Whitepaper written by ComAp a.s. in cooperation with GWL a.s.
https://www.comap-control.com/
https://www.gwl.eu/

1. Reasons for Using Batteries in Hybrid Microgrids

The installation of a battery system is not critical for microgrids that are connected to the electricity network. The ComAp control system automatically starts and synchronizes backup motor generators, so in case of an impending mains failure and subsequent limited power supply, a backup power supply would be switched on as soon as possible, without the power outage. However, in parallel applications, backup batteries can also play a role of uninterruptable power supply (UPS), where they feed the load until the motor generator starts (can be used in critical applications, such as hospitals, data centers, etc.).

For island systems (so-called off-grid) with renewable resources, where diesel fuel consumption reduction is a priority, the usage of batteries is crucial. A high-capacity battery (not only in the role of UPS) is then available to cover power peaks and to eliminate the need for dynamic spinning reserve. Thanks to that, the load of the motor generator remains constant, thus extending its life and reducing fuel consumption.

The above mentioned can be applied only on condition that the battery is dimensioned correctly, and a suitable technology is chosen which can reduce diesel consumption by limiting the operation of diesel engine generators. During the project design an economic return on investment has to be taken into account though since the battery storage is still relatively expensive.

 
2. Overview of Technologies Currently Available in Power Generation Applications

• Li-ion (lithium-ion) batteries are currently the most widely used technology for small home appliances, mobile phones, and other consumer electronics.

• NMC cells are based on nickel-manganese-cobalt (a new technology of the Li-ion category, which was created by enriching the cathode and anode)

• NCA batteries based on nickel-cobalt-aluminium are an improved version of LCO (lithium-cobalt) and LNO (lithium-nickel) batteries.

Due to their properties, NCA batteries are used primarily in cars, power generation applications are often their secondary use as second-hand battery packs from cars.

• LMO (lithium-manganese batteries) are used in electromobility and power generation applications due to their properties.

• LTO (lithium-titanium anode (carbon replacement), cathode NMC or LMO) has good parameters for use in electromobility (used by Honda) and in power generation applications.

• LiFePO4 / LFP (lithium-iron-phosphate) is a type of lithium-ion battery with LiFePO4 cathode and carbon anode (graphite), which has very advantageous properties for use not only in electromobility, but also in power generation applications.

Conclusion of Technical Properties of the LiFePO4 (LFP) batteries:

LFP batteries are currently the most suitable for use in microgrids for the following reasons:
1) High safety: LFP cells provide high safety due to low risk of spontaneous combustion
 
2) Chemical stability and long service life: LFP cells have a service life of more than 10 years and a high number of operating cycles
 
3) Proven and available technology: LFP cells have been widely used for more than 15 years
 
4) Cells have a large capacity: LFP offers a capacity of up to 1000Ah per cell, which greatly simplifies installation and increases the reliability and robustness of the solution unlike others, where it is necessary to use up to thousands of small cells to achieve higher capacities
3. Features of LiFePO4 Batteries

Thanks to their properties, battery cells based on the LiFePO4 principle have quickly spread to almost all areas of industry and have become commonly available on the market. While other types of lithium batteries can easily ignite or even explode if short-circuited or overcharged, with LiFePO4 this reaction is almost ruled out. Conventional lithium polymer (Li-Pol) or lithium ion (Li-Ion) batteries chemically degrade after approximately 300-1,000 charging cycles and its capacity begins to decline sharply, LiFePO4 cells still retain up to 80% of their original capacity after 10,000 cycles.

The biggest advantage over other types of lithium batteries (lithium ion or lithium polymer) is therefore high safety and an order of magnitude longer service life. Therefore, LiFePO4 cells are designed for demanding use in photovoltaics, electromobility and energy storage, where even with daily cycling they can reach a lifespan of 15 years or more.

LiFePO4 batteries have a very low internal resistance, so they can be charged and discharged with high currents without shortening their life or overheating. The maximum discharge current is 10C and the charging current is 2C. A cell with a capacity of 100Ah can therefore be discharged with a current of up to 1,000A and charged with a current of 200A. A fully discharged cell can be charged in as little as 30 minutes and a wholy discharged can be in just 6 minutes.

