Operating a Battery Bank with a Failed Battery

When a battery fails and you’re waiting for a replacement, continuing to operate the system can affect how the rest of the battery bank is loaded and charged.

In some situations, you may want or need to leave the failed battery installed temporarily. Whether this is practical depends on the system design, nominal voltage, battery configuration, and the type of failure.

Battery failures generally fall into three categories: an open cell, a shorted cell, or a transitional failure that progresses from an open cell toward a shorted cell.

With an open-cell failure, the rest of the battery bank may have difficulty reaching a full charge. The failed cell changes the battery’s resistance and affects how the bank accepts charging current. As a result, the bank may appear to charge normally without actually reaching a full state of charge.

A shorted cell creates a different problem. Because the failed cell no longer provides the normal voltage drop of a healthy cell, the remaining cells must absorb more of the charging voltage. This can lead to overcharging and damage to the other batteries.

The third condition is typically a progression from an open-cell failure to a shorted cell as the cell continues to deteriorate. This type of failure can go unnoticed, particularly when routine maintenance and testing are not being performed.

Removing a failed battery also changes the bank’s total capacity and the amount of current the remaining batteries must supply. Depending on the configuration, it may also require changes to the system’s charging parameters.

If a battery is removed and the system continues operating exactly as before, the remaining batteries can be subjected to considerably more stress. In some cases, one failed battery can eventually lead to several failed batteries because the remaining bank is expected to support the same loads with less capacity.

Make Sure the System Can Operate at a Lower Voltage

In some 24V and 48V systems built with individual 2V cells, it may be possible to remove a failed cell and temporarily operate at a lower system voltage. If this is done, the DC voltage settings in the inverter and charger must be adjusted accordingly.

A 24V system normally contains 12 cells in series, while a 48V system contains 24. For a 48V system reduced to 23 cells, an approximate revised setting can be calculated by dividing the original voltage setting by 24 and multiplying the result by 23.

For example, if the original settings are:

• Bulk/Absorb: 59.6V

• Float: 54.6V

• Equalization: 61.0V

• Low Battery Disconnect: 45.2V

With 23 cells in series, the approximate settings would become:

• Bulk/Absorb: 57.1V

• Float: 52.3V

• Equalization: 58.4V

• Low Battery Disconnect: 43.3V

This approach is only suitable if the connected equipment can operate safely at the reduced voltage and the charging equipment allows the required adjustments.

Systems using 6V batteries offer fewer options because each battery contains three 2V cells; an individual failed cell cannot simply be removed.

The same limitation applies to 12V batteries connected in series. Removing one 12V battery from a 48V string results in a 36V bank, and the inverter or other equipment may not be designed to operate at that voltage.

A bank consisting of multiple parallel strings is different. If one complete string is removed, the remaining strings can generally continue operating at the correct system voltage, provided they are properly configured for the load.

Before making any changes, confirm exactly how the batteries are connected and verify that the remaining bank provides the voltage required by the equipment.

Reduce the Loads

Removing a battery or string from a parallel bank reduces the bank’s total available capacity.

For example, if a bank originally contains four identical 12V batteries connected in parallel and one is removed, approximately 75% of the original amp-hour capacity remains. The system should not continue to be operated as though its capacity were unchanged.

Reduce unnecessary loads wherever possible. High-demand equipment such as electric heat, air conditioning, water heaters, pumps, power tools, large inverters, and other significant loads can place considerable stress on the remaining batteries.

If a backup generator is available, consider using it during periods of high demand. With fewer batteries supplying current, the bank may also experience greater voltage drop when large loads are switched on.

Check the Low-Voltage Disconnect

If the inverter or other equipment has an adjustable low-voltage disconnect, review its setting. With reduced battery capacity, you generally do not want the system attempting to draw the same amount of usable energy from the remaining batteries as it did from the full bank. A more conservative low-voltage disconnect setting can help protect the batteries while you wait for a replacement.

Keep in mind that voltage under load is not a perfect indication of state of charge. A large load can temporarily pull battery voltage down even when the battery is not deeply discharged.

Review the Charging Settings

Removing batteries from a parallel bank reduces its total amp-hour capacity. Charger settings that were appropriate for the original bank should therefore be reviewed to ensure they remain appropriate for the smaller bank.

This does not necessarily mean changing the charging voltage. The battery manufacturer’s recommended charging voltages should remain the starting point.

Review the:

• Bulk voltage

• Absorption voltage

• Float voltage

• Maximum charging current

• Absorption time

If cells have been removed from a series bank, the charging voltages may also need to be adjusted to match the reduced number of cells.

If the charging equipment does not allow the necessary voltage adjustments, operating with cells removed may not be practical or safe.

Don’t Overlook Absorption Time

Absorption time is particularly important with lead-acid batteries.

