Hmmmm, ..., NEVER put two batteries of unequal voltage in parallel! The current goes to infinity and something either melts down or explodes. Do the math! You'll see The current goes to infinity.
The entire voltage can be measured across the batteries, BUT this is very DANGEROUS. The 12 v battery could explode due to the excessive heat produced, and possibly the 24 V battery as well. Just imagine you doing the jig with sulfuric acid on your face and in your eyes. I CANNOT RECOMMEND DOING THIS FOR ANY REASON.
Not sure exactly what you are getting at. Is this supposed to be a charging circuit, with the 12V. battery totally exhausted, or a "flat" 12V. battery which might have 10V. across its terminals, and what is the 25 ohm resistor, is it a circuit load, or meant to be the internal resistance of the 12V. battery, or what?
