Batteries Not Discluded

d03a.jpg (11645 bytes)The nickel-cadmium, or Ni-Cad, battery, was the mainstay of defibrillators, infusion pumps and other medical devices for many years, but lately a blast from the past emerged to challenge the Ni-Cad’s dynasty — the lead-acid battery.

A great deal of concern has been expressed over the variability of sealed lead-acid batteries. First year failure rates exceeding 50 percent have been reported for certain makes and models. This means a thorough quality control program is essential to monitor your stock and a battery maintenance program is essential to keep healthcare devices operating reliably once the plug is pulled out of the wall.

Lead-acid batteries fall into three design categories: conventional, which have vents and holes adding distilled water to the liquid electrolyte to compensate for losses due to evaporation or during high-rate discharge and charge; “maintenance-free,” with vents to allow gas to escape, but without filling holes; and sealed.

The most common form of lead-acid battery found in medical equipment uses a gelled electrolyte — the “gel cell.” Most are sealed, but occasionally you may find a gel cell that is vented and must be mounted upright.

Batteries are built up from individual cells to create a unit with the desired voltage and capacity. Gel cells provide 2.4 volts per cell when fully charged. The amount of energy the assembled battery can store is measured in ampere-hours (ah).

The rated capacity, which is what the manufacturer uses for warrantee purposes, will be stated in either amps or milliamps and is often expressed as the “C/20”, or the current flow that will drain the battery over 20 hours. This figure can be used to estimate how long the battery will run a device, but it must be adjusted — “derated” — if your application draws more than the C/20 current.

A good service procedure will acquire the same information the battery manufacturer uses to consider warranty claims. After initial diagnostic checks the battery is “conditioned.” A cycle of deep discharges and recharges are applied to the battery to equalize its cells. Then additional tests are performed to prognosticate the batteries’ fitness for your application.

When a discharged lead-acid battery is left idle, lead sulfate crystals will form on the negative terminal of each cell. This is called “sulfation” and it lowers the capacity of the battery. These crystals can grow large enough to destroy the battery. Conditioners for lead-acid batteries employ tricks to break up the crystals, such as deliberately overcharging the cells or hitting the battery with a voltage spike.

Gel cells can be damaged by overcharging and should not be trickle charged unless the battery voltage is closely monitored. The best charging systems will deliver a safe charge current until the cell voltage reaches approximately 2.3 volts. Then the charge current will taper off until a full charge is reached. Gel cells used in stand-by applications may be “floated” on a supply regulated to deliver 2.3 volt per cell.

The minimum battery capacity needed for various medical devices is based on how much time the device must operate. This number may be used as your threshold for passing a conditioned battery. For example, one defibrillator manufacturer permits a battery to be used until its capacity is 85 percent of new. In less demanding applications, a 60 percent cut-off may be appropriate.

One battery analyzer manufacturer claims 60-70 percent of apparently useless Ni-Cads can be restored to 85 percent of rated capacity by reconditioning. Attempts to reanimate sealed lead-acid batteries are less successful. Conditioner manufacturers claim 15 percent can be restored, although a more realistic figure for batteries used in healthcare is about five percent. Even so, if you use a lot of batteries, you can recoup the $2-3,000 investment in a good conditioning system within a couple of years.