Calculation of battery life. Automatic selection of the UPS model

An uninterruptible power supply is a device designed to protect equipment connected to it from short-term and long-term power outages, as well as from unacceptable power surges in the network.

The battery life of an uninterruptible power supply is affected by many factors, the main of which are:

• load power connected to the UPS;
• the number and capacity of batteries connected to the UPS;
• design of the UPS.

By design, uninterruptible power supplies are mainly divided into two types: UPS with built-in batteries and UPS designed to work with external batteries.

UPSs with built-in batteries are not designed to provide long battery life. They are used to quickly and correctly shut down and shut down equipment (for example, personal computers). The battery life of such UPSs is, as a rule, no more than 5 - 15 minutes.

UPSs designed to work with external batteries can provide long battery life, as a large number of high-capacity batteries can be connected to them. Such UPSs are used, for example, to build uninterruptible power supply systems for private houses and cottages, and the battery life can be up to two or more days.

There are several ways to calculate battery life, among which the main ones can be distinguished.

Method 1. Calculation using a simplified formula (This method is averaged and gives an approximate result).

The simplified formula looks like this:

, where:

- Number of batteries;
- Constant load power, W.

For example, a 100 W incandescent bulb with one 75 ampere-hour battery will work continuously for 9 hours (75*12*1/100).

Method 2. Calculation of the battery life of the UPS according to the refined formula.

The refined formula looks like this:

Battery life, hour;
- Capacity of one accumulator, Ah;
- Voltage of one battery, V;
- The number of batteries in the group;
- Number of battery groups;
- UPS efficiency;
- The battery discharge depth coefficient is assumed to be 0.8 - 0.9 depending on the type and wear of the batteries;
- Coefficient depending on the temperature at which the batteries are operated (at a temperature of 25 °C it is assumed to be 1, at a temperature of 0 °C it is assumed to be 0.88);
- Coefficient depending on the battery discharge time. With a 10-hour discharge, it is taken equal to one. The dependence of this coefficient is shown in the graph below:

- Constant average load power, W. Here it is important to understand what constant average power loads. For example, if the load is a TV with a power consumption of 100 W, and if it is on 30% of the time, then the constant average power of such a load is assumed to be 30 W.

In the general case, if, for example, an uninterruptible power supply system for a private house is calculated, then the rated power of all loads is summed up, and average continuous power is taken equal to 20-25% of the resulting value.

Method 3. Asking for help in calculating the battery life from the specialists of our store.

If you do not want to deal with formulas and numbers for calculating battery life, you can always get qualified advice from our specialists.

Our experienced specialists will not only calculate your loads most accurately, but also correctly calculate the battery life based on many years of experience in building uninterruptible power supply systems.

For simplicity, we have made calculation calculators:

And now we present the calculation algorithm:

1) Determine the total load power and constant discharge current.

2) We calculate the required battery capacity for a given autonomy.

3) Determine the type of battery

Example

Given: two LED strips with a power of 10W and operating from 12V. Required autonomy: 10h. Service life: one year with daily use. Operating conditions: constant room temperature 20 degrees.

Find: minimum allowable and optimal accumulators for solving the problem.

Solution

1) Total power W=10W*2=20W. Constant discharge current: I=20/12=1.67A. For accurate calculations, it is desirable to measure the current consumption using a multimeter.

2) To determine the required capacity, go through the points:

a) In order to keep the load at such a discharge current, it is necessary to determine the minimum calculated battery capacity: 1.67 * 10 = 16.7Ah.

b) It must be borne in mind that the capacity of batteries is indicated by manufacturers based on a certain discharge time. Usually it is 10 hours. But some manufacturers indicate 20 hours. Here we will be helped by the battery, which can be taken on our website. Let's see the specification:

In our case, the battery life is 10 hours, which means we can consider the capacity equal to the nominal one. However, if the task costs 5 hours, then you need to make an allowance for the fact that with such a discharge time, the battery capacity will be lower (we multiply the discharge current by hours - 4.8A * 5h = 24Ah instead of 28).

In the task, we can see that the planned number of cycles we have is 365. The estimated maximum discharge depth in our case is about 57%. It is advisable to take it with a margin, we will count on a 50% discharge (actual operating conditions are different from ideal laboratory conditions).

Thus, we introduce an amendment of 0.5: 16.7 / 0.8 \u003d 33.4Ah.

G) In case we are dealing with an operating temperature different from the optimal one (25 degrees), it is necessary to enter a correction factor, which we can also take from the specification:

So at a temperature of 10 degrees, a coefficient of 0.9 should be entered, i.e. +10% more to the calculated capacity.

