Tank circuits are nothing but the power circuits used in Military tanks to produce power it in dessert condition.
They were first devoloped in Germany by Hitler... during the II world war. and so on ...!
We have adapted this technology to produce power to keep are Carputer alive during cranking the engine!
Its nothing but just a 22000uf /30v cap and a MBR6045 diode ..!
attached to the +12 line of the carputer.!
so when you crank the car carputer stays alive with the help of the caps. and the diode prevents the current been pulled back during crank
Well i hope it the above information helps you.