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Lead-acid 12V-DC to 5V-DC for ATmega328 (SAFELY) Answered

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Hi all!

I am currently designing a simple electronic device which uses Arduino-software. It needs to be connected to a car (Lead-Acid) battery of 12V and be stepped down to 5V for my ATmega328 to be powered. I read that I must use a "Standard 7805 5V regulator" for this. My question is, will it be able to cope with the heat? Must I put a heat-sink on it and/or is the regulator even "big" enough for this? I figured that using two regulators in parallel, might halve the current and help prevent over-heating. (http://www.sparkfun.com/products/107)

Another thing is, how could I go ahead and setup a small NiMH battery charger for this device which works like this: It runs of the car battery at all times and keeps the backup NiMH battery fully charged - as soon as the car battery gets disconnected, it switches over to the NiMH cells. (I will be using 4 of these for my backup-battery-pack http://www.sparkfun.com/products/335)

See my attached schematic of what I have in mind, please tell me what you think about it.

I have too little knowledge of electronics to solve this problem myself, but if you can point me in the right direction at least, I will try and figure it out from there.

Thank you for your time!