This article discusses the gasses given off by Industrial Batteries and how to manage these gasses. The document primarily considers Standby Batteries but the overall principals apply equally to lead acid and nickel cadmium types both vented or VRLA.

Under normal operating conditions, the gasses evolved are hydrogen (H) and oxygen (O). However, under extreme conditions other gasses may be produced such as hydrogen sulphide (H2S). Some strange gasses are also given off in very small quantities such as carbon dioxide (CO2). This document only considers the evolution of hydrogen, oxygen and hydrogen sulphide.

Hydrogen will burn in air when in concentrations of between 4% and 75%. Oxygen in itself will not burn but does support combustion. Hydrogen sulphide is flammable and will explode and is very poisonous.

Lead-acid and nickel cadmium batteries only generate gases when on overcharge. However, gases can be seen to come from vented batteries with clear containers when on discharge. This is because gas will be present on the plate surface and within the active materials and when plates expand or contract when discharging, cells can appear to gas. VRLA batteries often “vent” on discharge because of the heat generated which causes the internal pressure to rise and the vent then opens to release the excessive pressure. In a quiet environment, a faint whistle sound can sometimes be heard.

Overcharge is a normal operating condition for all industrial batteries on float charge. The level of overcharge is very low and correspondingly, the volume of gas generated is also small but importantly it cannot be ignored. Batteries must not be placed in sealed rooms or enclosures.

The volume of the gas given off is, in the most part, proportional to the overcharge current. It should be noted that it is the charging current that produces gas and not the charging voltage. Normal float charge voltages result is low overcharge currents but a small increase in float voltage can result in a dramatic increase in float current. It is recommended that all float voltage systems are fitted with “over voltage shutdown control”. For VRLA batteries, a typical “over voltage shutdown control” voltage would be 2.40Vpc. At this voltage level the volume of gas given off is typically 7 to 10 times greater than the volume at the normal float voltage.

If the battery temperature is elevated, more gas is given off because more current will flow for the same float voltage. Typically, if the temperature increases by 10oC the volume of gas will double. In the extreme, a high battery temperature can lead to thermal runaway and hydrogen sulphide can be evolved in large quantities. This condition has to be avoided under all circumstances. Fortunately, hydrogen sulphide can be detected very easily because it has a foul odour of rotten eggs. If present, extreme caution should be taken when entering the battery room. It is recommended to isolate the charging source and allow the gas to disperse before entering the battery room.

For vented batteries, the charging current may be many times higher than for VRLA types. Vented batteries are often commission charged at constant current and a value of 10% C10 amperes as a constant value is normal. Comparing this to the current for a 100Ah VRLA battery, the current may be 0.2A at the “overvoltage shutdown control” voltage; similarly for a 100Ah vented lead-acid battery being commission charged at 10% C10A the current will be 10A. Considerably more ventilation will be required.

We need to manage these gasses to avoid disastrous consequences.

International standards give guidance to the ventilation requirements and people responsible for battery installations must be fully aware of the guidelines. Below you can find the formula for maintaining the hydrogen concentration below the 4% explosive limit. This formula is directly from BS EN 50272-2 and you MUST refer to the full document for the correct use of the formula.

Q = v . q . s . n . Igas . Crt . 10-3 [m3/h]

(The author does not accept any responsibility for the miss-interpretation or use of the above formula.)

Please follow our blog for continued technical information and advice relating to a full range of battery installations regarding both management, implementation and maintenance. Should you require particular assistance with a specific project then please do not hesitate to contact us directly for assistance. Our team are available to provide the necessary support you may need.

A frequently asked question is, should I use a GEL or AGM battery?

To answer this question we need to understand the differences between VRLA GEL and VRLA AGM and we must understand what a VRLA battery is. Simply, a VRLA battery is one which would not normally be topped up with water at periodic intervals which is what has to be carried out on a “vented” battery. Because of this characteristic they have often been miss-quoted as sealed. They are not sealed but have a one way valve to release excessive internal pressure. The design of the valve varies but they all have a pre-set value and when this is reached releases and maintains the pressure within design limits. This pressure is usually low and in the order of 0.5 BAR. Both AGM and GEL are VRLA batteries and have this valve arrangement.

