Since our car is mostly electric, we don't really need cranking amps. I'm wondering if folks replace normal lead acid 12V with some type of deep cycle battery. I'm mostly interested in longevity. On my small scooter (little motorcycle, honda C70) I replaced the lead acid battery with a gel deep cycle type that has lasted about 8 years. No issues with that.
Honda's using an AGM 12v battery in the new Insight. It was likely chosen as much for size as longevity because when Honda couldn't find space for a battery under the hood they stuck it in the center console: To save weight I put a tiny Braille racing battery in my 2006 Insight (interestingly, Honda never brought the small OEM Insight 12v batteries to the US for replacements). However, reducing the weight of the 12v battery won't make much difference in the 2-ton Clarity PHEV. I'm happy to test the longevity of the Clarity's OEM lead-acid 12v battery.
As an aside, I installed a Lithium-Iron battery in a Light Sport aircraft. It weighs barely more than 2 lbs and replaced a 15+ lb lead/acid AGM Odyssey. That 13 lbs in a Light Sport reflects about 2 more gallons of gas to stay in limits. Anyway, that battery is now over 3 years old and still cranks my 100hp ROTAX with real authority. I wonder if such a battery would do in a Clarity - they do make larger sizes which are still quite light. Mine is the middle one, top row here: https://www.aerovoltz.net/lithium-experimental-aircraft-batteries/
Flooded, AGM, and gel are all lead-acid batteries. Deep-cycle vs. starting refers to construction of the plates inside a battery. For example, it’s possible to use either deep cycle or starting type plate construction in either a flooded or an absorbed glass mat type of battery. It seems like a deep cycle battery makes a lot of sense for a Clarity. If I were to do that someday, I would stick with a flooded battery, since the car’s charging system is designed for a flooded, not sealed, battery. Lithium batteries are a different creature altogether, as we know from the HV battery in the Clarity. 12V lithium batteries are coming on strong in the RV market and we may begin seeing them in the vehicle starting market soon. They do like a different charging profile than lead-acid batteries.
