May 21, 2019

10 mistakes in energy storage

Is it really that the batteries are not blamed on energy storage faults in stand-alone systems.

Read the box below for the good use and maintenance of a battery.

In every Autonomous Renewable Energy System, our batteries allow us to store and use electricity when there is no sunshine or wind. They are often called a “weak link” in renewable energy systems, but their problems are almost always due to poor equipment, installation errors, and lack of attention – as a human factor.

Over the past few decades, serious recurrent errors have been observed regarding batteries, equally from amateurs and professionals. The results can be expensive, dangerous and especially detrimental to the reputation of RES. That is why we present the following classic errors, and their proposed solutions. High-quality batteries can last for more than ten years but may die in one to two years if misused. This article only applies to lead-acid batteries and applies to energy storage facilities powered by photovoltaic systems, wind energy, hydropower, liquid fuel generators, utility networks, or any combination of sources. Applies to both non-interconnected systems and interconnected systems with a backup battery.

1: Incorrect battery type selection

Batteries are manufactured with a variety of structures and materials, depending on the application. RES applications require accumulators that can discharge more than 50% of their storage capacity repeatedly. This is called a “deep circle”. A fully autonomous home system will require 50 to 250 discharge cycles per year at 30-80% discharge depth (depending on the occasional or permanent use of the house). In RES applications, high-quality, deep-discharge batteries should always be used. Starter batteries (cars or trucks) are designed for fast, high-power discharges, and can only hold for a few deep-charge-discharge cycles.

Batteries used in interconnected Emergency Backup Systems (which are in standby mode) should be deeply discharged only occasionally when there is a power failure. Frequently, liquid-type batteries need to be charged to mix the electrolyte in order to avoid stratification. Because high repeat battery charging rate can be very rare in standby applications, it is preferable to use batteries designed specifically for back-up applications. They may not be good for hundreds of cycles but will remain in good working order for many years with light use.

Another distinction of accumulators is between closed (no maintenance – VRLA) and open-liquid type. Most deep discharge batteries are liquid type. They require frequent filling of liquids, but they tend to last longer. Emergency batteries in standby mode are often closed type, and do not require regular maintenance. VRLA batteries are chosen because they eliminate the need for ventilated space and are easy to access, cost more expensive than liquid electrolyte and require more careful recharging, but they especially excel in unattended applications.

The absorbed electrolyte (AGM) accumulators are designed for float (standby) applications, and are the best choice for interconnected photovoltaic systems that include a backup battery. GELelectrolyte battery packs are designed for circular and deep discharges, therefore they are ideal for Full Autonomous Photovoltaic Systems. 2: Incorrect battery capacity selection

In order to design an autonomous RES system, it is first necessary to calculate the “daily energy profile”. This is the number of Whrs that will be consumed per day. Then the necessary autonomy should be determined. This variable may vary between one and five days, depending on the average daily electricity consumption, power load performance and seasonal availability, and the user’s ability to use a backup generator.

Over time most home systems are growing. New loads are added, the photovoltaic array is magnified, but a battery pack can not be extended easily. Batteries usually work as a whole. After about a year, it is not wise to add new batteries to an installed system. If there is an increase in the system, it is best to start with a larger energy storage than is usually needed. At the same time, however, sufficient charging capacity must be ensured, otherwise the battery will be charged, which will lead to slagging of the plates and premature failure.

3: Insufficient maintenance

Liquid batteries require the addition of exclusively distilled water every one to four months depending on the type, battery temperature, charging control settings and system usage. Many forget to conserve their batteries, causing the low electrolyte level to cause excessive gas release and consequently deformation of the plates, short circuit or even explosion. On the other hand, accumulators should not be overcharged. There is no need to be filled more often than is required to keep the plates wet. Fill the batteries only to the level recommended by the manufacturer. Otherwise, during the final charge, the bubbles will cause a possible overflow, leading to corrosion of the poles and battery cables. Although it is an extra charge, an automated battery charging system (on whatever technologies is available) simplifies the battery filling in water. Of course, we never add electrolyte solution to the accumulators.