VRLA batteries, both AGM and GEL, are often referred to as Gas Recombination Batteries because they do not need topping up and a large percentage of the gas generated on overcharge is recombined into water and in this respect they are maintenance. However, it must be remembered that when being overcharged both types will give off explosive hydrogen and oxygen and they must never be installed in sealed enclosures or where ventilation is restricted.

Looking at the different characteristics between AGM and GEL batteries shows that they both have some advantages over the other and the choice is not easy. Looking at the designs, in an AGM battery the electrolyte is held in an absorbent glass mat which also acts as a separator between the positive and negative plates. In a GEL battery the electrolyte is “immobilised” in a GEL and a separate separator similar to that used in “vented” batteries has to be fitted. Both types may be considered as “spill proof” but the manufacturer should be consulted if they are to be operated in anything other than a conventional orientation.

Most AGM batteries use high quality very absorbent glass matt separators which in many respects look feels and have the same “mopping up” characteristics as blotting paper. However, it is very different in characteristics when used in a battery. In a GEL battery the electrolyte is “immobilised” by mixing the filling acid with fine silica which creates a gel. In the first instance this looks like thick milk or “runny” cream. It is thixotropic and has to be continually stirred to stay as a liquid, once inside the battery it “sets” into a “wet” GEL format.

In an AGM battery, typically 10% of the electrolyte is in the plates and 90% in the separators. The GEL battery has a greater volume in the plates because they are usually thicker and also have a larger volume of electrolyte in the gel. Consequently, the GEL battery can have twice the volume of electrolyte than the AGM type. This higher quantity of electrolyte in the GEL battery can help the characteristics in many ways but also has drawbacks. The electrolyte specific gravity (s.g.) in the AGM battery is generally higher at typically 1.310s.g. at 20ºC compared with 1.250s.g. for GEL. The higher s.g. results in a higher initial on load voltage for the AGM battery. Because the GEL battery has a greater electrolyte volume it has a better voltage profile towards the end of the discharge and it does not drop sharply as with the AGM battery. The AGM battery has often been describes as “acid limited”. Again we have to put this in context because the fall off in voltage as the result of acid depletion is only relevant for long duration discharges and is rarely noticed for discharges shorter than 1 hour.   The construction of AGM batteries with its thinner plates and no requirement for a conventional type of separator means that a much more compact design can be achieved. The space between the plates (plate pitch) is much smaller in AGM batteries which reduce the internal impedance resulting in a higher on load voltage for the same current drawn. Overall, this improves the power to volume and weight ratio and gives considerably better high current characteristics. AGM batteries are usually smaller in volume for the same ampere-hour performance.

GEL batteries are generally more tolerant to the influences of the surrounding ambient temperature. Experiments have shown that the recombination reaction generates internal heat and this is more easily dissipated into the surrounding air with GEL batteries. It is argued that this is because of the greater volume of electrolyte in the GEL battery and because this is in contact with 5 sides of the “cube” design they have a better heat dissipation. For the AGM battery, frequently none of the electrolyte is in contact with any of the sides which then forms a “double glazing” effect and retains the heat. Excessive heat within batteries can cause thermal runaway and although this is not a problem with either type when installed correctly, in very high ambient conditions, typically over 40ºC, the GEL battery is better at coping with this abuse condition.

Some GEL batteries have tubular positive plates which makes them better for cycling applications. The AGM design will not work with tubular plates, therefore thicker more durable flat plate are used by some manufacturers which come some way to matching the tubular plate GEL battery characteristics. Tubular plate or thick flat plate GEL designs do not have the high rate performance of the thinner plate AGM battery. Plates having a thickness of less than 1mm are now available from some AGM battery manufacturers and with modern lead alloys for the plates, corrosion is low and the life is long for float charge standby applications. In some designs it has been shown that the life of thin plate AGM batteries is as good as or even better than tubular plate or thick flat plate designs in standby applications.

It has often been stated that VRLA batteries, both GEL and AGM, do not cycle well. This is not true. It has been shown that VRLA AGM batteries of thin plate designs can easily achieve over 750 deep cycles when operated with the correct charging equipment. Some GEL batteries have achieved twice this for true cycling applications with sophisticated chargers which reduce overcharge to a minimum.