LiFePO4 batteries therefore have a high current carrying capacity and service life. They are safe and resistant to charging to nominal capacity or to prolonged high voltage or temperatures. They are non-toxic and do not contain corrosive substances.

LiFePO4 cells and batteries can be easily damaged by exceeding the charging (Vmax) and discharging (Vmin) voltage limits. The installation recommendations with simple protection electronics that monitor the voltage of the cells during operation apply.

The disadvantage of LiFePO4 batteries is the higher weight in terms of Wh stored energy. For Li-Ion and Li-Pol cells, the energy density reaches approximately 150-200 Wh/1 kg, for LiFePO4 batteries the standard is 80-100 Wh/1 kg. This technology is therefore not suitable for the aerospace industry and small cars.

The safe voltage of standard battery systems is 48V (i.e. 16 cells). The solution is also suitable for systems up to 100V.

Note: There are also industrial systems with a voltage of 750V (requires its own electronics for high voltage), which, however, require the use of special components.

4. LiFePO4 Battery: Basic Information on Recommended Operation

All batteries generate heat during charging and discharging. The higher the discharge current, the more heat is generated, and therefore it is necessary to prevent extreme discharge of the battery in the form of a short circuit (e.g. fuse). The BMS has a current sensor that monitors the maximum value of the discharge, respectfully the charging current. Although LiFePO4 batteries have a wide  operating temperature range (the theoretical limit is from -45°C to +85°C, while practically the range is rather -20°C to +60°C), to achieve full cell performance, it is best to operate the battery in a narrower temperature range in such limits required by the application.
 
a. Low Temperatures:
 
In cold climates it is recommended to keep the battery temperature above+5°C, in locations with extremely low temperatures it is possible to use thermal and alternating current thermal insulation pads, which provide sufficient heat energy to keep the battery warm during cold winter days and nights. More information is available on the following links on the GWL Power blog:
 
http://gwl-power.tumblr.com/post/77599670684/faq-low-temperature-operation-what-is-the
http://gwl-power.tumblr.com/post/77627582207/simple-solutios-to-keep-your-battery-warm-in
 
b. High Temperatures:
 
All types of generic lithium cells have a maximum operating limit of around 60 C. However, it is generally strongly recommended to keep the temperature below 50°C. At temperatures above 40°C, charge and discharge currents should be reduced to prevent further heating of the cells inside the battery. However, batteries are prone to very high temperatures, if the temperature inside the battery rises above 80°C degrees, the grid starts to melt and damage will occur. The maximum recommended battery temperature is therefore 40 - 45°C.
 
http://gwl-power.tumblr.com/post/142719866121/faq-can-the-lithium-technology-be-used-at-85-c

5. Battery Life

The technical service life of LiFePO4 batteries is up to 30 years, provided that the battery is regularly charged and discharged with small currents up to 1C in a mode of 1 full cycle in 1 day, i.e. 8,000 cycles = 22 years of operation. A full cycle here means discharging up to 70% DOD.

In real operation, the service life is usually lower, about 10 years, due to greater battery wear, due to faster cycling. For example, in a mode 3 full cycles/a day, 8,000 cycles correspond to approximately 7.4 years of operation. Since the actual service life depends on the mode of operation, operating conditions and handling, the battery life given in the number of cycles on the datasheets is only a theoretical indication.

The standard service life of large prismatic LiFePo4 batteries is 8,000 cycles. The capacity does not change even if the battery is not used for some time, because self-discharge and internal degradation are very slow with this type of cell. A fully charged battery will self-discharge in approximately 6 years. LiFePO4 batteries have no memory effect. They do not require reformatting, nor do they need to top up the electrolyte as  other types of batteries. They do not require any operational maintenance per se.

Nowadays, batteries are at their design limits and other requirements for longer life, less weight, smaller dimensions, impact resistance and so forth always require the use of compromise solutions and the more advantageous parameters obtained are often negatively reflected in faster aging or increasing its production cost. Reports of revolutionary batteries are always bent. Either the battery will charge quickly in minutes, but then its life will be relatively short, or the battery life will be long, but then it cannot be charged very quickly. The natural legality of "either/or“ applies.

