Quick background first - I'm planning to build a small box with some relays and circuitry to cut off power to a load (laptop, inverter, mini-fridge, etc) when the battery gets low and re-energize power when the car is running again.
To do this, I need to know what the operating voltage range is on a typical gasoline car within about 0.1v - because I don't want it to cut off if while the engine is ideling...but I don't want to drain the battery so far it can't start the engine. I found on my car that if the loads (heated seat, headlights, computer, radio) cuts off at about 11.8v there's enough power to crank the engine over when the battery rebounds to about 12.2v but if it fails to start within about 2 seconds of cranking it needs a charge/jump.
I'm thinking because there would be a load that 11.8-12.0v would be the low cutoff and 12.6-12.8v would be a good re-energize point...but I'm concerned the electrical system may drop to 11.8v if it's ideling (~600rpm) for an extended time with a heavy load...I've measured this occuring on my own car running a laptop charger (drawing 12v@9A), headlights, A/C or heating blower, and dual heated seats while ideling for about 5 minutes, then it comes back up to about 14v if I rev the engine to 1200rpm for almost a minute.
Anyone got suggestions I'd be thankfull - measurements would be nice too...
Tags: