What is a safe margin for the battery for daily use. I mean when you drive stealth and the battery is allmost empty, when do you decide to switch on the petrol engine? How many Miles is a safe margin for the battery? 5 to 10?
I let mine go down all the way and the gas engine will kick in as needed. The system keeps a reserve in place at all times to prevent the battery from discharging too far. With these types of batteries you can do more harm by leaving them fully charged for long periods of time.
When the car tells you that you have 0 stealth miles, it really means that that battery is still at 15% or higher. The computer never lets it get below 15% without turning on the Engine and recharging it to there. So, you don't have to worry about getting to 0 -- it's not really "0". However, on the other side of the coin, you also aren't supposed to worry about driving it and charging it each night. According to a Fisker Owners Video, they show better than 85% average capacity remaining after 3000 charging cycles -- which would be basically 10 years of charging it each night if you drove it nearly every day. So, they recommended charging the Karma nightly (assuming you drove it that day) for optimal electric range/gas mileage and starting each day with 50 miles available. Unless your habit is to regularly drive less than 20 miles a day, I'd do that. If you really are only going down to the store or church and back home ... then you would probably be best to go a few of those drives before you recharge.
Supposedly, that recommendation was based on a battery efficiency after 3000 cycles (that's basically 10 years) still being over 85%. So, you might as well enjoy the cheaper electric miles if your battery is robust enough to handle charging that often. There are certainly people who have more technical credentials than me whom I would love to have comment here, but this is my current understanding.