4: Failure to prevent erosion

During the final phase of the charge, the electrolyte in the liquid batteries causes gasses. When liquid batteries are used, a quantity of acid escapes and accumulates at the top of the battery. This can cause corrosion of the poles, especially in exposed copper, which in turn causes electrical resistance with potential hazards. The best prevention is to apply a suitable insulating cover to all the metal parts of the poles before assembly, by completely covering the battery poles, cable elbows, and screws separately. If the insulator is installed after assembly, the voids will remain, acid droplets will be dispersed and erosion will occur. Special products are sold to protect the poles, but many installers prefer petrolatum. Exposed cables or poles should be sealed using a special plastic heat seal tape. It is possible to insulate the cables by heating it, and immersing it in the petrol, which will melt and flow into the wire. Whatever the method to follow, the links should be very strong. Batteries that are protected in this way show very little corrosion, even after many years. It is also important to keep the top of the battery clean of acid droplets and dust. This helps prevent corrosion and current leakage across the surface of the battery. One good way is to wipe the batteries with a cloth or kitchen towels dampened with distilled water each time the battery is filled with water. Do not use baking soda for cleaning as it can penetrate inside the battery and neutralize electrolyte.

5: Parallel connections

The ideal energy storage complex is the simplest, consisting of a single array of accumulators in series that have been calculated for the application. This design minimizes the maintenance and likelihood of random manufacturing imperfections. Suppose we need a 24V – 600Ah depot. We can achieve this with a single 12-cell array of 2V-600Ah units connected in series, or with two parallel 24 series sets of 2V -300Ah cells or three sets of 12V-200Ah battery pairs. A common mistake is to choose smaller batteries because this approach seems less expensive. The problem is that when the current is branched between parallel arrays, it always chooses the street with the lowest electrical resistance and is never exactly equal. Frequently, a slightly weak cell or polar corrosion will lead a whole series of batteries to less charge. The result will be to degrade and fail faster than other industries. Because partial replacement exacerbates inequalities, the only practical solution is to replace the entire battery system. One way to reduce or avoid parallel battery connections is to use the highest DC voltage standard that is feasible. The same batteries that will form two branches at 24 V may be connected to a series for a 48 V system. The amount of energy storage is the same, but the layout is simpler and the current at critical points is split in the middle. If we need to have multiple battery ranges, we must avoid stacking the cables or terminals at the battery terminals, as well as connecting different capacity, voltage, manufacturer, technology and production batteries. The technically correct paralleling method is on copper bars, where this is not possible and the parallel connections of the accumulators have to be made on the poles, the final departures of the array must be made symmetrically diagonally. This reduces the possibility of corrosion and contributes to electrical symmetry.

6: Lack of monitoring devices

Have you ever driven a car without a fuel gauge? It can be frustrating! Many storage systems do not have a corresponding device that indicates the charging state (SOC) of the batteries and the level of stored energy. The measurement provides important information on energy management, resulting in a significant increase in the lifetime of the energy storage facility. These devices monitor the accumulated Ampera (AH) and display the charging state of the battery pack. They also show other items that may be useful for maintenance and troubleshooting. The installation of the controller should be centralized and programmed correctly.

7: Inappropriate operating environment

Lead-acid batteries temporarily lose about 20% of their actual capacity when their temperature drops to -1 ° C relative to their 25 ° C output. At higher temperatures, the percentage of permanent damage increases. It is therefore desirable to protect the batteries from extreme temperatures. If low temperatures can not be avoided, a larger energy storage capacity should be budgeted to offset their reduced capacity during the winter. A direct radiant heat source should be avoided and that causes some cells to rise (an ideal operating temperature range is between 10-25 ° C). Install the batteries so that they all stay at the same temperature. If it is on an outer wall, it should be insulated while ensuring space for air circulation. There should be gaps between the accumulators so that those in the middle do not heat up more than the others. A protection structure must allow easy access for maintenance, especially for liquid batteries. No switches, or other spark-producing devices in the housing, should be fitted.

8: Incorrect charging controller setting

When installing new charge regulators or inverters in the system, the appropriate charging values ​​for this type of battery should be programmed. If incorrect charging settings are selected, VRLA batteries may become overloaded and lose their internal moisture. Liquid batteries will be deprived of full charge and this will get worse if the charge setting values ​​are too low.

When the batteries are cold, they require an increase in the maximum charging voltage to fully charge them. When they are hot, they require a voltage drop to avoid overload. A charge regulator and inverter for the system should be selected that includes temperature compensation. To use, there should be a temperature sensor mounted on the batteries. A temperature sensor may still be needed for each charging device (including the inverter), but modern network systems transfer the temperature data to all chargers from a single sensor. Some small charge controllers have a built-in temperature sensor. In this case, we should make sure that the controller is positioned where its temperature is similar to that of the batteries. The evolution of electronics technology now enables the choice of a M.P.P.T Charging Capacity Charger (Charging Point Maximum Power). This does not mean that P.W.M (High Frequency – Pulse Range) Charge Controllers when combined with the correct panel type (sufficient number of strings) will not perform well. Trust your experienced installer!

