Battery Lifetime and UPS Reliability
The battery is the least reliable part of most well designed UPS systems. This is very important to the user, because UPS battery replacement is expensive (as much as 30% of the original cost of the UPS). Failed batteries reduce system reliability and are a source of nuisance and downtime.
Battery temperature affects reliability. The natural processes that cause battery aging are strongly affected by temperature. Detailed test data provided by battery manufacturers shows that battery life is reduced by 10% for every additional 10 degrees F (5 degrees C). This means that the design of the UPS should be such that the batteries are kept as cool as possible at all times. All on-line and all standby/on-line hybrid type UPS systems run hotter than standby or line interactive type UPS systems (that is why they all need a fan). This is the single most important reason why standby or line interactive type UPS systems last longer between battery replacements when compared with on line type UPS systems.
Battery charger design affects reliability. The battery charger is a very important part of UPS. The charging conditions of the battery have a major effect on battery lifetime. UPS battery lifetime is maximized if the battery is always powered from a constant voltage or float type battery charger. In fact, battery lifetime is much longer under charge than if the battery is simply left in storage. This is because maintaining the battery under continuous charge arrests some of the natural aging processes of the battery. Therefore, it is essential that the UPS charge the battery whenever it is plugged in, even if the UPS is switched off.
Battery voltage affects reliability. Batteries are made up of individual cells, each a source of approximately 2 volts. To make up a battery of higher voltage, individual cells must be connected in series. A 12-volt battery has 6 cells, a 24 volt battery has 12 cells, etc. When batteries are kept on constant charge as they are in an UPS system, the individual cells are charged in series. Slight manufacturing variations in battery cells cause some cells to take a larger percentage of the charging voltage than others. This causes premature aging of those cells. The series connected group of cells is only as strong as its weakest link, so when any individual cell becomes weak, the whole battery is weakened. It has been proven that magnitude of this aging problem is directly related to the number of cells in the string and therefore increases as the battery voltage increases. For a given UPS capacity, the UPS design that will have the longest battery life is the unit with the lowest battery voltage. This preferred type of UPS use a small number of large cells in series instead of a large number of small cells. Some manufacturers have chosen to use higher voltage batteries because for a given power level, a higher battery voltage allows the UPS to use smaller wires and semiconductors that can reduce the cost of the UPS. The battery voltage of some typical UPS systems at the power level of approximately 1kVA ranges from 24 volts to 96 volts. Over a 10-year product lifetime, the users of some brands of UPS systems can expect to pay more than twice as much for batteries than was originally paid for the UPS system. Although it is easier and less expensive to design a UPS system with higher battery voltage, hidden costs in the form of reduced UPS life are delivered to the user.
Battery ripple current affects reliability. Ideally, a UPS battery should be permanently maintained in a float or constant voltage charging situation in order to maximize service life. In this state, a fully charged battery draws a small amount of current from the battery charger, called the float or self-discharge current. Despite battery manufacturers recommendations to the contrary some UPS designs (including many on-line designs) subject the battery to an additional current, called a ripple current. Ripple current results when a battery continuously supplies power to an AC inverter, because the conservation of energy requires that an inverter supplying an AC load must draw an AC current from the DC source that supplies it. This current causes miniature charge and discharge cycles to occur within the battery, normally at a frequency equal to twice the UPS output operating frequency (50 or 60 Hz). These cycles exercise the battery and cause premature aging of the battery. Standby UPS designs such as the classical standby UPS, the Line Interactive design, or standby-ferro UPS do not subject the battery to ripple current. Other designs subject the battery to various amounts of ripple depending on the exact nature of the design. To determine if a given on-line design subjects the battery to ripple current, the topology of the UPS must be examined. If on-line UPS have the battery placed between the battery charger and the inverter, then the battery will be subject to ripple current. This is the classical double-conversion style UPS. If on-line UPS have the battery isolated from the input to the inverter by a blocking diode, relay, converter, or controlled rectifier, then the battery should have no ripple current. Of course, in these designs the battery is not always on line, and these types of UPS are therefore properly referred to as hybrid standby/on-line UPS.
Intelligent Discharge Management in UPS Batteries
Both vented type Lead-Acid batteries and Sealed Maintenance Free (SMF) Lead-Acid batteries are employed in UPS applications. The ensuing discussion will be equally applicable for both. Differences, if any, will be highlighted at the appropriate places.
Battery Capacity is usually quoted on 20hr basis i.e the rated capacity of the battery is the AH it will deliver if the current drawn from battery is such that it will be discharged completely in 20hrs.For example a 80AH Battery will last for 20hrs if it is delivering a current of 4Amps. However, it is well known that the available capacity from a battery is a strong function of discharge rate. When the discharge proceeds to the point at which all the available capacity has been taken out, the Battery voltage drops considerably in a small time indicating an over discharge situation. That is when the UPS should issue a low Battery warning and shut down the UPS. But what is the voltage at which this should happen?
