How Often Should You Replace a PLB Battery, and Does Age Affect Reliability?

Key Takeaways: A PLB battery might be rated for five to seven years, but that number is more of a ceiling than a guarantee. Heat, frequent self tests, and simple chemical ageing all nibble away at the available capacity long before the expiry date. A PLB with an ageing battery may still switch on, but it’s unlikely to deliver its full rated transmission time. If you sail offshore or rely on a PLB as your last line of defence, replace the battery earlier rather than later.

Why PLB Battery Life Is Not as Simple as the Sticker Suggests
How Battery Chemistry Ages on a Boat
The Hidden Impact of Storage Temperature
Self Tests and the Slow Creep of Battery Depletion
Why an Ageing Battery Reduces Transmission Duration
When to Replace a PLB Battery in Practice
Signs Your PLB Battery Is Approaching the End of Its Useful Life
Summing Up

Why PLB Battery Life Is Not as Simple as the Sticker Suggests

The printed expiry date on a PLB battery gives the impression of a firm boundary, but it’s really a controlled estimate based on ideal storage conditions. Manufacturers test their units under laboratory temperatures, stable humidity, and minimal handling. Life aboard a cruising yacht is anything but controlled.

The battery inside a PLB is designed to sit dormant for years and then deliver a long, uninterrupted burst of power when activated. That’s a tall order for any sealed lithium cell, and the moment it leaves the factory the clock starts ticking.

How Battery Chemistry Ages on a Boat

Lithium primary cells age even when they’re not being used. The internal resistance slowly increases, and the available capacity gradually falls. On a boat, this natural ageing is accelerated by:

Constant temperature swings
High humidity
Vibration
Occasional knocks or drops
Long periods of storage in lockers that heat up in the sun

None of these factors will kill a PLB outright, but they do shorten the useful life of the battery.

The Hidden Impact of Storage Temperature

Heat is the single biggest enemy of PLB battery longevity. A locker that sits in direct sun can easily reach temperatures well above the comfort zone for lithium cells. Even a few months of hot storage can shave meaningful capacity off the battery.

Cold temperatures are less damaging in the long term, but they do reduce performance during activation. A PLB that’s been stored in a chilly forepeak will still work, but the battery will deliver less energy and the unit may take longer to acquire a position.

Self Tests and the Slow Creep of Battery Depletion

Every self test consumes a small amount of battery power. The drain is tiny, but it adds up over the years. Some sailors run a self test before every passage, which is sensible from a safety perspective but does shorten the battery’s lifespan.

A PLB that’s been tested frequently may still pass its self test but fail to deliver the full rated transmission duration when activated in anger.

Why an Ageing Battery Reduces Transmission Duration

A PLB needs to transmit a strong, stable signal for many hours. As a battery ages, its internal resistance rises. That means the voltage sags more under load, and the unit has to work harder to maintain its output.

The result is predictable:

The PLB still activates
It still transmits
But it cannot sustain the rated duration

In cold water, the effect is even more pronounced. A battery that’s marginal on deck may fall short once submerged.

When to Replace a PLB Battery in Practice

Most manufacturers specify a five to seven year replacement interval. In practice, offshore sailors should treat that as the upper limit, not the target.

A sensible replacement schedule looks like this:

Coastal cruising: Replace at the printed expiry date

Regular offshore passages: Replace one year before expiry

Ocean crossings or high latitude sailing: Replace two years before expiry

If the PLB lives in a hot locker or has been through heavy use, shorten the interval further.

Signs Your PLB Battery Is Approaching the End of Its Useful Life

A PLB won’t give you a percentage readout, but there are clues that the battery is ageing:

Self tests take longer than they used to
The unit feels warm after a test
The strobe appears dimmer
The PLB has been stored in high heat
The battery is within 18 months of expiry
The unit has been dropped or knocked

None of these signs guarantee failure, but they’re all nudges towards early replacement.

Need help choosing the right PLB for you and your crew?
Click here to see my recommended models and comparison table.

Summing Up

A PLB battery is a long-life component, but it’s not immortal. Heat, age, and repeated self tests all chip away at its capacity. If you sail offshore, the cost of a battery replacement is trivial compared with the reassurance of knowing your beacon will transmit for as long as it’s supposed to. Treat the printed expiry date as the outer boundary, not the goal, and err on the side of caution.

This article was written by Dick McClary, RYA Yachtmaster and author of the RYA publications 'Offshore Sailing' and 'Fishing Afloat', member of The Yachting Journalists Association (YJA), erstwhile member of the Ocean Cruising Club (OCC) and owner/administrator of 'Liveaboard Sailboats for Sale'.