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If you wanted to be EXTREMELY el-cheapo, you can add 3 silicon diodes to the output to drop it by approximately 2.1V This however will vary with the current being drawn and isn't recommended. If you don't care about proper regulation, then this is okay, otherwise do it properly....
Hero may be right here. But if you still want to do this, go to National's website and run their WebBench software. You can design and simulate your circuit before building it. It will assist with all the parts values.
Hey there! need some help. I have a 12V/2.3AH RESERVE-battery and I have a 12V battery in my vehicle. Can I charge me 12V/2.3AH RESERVE-battery with my cars battery using this circuit i.e. stepping up the DC voltage and then charging me RESERVE battery. Will it be OK?
The best way is to use a 12v relay , connect a live from the switched side of the fuse box to one of the trigger contacts & the other to the thin wire leaving the alternator , that trips the warning light on the dash . When the alteranto begins to charge the wire goes to ground & completes the circuit energising the relay .
You then simply use a suitable guage fo wire from the vehicle battery to the relay WITH A FUSE IN LINE ! & continue from the relay to your second battery .
If you want to be really lazy car accessorie shops & caravan/camping stores sell split chrge relay kits that are pre wired .
Use of he relay stops your secondary battery supplying power when starting the vehicle
hope it helps
Oh 35+ years working on cars / trucks / boats Etc.