IEC Standard — Why is it important for solar batteries? IEC Standard Background In order to get the most life from a solar battery used in photovoltaic systems PV , it must be discharged and recharged properly. Unfortunately, we have experienced many PV installers regularly undersize batteries to either reduce costs or because they fail to properly underestimate the load for the system. Because of this, life expectancy of PV batteries has not been easy to calculate — until now. How to Define Life Expectancy in PV Systems Battery manufacturers traditionally define battery life either as float life or cycle life.
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IEC is not responsible for any services carried out by independent certification bodies. Use of the referenced publications is indispensable for the correct application of this publication. IEC shall not be held responsible for identifying any or all such patent rights. This first edition cancels and replaces the second edition of IEC published in This edition constitutes a technical revision. This edition includes the following significant technical changes with respect to the previous edition: a a restructuration of the previous edition of the document; b a clarification of the different clauses with regard to conditions of use, general requirements, functional characteristics, general tests conditions, test method and recommended use of tests, the aim being to ensure a better understanding by the end user; c a clear distinction between on-grid and off-grid applications for future markets needs.
A list of all parts in the IEC series, published under the general title Secondary cells and batteries for renewable energy storage — General requirements and methods of test , can be found on the IEC website. At this date, the publication will be? This part deals with cells and batteries used in photovoltaic off-grid applications.
This International Standard does not include specific information relating to battery sizing, method of charge or PVES design. This standard is applicable to all types of secondary batteries. For dated references, only the edition cited applies. For undated references, the latest edition of the referenced document including any amendments applies.
IEC —7— IEC , Secondary cells and batteries containing alkaline or other non-acid electrolytes — Portable sealed rechargeable single cells — Part 2: Nickel-metal hydride IEC , Secondary cells and batteries containing alkaline or other non-acid electrolytes — Secondary lithium cells and batteries for portable applications IEC , Secondary cells and batteries containing alkaline or other non-acid electrolytes — Nickel-cadmium prismatic secondary single cells with partial gas recombination 3 Terms and definitions For the purposes of this document, the terms and definitions given in IEC concerning secondary cells and batteries, and those given in IEC concerning photovoltaic generator systems apply.
The cells and batteries are normally delivered in the following state of charge: d discharged and drained vented nickel-cadmium batteries only ; e charged and filled; f dry charged and unfilled vented lead-acid batteries only ; g discharged and filled nickel-cadmium batteries only. Other secondary cells and batteries such as based on sodium or vanadium electrochemical systems can be potentially used for such an application.
Due to the fact that they are in a phase of adaptation for a possible use in PV systems, it is recommended that their respective supplier be contacted for the necessary planning, test and operation details.
IEC Batteries in a typical PV system operating under average site weather conditions may be subjected to the following conditions. When selecting the required battery capacity, the following items should be considered, e. Typical charge and discharge currents 4.
In some systems the load current must be supplied at the same time as the battery charging current. NOTE 1? This arises from varying average-charging conditions as follows: ?
Period of high state of charge? A voltage regulator system normally limits the maximum battery voltage during the recharge period. The system designer normally chooses the maximum charge voltage of the battery as a compromise allowing to recover to a maximum state of charge SOC as early as possible in the summer season but without substantially overcharging the battery.
The overcharge increases the gas production resulting in water consumption in vented cells. In valve-regulated lead-acid cells, the overcharge will cause less water consumption and gas emission but more heat generation.
Typically the maximum charge voltage is 2,4 V per cell for lead-acid batteries and 1,55 V per cell for vented nickel-cadmium batteries at the reference temperature specified by the manufacturer.
Some regulators allow the battery voltage to exceed these values for a short period as an equalizing or boost charge. For the other batteries the battery manufacturers shall give the most adapted charge voltage values. Charge voltage compensation shall be used according to the battery manufacturer instructions if the battery operating temperature deviates significantly from the reference temperature.
The expected lifetime of a battery in a PV system, even kept regularly at a high state of charge, may be considerably less than the published life of the battery used under continuous float charge conditions. The state of charge will then decrease and cycling will take place at a low state of charge. The low solar irradiation yield of the photovoltaic array may be a result of the geographical location combined with the winter periods, heavy clouds, rains or accumulation of dust on the photovoltaic array.
