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It may be - depending upon what kind (or, 'technology') of batteries you put in the Sterling stairlift - that the batteries really have failed, or it may be that the charging of them is inadequate or faulty.
Batteries of kinds other than lead-acid, might fail because of over-quick charging and overheating.
If you have the ability to measure the charging voltage on your batteries' charger, you can test for excessive, or insufficient, voltage, which if you can do it would be a good idea.