Battery in a Photovoltaic Power Supply System - Standards

Overview

The storage batteries are still the weakest, most vulnerable component in a photovoltaic power supply system. This might also be the reason why different types of batteries, ranging from automotive starter batteries and so-called “Solar Batteries”, all the way to high-quality industrial tubular plate (OPZS) batteries, and also sealed maintenance-free batteries, are used in photovoltaic (PV) systems.

Universal Standards for Solar Home Systems (SHS)

The “Universal Standard for SHS” gives a brief overview of the various aspects, advantages and disadvantages of the different battery types and their useful application in Solar Home Systems (SHS).

Some of the following observations may serve as an introduction for planners of subsequent specifications:

The most important feature of battery operation in SHSs is cycling. During the daily cycle, the battery is charged over the day and discharged by the night-time load. Superimposed onto the daily cycle is the seasonal cycle, which is associated with periods of reduced radiation availability. This, together with other operating parameters (ambient temperature, current, voltage, etc.), affects the battery life and maintenance requirements.

In order to maximise the lifetime of lead-acid batteries, the following operating conditions must be avoided:

• High voltage during charging (to prevent corrosion and loss of water)
• Low voltage during discharge (corrosion)
• Deep discharge (sulphation, growth of dentrites)
• Extended periods without full charging (sulphation)
• High battery temperature (all aging processes are accelerated)
• Stratification of the electrolyte (sulphation)
• Very low charge current (sulphation)

These rules lead to specifications for sizing (both battery and PV generator) and for battery protection procedures (charge regulator). However, it must be pointed out that some of the rules generally contradict each other (e.g. full charging requires high voltages but high voltages accelerate corrosion), so compromises must be found that take the particular local conditions into account: solar radiation, PV module and battery prices, duties and taxes, local manufacturing, recycling infrastructure, etc. Perhaps this explains the lack of consensus on this issue among the various sources of information (standards, experts, etc.) that have been consulted during the preparation of this standard; therefore, the requirements given below should be adapted to suit the local circumstances.

The need to prevent excessive discharge leads to the need to limit the maximum depth of discharge to a certain value, PD_MAX, which usually ranges from 0.3 to 0.6, but can approach 0.8, depending on the type of battery. The supply to the load must be cut off when this limit is reached. The available or useful capacity, C_U, is therefore less than the nominal capacity, C_B (which refers to the whole charge that could be extracted from the battery if no particular limitations were imposed) and equal to the product C_B * PD_MAX, such that:

C_U = C_B * PD_MAX

A good compromise between cost and reliability is typically obtained with a battery whose useful capacity ranges from three (in regions where extended cloudy periods are not expected) to five (in regions where cloudy periods are expected) times the total daily energy consumption in the house, so that the depth of discharge in the daily cycle, PD_d ranges from 0.06 to 0.2. The selection of a particular capacity mainly depends on the battery type. High-quality batteries are better able to resist deeper cycling than low-quality batteries. Hence, for the same application, high-quality batteries can be smaller than low-quality batteries, in terms of nominal capacity.

The highest-quality PV batteries are made with tubular plates and grids with low Sb-Se content. More than 8 years life, with PD_d = 0.2 and a maintenance period of 1 or 2 times per year, are attainable with such batteries. A particular disadvantage of tubular batteries for SHSs is that they do not readily accept low rates of charge. They are also expensive and are rarely available in the current markets in developing countries. Nevertheless, they should not be excluded from SHS programmes. On the contrary, it is recommended that large rural electrification programmes consider encouraging manufacturers to put these products on the market.

In contrast, automotive batteries, usually referred to as SLI, have a number of advantages. They are usually the cheapest batteries when compared in terms of nominal capacity (the difference in cost can be 4 or 5 fold), they are often locally produced and are widely available. Local production is not only convenient for economic and social reasons, but also because it represents the best means of recycling old batteries and avoiding environmental damage. Their main drawback lies in their relatively short lifetime. Because their cell design is optimised to deliver heavy currents during short periods of time, they have large areas of thin plates, and are poorly suited to supplying smaller currents for many hours before being recharged, as is required by SHS. It is therefore necessary to use larger battery capacities leading to PD_d £ 0.1, and a density of electrolyte which is lower than would normally be used in this type of battery (for example, 1.24 instead of 1.28 g/cl). This is necessary to reduce grid corrosion and hence to lengthen battery life. The associated increase in internal resistance in the battery does not pose any problems in SHS, because the charge and discharge currents are relatively low by comparison to conventional battery charge and discharge regimes. Classical SLI batteries use lead grids alloyed with antimony and require periodic topping up with water.

