Depends where you live, if the temps in winter dont go much below 0*C(32F) maybe the 500CCA is enough but that's not the main factor, it has to be at least 75AH like the factory fitted one, never fit weaker than it was from factory cos beside the starting issue a ''smaller'' one might have greater internal resistance and it can kill the alternator, the stronger the better but never fit weaker than from factory cos if you let it unstarted for many days and it's cold outside you might get immobilised too
My friends call me Fery
2000 Discovery Td5 ES manual
Hawkeye v.6, Nanocom EVO,
VDM UCANDAS, oscilloscope
if you think my english is peculiar it means you didnt hear my japanese
ALL MY ADVICES ARE BASED ON MY OWN KNOWLEDGE AND MY WAY OF UNDERSTANDING THE DESCRIPTION AND OPERATION OF SYSTEMS... I'M NOT A LAND ROVER TRAINED SPECIALIST JUST AN ADDICTED ENTHUSIAST.