9: Inappropriate charging

The surest way to destroy batteries within one or two years is to stay at low charge for many weeks. The battery electrolyte will crystallize, covering the plates, which will become permanently inactive. This is called “sacrifice”. Ideally, batteries should receive 100% of full charge about once a week for better and longer life. Using a monitoring system is necessary to know when full charging has been achieved. If there is no Amperometer (AH), the voltage will have to reach the maximum and the current will drop to a low level. This means that the batteries are unable to receive more energy, and they only accept a final charge.

In winter, some use their backup generator for one hour a day – as much as needed to prevent the system from closing. Bad idea! It may be better to use it for ten hours, once a week, or as long as it takes to fully charge the batteries instead of being partially charged and more frequent. The final charging of a power depot with a generator is an inefficient use of fuel, resulting in an extremely long use of the generator. As a result, generators usually close when the charge absorption stage is over. But at this point in the charging process, the battery pack will be only 85% of the SOC. Because, as we know, full battery charging is important for battery longevity, we need to make sure that RBCs will complement the charging of the battery system after the generator has completed most of the charging. Based on the photovoltaic system, providing full charge can be difficult during the winter months. Another option is to set the inverter-charger to the balancing mode during charging with a generator about once a month to ensure that the battery is fully charged. The underload limit is called “overload”. The voltage must never be below about 10.8V (for a 12V system), or21.6V (24V system), etc. System regulators and inverters usually include low voltage cut-off (LVD). It is better to lose power than to consume another (Wh) and destroy the batteries. Finally, liquid batteries must be equally charged at least four times a year. The exact frequency depends on various factors, including the size of the energy storage system relative to the charging sources and the average discharge depth during cyclic use. During normal discharge / charging, the individual elements of an array will exhibit a deviation of density & voltage values ​​between them. The balancing load can be considered as a controlled overload of the system that serves both to equalize cell voltage and to provide a necessary mixing of the electrolyte. It can be done with the photovoltaic system if it is large enough, either with a generator or with the grid. Most charging regulators and voltage converters – A.C chargers have a battery charging function.

10: Excessive energy balance If you consume more energy from your battery system than you enter in, your energy storage will suffer. It’s not a mistake of batteries, though this is the most common cause of complaints about batteries:

“they are not charged or last a little”. A simple computing load rule dictates a total amount of incoming energy to the storage system by 30% greater than the amount we consume. Here is also a simple scenario: we buy a device that consumes “minimal electricity”. However, there is no reference to the initial cost of solar electricity. By adding solar panels and increasing the battery capacity to cover this load, there is a risk that we will be faced with an investment of several hundred (or thousands) euros! The same happens when a user decides that it is insignificant to leave a coffee maker or large TV in operation throughout the day. Even low energy loads will work additively if they work 24 hours a day, 7 days a week. When people do not accept this reality, they exceed their energy balance and often blame the batteries.   Conclusions As a conclusion, it is useful to keep the following: Maintaining a high charging state is critical to maintaining the expected battery performance and life cycle in small photovoltaic systems. Renewable Energy Sources should be dimensioned at a rate of at least 1: 1.3 consumed: generated energy in any case for the worst time of the year to ensure that energy is available from the panels to recharge the battery properly. The size and duration of the electrical consumption loads should be carefully evaluated and checked periodically. Systems where the user handles the load are more susceptible to battery-related problems than systems with automated load operation. Special precautions should be considered in such applications, including the higher Load Disconnection & Reconnection points. The Charging Charge Charging Points should be determined based on the type of battery used, the regulator algorithm, and system operating characteristics. It is useful to be accompanied by a monitoring system. Temperature is the biggest enemy of the battery! The temperature compensation of the Charge Voltage Adjustment points should be used as an option, especially when using VRLA batteries. In addition, the temperature should be measured in the battery, except for the Charge Controller. In liquid-lead-acid batteries, a weekly – fortnightly maintenance (depending on the season and use) and water filling can maximize performance and service life. Talk to experienced market people from the beginning of designing a RES system. It will save you from many problems and runs in the future.

Article by Mr. Manos Savvakis, Head of Static Appliance & Residential Unit of the Industrial Batteries Division of BIOS SA