In the absence of such information, the storage period may be estimated according to the climatic conditions as shown in Table 1 as below. Lead-acid or nickel-cadmium batteries with electrolyte shall be stored starting from a state at full charge. A loss of capacity may result from exposure of a battery to high temperature and humidity during storage.
Choice of a shaded location or cooling should avoid this risk. In the absence of such information, operating temperatures and humidity may be those shown in Table 2. Typically the life expectancy of batteries will decrease with increasing operating temperature. IEC — 11 — Low temperature will reduce the discharge performance and the capacity of the batteries.
For details, the manufacturer should be consulted. Instead, overcharge affects the water consumption in vented batteries and consequently the service interval. Overcharge can be controlled by the use of proper charge controllers. Most non-aqueous systems, such as lithium-ion batteries and similar, will not accept any overcharge without damage or safety problems. Such batteries are normally supplied with a BMS battery management system that prevents, independently from its charge controller, that such overcharge happens.
The parameters of the regulator shall take into account the effects of the PV generator design, the load, the temperature and the limiting values for the battery as recommended by the manufacturer.
Vented lead-acid or nickel-cadmium batteries including those with partial gas recombination shall have sufficient electrolyte to cover at least the period between planned service visits. Overcharge in valve-regulated lead-acid batteries shall be carefully controlled to be able to reach the expected service life.
The water consumption is measured during the cycle test see 8. Additional packing or protection shall be provided for off-road conditions. Particular care shall be taken while handling unpacked batteries. In case of specific requirements regarding mechanical stresses, such as earthquakes, shock and vibration, these shall be individually specified or referred to in a relevant standard.
IEC The charge efficiency is the ratio between the quantity of electricity delivered during the discharge of a cell or battery and the quantity of electricity necessary to restore the initial state of charge under specified conditions see IEC , Where no data are available from the battery manufacturer, the following efficiencies as given in Table 3 may be assumed.
Vented and partial gas recombination Nickel-cadmium batteries do not normally require this type of protection. The manufacturer shall advise if there are special considerations to be observed for the initial charging of batteries when only the photovoltaic array is available as the power source.
IEC — 13 — 6 Functional characteristics The batteries shall be characterized by their: — — — — rated capacity see 8. The parameters and accuracy values shall be in accordance with relevant clauses of the applicable standards listed in 7. IEC for stationary lead-acid batteries vented types ; IEC for stationary lead-acid batteries valve-regulated types ; IEC for portable lead-acid batteries valve-regulated types ; IEC for sealed nickel-cadmium batteries; IEC for vented nickel-cadmium batteries; IEC for nickel cadmium prismatic rechargeable single cells with partial gas recombination; IEC for portable nickel-cadmium batteries; IEC for portable nickel metal hydride batteries; IEC for portable lithium batteries.
The IEC for lithium batteries for use in industrial applications is under development. NOTE 8 8. The verification of the rated capacity shall be performed by using a current of I 10 A for leadacid batteries, 0,2 I t A for nickel-cadmium, Ni-MH, and Lithium batteries and I 10 A for other batteries according to Table 4 and the relevant clauses of the applicable standards listed in 7.
The verification of the long duration capacity shall be performed according to Table 4, by using a current of I A and the relevant clauses of the applicable standards listed in 7.
IEC The charging shall be carried out according to the relevant clauses of the applicable standards listed in 7. NOTE One set of aggregate cycles is approximately equivalent to 1 year service in a PV energy storage application. The cells or batteries shall therefore comply with the requirements of the test below, which is a simulation of the photovoltaic energy system operation: a the test battery shall be selected, prepared and installed according to the applicable standards listed in 7.
IEC 8. For other batteries the charge voltage shall be limited to a safe level as specified by the manufacturer? For Ni-MH and lithium batteries the charge voltage shall be limited to a safe level as specified by the manufacturer.
The steps a and b shall be repeated 99 times. At the termination of the 99 execution of step b the test battery shall be submitted to a capacity test according to 8. The amount of water added shall be measured and reported. IEC — 17 — 9 9.
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