In the world of float charge standby batteries, the charging source is usually a constant voltage type. This will give a constant voltage at the charger design maximum current up to a preset voltage which is usually in the order of 2.23Vpc to 2.28Vpc, depending on the VRLA battery design. These chargers are very efficient in providing power to the battery following a discharge but will inevitably result in taking several days before the battery is truly fully charged. A “nominal” 100% recharge can usually be achieved within 72h even following a 100% depth of discharge. All too often, cycling applications require the battery to be recharged in a short time such as 8 or 12 hours and this is insufficient time when using a standby system battery charger. For this type of industrial application it is essential to oversize the battery by at least 20% and to have a higher recharge voltage available. It is also desirable to have a higher recharge current. Typically after adding 20% to the required capacity to carry out the discharge, a voltage of typically 2.35Vpc and a current of 10% C10 amperes is desirable. Some charger designs have multiple voltage steps and consider battery temperature to achieve the shortest recharge time with minimal overcharge.

'Off grid' applications using solar and / or wing generators often have a generator back up to cater for the inevitable shortcoming of the “renewable” charging source. The GEL battery is generally the better choice for this application. The user should always seek professional advice from the battery manufacturer. If the design is flawed, the battery will fail prematurely.

For mobility scooter and golf cart applications the first choice is usually a GEL battery because it offers a better whole life cost. Battery failure due to abuse is frequently seen for these applications. The battery is often left it in a discharged condition for many days or even weeks. The GEL battery is much more tolerant to this abuse but it is not immune. Users should be aware of this because there is no point in buying a higher cost GEL battery and then destroying it by abusing it. For this application, the scooter or golf cart will have its own charger and it is essential to use this if a satisfactory life is to be expected. Interestingly, lithium batteries are now finding their way into this application but the cost is high. When we understand the characteristics of AGM and GEL, we are in a better position to make our choice for the application. However, we often find that the initial cost is a major factor and this will make the choice simple. If initial cost is not a deciding factor then whole life costs may make the choice easier. The lowest initial cost very often turns out to be the highest cost over even a 5 years life. Space considerations will also influence the type purchased and principally, the AGM battery will be a better choice where this is important. Where high power outputs are required from the battery such as 15 minute UPS applications, the AGM battery is the first choice. They offer very good power to volume and power to weigh ratios and they offer a lower cost both initially and as a total life cost. There are very few UPS applications having a required run time of less than 1 hour where the GEL battery has been chosen.

For telecommunication applications, the choice is much harder to make. Small batteries are generally AGM but in high ambient conditions the GEL battery has advantages. The temperature in street side cabinet can be alarmingly high in the summer months and the GEL battery is generally better suited to this application. Where batteries are in buildings with good temperature control, the AGM is usually installed. There are large ampere-hour batteries in both GEL and AGM design and the costs are often very similar. The discharge voltage profile can favour the AGM battery in some cases but the more stable profile of the GEL battery can also be of benefit. Each application should be considered on its own requirements to ensure the correct choice is made.

Many factors will influence the choice between GEL and AGM and sometimes no rational logic is used in the decision.

This paper discusses techniques that will improve the service life and reliability of VRLA batteries. There are many facets that influence this and here we look at the most important items.

By improving the service life the reliability will be enhance. In a standby emergency battery system it is more important to improve the reliability rather than the service life. There are some parameters that will marginally reduce the service life but will enhance the reliability. Some care needs to be applied to obtain the correct balance.

It is imperative to start right if a long life and reliability are to be expected from the battery. To this end, competent persons should only carry out the installation. This is particularly important from a Health & Safety point. In this “time conscious world” that we live in we continually look to faster ways of carrying out tasks but this must not be at the expense of reliability. A VRLA battery is often an expensive item and the last line of defence and it is important to safeguard the Health & Safety and investment from the start. Without compromise, the following procedures should be followed at the installation stage: -

a) Upon receipt, move the consignment to a safe and secure location.

b) The storage area should be dry with an ambient temperature between +5ºC and 20ºC. The storage time is halved for every 10ºC increase in temperature. Consult the supplier for the maximum storage time that should not be exceeded.