Prismatic batteries fully meet the requirement for the longest possible service life. These batteries are basically indestructible and are therefore very suitable for applications in the power generation segment.

Note on building packs from multiple separate cells:

It is a common phenomenon that one to two cells within tens or hundreds of pieces in one pack do not qualitatively correspond to others from the same battery pack, which manifests itself after installation (1-6 months). Although these cells are within the production tolerance, the operating conditions of the entire pack should be unified, therefore it is good to identify and replace these cells. However, such a cell is usually not defective, it only shows relatively worse parameters compared to others.

6. Battery Management System (BMS)

Battery limit values (temperature, voltage, charging and discharging currents and so on) are monitored by an electronic control system (Battery Management System), which itself does not have other control functions (for example to control the battery charger). Therefore, it is necessary to control the charging and discharging parameters from the superior control system. The BMS often functions primarily as a monitoring tool and communicates with the control system via CAN or Modbus protocol.

The control system should also provide a protection function and thus a safe disconnection of the batteries. Although the BMS can disconnect the batteries once with a contactor, this can lead to the burning of power transistors, which should not be switched off under load. More information on control systems for hybrid battery microgrids is available here.

Each battery cell differs in capacity and internal resistance. If there are frequent discharges and charges, then the cells, which have a greater internal resistance, begin to deviate from others (it can occur after as little as 100-200 cycles) and reach the voltage limit values in a few months. The deviation of the cells is solved by the so-called balancing and recharging of all cells to full capacity, i.e. to 100% SOC.

For industrial applications with large capacity batteries, a continuous cell/battery check should be performed due to the high investment. Installations that are connected to a control system with remote management can be monitored continuously, but for autonomous systems (with limited communication) it is necessary to check up to twice a year (e.g. with a infrared camera during discharging/charging to verify whether some cell  does not heat up more than another).

For guaranteeing battery life, it is highly recommended to have detailed information on the status of operating variables. A BMS with a superior control system should therefore be a clear and only choice for all industrial installations.

7. Uninterruptible Power Supply (UPS)

Uninterruptible Power Supply (UPS) are backup power supplies, the function of which is usually a short-term (in the order of minutes to hours) power supply in case of an outage, network voltage instability or unforeseen network events such as noise, shock, voltage spikes, voltage dips, etc. UPS is not only protecting data and sensitive equipment from damage. It is also used in healthcare, transport, or for military purposes, where a power outage can mean a threat to health and life or a significant material loss.

In hybrid systems (PV-Diesel) with high renewable energy penetration, UPSs can significantly reduce diesel consumption and extend the life of motor generators. In this case, the UPS can serve as a load reserve, so it is not necessary to run an unloaded motor generator and this leads to great savings in diesel and emissions. The UPS will also serve very well in the event of a sudden drop in power from the PV plant due to clouds or a jump in load when the PV plant is not able to cover the load -  UPS will cover power peaks without having to start the motor generator, thus extending its service intervals and life.

Uninterruptible Power Supplies can be divided into groups according to the technology they use.

• VFD  (Voltage and Frequency Dependent): "Off-line", which switches the load to a backup converter powered by batteries using a relay. There is a short-term (approximately 5ms) power supply cut
• VI (Voltage Independent and Voltage-independent Sources): "Line-interactive" via the control transformer compensate for extraordinary anomalies of the electrical network
• VFI (Voltage and Frequency Independent): "On-line" supplies the appliances continuously via the inverter from batteries that are simultaneously charged from the mains.The UPS stabilizes and filters the voltage. In the event of a power failure or drop, the batteries supply power without any interruption

8. Suitable Capacity and Battery Type Design

As mentioned above, battery life is given in a number of cycles. LFP batteries do not mind regular lighter discharges in smaller cycles. However, if the discharge mode is fast and full  - i.e. more than 70% DOD (e.g. 3C continuously) -  the battery should then be charged more slowly to allow time to cool down.

