Beeping sound from Stena stairlift control box...

The control box on my elederly parent's stairlift started beeping every three seconds. Does anyone know what this means? I opened the box thinking that the PP3 battery inside might be running down, but the beeping continued after I replaced it.
I wonder if the beeping is a warning that the main gel-cell battery powering the motor is on its way out. Anyone know? The gel-cell is a bit of a pain to get at, otherwise I'd just test it with my multi-meter rather than posting this question. The gel-cell bettery has not been changed for about ten years, I believe. It is supposed to keep itself charged up from the mains while the stairlift's chair is stationary.
Thank you..
JD
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According to the site the battery last 3 to 5 years depending on use?
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Best way is to locate the gel-cell and see what volatge it is? then apply a voltage to the gel-cell terminals on the stair lift and see if this is the problem and if its not? then you have saved yourself gel-cell money.
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I have a redundant Stena system but we left it installed. When my wife was on crutches recently (riding accident) we found the batteries had died: one of the kids killed the power 'because it was bleeping' and didn't tell us. I can check the voltage OK, but I'm blowed if I can see how to get it apart to the stage where I can remove them.
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