c) When the battery is being installed, all individual cell or monobloc (unit) voltages should be measured and examine them for damage before they are installed into the enclosure or on the battery stand. Look for abnormalities such as low voltages and consult your supplier if in doubt about any values.

d) At the installation stage, ensure that the terminal connection faces are clean, free from corrosion and not damaged. Some manufacturers recommend a small quantity of corrosion resistant grease be applied to the terminals prior to assembly. Contact your supplier for further details. The units can now be positioned and connected. It is important to tighten bolts or nuts to the correct torque setting. Fires have been known to start because of incorrectly tightened connections.

e) Still isolated from the charger, but fully connected measure and record the open circuit voltages of all units. This is the “base” number used in the future. There is now a definitive voltage for the unit position in the string and its voltage that can be cross related in the future. Check that the typical unit voltage multiplied by the number of units reasonably matches the total string voltage. A 54 cell battery of VRLA cells may have a typical voltage of 2.13Vpc x 54 cells = 115V. If one cell is reverse connected, a voltage nearer 2.13Vpc x 52 = 111V will be measured.

Clearly a differential of 4V indicates a problem. The polarity should be checked if more than 2V difference is found between the typical unit voltage x the number of units and the voltage measured across the battery terminals.

f) The battery can now de connected to the charging source and all individual unit voltages should be measured and recorded typically about 1h after connecting to the charger. Also measure and record the charging voltage and if possible, the charging current. It is also a good idea to measure and record the typical unit temperature and ambient temperature round the battery.

g) It is recommended that a test discharge be carried out on the complete battery system before putting into service. This will not only prove that the battery can supply its designed power but will also prove that the charger is operating correctly when recharging.

It is important to set the float voltage to the correct value in accordance with the manufacturer’s recommendations for the battery temperature.

Significantly, the battery temperature may not be the same as the ambient temperature. If air conditioning is used to control the room temperature air circulation should be considered. It has been shown that in some instances, air conditioning may result in a temperature gradient across the battery by up to 5ºC and in some extreme cases more than this. An actual example of this was a 3-string battery system installed on a 6-tier stand where the bottom tier was 7ºC lower than the top tier. In this case the only solution was to revise the air conditioning. No adjustment of the float voltage could compensate for this difference. Typically, the temperature across the battery should be within ±2ºC of the average.

The charging source must be capable of maintaining the correct voltage within the recommended limits and preferably with temperature compensation. Ripple current should also be within the manufacturers limits. It has been shown that batteries fitted to some static UPS machines will “wear out” sooner than those connected to a typical “telecoms smoothed” charging system.

An overcharged battery will “wear out” faster and have a shorter life. However, the effects of undercharging may be even more dramatic because an undercharged battery may not supply the required power in the event of a critical supply incident and it may not be fully recharged following a discharge.

Float voltages should be checked at regular intervals and modern charging equipment often has this “built in” with the facility to send an alarm via the building management system.

The operating temperature has a significant effect on service life. Typically, for every 10ºC increase in battery temperature, the service life will be halved.

Temperature compensation of the charging voltage will mitigate some of the life lost if the temperature is greater than the optimum. However, when the battery temperature exceeds about 50ºC, depending on battery type, a significant reduction may take place, which the charging voltage may not be capable of countering. These include container-softening leading to loss of element compression because of swelling and dry out.

At the other end of the temperature spectrum, when the battery temperature is below 0ºC, the relationship between voltage compensation and temperature will not be linear and significant undercharging can result.

It is tempting to set the float voltage marginally low in the assumption that this will enhance the life because the charging current will be lower. Providing the voltage is set to within the manufacturers parameters, this is acceptable. However, a lower that recommended float voltage could lead to severe undercharging that will reduce life. Taken further, if the voltage is too low, the battery may not fully recharge after a discharge, leaving the available standby time lower than the design. Taken to the extreme, for short duration standby batteries, this can lead to no available protection because the voltage will collapse as soon as it is connected to the load.

It has often been claimed that the installation of battery monitoring will increase the life of the battery. In part this is true because faults may be detected before they become critical and suspect units may be replaced and the life of the system may be extended. The most important point in this subject is that the reliability will be enhanced.

