But I also give the battery a helping hand. When the temperature gets near freezing, I plug in the inline heater. A warm engine starts easier than a cold one.

 

You didn't say how many miles and what electrics were running?

Also Stuart is correct re hot/cold but he may not appreciate that VERY high temps are unheard of in the UK (despite me saying earlier that over 75f was rare in the UK) whereas some US states see regular 100f temps, so I don't believe that train f thought holds here.

Since your battery is under warranty change it out, since it's at n cost to you

My 4yr old Bosch is running at 12.9v tested across the terminals,,, but your CTEK supposedly already ordered should be with you soon

 

I hope you carry a jump starter with you when you go out...

 

The battery may be fine and you just have the very common slow battery drain. I have a battery about the same age, when it gets a bit tired I throw it on for an overnight trickle maintainer. I try to put it on at least once a week as a preventative action, but there are times I may go a month and the car will tell me it is thirsty for electricity by choking at startup a bit. If it has a slow turnover I shut it off, stick it on the maintainer, and take the SUV instead. After being fully green on the maintainer it is perfect for a period of time until it is no longer perfect.

My slow drain is the consequence of our unfavorable driving profile, very short trips locally that don't replenish the battery on it's own.

 

might be your alternator about to go.I had to get mine replaced in my 2007 XKR that i no longer have at around 50,000 miles in the summer of 2016