Principle of operation
Rechargeable batteries (batteries) are used in the UPS to accumulate and use electrical energy to maintain equipment operation in the event of a power failure from the main power source. Accumulators under the trade mark “Sail Electro” are maintenance-free lead-acid sealed with valve regulation and mainly use AGM technology (electrolyte bound in a fiberglass mat with additional separators) in order to ensure battery safety during vibration, shock loads and in any position except inverted.
The principle of operation of lead-acid batteries is based on an ongoing electrochemical reaction between the plates of lead (Pb) and its dioxide (PbO2) through a glass fabric soaked in a solution of sulfuric acid (H2SO4).
Fig. 1 – diagram of charge (a) and discharge (b) AGM battery, where:
- fiberglass cloth absorbed with electrolyte
- lead plate anode
- lead plate cathode
- current direction
- positive and negative conclusion
During battery charging (Fig. 1a), lead sulfate under the influence of a large number of reactions causes lead sulfate (PbSO4) to decompose and distilled water (H2O) is converted into electrolyte (H2SO4), which results in the accumulation of free electrons inside the electrolyte absorbed in the glass fiber matter.
When the battery is discharged (Fig. 1b), free electrons are concentrated on the cathode plates of the battery when lead dioxide (PbO2) is converted to lead sulfate (PbSO4) and electrolyte to water. Free electrons through an external electrical connection are sent to the anode, thereby creating an electric current.
The period of charge-discharge of the battery is called the cycle. With each cycle the battery wear out. The durability of the battery is estimated by the number of cycles and depends on the resource embedded in its electrochemical system and design, as well as the conditions of installation and operation.
In the manufacture of batteries according to AGM technology, a separator consisting of glass fibers of various thickness from 0.25 μm to 3 μm fits tightly between the plates to ensure the contact of the plates with the electrolyte. A system of pores with a diameter of 1 to 10 microns is formed between the fibers, in which the liquid electrolyte is retained by capillary force like a sponge.
In gel batteries, the electrolyte is in a bound state due to the use of a substance that contains dispersion of silicic acid particles with a large surface (200 m 2 / g). After the battery is filled, it thickens to a jelly-like state with the formation of a system of pores with a diameter of
With the use of absorbed and gel electrolyte, it is possible to obtain a sealed battery that can work in any position except the inverted one.
Under the conditions of operation, lead sulphate formed on the plates as a result of the discharge of the battery discharges without problems when charged. However, if you leave a discharged battery for a long time, lead sulphate forms an insoluble crystalline form with increased electrical resistance (this process is called sulphation or sulphation). Thus, the interaction area (electrolyte-impregnated glass fiber material with plates) decreases and the battery becomes unusable. For this reason, batteries must always be stored in a charged state. Maintaining charge in the battery can be problematic due to the so-called self-discharge effect. With this effect, the battery is discharged without affecting the external load. Storage temperature has a direct impact on this process (Fig. 2). The following table shows the dependence of the time between the full charge of the battery and the storage temperature to maintain the required minimum charge level:
Fig. 2 is a graph of loss in capacity of an AGM battery over time and storage temperature.
The nominal capacity of the battery is the capacity guaranteed by the manufacturer under the specified discharge conditions. Charging capacity is the amount of electricity that a battery gives when it charges. The charging capacity is approximately 10-12% more than the discharge capacity due to irreversible processes occurring during charge and discharge. After the end of long-term storage of batteries, their capacity may fall below the nominal. Full capacity can be restored with a few cycles "charge discharge".
Temperature also affects the performance of lead-acid batteries (Fig. 3). Room temperature of 20-23 ° C is optimal for their operation. A higher temperature increases the intensity of corrosion of the plates, which reduces the battery life (every 10 degrees shorten the service life by 2 times, but at the same time increase the capacity). At low temperature, the electrolyte is cooled in the battery, which leads to a decrease in the rate of flow of electrochemical processes and, accordingly, a decrease in the battery capacity (approximately 1% of the capacity with a decrease in temperature from the norm by 1 ° C).
