Piston Slap: Foiled by a Dome Light?

Peter writes:

I drive a 2010 Toyota Prius. From top to bottom, I’m thoroughly impressed by the technology in this car. And yet this engineering marvel is so easily disabled by its inferior owner leaving a dome light on overnight and draining the 12V battery.

Is there any technical reason cars allow the 12V battery to be drained down beyond the point where the car will start? Who needs that extra 6 hours of dome lighting?

Sajeev answers:

And here I thought every modern vehicle implemented a battery saver relay/switch/doohickey, eliminating human error! The Best and Brightest better find us a reason not to isolate such “human-error only” items, as hybrids aren’t exempt in my book.

Since when did a map/dome/cargo light play such an influential role in an electrical system’s design?

Let the people have an always-on USB port/cigarette lighter for an emergency phone charge (they stop charging at 100%) or for a hit of that sweet, sweet nicotine monster. Everything else open to human errors should shut off after 10-30 minutes of inactivity from a human or proximity key.

I’d recommend one of these:

Easy: change the offending (reading/dome/cargo) interior lights to LED bulbs. Not all are created equal in terms of light quality, but that’s a bigger concern for exterior lighting. LED’s lower power consumption could be the difference between a start or a no-start on that tiny 12V battery.

Hard: Install a battery saver relay intended for another application, a battery saver system, or a trigger module: all require extensive knowledge of the Prius’ chassis wiring to make sure the 12V system won’t go berserk over such a change.

[Image: By Mariordo Mario Roberto Duran Ortiz (Own work) [ CC BY-SA 3.0], via Wikimedia Commons]

Send your queries to sajeev@thetruthaboutcars.com. Spare no details and ask for a speedy resolution if you’re in a hurry … but be realistic, and use your make/model specific forums instead of TTAC for more timely advice.