The short lifetimes of automotive batteries can also be compensated to some extent by introducing relatively simple modifications to the battery design but not to its technology. The most common modifications are thicker electrode plates and a larger quantity of acid solution in the space above the plates. Such modified SLI batteries are sometimes marketed as "solar" batteries and represent a promising alternative for the future of SHSs. Wherever possible, modified SLI batteries should be selected (and local manufacturers should be encouraged to make them) in preference to conventional SLI batteries.

Certain conditions must be met in order for a battery to be categorised as "modified SLI", as follows:

• The thickness of each plate must exceed 2 mm.
• The amount of electrolyte must exceed 1.15 l per 100 Ah of 20-hour nominal capacity and per cell.
• The separator must be made of microporous polyethylene.
• The density of electrolyte must not exceed 1.25 g/cl.

"Low-Maintenance" SLI batteries, sometimes marketed as maintenance-free batteries, often employ grids containing calcium alloys. The calcium increases the voltage at which gassing begins but reduces the cohesion of the active material to the grids. Hence, it cuts down the loss of water but also reduces the cycle life. Such batteries are particularly vulnerable to damage from deep discharge. In addition, they are also liable to be damaged by high temperature variations. Hence, many PV system designers strongly recommend that they not be used in PV applications in hot countries. However, the maintenance-free feature is still attractive, and extensive use has been made of these batteries in some countries like Brazil.

"No-maintenance" batteries of various kinds are also made for professional applications by using a semi-solid electrolyte (gel or malting). Such batteries, referred to as VRLA (valve-regulated lead acid), are more often resistant to deep discharges, but they are usually very expensive for SHSs, and they require specific recycling facilities. They are not considered in the present standard although they represent a legitimate technology choice in some cases. The same is valid for NiCd batteries.

The 20-hour nominal battery capacity in amp-hours (measured at 20 W and up to a voltage of 1.8 V/cell) should not exceed CR times the PV generator short-circuit current in amps (measured at Standard Test Conditions).

CR values are proposed for each type of battery in the table below:

Battery type	CR
Tubular	15-20
SLI (automotive): - Classical	30-40
- Modified	35-40
- Low-Maintenance	30-40

The maximum depth of discharge, PD_MAX (referred to as the 20-hour nominal battery capacity) should not exceed the values proposed in the table below:

Battery Type	PDMAX
Tubular	70-80
SLI (automotive): - Classical	30-50
- Modified	40-60
- Low-Maintenance	20-30

The useful capacity of the battery, C_U (20 hours nominal capacity, as defined above, multiplied by the maximum depth of discharge) should allow for a three to five-day period of autonomy.

The cycle life of the battery (i.e., before its residual life drops below 80% of the nominal capacity) at 25°C must exceed NOC cycles when discharged down to a depth of discharge of 50%.

A NOC value is given for each type of battery in the table below:

Battery Type	NOC
Tubular	600
SLI (automotive) - Classical	200
- Modified	200
- Low Maintenance	300

Comment: As the discussion of the "Universal Standard" above already shows, selecting the “right” battery type for PV systems is a difficult task, considering all of the different aspects of cost, lifetime, local availability, maintenance, recycling, etc. Another fact is, that no international standards for type-testing of batteries for PV applications are available as yet.

The IEC standards 60896 Part 1 and 2 “Stationary lead-acid batteries - General requirements and methods of test. Part 1: Vented types, Part 2: Valve-regulated types” give the general test methods for stationary batteries, but also include the comment that special test procedures for PV applications will be worked out by IEC group TC 21/TC 82. As confirmed by IEC, Geneva, a new standard on solar batteries is still being written and will have the number IEC 61147.

