Our Swedish car was stored for 6 months each winter, so the battery was not used continuously. I usually kept the battery inside our apartment while the car was being stored so the battery didn't experience Sweden's winter temperatures.TOEd wrote:In my experience batteries last longer if used frequently. So I use the agm battery from my snowmobile in my boat during the off season. I live in Canada where the cold is very hard on batteries. I would guess that Hawaii doesn't get -40 C very often . So 1 to 2 years is what is expected for lifespan here. I also use a battery maintainer to top up batteries. It helps.
One nice feature of AGM batteries is that their self-discharge rate is very low which makes keeping them on a battery maintainer not necessary unless the storage period is longer than a year. In fact, keeping a battery maintainer on an AGM battery could shorten its life. Just fully charge it and let it sit.
We have stored our i3 for 6 months each year since 2014. I didn't remove its AGM battery during storage and its voltage did not drop much. However, something has apparently caused it to degrade unlike my experience with the AGM battery in our Swedish car. BMW of Honolulu will be testing it on Monday, so I'll know more then.