Battery monitoring may be permanently connected or “hand held” and there are several different types available. Some measure the internal resistance whilst others are more sophisticated and measure impedance or conductance. This paper makes no recommendation for the type that should be used.

Battery manufacturers have their own recommendations for servicing batteries made by them and below are typical of what can be found in their literature. In all cases, the user should consult the supplier for specific requirements.

Ensure that the battery float charge voltage is within the recommended limits for the operating temperature. Record the voltage and date. See Additional Information below.

Ensure that the battery float charge voltage is within the recommended limits for the operating temperature. Record the voltage and date. See Additional Information below. With the battery connected to the charger operating in the normal float charge mode, measure and record all unit voltages, temperature, and as far as possible the charging current.

In addition to the Three Monthly Service Recommendations detailed above it is further recommended to carry out the following: -

If the battery has been subjected to vibration, check the unit connectors, inter-row, inter-tier and end connections for correct torque tightness.

Ensure the battery is in a clean and dry condition. For cleaning and drying, use only soft cotton cloths moistened is a solution of soap and water.

Examine all units for abnormalities.

Record all service requirements that have been carried out along with any abnormalities. Report all information to the person responsible for the battery and emphasise any abnormalities and recommendations.

After a minimum of six months of satisfactory monthly inspections, the frequency of monthly requirements may be extended to three monthly.

Similarly, after six months (2 service operations) of satisfactory operation, the three monthly service recommendations may be extended to six months. The six monthly service recommendations may be extended to annually after one year of satisfactory operation.

This article discusses the use of parallel string batteries in high voltage applications but the contents may be considered for any battery voltage.

The subjects discussed in the paper are: -

A) What do we mean by Parallel String Batteries?

B) What do we mean by High Voltage Batteries?

C) History of Parallel String Batteries and Past Problems.

D) How many Parallel Strings in One System?

E) Inter String and Main Cables.

F) Circulating Currents on Charge & Discharge.

G) Differential Currents on Discharge & Recharge.

H) Battery Reliability.

I) Paralleling Batteries of Different Capacities and from Different Manufacturers.

J) Advantages of using Parallel String Batteries.

In a parallel string battery arrangement one string is connected in parallel with another string. It may consist on one additional string or many additional strings. It is not uncommon for a high voltage system supporting a UPS system for 6 strings to be in parallel. Some 48V systems have 50 strings in parallel and in rare cases, even more.

When cells or monoblocs are connected in series the voltage of the system is increased. For example, 2 lead-acid cells of 50Ah each connected in series would be a battery having a nominal voltage of 4V and a capacity of 50Ah.

Fig 1 below shows a simple 2 string arrangement.

National and International standards define High Voltage Batteries as any battery where 60 or more cells are connected in series; i.e. greater than 120V nominal In this paper we consider a parallel battery to be more than one string of cells or monoblocs connected to the same charging source and load over 120V. However, much of the content of this paper may be applied to any battery voltage.

Until VRLA batteries became popular the design philosophy was to use large vented cells and if the required Ah capacity exceeded about 2000Ah, another string was added.

When VRLA AGM product first found its way into the market place, only a few types were available in 2V spiral wound format up to about 25Ah. This capacity was not practical for many applications. In the early 1980’s prismatic types were introduced from 2 or 3 manufacturers from about 6V 50Ah up to 2V 450Ah. However, only about 6 capacity / voltages were available. If demand required a larger Ah capacity then parallel strings were used. Initially this was for 48V Telecom Applications but demand soon required larger capacities for UPS applications.

Design Engineers were concerned about the distribution of cell voltages over a large number of cells connected in series and many UPS batteries required typically in the order of 180 cells. In any string, the current will be the same in all cells providing no inter connections are made. However, with 2 or more strings the current flow and voltage equalisation was a concern because each string may have a different charging and discharging current. It had been shown that for 48V batteries, the current sharing was not a problem. Some early high voltage parallel string batteries had “equalising” connections typically every 20 cells. This proved to be a serious error. See Fig 2, Fig 3, Fig 4 and Fig 5 below illustrate what can go wrong with equalising connections.

