How much current is needed to maintain the battery at it's charge?

 

Since noone has given you a scientific answer, I'll give it a stab.
My old Battery Tender is a 1.5 A/h smart charger. Assuming a "normal" modern day car that has a small ECU draw and a clock and alarm system AND a healthy battery, I'm guessing a draw of 1 A/h or less because my smart charger could easily top up a battery and then go on standby mode for several hours before going into "top up" mode again. An older battery or a car with lots of accessories that draw constant juice, the smart charger would be on continuously and as far as I could tell, was able to keep up. I knew this because the thing would actually go on standby but only for a few miuntes before coming back on again.

 

OK, after doing somewhat extensive research, here's what I put together.

Last Christmas, somebody gave me a nice lighter-to-lighter socket battery jumper that I never used:

http://www.securityworld.com/auto/chargenstart.html

And I had one of these cool AC-DC lighter-plug adaptors (13.5V, 1000mA):

http://www.radioshack.com/product.asp?cata...t%5Fid=22%2D501

Used in combination, I measured 940mA delivered when the battery is at 12.80V. This is after few hours charging, the voltage went up from where it was after sitting a week not driven (12.15V). Also, the blue in the indicator of the battery is coming back (when there is no blue, it indicates charging required).

Note that on our cars the ignition switch needs to be at the ACC position to connect the "lighter" socket to the battery. So the radio and stuff should be off, then all the current goes to the battery.

For our relatively small 28 A*h battery, this charging current is "C/30", considered a slow recovery charging rate. However, a recommended constant maintaining trickle charge rate (for a battery that's already fully charged) is "C/100".

I think what I might do is find a timer laying around and set it to run the charger at nighttime for 8 hours (1/3 of the day). Or just plug it in during the bad-weather weekends when I don't take it for a spin.

The cool thing is that somebody going out to buy these same components to make a battery charger spends less than $40, and now has an emergency car battery jumper and portable 12V power source at no extra charge

Update: I found a DC adapter that provides about 400mA at 12.8 Volts... I soldered on a lighter receptacle from my box of spare parts. I'm going to use that instead of the one listed above, and keep it plugged in all the time.