thanks everyone,

We have year round mild weather here so the cold cranking issue is not really a worry. I was just sort of thinking of the electrical loading on fans, blowers and the like with the air con running full in mainly town driving. What also bought it to my attention is that my 500SEL has a whacking big 100 amp/hr battery that looks non standard and certainly fills every last inch of the battery tray and then when I look at the 300 I see I could easily squeeze a bigger one in that too. My new car and I are still going through that wonderful, honeymoon type " getting to know you " phase and everything works just fine. I don't want to start worrying about a problem that does not exist, as I see everything is working good so I don't see the need. I have always favored paying over the top a bit for my batteries, a good one last for years and to me is a wise choice..

.....The only job I have to do today I think is change the oil and a filter

again, thanks to all for your ideas