Fig.1 shows the discharge characteristic of a Lead-Acid Battery operating at room temperature at various discharge rates. The curves assume that the particular discharge rate is maintained constant till the Battery reaches the over discharged state. Obviously variable discharge rates in one cycle of discharge itself is going to complicate the nature of curves too much to be tractable.
In overdischarge, the sulfuric acid electrolyte can be depleted of the sulfate ion and become essentially water. This lack of sulfate ions to act as charge conductors will cause cell impedance to rise and little current will flow. This will necessitate a longer charging time in the case float voltage charger being employed. If the UPS battery is actually on a cyclic discharge mode (as all UPS units are in Indian conditions) and the battery charging is done by constant voltage charger mode (as it is in the case of a majority of UPS models available in Indian market) this rise in cell impedance due to overdischarge will make charging insufficient and UPS will fail to deliver the specified backup time. A second potential problem arising from overdischarge can occur due to increased solubility of lead sulfate in electrolyte as the acid concentration decreases. Overdischarge brings down the acid concentration to low levels and as a consequence lead sulfate gets into electrolyte. This dissolved lead sulfate is made to deliver the sulfate ion to the electrolyte on recharging. But the released lead does not get back to where it should be; rather it gets deposited at the bottom of the cell or it gets deposited in the separator plates. In either case the active material in the cell comes down and its available capacity goes down with every such deep discharge or over discharge. Also, if the lead gets deposited on separator plate, resistive paths will develop between separator plates in the long run thereby damaging the battery.
Thus we have insufficient charging and coupled to that a reduction in Capacity also due to over discharge. Hence over discharge should be prevented by shutting down the UPS when it is about to happen. And over discharge commences at the knee of discharge characteristic shown in Fig.1. Obviously the cut off point will depend on the rate of discharge. Specifically, if the cutoff voltage is set as shown (based on the discharge rate when UPS is on rated load) low battery shut down will prevent the deep discharge only if the UPS is on full load. If it is on low load and the low battery sense voltage continues to be at the same value, by the time low battery shut down takes place considerable over discharge would have taken place. Thus the Battery life and backup time of UPS will be heavily compromised if an UPS is routinely used at low loading levels in the case of units in which the low battery sensing is based on a fixed voltage set point with no regard for discharge rate and temperature. The discharge characteristic is temperature dependent and discharge voltage is higher at higher temperature. If low Battery sensing is done at the same setting the Discharge State of Battery will be deeper at higher temperature.
One does not have wait till the knee point to shut down the UPS. One can set the trip point at higher than knee point and that will partly solve the problem of possible over discharge when the UPS is on low load. But then, we will require a higher AH rating battery to obtain the specified back up time. This implies higher initial cost. The returns will be longer Battery life and excellent backup time performance throughout the Battery life period.
Intelligent discharge management envisages modifying the low bat threshold based on the actual discharge rate of the Battery. The relevant electronics senses the actual Battery current and suitably sets the threshold for low battery warning and thereby avoids deep discharge at all discharge rates. Also such intelligent Battery management systems adjust the low battery threshold with the Battery temperature also. The Battery temperature is sensed and this information along with the Battery characteristic curves supplied by the manufacturer will be used to adjust the threshold. Though all this can be done by analog electronics using off the shelf components, the current industrial practice favors microcontroller based system due to their higher reliability, versatility and programmability. Such Battery Management systems make it possible to use smaller Battery without any compromise in performance over an extended lifetime.
Overdischarge Problem in Long Battery Strings
Even two Batteries of same type and make can not be identical in their characteristics at all discharge rates.Fig.2 shows the characteristics of two similar batteries at different discharge rate.
The small differences in characteristics get enhanced at high discharge rates. If these two Batteries are put in series and discharged at a high rate –say at C3 rate in figure- the battery B reaches the end of available Capacity earlier and proceeds to deep discharge before low battery shut down takes place. The degree of deep discharge that takes place in the weak cell will depend on the difference between the curves. A deep discharged cell has low charge acceptance during charging compared to the other with the result that in the next discharge cycle the difference between the two will worsen resulting in deeper discharge of the weak one. This process will gradually lead to loss of Capacity in the string and eventually to the damage of both. Especially so if the UPS is on cyclic duty rather than of float duty. In real float duty, the Batteries will get enough charging time for equalization charging before next discharge cycle and that will save the situation somewhat.