3) If we need long discharge modes, we should pay attention to the AGM series of batteries from manufacturers popular on the Russian market:

• Battery Delta - series
• At CSB -

To calculate the battery life of a UPS UPS, you can use the average data for the UPS of most manufacturers. For example, when the UPS load is 100% - the autonomy time is 4...8 minutes, 75% - 7...12 minutes, 50% - 12...20 minutes. Or special tables that indicate the battery life of the UPS uninterruptible power supply for various values ​​of power in the load and the capacity of the built-in rechargeable batteries. It is important to take into account that the battery life values ​​indicated by the manufacturer are estimates and are not the basis for the emergence of obligations of the supplier or claims of the buyer. It should be remembered that manufacturers of UPS uninterruptible power supplies indicate the values ​​​​of UPS power, battery capacity and battery life for operation at a temperature of 20 ... 25C. It is this temperature that is optimal for the operation of the UPS and batteries. But the real operating conditions of UPS uninterruptible power supplies are different from ideal ones.

Determining the exact autonomy time of a UPS is not an easy task, taking into account many parameters that are different for each calculation case. Simplified-approximately, the battery life of the UPS uninterruptible power supply when operating on battery power can be calculated by the formula:

T=E* U/ P(hour.)

E- storage battery capacity (Ah)

U- battery voltage (V)

P- UPS load power (W)

If the technical task of the buyer allows errors during the operation of the UPS uninterruptible power supply, then it is possible to calculate using this formula.

In the event of interruptions in the supply voltage to a critical load, it is required to ensure its autonomous operation. The use of UPS (uninterruptible power supplies) in the power supply circuit allows solving this problem. The battery life of a UPS is the main indicator when choosing such devices for a particular equipment. The battery life of the UPS depends on the load power and battery capacity. Responsible consumers include servers, heating boiler control circuits, complex laboratory equipment for cyclical studies, medical equipment for life support systems. For a more accurate calculation of the battery life of the UPS uninterruptible power supply when operating on batteries for responsible consumers, the calculation formula should take into account the efficiency of the inverter (usually this value is 0.75 ... 0.8), the number of batteries in the battery , the degree of battery wear, the depth of discharge of the battery (0.8 ... 0.9. Batteries reduce their capacity by up to 5% for each degree of temperature increase after 40C.), the coefficient of available battery capacity (it is determined from the ratio of capacity values ​​in battery discharge mode and ambient temperature), ambient temperature (when the ambient temperature rises above 25C, it is necessary to reduce the load power of the UPS by 20% for every 10C of temperature rise.).

When choosing an uninterruptible power supply, it is better to buy a UPS with additional features, for example, with the ability to connect a stabilizer, additional charging boards. This configuration of the UPS will save in the future when the load power increases.

The calculation of the individual configuration of the UPS uninterruptible power supply is best left to specialists.

It is an essential guarantee of the reliability of the power supply system. The UPS parameters must be strictly comparable with the load that will be connected to the UPS. Otherwise, the uninterruptible power supply will not bring the desired benefits, and the money will be spent in vain.

How to calculate an uninterruptible power supply? To do this, it is necessary to take into account a number of parameters, the key of which is power. If you buy a UPS that has less power than the load, then it simply will not work. To accurately calculate the power, you need to remember a little physics.

The load power factor or otherwise Power Factor is very important when calculating the power of an uninterruptible power supply. This figure shows what proportion of power the load actually consumes, that is, active power. If we consider the load as an ideal resistance, then in this case the value of the coefficient will be equal to one, which is the maximum value. Capacitors and coils are not power consumers, so for them the value of the coefficient is zero. In the equipment, the predominance of both capacitive and inductive components is possible.

Equipment with a capacitive component includes computers and servers. The inductive component is present in devices with electric motors, it can be a pump, an air conditioner, etc. This information is necessary when the UPS will protect equipment of various types, since the first power factor tends to unity, and the second is in the interval from 0.8 to 0.9. In such a case, it is necessary to find the average power factor in order to obtain an accurate result.

How to calculate the power of the UPS, knowing the power factor of the load? To calculate the power, multiply the rated power of the UPS by the power factor. As a result of the operation, a number is obtained that shows the maximum active power that the uninterruptible power supply can serve. For example, the UPS power is 100 kVA and the load power factor is 0.9. In this case, the active power of the load will be 90 kW. The total load power should not exceed 90 kW, and it is better if it is slightly less.

