Life of the Maintenance-Free Battery

By battery life what are you referring to? The age of the battery or the AH output? By maintenance free is it a gel cell or sealed lead acid? Anybody ever check the charger output to ensure it's charging to spec? You said batteries are they charged in series or parallel? If in series just one bad battery can reduce the total output. How old are the batteries?

My Ans are

1 )The designed life of the battery can be achieved by following manufacturers O & M.

2) The life of the Battery decide the following factors,

a)Number of cycles (charge/discharge).

b)In each cycle depth of the discharge.

c)In each cycle capacity of charging.

d)Charging current limits.

e)If any over charge.

f)Float to boost and boost to float charging current setting.

and somany

The reasons that most batteries fail prematurely are related to one or more of the following:

1. Excessive cycling

2. Improper charging

3. Lack of temperature control

4. Installation

5. Manufacturing problems

6. Operational issues

Note: User has control over most of the conditions that lead to premature failure.

1. Excessive Cycling:

Every time a battery cycles (a discharge followed by a recharge), the electrochemical generator has to go to work, which involves converting acid and paste. As the paste on the positive grid changes from PbO2 to PbSO4, there is a large increase in volume, which puts pressure on the paste. The more the paste is expanded and then later contracted, the more the wear and tear on it. This means that deeper discharges are more harmful to the battery. Also, cycling a battery causes accelerated corrosion of the grid structure, which leads to shorter life. This is especially true for lead calcium batteries, which happens to be the most popular technology in use today.

The lead calcium battery's cycling capability depends on the depth of discharge. For example, it is only capable of 50 deep cycles (the removal of more than 80% of energy), but can deliver 300 cycles for a 25% depth of discharge cycle. A UPS battery which normally only delivers about 25% of its stored energy during its 15 minute rated reserve time can deliver 300 such cycles. If the load on the battery is less than 30 seconds (momentary power glitch), it can handle thousands of these short cycles.

2. Improper charging

Battery manufacturers specify a voltage range for their various cells that must be adhered to. If the voltage on a given cell is allowed to go either higher or lower than the recommended value, it will have a detrimental effect on the life of the battery. It should also be noted that the specified voltage range is very temperature dependent. The right voltage for a battery at 77°F would be too high if the battery was operated in an ambient temperature of 90°F. It is important for a user to understand the interaction between voltage and temperature. Low float voltage

(Undercharging) - Undercharging causes sulfate crystals to form on the plate surfaces, since there is not enough current flowing to keep the battery fully charged. Sulfate crystals that harden over a long period of time will not go back in solution when proper voltage is applied and, therefore, result in a permanent loss of capacity. Extended undercharging will also cause a loss of active material from the negative plates.

High float voltage (Overcharging) - Overcharging causes excessive gassing of hydrogen and oxygen. This leads to loss of water in flooded cells and dryout in VRLA cells. High float voltage also causes higher float current, which in turn causes accelerated corrosion and shedding of active material from positive plates. The recombination of gases to form water in VRLA cells generates heat, and heat causes higher float currents. Therefore, excessive gassing in VRLA cells can lead to thermal runaway.

3. Lack of Temperature Control

Batteries are very temperature sensitive, and efforts should be made to maintain the operating temperature near 77°F. The proper temperature will optimize battery life and is especially critical for VRLA cells. The recombination of gases within a VRLA cell can only take place at a certain rate. If this rate is exceeded, gas pressure will build
up beyond the safety valve level, and gases/water will be vented out and permanently lost. At 77°F, the highest float voltage at which a cell can still recombine all the gases driven off the plates is approximately 2.32 volts. If the cell temperature increased to 90°F while holding the voltage constant, the cell would dry out and possibly
go into thermal runaway. Thermal runaway leads to a melting down of the jar and, under worst-case scenario, will lead to an explosion and fire.

It should be obvious from the above discussion that all VRLA applications should have tight temperature controls and/or temperature compensated chargers.

Low temperature - Battery capacity is diminished at low temperatures. For example, at 62°F, capacity is approximately 90% versus 100% at 77°F. At low temperatures, a higher float voltage is required to maintain full charge and, if the charger is not adjusted properly, cells may be undercharged, leading to the problems described under low voltage.