Fig. 3 is a graph of AGM battery capacity versus operating temperature, where CA is the nominal capacity from which the charging current xА (x is a coefficient) is calculated
When the lead-acid battery overcharges with an increased current after the end of the process of lead sulfate decomposition, electrolysis of water begins to occur. At the same time, hydrogen and oxygen acquire a gaseous state and the so-called “boiling” process takes place. During this process, water is converted to steam and the electrolyte density increases, which can lead to deformation of the battery case and its failure. The voltage level of 2.4V per element at which the process of electrolysis of water begins is called the “gassing voltage”. In order to prevent such destructive processes in AGM batteries are used:
- automatic voltage regulation chargers that reduce the charging current when approaching full battery charge
- limiting the value of the maximum charge voltage at 2.35 V per cell
- thickened electrolyte enclosed between plates in a glass fiber material that has a significantly lower gas generation rate at boiling
- sealed plastic case with an adjustable valve that holds the formed gas and guarantees its recombination (in case of formation of excess pressure of hydrogen, the automatic valve releases excess gas)
Due to the above-described properties, water consumption from the electrolyte is very limited, therefore, during the normal service life of the battery, electrolyte viscosity measurements and addition of water are not required. This ensures ease of operation, as well as the ability to install a UPS with such batteries in the room where people work or live. It is safety of use that makes batteries built using AGM technology or gel an excellent choice for use at home and in the office.
Battery life is one of the main indicators and is characterized by the number of charge-discharge cycles. The number of cycles during operation primarily depends on how deep the battery is subjected to a discharge, on the operating temperature, on the voltage and charge current.
The permissible battery charge voltage is characterized by the value: “charge limit voltage” is the number of volts per cell that can safely charge the battery to maximize its service life. When charging with high current and short duration of operation in the discharge mode, a lower value of the charge voltage limit is allowed, while with a low current charge and continuous discharge, a higher value of the charge voltage limit is required.
You must ensure that the proper voltage is set in accordance with the recommendations of the manufacturer of the battery. Too high a voltage enhances the corrosion of the positive plates and shortens the life of the battery. Too low a voltage leads to sulphation of the plates, which, in turn, causes a decrease in capacity and, ultimately, reduces battery life.
As it was said earlier at normal operating parameters, namely: at 20
30 ℃ voltage compensation for operating temperatures is not required. However, in order to maximize the life of the battery, temperature compensation should be taken into account for operating temperatures outside this range.
Temperature compensation of the battery charging voltage:
- When operating in buffer mode: Vt = V-0.003 (t-25)
- When working in charge-discharge mode: Vt = V-0.005 (t-25) (V is the voltage used for the charge at 25, t is the temperature, Vt is the charge voltage at t)
The battery life used in the charge-discharge mode (characterized by disconnecting the charger after the end of the charge) largely depends on the depth of the discharge to which it is subjected during each cycle. The ratio of the different number of cycles to the depth of the discharge is shown in Fig. four.
Fig. 4 – the ratio of the number of cycles and capacity depending on the depth of discharge.
The period of operation in the buffer charge mode (when the charge process does not stop, constantly maintaining the battery charge level close to full) largely depends on the temperature at which it is carried out. The period of operation in the buffer mode will be very long at low temperature charge (10
20 ℃), however, at high temperatures, battery life is shortened.
Fig. 5 – The ratio of the period of operation to temperature
The classification of the EUROBAT association, which unites the leading battery manufacturers, implies the following categories for the estimated service life:
- service life 3-5 years – Standart Commercial (standard commercial)
- service life 6-9 years – General Purpose (general purpose)
- service life of 10-12 years – High Performance (high performance)
- service life 12 years and above – Long Life (long service life)
General purpose UPSs use standard commercial batteries. In this case, the design of the UPS often provides for their own replacement by the user with the observance of precautions when working with batteries.
In order to avoid performance degradation when replacing, identical batteries of the appropriate type and capacity should be used. Since during operation, the technical characteristics of batteries, in particular the internal resistance, vary significantly, it is not allowed to use old batteries with new ones in the same group. It is not recommended to connect more than 4 groups of batteries in parallel, because of the variation in characteristics, charging currents vary significantly, which leads to a reduction in battery life.
Calculation of battery life from batteries
The most important characteristic of battery for UPS is the battery life that they can provide as part of an uninterruptible power supply. To perform such a calculation, it is necessary to accurately understand the operating conditions of the UPS.
