On 24 mayo, 20:23, Lobster wrote:

I've opened it up and the batteries (AA or AAA size)

you will need to open it up with a stanley knife and see if you can
get an idea of the voltage and capacity of the batteries. chances are
you'll be able to find a replacement. if need be, be sure to order
ones with solder tags spot- welded on!

http://www.chsinteractive.co.uk/replacement_batteries/ might give you
an idea.
-B.