SLA Battery Testing Challenges

Is my battery still good?

It is a straightforward question that should have a straightforward answer, but it doesn’t. The State of Health (SoH) of your battery is complicated.

There are three indicators that ideally should be evaluated to determine if the battery is still good:

• Capacity – the ability of the battery to store energy
• Internal resistance – the ability of the battery to deliver current
• Self-discharge – the rate at which the battery loses charge when not in use, reflecting mechanical integrity and stresses on the battery

How to test a battery

The best way to test the charge of a battery is with a multimeter. This device gives a good indication of how high or low a battery’s charge is.

Of the three indicators, capacity is the leading measure of the state of health of a battery. When new, a battery should deliver 100 percent of its rated capacity. If a battery rated at 5 Ah delivers 5 amperes for one hour, its capacity is 100 percent. If it delivers 5 amperes for only 30 minutes, then the capacity is 50 percent. However, there is no convenient test for capacity without subjecting the battery to controlled charge and discharge cycles, which makes it difficult to know exactly when to replace a battery.

In addition, lead-acid batteries often start out at only about 85 percent capacity when new and reach 100 percent capacity after some use. Once they hit 100 percent, they begin to lose capacity again, further complicating the picture.

When to retire a battery

Without a simple, accurate SoH test, deciding when to retire a battery depends heavily on the application. If a battery is running critical medical equipment or essential military devices, it will typically be retired well before end of life. Where device failure could endanger life or safety, squeezing extra use out of a battery is not worth the risk.

For some applications, equipment owners or managers set cycle-count or age-based thresholds for battery retirement. In systems where a weak battery means the equipment simply will not function, the battery will be swapped out much earlier than in applications that can still operate acceptably on reduced capacity.

Some batteries and portable devices have a fuel gauge that shows how much energy remains in the battery. A full charge will show as 100 percent regardless of the battery’s age. This means a fully charged battery that has lost capacity will look the same on the gauge as a new battery, even though the device will stop working sooner. These gauges show only the state of charge, not the remaining capacity, and can give a false sense of how long the device will run.

Because sealed lead-acid (SLA) batteries are sealed and maintenance-free, there is no single, simple test to determine SoH. If the battery powers a critical function, err on the side of caution and replace it early. The older battery may still be usable in a non-critical machine or vehicle.

The battery can be recycled. It will be taken apart and its components can be re-used in new batteries or treated and returned safely to the environment.

The battery can be recycled. It will be taken apart, and its components can be re-used in new batteries, or neutralized and put back into the environment.