This problem of progressive weakening of the weakest unit in the string, consequent loss of Capacity, reduction in backup time of UPS etc. will be severe if the string is long and the string is being discharged at a high rate. For example consider a 5kVA UPS with a backup time of 15 minutes using 120V, 24AH battery. The battery string has 10 units of 12V each. The full load battery current will be about 48 Amps and hence at full load the battery is being discharged at 40C rate. Assume that one 12V battery is weak (i.e it has lower available capacity) and that it reaches end of charge at 12th minute and reaches the unit threshold of 10.5Volt.The string threshold will be set at 10.5 x 10 = 105 Volt. Say at the 12th minute all other units have reached only 11 Volt. Then within the next minute or so the weak cell will discharge deeply and its voltage will dip to 6 Volt and at that point shut down will take place. Deep discharge to 6V level will seriously affect the charge acceptance capability of that unit, thereby resulting in more severe deep discharge (and possible cell polarity reversal) in the next discharge cycle. Obviously this problem will be serious in a UPS which uses high d.c voltage and is designed for high discharge rates i.e for small back up duration. Equally obvious that in such a situation the deep discharge that takes place due to low load on UPS is more harmful. The solutions are (1) to avoid using long strings as far as possible (2) to use higher AH rating when long string of batteries is used to avoid high rate discharge (3) to employ intelligent Battery management system in any case.
Optimum Battery Charging
(References – Application Notes U104 and U155 from M/s Unitrode Corporation)
Lead-acid battery chargers typically have two tasks to accomplish. The most important is to restore capacity as quickly as possible. The second one is to maintain capacity by compensating for self-discharge and ambient temperature variations.
There are two fundamentally different charging methods for lead-acid batteries. In constant voltage charge, the voltage across the battery terminals is constant and the condition of the battery determines the charge current. Constant voltage charge is most popular in float mode application. The charging process is usually terminated after a certain time limit is reached. Another technique is constant current charge, which is often used in cyclic applications because it recharges the battery in a relatively short time.
As opposed to constant voltage charge, the constant current charge automatically equalizes the charge in the series cells. There are many variations of the two basic methods, well suited for switched mode battery charger circuits. Considering that well designed switched mode power converters are inherently current-limited, the combination of constant current and constant voltage charge is an obvious choice.
The best performance of the lead-acid cells can be achieved using a four-state charge algorithm. This method integrates the advantages of the constant current charge to quickly and safely recharge and equalize the lead-acid cells, with the constant voltage charge to perform controlled over-charge and to retain the battery’s full charge capacity in float mode applications. The carefully tailored charging procedure maximizes the capacity and life expectancy of the battery.
The four states of the charger’s operation are trickle charge, bulk charge, over-charge and float charge, as they are shown in Figure 3. Assuming a fully discharged battery, the charger sequences through the states as follows:
State 1: Trickle Charge
If the battery voltage is below the cutoff voltage, the charger will apply the preset trickle charge current (Itrickle ). In case of a healthy battery, as the charge is slowly restored, the voltage will increase towards the nominal range until it reaches the cut-off voltage. At that point the charger will advance to the next state, bulk charging.In the case of a damaged battery, e.g. one or more cells are shorted or the internal leakage current of the battery is increased above the trickle current value, the low value of trickle charge current ensures safe operation of the system.In this case the battery voltage will stay below threshold
(Vcutoff ) preventing the charger from proceeding to the bulk charge mode. When the battery voltage is above the cutoff voltage at the beginning of the charge cycle, the trickle charge state is skipped and the charger starts with the bulk charge mode.
State 2: Bulk Charge
In this mode the maximum allowable current (I bulk ) charges the battery. During this time, the majority of the battery capacity is restored as quickly as possible. The bulk charge mode is terminated when the battery voltage reaches the over-charge voltage level (Voc ).
State 3: Over-Charge
Controlled over-charge follows bulk charging to restore full capacity in a minimum amount of time. During the over-charge period, the battery voltage is regulated. The initial current value equals the bulk charge current, and as the battery approaches its full capacity the charge current tapers off. When the charge current becomes sufficiently low (I oct ), the charging process is essentially finished and the charger switches over to float charge. The current threshold, Ioct, is user programmable and is typically equals I bulk /5.
State 4: Float Charge
This mode is only applicable when the battery is used as a backup power source. The charger will maintain full capacity of the battery by applying a temperature compensated DC voltage across its terminals. In the float mode, the charger will deliver whatever current is needed to compensate for self-discharge and might supply the prospective load up to the bulk charge current level. If the primary power source is lost or if the load current exceeds the bulk current limit, the battery will supply the load current. When the battery voltage drops to 90% of the desired float voltage, the operation will revert to the bulk charge state. The ultimate lead-acid battery charger will combine the above-described four-state charge algorithm, and particularly at higher output currents, a switched mode power converter. The Lead-Acid Battery Charger Controller IC UC3909, manufactured by M/s Unitrode Corporation,is tailor made to implement this four state charging algorithm.
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