That seems like an ideal solution for people who are willing to do very minor mod to their car. But I think there's probably a lot of people for whom it would be too much trouble to buy a suitable 12V PSU/charger, mod their car, and plug in 2 plugs every day. I wonder if there's a better solution for the masses which is technically almost as good.
Problem space: vampire load seems to be about 20Watt (W), drawing down on the sealed lead-acid battery (SLA). This battery is a adsorbent glass mat (AGM) 12 volt (12V), 33Amp-hour (Ahr) battery. 20W is about 1.5Amps (A). Every 4-5 hours, there has been about 7Amps taken out of the battery, and something closes contactors, the high voltage DC to low voltage DC charger powers up, and charges the SLA 12V battery from the main battery. Once the SLA battery is charged to a certain level, the contactors open, and the whole process starts again. While the DC-DC charger is running, some other stuff is also running, so the net effect of losses in the DC-DC process, and other unnecessary load is that the vampire load is ultimately served by the main battery, but by the time it's served at that level, it looks more like a 40W load. If the car is plugged in to a charger, every day or two it will turn on the main battery charger to boost it back to where it was.
The SLA is charged in the following situations:
* Car actively 'on' (driving)
* Car actively charging (not just plugged in)
* dedlcated 'recharge' cycle
How's this fit for a solution (manual solution for now, but could all be done automatically in software in theory):
I saw a vid of a guy charging his model S (MS) off a small 110V AC petrol generator. In the center console he was able to specify how many amps the MS should draw, right down to 1A. If you want to charge to 80%, you'd plug in at home as normal, charge to 80, then go in to the car just before it hits 80%, and set the charge rate down to 1A and the charge limit to 90%. That's about 120W going in to the car, enough to maintain the vampire load, without draining the SLA, and put a tiny dribble in to the main battery. That dribble could continue until the battery hit 90%, about 3-4 days later, or never if you drive you MS at least every 3 days.
Here's the question - what's the downside I'm not seeing? Is there a downside to charging the main battery (very slowly) for long periods of time? Is there other equipment active when the main battery is charging that will wear out if left on too long?
If this is a viable option, then software changes are all that's needed to make this work for most people - TeslaMotors could change the software to have an extra setting 'dribble charge %', default it to 5 or 10%, and most of the teslas out there would suddenly only be cycling their 12V battery when they're parked up away from their home charger. Average cycles per day would go from 5ish to 1ish, making tens of thousands of batteries last 5 times longer.
Too good to be true - what am I missing?