If there is often a short-term peak discharge with large currents (up to 10C), it is necessary to take measures that dissipate the generated heat, such as so-called blowers, metal cooling belts, or fans that dissipate excess heat that otherwise tends to accumulate in the mass of the cell. Another way to reduce the temperature rise is to increase the space between the individual cells. The cells then do not heat each other up and the heat can be better dissipated.

If the batteries have a temperature of up to 20°C, they have a discharge peak up to 10C (generally up to 10°C can be discharged up to 10C without serious problems), at 40°C then the peak load up to 10C can be operated only in fractions of minutes. When the temperature rises to 50°C, it can only be discharged by 1C. The battery must have the ability to cool down. Temperature is the limitation of the maximum current consumption. Cooling must be reflected only during maximum operation (10C), during normal operation (up to 1C) the batteries do not need cooling.

Precisely for the above reasons, it is very important not only to select the appropriate type, but also to design the appropriate capacity of the battery with regards to the size of the required power supplied from the battery whether peak or long-term performance is needed.

GWL a.s. in cooperation with ComAp, can design a suitable battery size for the customer for the given project and the particular location (important due to ambient temperature), supply battery cells including BMS, and specify the requirements for the selection of suitable inverters. Integral part of the design are wiring diagrams, and configuration of the control system.

An Example of a Battery Capacity Calculation
The capacity is typically given in Ah (ampere-hours) and to convert it to Wh (watt-hours) the capacity value must be multiplied by the battery voltage. The reason for indicating the value of the capacity in Ah is that the battery does not work only at the nominal voltage, e.g. 12V, but works in a certain range, e.g. 10.5V - 14.4V.
From a 1,000Ah battery with a voltage of 48V, 1,000A can be pumped continuously for an hour, and the total energy supplied per hour will therefore be 48 kWh.
 
 Top Performance
At peak power, the amount of power supplied depends on the C-rate (current height) and the time for which the battery is to supply power.

Example: If we draw 3C current from a 48V/1000 Ah battery, then the battery will deliver 144 kW for less than 20 minutes. The same battery can deliver a peak power of 10C (480 kW), but only for less than 6 minutes. In both cases, however, the energy supplied is the same, i.e. 48 kWh.
Note: since any losses increase with the square of the current, it is advisable to avoid long-term peak currents. In real use, the peak load currents up to 10C is intended only for the start-up of technological equipment and/or for starting motor machines, that should not take more than tens of seconds.

Detailed Specification
Technical specification of the battery, and in particular the UPS, should be selected according to the required peak performance and the time for which the battery must be able to supply it. At the same time, the medium (average) performance and the time for which the battery is supposed to  deliver it, should be determined.

9. Additional Information - Battery Charging and Discharging

The battery voltage is not an accurate indicator of the state of charge, because the voltage value remains  constant for a long time, however the actual state of the battery is derived from  number of cycles and as such, it always differs from the calculated state (voltage), e.g. due to losses. In addition, the number of cycles during which there are differences is directly proportional to the way the battery operates. For scenarios with drastic discharge with large currents (3C and more) the difference can occur after tens of cycles and with slight discharge (up to 1C) after hundreds of cycles (e.g. the discharge cycle from SOC 90% to 10% corresponds to 80% DOD, i.e. with current up to 0.5C, a service life of 8,000 cycles can be expected. The battery service life can be extended if it is operated in a cycle from 85% to 15% SOC, i.e. 70 % DOD).

SOC (State of Charge): charge level, the value of available energy in the battery. 100% SOC is a fully charged battery, 0% SOC is a discharged battery.

DOD (Depth of Discharge): discharge depth, e.g. DOD 70% means that a maximum of 70% of the rated energy is taken from the battery in one cycle.

Recycling
Due to the service life of 10 or more years, recycling of LiFePo batteries currently only occurs if the customer accidentally destroys the battery. If the battery must be recycled, the currently most commonly used recycling method is crushing in selected electrical waste lines. Lithium recovery is expensive and not worth it. Due to the fact that LiFePO4 is not toxic, no hazardous waste is generated (unlike cells with lead, cadmium or cobalt).

