Why you should not trust your EPIRB test button

7:54 AM Tue 26 Jan 2010 GMT

'Never trust your EPIRB test button to tell you that the EPIRB will save you on your next voyage' .

You've bought the EPIRB, put it in a good position, instructed the crew on its use and care, and before you venture out on a voyage, you always test it to make sure it's still working. You're doing the wise thing, right?

The answer is 'NO! you're not doing the right thing,' and Peter Forey, of Sartech, search and rescue technology consultants, tells why:

Any safety device that you've bought to save your life in a 'worst case scenario' should be treated with the utmost respect. You understand what it does, when you need to use it - you trust it with your life.

Yet some sailors will still ignore critical information about the battery life in their documentation.

In buying an EPIRB you have already taken the time and money to invest in it because you understand that it is a vital life-saving piece of equipment. Just like your life-jacket or life-raft, its sole function depends entirely on the unit working properly, exactly when you need it to. Otherwise it was pointless buying it in the first place.

The truth is batteries are manufactured from chemicals that break down over time. They will even degrade without being used and the rate of deterioration depends a lot on the ambient temperature. In most cases replacement is required after 5 years to ensure that there is enough capacity remaining to give the necessary operating time.

With regular alkaline batteries (see Fig. 1 below), it's pretty easy to determine how healthy they are because they decay steadily with time, as illustrated when switching a torch on and getting a progressively dimmer light. Mobile phones batteries are more complicated, in-phone software monitors your usage and charging history to predict how much battery charge you have left, but it's still just a best-guess.

However, with primary lithium batteries (the type of power source used in most EPIRBs) their decay is non linear (see Fig. 2 below).

No 'fuel gauge' has ever been developed to accurately measure Lithium cells, and their chemical composition makes it very difficult to determine how much battery life is actually left. Even years after manufacture the terminal voltage may remain relatively high, but there is no way of determining where you are in the battery's lifecycle or how much longer this voltage will remain stable enough to operate the device.

Batteries do not all lose their charge in the same way - .. .

Paranoid pushing of the 'test' button will simply decrease battery life further. To ensure that your EPIRB will work perfectly when you need it most, it is best to operate the 'test' function no more than once a month and to get the EPIRB serviced in accordance with the device's guidelines.

But, just like your car, you don't have to go back to the manufacturer and prepare yourself for a hefty bill. What you need is a Registered Shore Based Maintainer such as our company Sartech, qualified to maintain all makes of SART and EPIRB.'

It's probably going to be OK to take a gamble on the best-before date on that tin of tuna that's been knocking around in the galley for three years; but don't apply the same thought process to the one thing on your boat that could ensure your safety when you need it most.

If the worst happens and you do get into trouble at sea, we guarantee you will not be thinking about the money you saved by not servicing your EPIRB.