Manger Elk emergency lifting cushion - not working (Olton B92)

I am offering my mother-in-law's Manger Elk emergency lifting cushion. If someone falls over, they can be rolled onto the deflated cushion. The pump is then activated and the cushion inflates to about the height of a dining chair. The person can the get up from the seated position.

The cushion hasn't been used for many years. The pump has batteries and is designed to be charged up before use. The power light illuminates on the charger but the batteries don't appear to be charging. The pump does make a brief noise suggesting that the battery is flat. Replacement batteries are available, but I don't know whether this would solve the problem.