New Battery Technologies
Despite frequent reports of new battery system technologies, these technologies cannot be considered suitable for widespread use, as they are almost always laboratory experiments only; the development path from technology to product takes 8 years on an average. Smaller batteries have a higher energy density, but this advantage is decompensated by fragility and easy overheating. With LiFePo4 technology, GWL has very good experience from many practical applications installed over the last 14 years throughout Western Europe, but also in many other places outside Europe.

Additional Links And Other Useful Information And Test Results of LiFePO4 Batteries:

LiFePO4 information regarding the number of cycles:

FAQ: What is the real cycle life for lithium LiFePO4 cells?
http://gwl-power.tumblr.com/post/97128890296/faq-what-is-the-real-cycle-life-for-lithium
 
FAQ: Number of Cycles versus Depth of Discharge for LiFePO4 cells
http://gwl-power.tumblr.com/post/104324572271/faq-number-of-cycles-versus-depth-of-discharge

FAQ: Depth of discharge (DOD)
http://gwl-power.tumblr.com/post/26696964746/depth-of-discharge-dod-all-battery
 
 FAQ: LiFePO4 cycle-life based on DOD
http://gwl-power.tumblr.com/post/130701906811/faq-lifepo4-cycle-life-based-on-dod-the-graph

Information on the difference between LFP and Lead-Acid Batteries (according to DOD): 

FAQ: Cycle life of LiFePO4 versus Lead-Acid
http://gwl-power.tumblr.com/post/97417079781/faq-cycle-life-of-lifepo4-versus-lead-acid
 
FAQ: The Cycle life of Winston Batttery cells - versus lead acid
http://gwl-power.tumblr.com/post/140925159571/the-cycle-life-of-winston-batttery-cells-versus

Information on temperatures and functionality of LFP cells: • SOC vs Temp
http://gwl-power.tumblr.com/post/24765132732/soc-vs-temp-the-state-of-charge-soc-versus
 
Cold temperature tests of SE100AHA and CA100AHA
http://gwl-power.tumblr.com/post/41026012669/cold-temperature-tests-of-se100aha-and-ca100aha
 
Low temperature operation
http://gwl-power.tumblr.com/post/77599670684/faq-low-temperature-operation-what-is-the
 
http://gwl-power.tumblr.com/post/77627582207/simple-solutios-to-keep-your-battery-warm-in
 
High temperature
http://gwl-power.tumblr.com/post/142719866121/faq-can-the-lithium-technology-be-used-at-85-c
 
http://gwl-power.tumblr.com/tagged/temperature

Test Information: Graphs:

Winston Battery tests: 1C at different temperatures
http://gwl-power.tumblr.com/post/140959269546/winston-battery-tests-1c-at-different
 
Winston Battery LiFeYPO4 discharge characteristics
http://gwl-power.tumblr.com/post/98740855201/winston-battery-lifeypo4-discharge
 
High Current Charging and Discharging Report for Lithium (LiFeYPO4) Cells
http://gwl-power.tumblr.com/post/17216754627/high-current-charging-and-discharging-report-for
 
Discharge of 90AH and 100AH batteries - Test Report at 102A (1C)
http://gwl-power.tumblr.com/post/12654029932/discharge-of-90ah-and-100ah-batteries-test
 
Discharge of 90AH and 100AH batteries - Test Report at 220A (2.2C)
http://gwl-power.tumblr.com/post/13362427399/discharge-of-90ah-and-100ah-batteries-test
 
Test report: 4 pcs of LFP200AH cells discharged at 280A (1.4C)
http://gwl-power.tumblr.com/post/2583572591/test-report-4-pcs-of-lfp200ah-cells-discharged-at
 
TS-LFP160AH TEST REPORT
http://gwl-power.tumblr.com/post/226077441/ts-lfp160ah-test-report-please-download-an
 
TS-LFP400AH TEST REPORT
http://gwl-power.tumblr.com/post/226076250/ts-lfp400ah-test-report-please-download-an

Test and measurement results:

http://gwl-power.tumblr.com/tagged/testdata

Links to specifications and parameters of articles of various technologies (LFP, LTO):

http://gwl-power.tumblr.com/tagged/spec