Fig 2: Shows Equalising Connectors.

Fig 3: Illustrates what can go wrong when equalising connections are made.

Fig 4 below shows what can happen if a cell goes to open circuit or if an inter connector develops a high resistance which may be due to poor instalation or corrosion etc.

Fig 5: Illustrates the effect of a short circuit cell.

With a large difference in the ohmic value of the two strings the complete battery system would eventually fail because the strings would not have the required charging current.

Equalising connections as shown in Fig 2 above should not be fitted. The only paralleling connections should be fitted to the end of string terminals.

It is fair to say that the industry has learned from its mistakes and equalising connections are no longer fitted.

How many would you like?

The first published Standard stated 4 as a maximum. Why 4? This was the maximum of experience and the industry was cautious following the problems encountered with equalising connections. Today the industry has much more experience and we know that 10 string high voltage batteries are operating without problems.

It is recommended that “daisy chains” are avoided, see Fig 6 below. Note that cable lengths from the charger and to the load are different for each string.

Cable CSA should also be the same from charger to strings and from strings to load.   Fig 6: Avoid “daisy chains”.

Although it is desirable to keep the cable lengths and CSA the same, it has been shown that this is not essential. When the discharge current or recharge currents are high, different voltage drops may occur. On discharge this is not a problem but on charge it is argued that this may result in different string charging voltages which may lead to string “balancing” problems. It is further argues that this is not an issue because as the battery becomes more and more charged, the current reduces and the voltage drop also reduces and the string voltages will stabilise within acceptable limits. Ultimately, the string voltages will equalise and the system will automatically “balance”. Fig 7 below illustrates the “desirable” but not essential situation.

Fig 7: Preferred cabling.

What happens if we have one string discharged and one charged and what will be the value of the current flowing between the strings?

This scenario will occur if one sting has been test discharged and is then reconnected to the other string or strings.

A problem encountered on several occasions has been seen during capacity testing of batteries having parallel strings where each string was tested individually. When one sting (A) was tested it passed the test and then reconnected to other strings but this was before being fully recharged. The second string (B) was then tested and failed. Then string B was reconnected before being fully charged. The third string (C) was tested and this failed.

Evaluation of the test results of this real case showed that in string A no cells failed the test; 3 cells in string B failed; 8 cells in string C failed and all 24 cells in string D failed. It was concluded that because the individual strings were not fully charged prior to being reconnected to the other strings, they progressively drew current from the others which reduced their state of charge. As the tests continued, the situation became more acute to the point that all 24 cells in string D failed the test.

Fig 8: With string A fully charged and string B fully discharged, switch 2 is closed. In a real test the resultant circulating current was 10 x C10 amperes for 0.2 seconds reducing exponentially to 0.2 x C10 amperes after 3h.

Most battery manufacturers would consider a discharge current differential of 10% from string to string to be within acceptable limits However, in real terms any differential of more than 5% should be investigated. On charge, the differentials are less noticeable because of the low currents flowing in the strings but the same 10% differential limits should be considered.

It has been argued that having more than one string will increase the total reliability. Caution has to be made because in a two string configuration, if one string is lost the remaining string may not provide power for even a few seconds. On the other hand, if a single string battery fails the outcome is the same. Even in a 3 string system, for short standby times such as 5 minutes, if one string is lost the remaining 2 strings may not provide any power. In some extreme cases even when using 4 strings, the loss of one string may still result in no available standby time.

There should be no issues with paralleling batteries of different capacities providing the same numbers of cells are in each string. For example a 330 cell string of 175Ah may be paralleled with a 330 cell string of 25Ah giving a total battery system of 330 cells of 200Ah. Some manufacturers may disagree with this practice but in reality, providing the correct float voltage is presented to each string the system will work without problems. On charge, circulating currents are avoided because each string will have the same float voltage applied. On discharge, each string will deliver power to the load in proportion to the capacity. Some small differences may be found because of variations in the internal ohmic value of each string due to connections and connector type.

In a system with strings from different manufacturers, providing each battery type requires the same nominal float voltage, no issues are likely to be experienced. Again, battery manufacturers will discourage this practice but there are many systems, particularly of 48V where it is common practice to parallel different manufacturer’s product. In most cases, the product is not of the same age and not of the same Ah capacity. These systems have been running for many years without problems.  