Up to now, the only standard available on solar batteries is the French standard NF C58- 510 “Lead-acid secondary batteries for storing photovoltaically generated electrical energy”, which will be used temporarily by PV GAP and the IEC SHS standardisation group. Therefore, the type-test procedures described in this standard will be the basis of the following battery specifications:

In addition to the above-mentioned standardisation activities of IEC, the CENELEC committee BTTF 86-2 has also drafted a standard proposal entitled "Accumulators for Use in Photovoltaic Systems, Safety-Test Requirements and Procedures", which was kindly made available by TÜV-Rheinland. As this proposal is not yet complete (no cycle tests, etc.) and all tests described here are also included in the French NF C58-510 standard, this standard-proposal is not considered in the following specifications.

The rechargeable battery shall consist of one 12 V_DC vented type lead-acid “solar” battery.

Optional: The rechargeable battery shall consist of one 12 V_DC valve-regulated type maintenance-free lead-acid battery.

Health: The rechargeable battery shall consist of a 12 V_DC (24 V_DC) vented-type “heavyduty” tubular lead-acid battery.

Type-tested and Certified Batteries

The battery must be type-tested and certified in accordance with NF C 58-510 “Lead acid secondary batteries for storing photovoltaically generated electrical energy”, and/or IEC 60896-1 or -2 “Stationary lead-acid batteries - General requirements and methods of test. Part 1: Vented types, Part 2: Valve-regulated types” (will be replaced by IEC 61147).

The following tests must be performed, documented and certified as described in NF C58-510:

• Nominal capacity C_n in Ah (generally C₁₀)
• Rated capacity C_t (10-hour capacity C₁₀, 20-hour capacity C₂₀ or 100-hour capacity C₁₀₀ given by the supplier
• Number of cycles in a constant average state of charge (DOD=40%) Minimum requirement: 400 cycles
• Number of cycles in a changing average state of charge (DOD=20%) Minimum requirements: 1500 cycles for vented tubular plates, 900 cycles for sealed and vented flat plates
• Suitability for operation under increasing and decreasing “state of charge” conditions. Minimum requirements: 95% of C10 after 60 cycles
• Suitability for overcharging for 400 days at 2.35 V/cell. Minimum requirements: Vented cells - no dangerous gases escape from the cells; valve-regulated cells: recombination of oxygen and hydrogen > 95%
• Ampere-hour efficiency at discharge until 0.75 C₁₀₀ Minimum requirement: not given.
• Enclosure test: 4 hours at 65° C and alternating temperature 30° C without any deformation
• Cell-sealing test. Minimum requirement: No seepage of electrolyte at an inclination of 30° and under a pressure of 0.1 bar
• Vent plug efficiency test. Minimum requirement: During the overcharge test no sulphuric acid and no explosive concentration of hydrogen escapes from the cells.
• Drop resistance test. Minimum requirement: 10 cm drop with all edges on concrete

Optional (World Bank specification): Cycle life of the battery (i.e., before its residual life drops below 80% of the rated Ah capacity), at 25° C must exceed 200 cycles when discharged down to an average depth of discharge (DOD) of 75%.

The minimum rated battery capacity C_t shall be calculated with the following formula:

C_t = (E_d / V_s)* (DOA / DOD)

with:
C_t = Rated Capacity [Ah]
E_d = Daily Energy Consumption [Wh]
V_s = Nominal System Voltage [V]
DOA = Days of Autonomy
DOD = Depth of Discharge (maximum)
t = Discharge Time = DOA * 24 h

For a Solar Home System, the following values might be assumed:
E_d = 180 Wh
V_s = 12 V
DOA = 3
DOD = 0.5 (50%)
t = 3 * 24 h = 72 h
⇒ C₇₂ = (180 Wh / 12 V) * (3 / 0.5)
= 90 Ah

In order to calculate the nominal capacity C₁₀ from the rated capacity, the supplier must submit a capacity curve related to the discharge current.

The minimum nominal capacity C₁₀ should not be less than 55 Ah.

Optional (World Bank specification): The battery capacity shall be at least 1.4 times the rated Ah per W_peak of the PV array power, but not less than 70 Ah. The rated Amperehour capacity is measured at 25° C at the C₂₀ discharge rate down to a voltage of 1.75 V per cell.