Battery life depends on many factors, but the main ones are:
- C is the total capacity of the battery, which is expressed in [А * hour] and is the time-to-time characteristic (depending on the number of batteries in the discharge current circuit, the temperature during the discharge, the charge voltage limit, the conditions of operation and storage)
- V is the voltage of the connected battery in volts
- η – inverter efficiency, coefficient depending on conversion efficiency for technologies used in the UPS
- P is the average power of equipment connected to the UPS in watts
- T – operating time in hours, characterizes the time interval for maintaining the power supply of the UPS using the battery until full discharge
To simplify the calculation of battery life, use the following formula:
T [hour] = C [A * hour] * V [V] * η / P [W],
Let us analyze the calculation method in more detail:
1. The calculation of the average power
First, the average active power of the equipment connected to the UPS (P) in watts is determined (denoted by W – watt, and not VA – volt-ampere). We need to know exactly the average (during battery life) consumption. It may differ significantly from the maximum or rated power indicated in the equipment descriptions.
Let us consider an example: the nominal power of a computer power supply unit is indicated 500 W – this is the maximum possible power it can produce. Actual consumption will depend on the installed equipment and may be about 150 watts.
Another example: an electric heater connected to the UPS operates on an element with an electrical power of 350 W, however, this heater is turned on every 30 minutes, and its operation lasts 5 minutes. In this case, the average consumption will be equal to:
350 W * 5 minutes / 30 minutes = 58.3 W
2. The calculation of the total capacity of the battery (battery) UPS
Standardly, the UPS battery monoblock consists of several identical sealed sections. As a rule, batteries are used with a nominal voltage of 12 volts, inside consisting of 6 sections of 2 volts each. In the UPS can be used from one to forty of these batteries as part of one group (chains of series-connected batteries). Groups can be connected in parallel to increase the total capacity.
It is necessary to find the total capacity of the battery. For this purpose, we multiply the total number of battery groups by the capacity of one group (only batteries of one capacity should be used).
Let us consider an example: the UPS has a built-in battery group consisting of a series of 3 sealed lead acid batteries with a capacity of 9 A * hour (at 10-hour discharge) and a voltage of 12 volts. In addition, one external battery pack (BB) with six of the same batteries is connected to the UPS. Then the total voltage on the batteries is 3 * 12 = 36 V, and the total capacity of the batteries will be equal to:
C10 = 9 А * hour * 3 parallel groups = 27 А * hour
3. Calculation of battery operation time
Now we are ready to calculate the operating time of the UPS from batteries:
T [hour] = C [A * hour] * V [V] * η / P [W],
where: C is the total capacity of the batteries, which we calculated earlier in ampere-hours; V – battery voltage in volts; η – UPS inverter efficiency (if it is not known, it is recommended to use the average value
Continuing the examples discussed earlier (a computer that consumes 150 W of power is powered by a UPS with three built-in 12-volt batteries with a capacity of 9 A * hour and an external external battery pack of six 9 A * hour batteries in two groups) we have:
T = 27 A * hour * 36 V *
As can be seen from the formula, the operating time of the UPS from the battery is not directly dependent on the power of the UPS. Thus, in order to increase the operating time of the UPS from the battery, it is necessary, instead of choosing a higher power UPS, to stop the choice on a UPS that has a larger total battery capacity. Or increase the total capacity by connecting external battery packs.
4. Bit Characteristic
The above formula is accurate enough for a long time of operation of the UPS from the battery (more than 8-10 hours). For short discharge times (high discharge currents), the battery is able to give only part of the capacity. Exactly this value is reflected in the battery specifications, and approximately shown on the graph (
Fig. 6 – Battery capacity from discharge time.
Thus, a more accurate calculation of battery life can be made using the UPS inverter efficiency at a given power and discharge curves for a particular type of battery.
You can briefly summarize basic battery information for UPS:
- Batteries should always be stored charged at room temperature with occasional recharging
- The battery capacity depends on the temperature: the higher the temperature, the higher, but when exceeding 20 ° C, the battery life begins to decrease
- AGM and gel batteries do not require additional maintenance, and due to the recombination of gas in a sealed enclosure, they are safe for use in places of work and residence of people while observing the operating rules
- Battery life depends on:
- battery discharge depth (the greater the percentage of remaining charge, the better)
- operating temperatures (every 10 ° С above 20 ° С shorten the service life by 2 times)
- voltage and charge current (too high increases corrosion, too low results in sulfation)
- When calculating the operating time of the UPS from the battery, it is necessary to take into account the discharge curves (dependence of the output power on the discharge time).