Medical Device Link.

Originally Published MEM Fall 2003

BATTERIES

Despite its age, no suitable replacement has been found for sealed lead-acid battery technology for use in some medical devices.

Isidor Buchmann

The lead-acid battery is the oldest type of rechargeable battery. Invented by the French physicist Gaston Planté in 1859, lead-acid serves stationary or wheeled applications when weight and size are of lesser concern. Today, the flooded lead-acid battery is used in automobiles, forklifts, and large uninterruptible power supply (UPS) systems.

During the mid-1970s, a maintenance-free lead-acid battery was developed that could operate in any position. The liquid electrolyte was transformed into moistened separators and the enclosure was sealed. Safety valves were added to allow venting of gas during charge and discharge.

Two designations of sealed lead-acid batteries have emerged: the sealed lead-acid (SLA) battery, also known as Gelcell, which serves predominantly in wheeled mobility applications, and the large-valve regulated lead-acid (VRLA) battery, which is used for stationary applications. In the medical industry, SLA technology is predominantly used for wheelchairs, ventilators, incubators, and some defibrillators.

The SLA battery is not subject to memory loss. Leaving the battery on float charge for a prolonged time does not cause damage. The battery's charge retention is best among rechargeable batteries, which makes it an attractive option for defibrillators that are only used occasionally. Whereas the nickel-cadmium (NiCd) battery self-discharges 40% of its stored energy in three months, the SLA self-discharges the same amount in one year. The SLA is relatively inexpensive to purchase, but the operational costs can be more expensive than the NiCd if full cycles are required on a repetitive basis.

The SLA does not lend itself to fast charging, which makes it unattractive for use in mission-critical devices. The typical charge time for SLA batteries is 12–16 hours. The SLA must always be stored in a charged state. Leaving the battery in a discharged condition causes sulfation, a condition that makes the battery difficult, if not impossible, to recharge.

Unlike the NiCd, the SLA does not respond well to deep cycling. A full discharge causes extra strain, and each discharge-charge cycle robs the battery of a small amount of capacity. Other battery chemistries wear down similarly in varying degrees. For devices that require frequent charge-discharge cycles, a large SLA battery is recommended to prevent the battery from being stressed from repetitive deep discharge.

The SLA provides from 200 to 300 charge-discharge cycles. Its relatively short life cycle is attributable to grid corrosion of the positive electrode, depletion of the active material, and expansion of the positive plates. These changes are most prevalent at higher operating temperatures. Applying charge-discharge cycles does not prevent or reverse the trend.

Among modern rechargeable batteries, the lead-acid battery family has the lowest energy density, making it unsuitable for handheld medical devices that demand compact size. In addition, performance at low temperatures is poor. The high lead content makes the SLA environmentally unfriendly if carelessly disposed.

Charging the Lead-Acid Battery

Figure 1. Charge stages of a lead-acid battery. A multistage charger applies constant-current charge, topping charge, and float charge. (click to enlarge)

The charge algorithm for lead-acid batteries differs from nickel-based chemistry in that voltage limiting rather than current limiting is used. Although the typical charge time of an SLA is 12–16 hours, with higher charge currents and multistage charge methods, charge times can be reduced to 10 hours or less. SLA batteries cannot be charged as quickly as nickel- or lithium-based systems.

A multistage charger applies constant-current charge, topping charge, and float charge (see Figure 1). During constant current-charge, the battery charges to 70% in about 5 hours; the remaining 30% is completed by the slow topping charge. The topping charge lasts another 5 hours and is essential for the well being of the battery. If never completely saturated, an SLA would eventually lose its ability to accept a full charge, and the performance of the battery would be reduced. The third stage is the float charge, which compensates for the self-discharge after the battery has been fully charged.

The charge-voltage threshold is critical. A typical voltage limit is from 2.30 to 2.45 V. If a slow charge is acceptable, or if the room temperature may exceed 30°C (86°F), the recommended voltage limit is 2.35 V/cell. If a faster charge is required, and the room temperature will remain below 30°C, 2.40 to 2.45 V/cell may be used. Table I compares the advantages and disadvantages of the voltage settings.

The charge-voltage limit indicated in Table I represents a temporary voltage peak when applying a full charge cycle. The battery cannot dwell on that level. Once fully charged and at operational readiness, the float charge maintains the voltage at a lower level. The recommended float charge voltage of most low-pressure lead-acid batteries is between 2.25 and 2.30 V/cell.

The optimal float-charge voltage shifts with temperature. A higher temperature demands slightly lower voltages, and a lower temperature demands higher voltages. Chargers that are exposed to large temperature fluctuations are equipped with temperature sensors to optimize the float voltage.

The voltage settings in Table I apply to a low-pressure SLA battery with a pressure relief valve setting of about 34 kPa (5 psi). By contrast, the cylindrical Hawker SLA requires higher voltage settings. The voltage limits should be set according to the manufacturer's specifications. Failing to apply the recommended settings causes a gradual decrease in capacity due to sulfation. Typically, a Hawker cell has a pressure relief setting of 345 kPa (50 psi). This pressure relief allows some recombination of the gases during charge.

The price of a Hawker cell is slightly higher than that of the plastic equivalent, but lower than the NiCd. Also known as the Cyclone, the Hawker cell is wound similar to a cylindrical NiCd. This construction improves the cell's stability and provides higher discharge currents when compared with a flat-plate SLA. Because of their relatively low self-discharge, Hawker cells are suitable for defibrillators used on standby mode.