The maximum permissible self-discharge rate is 6% of rated capacity per month (30% per 6 months) at 25° C.

Battery connections in parallel shall be avoided, if possible. It is definitely not allowed to connect batteries of different type, size or age in the same battery bank. Where parallel connections are unavoidable, no more than two identical 12 V_DC lead-acid batteries shall be connected in parallel. Batteries of larger capacity shall preferably be selected, rather than a combination of batteries in parallel.

The amount of electrolyte must exceed 1.15 l per 100 Ah of 20-hour nominal capacity and per cell. The electrolyte reserve in the battery must be sufficient for 100 days of operation under overcharging conditions without addition of distilled water.

The density of electrolyte must not exceed 1.25 g/cl.

Information and Specifications

The following information and specifications must be available from the battery manufacturer, supplier or recognised test laboratory:

• Make, type, nominal voltage and nominal capacity
• Capacity at 10-hour discharge rate (at specified temperatures)
• Electrode type and type of electrolyte
• Specific gravity ranges for the electrolyte (for flooded batteries)
• Graphical information on charge and discharge characteristics, at different current rates and specified temperatures
• Graphical information on cycle life versus depth of discharge (at specified temperatures)
• Information on any particular regulation requirements for optimal cycling in a PV environment

In general, the data must be sufficient to establish the suitable method of battery regulation and suitable set points.

Additionally, the tenderer shall submit information in respect of:

• battery safety requirements
• battery maintenance requirements
• battery replacement requirements

Protection

Optional: Batteries shall be protected from the weather and installed in boxes or suitable enclosures which fulfil the following requirements:

• Access to the terminals and electrolyte shall be restricted to authorised and responsible persons only. Batteries shall be protected from access by children.
• However, the batteries shall be accessible for inspection by authorised persons.
• The terminals shall be protected from accidental short-circuiting from a tool being dropped across the terminals, for example.
• There shall be sufficient ventilation for hydrogen to escape. Hydrogen shall not be routed to any location where there is a danger of sparks or flames.
• There shall be at least 20 mm free space between the batteries and the walls and top of the battery box.
• The battery box or enclosure shall be made of suitable, durable and acid-resistant materials.
• A permanent notice shall be fixed to a battery box or suitably placed at a battery enclosure as a warning of the hazard from a release of gases, the danger of explosion, and that smoking or flames should be prohibited.

Health: Provision shall be made for the storage of distilled water. Further supplies for routine maintenance shall include rubber gloves, goggles and bicarbonate of soda solution. Clear maintenance instructions shall be provided.

Health: Small batteries or battery banks, up to a couple of hundred Ah at 12V or 24V, are normally not considered an undue hazard in large, well-ventilated rooms, provided the above precautions are observed. Larger battery banks should preferably be installed in separate battery rooms on insulated racks (safe for loading), with good ventilation at the highest point in the ceiling, a sloping floor with drain and a raised door threshold.

Optional: In general, moderate temperatures are desirable, between 15° and 30°C, and precautions shall be taken to protect batteries from extreme temperatures. In installations where temperature extremes cannot be avoided, manufacturers' data must be consulted to ensure that the type of battery is suitable for the conditions.

The batteries can be supplied in either a wet-charged or dry-charged condition. If drycharged, all chemicals and electrolyte must be supplied in accordance with the battery suppliers' specifications.

Installed batteries shall receive an initial conditioning charge, either by using an external (properly regulated) charger, or by charging the batteries from the PV array for at least two days with no loads connected.

The battery and associated containers should be packaged to withstand transport over rough dirt roads.

The supplier guarantees that it will take back and professionally recycle or dispose of old batteries and their toxic chemicals.

The warranty period for batteries shall be at least six months. Longer warranty periods granted by the manufacturer shall be passed on to the user.

References

This overview on standards for batteries in solar home systems is an extract of the publication: GTZ, Division 44, Environmental Management, Water, Energy, Transport: Quality Standards for Solar Home Systems and Rural Health Power Supply. Photovoltaic Systems in Developing Countries, February 2000.