Such difficulties in calculating power can be avoided by using an uninterruptible power supply as an indicator of output power. In this case, the calculation of the uninterruptible power supply will be performed without errors. It is a big mistake to compare powers expressed in volt-amperes and watts, since the values ​​differ significantly.

It should also be taken into account that the power consumed by the equipment may be slightly lower than the nominal one. This can happen in a variety of situations. For example, if we consider computers, then their power in most cases is determined by the power of the power supply. But not in all cases, such a calculation algorithm is correct. So, for example, a computer may have a power supply with a power of 450 W, but the total power of the computer components is only 120 W. There can be a lot of such features and they need to be taken into account when calculating the uninterruptible power supply.

Another situation that needs to be taken into account in order to calculate the operation of the UPS is related to the refrigerator. For example, it can have a power of 250 W, but it should be borne in mind that the refrigerator does not work all the time, but only at certain intervals. In this case, you need to find out the annual electricity consumption. In calculations, you must use this value divided by 9. It should be noted that the load power must be considered in watts.

On some sites you can find the calculation of the UPS power online, but they cannot give accurate data, because they do not take into account such nuances. If you still decide to use such services, then in addition to the result you need to add about 20%. It is important to think about the prospect of increasing the load power. When the load increases in the future, it is better to immediately purchase a more powerful UPS. A similar situation is with services that allow you to calculate the UPS operation time online.

Battery calculation

If it is required to calculate the capacity of the UPS for a given power and operating time, then a simple formula is applied:

Capacity= 100*time*load power

The battery life is expressed in hours, and the load power in kilowatts. Note again that power is not expressed in voltamperes. For example, an uninterruptible power supply protects a computer with 500 W (0.5 kW). The uninterruptible power supply must provide an operating time of 2 hours. Under these conditions, the formula that allows you to calculate the battery capacity for the UPS takes the following form:

100*0.5kW*8h=400Ah

Thus, for a load with a power of 500 W, a battery capacity of 400 Ah is required to ensure operation for 8 hours. This UPS battery capacity calculation is applicable for a 12 V battery. In addition, it must be taken into account that the formula is suitable for a long battery life, namely about 9-10 hours. This is due to the fact that the dependence of the battery capacity on the charge time is not linear throughout.

If the operating time is less, then amendments must be made. This is due to the fact that with a short time, the discharge current is large and the battery gives the load only some of its capacity. So, if you need a work time of 30 minutes, then the result must be divided by two, for 2 hours, reduce by 40%, for 4 hours - 30%, for 6 hours - 40%. To determine the exact value, it is necessary to use the exact value of the efficiency of the inverter that is installed on the UPS and compare the data from the discharge curve of a certain type of battery.

After the total capacity is found, it is necessary to calculate the number of batteries for the UPS. To perform it, you need to divide the total capacity by the capacity of one battery. In our case, the total capacity was 400 Ah. Let's assume that the capacity of one battery is 50 Ah. In this case, we need 8 of these batteries.

Working hours

Many users are interested in the operating time that one or another uninterruptible device can provide. How to calculate the operating time of an uninterruptible power supply? To do this, you need to know the power of the connected load to the UPS, the efficiency of the inverter and the total capacity of the battery.

The total calculation of batteries for the UPS is extremely simple. In most cases, uninterruptible power supplies contain standard batteries. To perform a total calculation of batteries for the UPS, you need to multiply their number by the capacity of one battery.

In order to calculate the battery life of the UPS, the efficiency of the inverter is recommended to be taken equal to 0.85. The total load power must be expressed in watts. We talked about how to find it at the beginning of the article.

The calculation of the UPS operating time is carried out according to the following formula:

Time = total battery capacity * battery voltage * (inverter efficiency / load power)

The value obtained is an approximation and may change during the life of the uninterruptible power supply. The calculation of the UPS time is approximate, since the time depends on the wear of the battery and the operating conditions, mainly on the air temperature. So, for example, a temperature increase of one degree after the 40 ° C mark reduces the battery capacity by 5%, which is very significant. For maximum service life, it is recommended to reduce the load on the UPS for every 10 degrees after 25°C by 20%. Or you can organize a good cooling system and not allow any temperature increase at all, for which the uninterruptible source will only be grateful.

If such calculations are incomprehensible to you, then you can contact specialists in this field or use a special calculator - a UPS calculation program. However, in this case, it is necessary to use proven software created by professionals in order to avoid mistakes and the wrong choice of UPS. The advantage of such programs is the calculation. When calculating, you can choose the type of core for the transformer. The calculations take into account the losses that are possible in the core and copper wires.