An SLA must be stored in a charged state. A topping charge should be applied every 6 months to prevent the voltage from dropping below 2.10 V/cell. The topping charge requirements may differ with cell manufacturers.

An approximate charge-level indication can be obtained by measuring the open cell voltage while in storage. A voltage of 2.11 V, if measured at room temperature, reveals that the cell has a charge of 50% and higher. If the voltage is at or above this threshold, the battery is in good condition and only needs a full charge cycle prior to use. If the voltage drops below 2.10 V, several charge-discharge cycles may be required to bring the battery to full performance. When measuring the terminal voltages, the storage temperature should be observed. A cool battery raises the voltage slightly, and a warm one lowers it.

Some buyers who inspect batteries during quality control reject SLA batteries arriving from vendors with less than 2.10 V/cell. Low voltage suggests that the battery may have a soft short, a defect that cannot be corrected with cycling. Although cycling may increase the capacity of these batteries, the extra cycles compromise the service life of the battery. Furthermore, the time and equipment required to make the battery fully functional adds to operational costs.

Restoring and Prolonging SLA Batteries

The SLA battery is designed with a low overvoltage potential to prohibit the battery from reaching its gas-generating potential during charge. Excess charging would cause gassing and water depletion. Consequently, the SLA battery can never be charged to its full potential.

Finding the ideal charge voltage limit for a sealed lead-acid system is critical. Any voltage level is a compromise. A high voltage limit produces good battery performance, but shortens the service life due to grid corrosion on the positive plate. The corrosion is permanent and cannot be reversed. A low voltage preserves the electrolyte and allows charging under a wide temperature range, but is subject to sulfation on the negative plate.

Once the SLA battery has lost capacity due to sulfation, regaining its performance is often difficult and time-consuming. Reasonably good results in regaining lost capacity are achieved by applying a charge on top of a charge. This is done by fully charging an SLA battery, then applying another charge after a 24- to 48-hour rest period. This is repeated several times, and then the capacity of the battery is checked with a full discharge. The SLA is able to accept some overcharge; however, too long an overcharge could harm the battery due to corrosion and loss of electrolyte.

Applying an overvoltage charge of up to 2.50 V/cell for one to two hours can reverse the effect of sulfation of the plastic SLA. During that time, the battery must be kept cool and it must be carefully observed. Extreme caution is required to avoid raising the cell pressure to a venting point. Cell venting causes the membrane on some SLA batteries to rupture permanently. Not only do the escaping gases deplete the electrolyte, but they are also highly flammable.

A Hawker cell can be stored at voltages as low as 1.81 V. Reactivation is relatively easy. However, when activating, the cell voltage under charge may initially raise up to 5 V while absorbing only a small amount of current. Within about 2 hours, the small charging current converts the large sulfate crystals back into active material. The internal cell resistance decreases, and the charge voltage eventually returns to normal. At a voltage between 2.10 and 2.40 V, the cell is able to accept a normal charge.

To prevent damage, current limiting must be applied to protect the battery. Always set the current limit to the lowest practical setting. If current limiting is not available, the battery should be observed at all times. Not all Hawker cells allow restoration after prolonged low-voltage storage.

Attempting to improve the capacity of an older SLA battery by cycling is futile. Such a battery may simply be worn out. Cycling would just deplete the battery further. SLA batteries are commonly rated at a 20-hour discharge. Even at such a slow rate, a capacity of 100% is difficult to obtain. For practical reasons, most battery analyzers use a 5-hour discharge when servicing SLA batteries. This typically produces 80–90% of the rated capacity. SLA batteries are normally overrated.

It is important to note that in case of rupture, leaking electrolyte, or any other exposure to the electrolyte, flush the affected area with water immediately. If electrolyte is exposed to the eyes, it is critical to flush them with water for 15 minutes and consult a physician immediately.

Conclusion

The SLA battery serves a market in which newer battery chemistries would be too expensive or the upkeep too demanding. A modern replacement may simply be too delicate and could fail prematurely if subjected to harsh environment. For applications such as wheelchairs, scooters, and small UPS units, no suitable option—one that is both rugged and cost-effective—has been found to replace the SLA battery in some applications.

But like any other battery, the SLA exhibits weaknesses and certain requirements must be met to obtain a long and reliable service. It is important to keep an SLA charged at all times. Never store it below 2.10 V/cell. It is also critical to avoid repeated deep discharges and to charge it more often than other chemistries. If repeated deep discharges cannot be avoided, use a larger battery to ease the strain. Finally, remember to prevent sulfation and grid corrosion by choosing the correct charge and float voltages.

Isidor Buchmann is the founder and CEO of Cadex Electronics Inc. (Richmond ,BC, Canada). He can be reached via e-mail at isidor.buchmann@cadex.com. This article contains excerpts from the second edition of his book titled Batteries in a Portable World—A Handbook on Rechargeable Batteries for Non-Engineers. The 300-page book is available from Cadex Electronics Inc. through book@cadex.com and at http://www.amazon.com. For additional information on battery technology, visit http://www.buchmann.ca or http://www.batteryuniversity.com.

Copyright ©2003 Medical Electronics Manufacturing