Advantages include: -

• Better cell or monobloc efficiency which reduces the overall battery cost.

• Better Ah battery efficiency reduces the total battery capacity required which reduces the overall battery cost.

• Easier manual handling at the manufacturing stage which reduces manufacturing costs.

• Easier to handle on site which reduces installation costs.

• Possibility of using monoblocs instead of individual cells which generally reduces cost due to the above reasons.

• Parallel string batteries are generally smaller in physical size which reduces the battery room requirements.

Disadvantages include: -

• More individual units which can, in some instances increases the installation costs due to more inter unit connections having to be made.

• More individual units will increase the maintenance time in checking voltages etc.

This concludes our latest technical blog post, make sure you continue to follow our blog posts for ongoing information and support on a range of battery related subjects and common issues experienced in the field, wether it be for large industrial battery systems or simple maintenance advice for smaller battery applications. We at Blue Box Batteries are available to assist on a wide range of requirements whatever they may be.

In order to meet with our ongoing success as a major industrial battery supplier Blue Box Batteries Ltd has now relocated to larger premises which will enable us to maintain increased stocks in line with the requirements of our valued client base. Our ability to maintain good availability of battery products is essential to further improve our fast response service to ensure that projects are completed within the best possible timescale, first time, on time, every time.

Based at Deer Park Farm, Horton Heath our new premises offer a modern and professional facility which will provide the extra space needed for our continued expansion. We will soon be adding additional options and choices to our extensive catalogue of products, further increasing the selection of solutions we can make available.

The team at Blue Box Batteries would like to take this opportunity to thank both our customers and suppliers for their ongoing support, we look forward to continued success together in the future.

Generally we talk about series connected cells or monoblocs and single or parallel connected strings. For example, a modern telecommunication battery system may have 24 lead-acid cells connected in series and 4 strings connected in parallel. This configuration is illustrated in Fig 1 below.

When cells or monoblocs are connected in series the voltage of the system is increased. For example, 2 x 2V lead-acid cells of 50Ah each connected in series would be a battery having a nominal voltage of 4V and a capacity of 50Ah. See FIG 2 below. The same 2 x 2V lead-acid cells of 50Ah each connected in parallel would give a total voltage of 2V and a capacity of 100Ah. See FIG 3 below.

By changing the configuration we can increase the voltage or capacity of a battery system with almost limitless possibilities. However, battery voltages over 1000V are likely to give insulating problems and in practical terms the highest voltage seen in normal commercial use is about 750V. Looking at the capacity, in theory there is no limit to the capacity that can be achieved. There are many examples of parallel connected batteries with a capacity of over 5000Ah.

During charging, for the configuration shown in FIG 4 above, the sum of the currents i1, i2 and i3 would total iT. In an ideal situation we would like the charging currents i1 and i2 to be the same. However, in practice we will always find a difference in the charging currents because cells are never identical in a float charge battery system. As the strings become more charged, the current will reduce to a very low value and the effects of voltage drop will be insignificant. Even in fully charged systems a small difference in charging current may be seen. This will be very small because the charging current will be in the order of mA for most battery systems and accurate measurements are difficult to obtain without special equipment.

During discharging, we cannot expect the current from string 1 to be identical to string 2. A good guide to a healthy battery system would show the discharge currents in string 1 and 2 to be within 5% of each other and 10% is considered to be the normal limit of variability. Anything more than 10% should be investigated.

In an ideal battery system, the cable lengths from the charger to the strings and from the strings to the load should be the same length and the same cross sectional area. This is represented in FIG 5 below. In this way, the volt drop will be very similar and better balancing of the system will be achieved. In practical terms this is not as important as may first be envisaged. During a discharge it is normal that the string currents will be different. At the commencement of the discharge string 1 may have a larger current than string 2 but as the discharge progresses, it is likely that the situation will reverse. In another application, one string may deliver more current than another throughout the discharge. Unless the difference at any time is more than the 10% guideline no corrective action would normally be required.

A further paper titled “Parallel String Batteries in High Voltage Applications” will be published within the near future.