High temperature - High temperature causes loss of life. For every 15°F rise in operating temperature, the life is cut in half. High temperature causes increased float current, which means increased corrosion and, therefore, the loss of life. High temperature also causes gassing, which means loss of water in flooded cells and dryout and thermal runaway in VRLA cells.

4. Installation
A lot of battery problems stem from improper installations. Some of the more common ones are the following:

Loose intercell connections - These can lead to abrupt failures, including fires.
Damaged post seals - Improper cell handling or not supporting cables can damage post seals. This allows acid to migrate up the post and corrode the post to intercell connection.

Not replacing shipping caps with vent caps - In flooded batteries, this creates internal gas pressures that will force gases to escape past the post seals, causing post corrosion.

5. Manufacturing Problems

Manufacturing problems actually represent a small number of the total. Some of the more common problems, which may not show up for years, are the following:

Faulty post seal design - A leaky post seal allows acid to migrate up to the post/intercell connection area, causing a connection problem. Sometimes a new design appears to work well, but then suddenly starts failing after six to eight years in the field.

Internal connection problems - Quality problems in the connection between grid tabs and the interconnecting bus have been reported from time to time. In multicell jars like six or twelve volt modules, the intercell connection between adjacent cells may fail as a result of a poor lead burn.

6. Operational Problems

· Discharge without recharge - A fully discharged or near fully discharged cell will be damaged and possibly ruined if not recharged within 24 to 48 hours. As a battery discharges, the electrolyte starts changing from an acid solution to almost pure water when the battery is fully discharged. Lead dissolves in water, and some of the plate material mixes with water to form lead hydrate. Lead hydrate causes the plate surfaces to turn white and, because it is conductive, it forms a short circuit between the plates, rendering the battery irreversibly damaged.

· Over discharge - Over discharge causes abnormal expansion of the active material in the plates, which leads to permanent damage and also recharges problems. This can happen in lightly loaded UPS systems that experience in extended power outage.

· Excessive discharging (same as excessive cycling) - Some users have local requirements that call for testing their critical backup systems either weekly or monthly. If this testing includes cycling the battery, it will severely limit the life of the batteries.

Failure Analysis Summary

Battery system failure modes can be broken down into the following two major categories:

1. Abrupt failure - This is a sudden loss of the battery system without any warning while the system is trying to perform its intended mission. This is the worst-case scenario, as it will lead to very expensive failures. In a data center application, even a momentary loss may result in millions of dollars worth of damage. An abrupt failure is cause by an interruption in the conduction path.

Typical failures are:
· Faulty intercell connection - This could be an installation problem or a severely corroded connection.
· Internal conductance path problems - Remember, the current has to flow through the post, to an internal bus, to the grids, through the paste and electrolyte, to the opposite polarity plate, and then back out through the other terminal post.

Abrupt failures can result from a totally corroded grid that is breaking apart and only able to pass a low current flow. It can also result from a terminal post that has lost its copper insert. High current batteries have a copper insert in their posts and, if this copper is exposed to acid through a small void in the lead coating, the copper will dissolve and leave only a small lead coating to carry the current. VRLA cells that have totally dried out can also be viewed as conductance path failures, since they have no effective path between adjacent plates.

2. Low capacity failure - This is a failure to support the load for the required period of time. Low capacity results from both mechanical conductance problems as well as electrochemical problems. As a battery ages and its conductance path (grid corrosion, paste to grid connection) starts to deteriorate, the internal resistance increases. When the battery is placed under load, the voltage drop across the internal resistance will cause the overall battery voltage to reach the end voltage before its rated time.

The VRLA battery has the additional problem that, as it loses water due to dryout, it loses capacity. In essence, it loses the energy storage required for a full capacity battery.
The capacity problem is a slowly developing problem that is easily detected and, most of the time, does not cause expensive outages, since it is rare that full capacity is required during an outage. Typically, an outage is a short momentary event, and an emergency generator is usually part of the backup scheme.

It should be obvious from the preceding discussion that almost all battery problems can be detected by an increase in a cell's internal resistance, and close monitoring of this parameter can avoid disastrous failures.

Resistance measurements are mandatory for applications that cannot tolerate a loss of power. The only battery test that can provide better information on a system's state of health is a full capacity test, which is also recommended on a scheduled basis.

(I think this information may help you to find out the solutions to increase your battery life).

Vinu Answers...Sure Answers.