Reaching the absorption voltage does not mean the battery is fully charged. Once that voltage is reached, charging current gradually decreases as the battery approaches a full state of charge. The battery must remain in absorption long enough for this process to occur.

If the absorption timer was configured for the original battery bank, review it after reducing the bank’s capacity. In many cases, a smaller battery bank may require less absorption time.

At the same time, repeatedly ending absorption too early can leave the batteries chronically undercharged. Over time, this can contribute to sulfation and reduced battery life.

For this reason, don’t rely solely on automatic charger settings. Verify what the charger is actually doing and compare it with the battery manufacturer’s recommendations.

For Flooded Batteries, Check Specific Gravity

For flooded lead-acid batteries, specific gravity is one of the most useful indicators of what is happening inside the battery.

Where the battery design allows it, check and record specific-gravity readings periodically. Watch for batteries that consistently show low readings or individual cells that differ significantly from the others. Voltage provides useful information, but specific gravity can reveal conditions that voltage alone may not identify.

Monitor Battery Temperature

Temperature is another important factor in charging lead-acid batteries. Charging voltage should be compensated for battery temperature. If the temperature sensor is disconnected, installed incorrectly, or providing inaccurate readings, the batteries may be overcharged or undercharged and potentially damaged.

Verify that the temperature sensor is:

• Properly connected

• Installed in the correct location

• Reporting a reasonable temperature

• Correctly configured in the charger

Temperature compensation becomes particularly important when batteries are operating in very hot or very cold environments.

Check the Connections

If a parallel battery bank contains multiple series strings and one string is removed, inspect the remaining connections carefully. Check the battery terminals, cables, busbars, fuses, disconnects, and cable connections. Make sure cables are correctly sized and connections are clean and tightened to the manufacturer’s specified torque.

Battery connections can be overtightened, so use a properly calibrated torque wrench. Avoid using an impact driver, as it can easily apply substantially more torque than intended. Current will favour the path of least resistance. If parallel connections are not properly configured, one string may carry more of the load than the others. Poor connections can also introduce resistance, resulting in voltage drop and heat that may eventually cause terminal or connection failure.

If possible, measure the voltage of each battery while the bank is operating under a known load. If one battery differs significantly from the others, investigate the cause rather than assuming the bank is operating normally.

When the Replacement Arrives

Don’t simply install the replacement battery and assume everything will immediately return to normal.

If the replacement is new and the existing batteries have already been in service for several years, differences in age, capacity, state of charge, internal resistance, and overall condition can affect how the batteries share current.

Ideally, batteries operating in the same bank should be reasonably well matched. Always follow the battery manufacturer’s recommendations regarding the installation of new batteries alongside existing batteries.

Before installation, verify the replacement battery’s state of charge.

A new battery may require approximately 10–20 cycles, including meaningful cycling of roughly 30–50%, to fully form and reach its rated capacity. If the entire bank is not being cycled during this period, the new battery may not fully form and may behave differently from the existing batteries.

For flooded batteries, specific gravity can be used to help verify state of charge. For AGM and gel batteries, follow the manufacturer’s recommended charging and verification procedures.

Avoid connecting a fully charged replacement battery to a significantly discharged bank and expecting the batteries to immediately balance.

Where possible, bring the existing bank close to full charge before installing the replacement. After installation, monitor the bank closely over the next 10–20 cycles as the new battery reaches its full capacity.

Document the System Before Making Changes

Before changing the battery configuration or charger settings, document the system’s current condition.

Record:

• Battery model and configuration

• Individual battery voltages

• Specific-gravity readings, where applicable

• Charger settings

• Inverter settings

• Absorption time

• Float voltage

• Temperature-compensation settings

• Battery temperature

• Charging current

• Major loads being used

After installing the replacement battery, take the same measurements again.

These readings provide a useful baseline. If something does not look right after the replacement is installed, you will have before-and-after measurements to help identify the problem.

Summary

A failed battery should not simply be removed from the bank and forgotten while you wait for a replacement. Although the remaining batteries may be able to keep the system operating temporarily, the bank should be treated as a reduced-capacity system.

Reduce unnecessary loads. Check the low-voltage disconnect. Review the charging settings, including maximum charging current and absorption time. Monitor battery temperature and, for flooded batteries, check specific gravity. Inspect the connections and watch for signs that one battery or string is carrying more of the load than the others.

When the replacement arrives, don’t assume it is ready to install simply because it is new. Verify its condition and state of charge, and follow the battery manufacturer’s charging and installation recommendations before adding it to the bank.

The objective is straightforward: protect the batteries that remain so that one failed battery does not lead to additional failures.

When in doubt, follow the battery manufacturer’s charging and installation recommendations or have a qualified technician evaluate the system before making changes.