Sopromar
A variety of batteries can be found at Sopromar Chandlery.
Futurpeças Lagos has a small auto part store, Futurpeças, that can order in batteries.
Address:
Urbanização Marina Sol,
R. Palos de La Frontera 13 Loja A,
8600-707 Lagos
Portugal
Algarbaterias
Algarbaterias comes highly recommended, although a bit further away in Faro.
Address:
R. Cunha Matos 6 - 8 A,
8000-262 Faro
Portugal
Farbateria
Also in Faro but still highly recommended is Farbateria.
Address:
Centro Comercial Horta Lj 25
8005-329 Faro
Portugal