I've had a Lifeline (sealed) battery for 3 years and very good it is. However, when lifting it out of the boat for the winter this year, I noticed that the sides of the casing are all dished inwards slightly as if the pressure inside the battery is less than outside. Has anyone any thoughts/experience whether this is normal?

I've seen plenty of batteries in my time but never observed the symptom you describe. It's plain that on a cold day the pressure inside the battery will drop, but I would expect there to be a breather to prevent deformation of the casing. A split battery could cause injury and/or other merry hell, best case you might not be able to start your engine when you need it, so best get in touch with the manufacturer for clarification methinks. You never know, maybe it's designed to be that way.

Hi, This is perfectly normal for a Lifeline Battery - see www.impossiblebattery.com or www.lifelinebatteries.com for more info