Batteries for solar panels – selection rules

Batteries are the heart of solar installations, they are responsible for the accumulation and storage of solar energy, its distribution in accordance with the needs of the consumer. But in order to use the energy of the Sun as efficiently as possible, knowledge of the main technical characteristics and rules for choosing a battery will be required.

You need to understand that conventional car batteries for solar installations are not an option. Solar power systems use deep cycle batteries that discharge up to 50-80%. Therefore, first you need to decide on the type of battery , as we talked about earlier. The next step is to find out its main technical characteristics.

Battery Selection Criteria

When choosing a battery for solar panels, the following parameters and their ratio are taken into account:

  1. resource of charge/discharge cycles;
  2. battery charging / discharging speed;
  3. self-discharge indicator of the device;
  4. maximum current value during charging / discharging;
  5. battery capacity, which helps to calculate the power of the device;
  6. overall dimensions and weight;
  7. terms of Use;
  8. the need for service;
  9. the ability to work under different environmental conditions (for example, at very low or very high temperatures).

How to calculate the capacity and number of batteries

To determine the technical parameters, you need to correctly set the task, understand what it is planned to use the solar installation for, the conditions for its operation, what will be the level of energy consumption that it must accumulate. To calculate the capacity of the batteries and their number, you must do the following steps.

Determine the peak load or maximum power of your house/apartment/object, which is supposed to be powered by a solar plant. To do this, add up the power of devices that can work simultaneously.

Include everything from microwaves and lighting to computers and digital clocks. The power consumed by appliances is usually stated in watts – this information can be found on the data sticker that most electrical items have (for example, the power of a small portable TV is about 20 watts). You can use the resulting value in further calculations to determine the energy that is planned to be received from the solar installation and used in the process.

Example: There are two 75 watt light bulbs and a 300 watt desktop computer in a room. The maximum required power is 2 ∙ 75W + 300W = 450W.

Determine your daily energy intake. Find out the time (number of hours) each electronic device is expected to operate during the day. After multiplying the power of each individual device by the time of its operation, the desired value of the required energy (in watt-hours per day) will be obtained. To get the total amount of energy needed to power your home, you need to add up all the received watt-hours.

This amount is likely to be somewhat underestimated, since it does not take into account the power losses of the entire electrical system (wires, transformers, etc.). You can get an approximate, but roughly approximate, idea of \u200b\u200ba real figure with system losses by multiplying the resulting value by 1.5.

For example: 2 light bulbs of 75 W work for 4 hours a day.

A 300W computer also works 4 hours a day. Total: 150 ∙ 4 + 300 ∙ 4 = 1800 watt-hours. 1800 ∙ 1.5= 2700 watt-hours or 2.7 kWh.

Determine the number of days during which you need battery autonomy without recharging. On average, it will be from two to five days.

Now you can calculate the required minimum battery capacity (A∙h) by calculation. To do this, you need to multiply the number of watt-hours per day by the number of days that was determined in the previous paragraph, which will be 50% of the discharge of your batteries. Therefore, multiply this value by two and convert the result in kilowatt-hours to ampere-hours (A∙h). This can be done by dividing the resulting value by the battery voltage.

Example: You want the battery to last three days without recharging and you want to use 3.15 kWh per day. That is, 2.7 ∙ 3 ∙ 2 \u003d 16.2 kWh – this is the energy that you plan to receive from batteries. To convert this to A∙h and find the required battery capacity, you must divide the resulting energy value by the system voltage, which is usually 12, 24 or 48 V in solar panels. If, for example, 48 V is used, the minimum capacity will be 16200 / 48 = 338Ah

Now, when you divide the capacity of the entire system by the rated battery capacity, you will determine the number of batteries you need.

To make everything easier and more accurate, you can use special online calculators to calculate technical characteristics or contact specialists.

Battery connection

Before buying batteries, you need to find out their number. An important role will be played by the way they are connected. The main challenge is to find the best configuration to produce the desired capacitance and voltage. There are two main ways to connect batteries in a circuit: parallel and series. When connected in series, the voltage of the battery increases, and when connected in parallel, the current increases. Series and parallel connections can be combined to obtain the required voltage and capacitance values (so-called combined connection).

We wrote above that you can determine the number of batteries you need by dividing the capacity of your system by the rated power of your batteries. In fact, a lot depends on how you connect the entire system. Also consider some nuances. For example, if a new battery is connected in parallel to an already working battery, this can lead to a deterioration in the operation of the circuit and a decrease in the life of the system as a whole. For the same reason, it is undesirable to connect batteries of different capacities to each other in a circuit. There is an opinion that, ideally, for the efficient operation of a solar battery, it is preferable to use a long chain of batteries connected in series. Unfortunately, this is not always possible due to voltage and capacitance requirements.

A few additions to the general installation rules

  • Obviously, in solar installations used in everyday life or industry, where a lot of electricity is required (to power a large number of electrical appliances, lighting, etc.), one battery is not enough. For such a load, whole arrays of batteries are usually used, the voltage of which, as a rule, is a multiple of 12 V. Depending on the operating power and voltage required “at the output”, as we wrote above, they can be connected to each other in parallel, in series, or both of these way.
  • Batteries are preferably placed close to each other so that the total length of the wires is minimal. This is due to the fact that as the cable length increases, the electrical resistance increases and, accordingly, the efficiency of the entire system decreases.
  • The assembled battery is connected to a controller that controls the charge of the battery, as well as to an inverter – it converts the accumulated solar energy into electrical energy. Such a system can be used for permanent (year-round and round-the-clock) power supply to houses or other objects, accumulating electricity for a long time, and can be used as a source of additional or backup power.
  • In order for batteries to have a maximum service life, they work efficiently and with minimal energy loss, they need to create certain conditions. It is desirable that moisture does not get on them, and the ambient temperature is in the range of 8-25ºС. Therefore, such systems, as a rule, are installed in buildings, and not on the street.
  • Batteries have a significant weight, so it is better to place them on the floor or well-fixed shelves, racks. It is important to avoid dropping the battery from a height – this can damage the device, and if the battery is with liquid electrolyte, and it is depressurized from falling, it can be dangerous to people’s health.

It is not recommended to install batteries in residential premises, even though modern technologies provide for increased environmental safety measures for batteries. At the same time, there is no need to take any special measures in the premises where the batteries will be located.

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