV, the battery pack should have at least 2 strings of 120 V / 40Ah modules in parallel (120V / 80 Ah) to provide a minimum of 10kWh of power. In terms of economies of scale, the 40Ah modules are the most cost-effective due to an internal configuration that allows one Voltage Monitoring System (VMS) to monitor all the cells in the entire module.
Regardless of the size of the pack, the battery module components will deliver at least 2000 cycles. After 2000 cycles, there is just a 15% loss in overall capacity that can continue to provide power to the EV for operation, resulting in longer term savings and return on investment compared to PbA batteries.
As far as variances within the modules of the battery pack, we can string together 12, 24, 36 and 48V modules in series at 40 Ah each for the same overall price, in order to achieve 120V total. Adding additional 12V modules onto the series strings can increase pack voltage up to the maximum input rating of the motor speed controller. Adding more voltage to the pack will reduce overall current requirements for the vehicle power train system but continue to provide about the same power over time .
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