* what will it cost to run this over every 24 hrs...assuming an avg electric rate?
* what is the efficiency of this small transformer using grid power vs the car's alt?
* will it keep the battery charged?
* will it last...or will it overheat?
The battery probably needs replacing...does fine in summer...but if it sits in winter it will go flat.
I've heard that a failing battery can reduce mpg due to the alt constantly having to charge it.
Assuming 120V input and 14V output, you are using 11.5W over 24 hours or 0.276 kwh. At 10c per kwh, it would cost less than 3 cents per day. You are only outputting 5.6W however so you are less than 50% efficient. The other 50% is the heat you feel from the transformer. It will probably last as long as your battery is able to hold a charge. It will probably overheat if it tries to charge a shorted battery. And yes, a failing battery can reduce FE due to the reason that you listed.
Also, the car battery is likely MUCH lower internal resistance than the drill batt. If you think the charger is getting too hot and you'd like to prevent it brom burning out, put a resistor in series with the 12V coming from the charger. Try a 1 ohm (give or take) and a high enough wattage.
Reality is merely an illusion, albeit a very persistent one. - Albert Einstein
the chargers output is less then an amp, trickle charges that you can leave on your battery for entire winters are normaly half to one amp output, so I would say that your car battery is fine as far as the chargers output, but your charge will most likely not take more then a few weeks of use befor it burns out because it's not designed to run at 100% like this, right now it's dumping all of it's energy in to a large battery, and the charge is never going to act like the battery is getting full, so it's going to stay at 100% output and burn out faster.
if your battery is weak, check the water leavle, toping it off with distilled water, chean all of the electrical contacts and ground connections between it and the alternator useing some dielectric greese if you have it.
if you are tight for money, while it is still nice out ride a bicycle as much as you can, keeping it in the car if you are going to make some alot of stops farther away from home, after a few weeks your savings should start to become enough to buy a new car battery.
I'm just comparing the transformer temp by "feel" when on the drill vs the car battery...it's only slightly warmer.
Will probably use this method of charging the batt in the winter mostly...the idea being to make it easy to plug in and use...and not to use a regular battery charger which won't charge below 2 amps and is not as efficient...and needs an open hood?
This is mostly about the feasibility of using one of the inexpensive wallwarts to maintain a battery for mpg purposes. Plus in winter.....
"HINT: Replace your battery if it is old/weak. A weak battery drains excess energy from the alternator and may waste the alternator. But in the meantime, the high current draw puts a load on your gas mileage. A new battery from Batteries Plus can be a nice little boost to mileage. We also like to charge the battery overnight with a trickle charger to improve MPG. How is this possible? We make the alternator do less work because if the battery is fully charged, the energy expended by the engine is reduced."
I thought I had too much batt drain when the car sat for a few days...not sure whether it was the battery or something was draining it.
I can afford a battery...I was just in the process of diagnosing the problem when spring hit and the problem went away.
Battery has been flatlined too many times to be useful now though.
Leading the perpetually ignorant and uninformed into the light of scientific knowledge. Did I really say that?
a new policy....I intend to ignore the nescient...a waste of time and energy.
If you want to bring your battery back to life, you might try a pulsating charger that is said to desulfate the battery, I've used them on batteries that wouldn't keep a full charge, and about 80% of the time I'm able to get them back when other chargers don't seem to work.