There may be cases where there is no need for absolutely accurate data. In this case, you can use special tables that show the battery life for various types of uninterruptible power supplies. These tables include the operating time depending on the capacity of the batteries and the total load power. Thus, you can compare your data with the table and find out the approximate time.

Knowing how to calculate the UPS, you can make the most correct choice of UPS. Now you know that battery life does not depend on the power of the UPS or on the total voltage of the battery, but on the capacity of the batteries. Therefore, when choosing a UPS, preference should be given to a larger battery capacity in accordance with the given power. This choice will ensure maximum autonomy.

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The power and type of connected load is the main factor that should influence the choice of a particular model of uninterruptible power supply. The correct calculation of the UPS power according to the load will allow you to buy a device that works effectively when the network is turned off, without overpaying for the extra watts declared by the manufacturer.

Calculation of UPS power by load

To avoid situations in which the UPS turns off under load for no particular apparent reason, consider the following electrical indicators of the connected devices and the uninterruptible power supply itself:

• Active power of connected devices (W).
• Power factor. For a conventional computer it is 0.6-0.7, for server equipment tends to 1 (the value of 0.95 is used in calculations), for household appliances with an electric motor - 0.7-0.8. The coefficient is needed to determine the total power consumption (VA).
• Starting current value. Relevant for devices with electric motors. This indicator can be 3-10 A. It is by this many times that the UPS power will have to be increased to ensure that the engine starts. Otherwise, get a situation where the UPS does not hold the load when the main power supply is turned off.
• Own efficiency of the uninterruptible power supply. Depends on the type of UPS, usually in the range of 0.7-0.9.

To obtain an approximate value of the power of an uninterruptible power supply, you simply need to multiply the values ​​​​of all the above characteristics. It will not be superfluous to add a 10-20% margin.

What are the consequences of incorrect UPS load calculation?

Selecting a source with a lower power will lead to the fact that at the moment the main network is turned off, the UPS will not be able to provide the serviced devices with electricity and will turn off. By the way, this situation is typical not only when switching to battery power. This is especially true for devices operating on the principle of double conversion and line-interactive UPS. If the power of the uninterruptible power supply is insufficient, a permanent transition to the "Bypass" mode will occur. That is, the load will be reconnected to the main network, and not receive stabilized power from the converter.

It is worth noting that the joint parallel operation of the UPS on the load will not solve the problem. Connecting 2-3 sources to one consumer is technically difficult to implement. The fact is that maximum synchronization of operating modes is required. And it is difficult to achieve this even when selecting devices according to their characteristics. The solution may be to share the load with the subsequent connection of individual consumers to different UPSs. If this is not possible, then you will have to purchase an uninterruptible power supply of greater power.

Features of the UPS without load

Another point that you should pay attention to when choosing is the own power consumption of an uninterruptible power supply. How much the UPS consumes without load depends on the mode of operation of the device at a certain point in time. There are 2 main situations in which energy consumption will differ significantly:

• With a fully charged battery, the amount of energy consumed depends solely on the no-load current of the device, that is, on the design features of the UPS. For high-quality devices, this figure does not exceed 5-10 watts. But devices of the budget price segment will spend 20 W or more for their own needs.
• The increase in own consumption occurs during the battery charging period. The standard current in this mode usually does not exceed 0.15 of the battery capacity. Multiplying this value by the charge voltage, you get the power consumed by the uninterruptible battery.

UPS load backup time calculation

It affects the power of the connected load and the overall autonomy of the device when the network is turned off. To determine the maximum backup time, you need to know the capacity (C) and voltage (V) of the battery, the amount of connected power (P) and the efficiency of the source. The formula looks like:

T \u003d C x V x efficiency / P

A more accurate calculation that allows you to determine the operating time of the UPS depending on the load requires additional factors to be taken into account. Take into account the ratio determined by the discharge curve of the battery. The higher the connected power, the greater the discharge current and the faster the battery will give all its power. It is from these indicators that the value of the clarifying coefficient depends. Note that the magnitude of this value varies non-linearly.

By the way, this is not the only clarifying factor. Accurate calculations will have to take into account the type of battery, its age, ambient temperature.

If electrical calculations are difficult for you, you can use the online calculator to determine the power of the UPS depending on the parameters of the connected load. But a more accurate result can be obtained by consulting a specialist. Call us now or request a free call back. We will help you determine the optimal parameters of an uninterruptible power supply for operation in your conditions.