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LFP (LiFePO4) Battery Cells

All Amvolt battery storage systems are assembled in the Czech Republic, modularly tailored from industrial prismatic cells of world-renown brands Winston and Elerix. These LFP (LiFePO4) cells are characterized by high safety and long service life, they are non-flammable and non-explosive. The usual lifetime of the cell is up to 8 thousand cycles, but even after that the cells can be used for many more years without any problems. In the event of damage to any of the cells in any Amvolt storage unit, the other cells can be reassembled and used or replaced – so the entire storage unit is not damaged, as is the case with sealed battery "black-boxes".

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Basic Information about LFP Batteries

High safety
LFP cells provide high safety due to low risk of spontaneous combustion
Chemical stability and long service life
LFP cells have a service life of more than 10 years and a high number of operating cycles
Proven and available technology
LFP cells have been widely used for more than 15 years
Cells have a large capacity
LFP offers a capacity of up to 1000 Ah 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
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Features of LFP Batteries

Thanks to their properties, battery cells based on the lithium iron phosphate (LFP, LiFePO4) chemistry 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 300–1000 charging cycles and their capacity begins to decline sharply, LFP 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, LFP 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.

LFP 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 10 C and the charging current is 2 C. A cell with a capacity of 100 Ah can therefore be discharged with a current of up to 1000 A and charged with a current of 200 A. A fully discharged cell can be charged in as little as 30 minutes and a wholly discharged can be in just 6 minutes.

LFP 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.

LFP 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 LFP 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/kg, for LFP batteries the standard is 80–100 Wh/kg. This technology is therefore not suitable for the aerospace industry and small electronic vehicles.

The safe voltage of standard battery systems is 48 V (i.e. 16 cells). The solution is also suitable for systems up to 100 V. There are also industrial systems with a voltage of 750 V (which requires its own electronics for high voltage), which, however, require the use of special components.

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LFP 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 LFP 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.

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 protection to keep the battery warm during cold winter days and nights. More information is available on the following links on the GWL blog:

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.

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LiFePO4 Battery Life

The technical service life of LFP batteries is up to 30 years, provided that the battery is regularly charged and discharged with small currents up to 1 C in a mode of one full cycle in one day, i.e. 8000 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 of three full cycles a day, 8000 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 presented in the number of cycles on the datasheets is only a theoretical indication.

The standard service life of large prismatic LFP batteries is 8000 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 six years. LFP 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 ageing 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.

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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. 3 C 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 10 C), 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 10 C (generally up to 10 °C, batteries can be discharged with a load up to 10 C without serious problems), at 40 °C then the peak load up to 10 C can be operated only in fractions of minutes. When the temperature rises to 50 °C, it can only be discharged by 1 C. 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 (10 C), during normal operation (up to 1 C) 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. through its Amvolt division 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. An integral part of the design is wiring diagrams and the configuration of the control system.

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 10 C are intended only for the start-up of technological equipment and/or for starting motor machines, which should not take more than tens of seconds.

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Additional Information

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Inverters in Amvolt Battery Containers

The Amvolt HV 750/200 inverters (AC mains – DC battery) manufactured in the Czech Republic are one of the main advantages of the Amvolt high-voltage container solutions. Thanks to the inverter custom developed for Amvolt, our Battery Containers do not need a transformer or DC/DC converter, which increases reliability and reduces losses.

The control processor electronics of the inverter ensure the quality of the required parameters for connection to the distribution network. The inverter is phase symmetrical and fully reversible, i.e. it can charge and discharge the battery at full power. The transition from charging to discharging mode is a matter of tens to hundreds of milliseconds. It is suitable for almost unlimited parallel wiring and thus unlimited power scaling.

The inverters are hydraulically cooled, reducing the thermal management requirements of the BESS internal environment. Waste heat is immediately removed from the battery compartment and radiated to the surrounding environment. With a low-temperature gradient of only 2 °C, the inverter can be easily cooled passively even in the summer months. The inverter is controlled by an internal PLC unit based on standardized WAGO modules.

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Low-Voltage Inverters for Amvolt Battery Packs

Amvolt Low-Voltage Battery Packs (48 V) are compatible with all inverters that support a system voltage of 48 V or a range of 46–58 V. In Europe, the most widely used are the Victron Multiplus I/II, Steca/Studer Xtender and many others. The inverters draw direct current (DC) from a 48 V battery and convert it into a high-quality sine wave of alternating current (AC) and 230 V voltage as in a normal socket.

The inverters communicate with Amvolt Battery Packs using three "dry", i.e. potential-free, binary status outputs. If the battery condition is normal, all three contacts are closed (3x NC). A fully charged state (100 %) is indicated by the battery reaching 56.5 V. The connected inverter or MPPT controller must support the function to stop charging when this voltage is reached.

Full discharge is signalled by the Amvolt Battery Pack when the voltage drops to 48 V or when the adjustable % SoC is reached (most often 20 %). In this case, the first signal contact is open. Therefore, the inverter shall support a discharge termination function based on the voltage dropping below a set limit and/or by opening a dedicated binary contact. The remaining two battery signal contacts are reserved for a fault (unbalance, undercharge, overcharge...) and can control an emergency contactor, relay, GSM alarm or anything else.

But which inverters do we really recommend for Amvolt Battery Packs?

Definitely the Victron Energy inverters. This Dutch family-owned company originally focused on the production and development of inverters for the harsh environment of maritime transport. It applies the same uncompromising quality to home installations, which is why it is our choice and recommendation. The development, final assembly and recovery are done in Europe, making the inverters highly serviceable. Victron inverters are always single-phase, so for a three-phase system, three units are required.

Key advantages of Victron Energy inverters:

The inverters are easy to service – thanks to the fact that the development, final assembly and recovery are done in Europe.

Unlike high voltage inverters (GoodWe, Solax...) they contain fewer components.

They operate at low frequency and low voltage.

The key component is a massive toroidal transformer, which allows the inverter to be used for starting more powerful rotating machines, such as a pump.

Among practitioners, they have a reputation as an "indestructible tank".

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