Now I am totally confused.
If you have two 12 volt batteries. One is say 11.5 volts the other is at 10 volts.
If you combine the two what do you get?
I know you will not get more than 12 volts. But will it give you any extra juice to start your engine?
Sd
I think of it this way: Voltage differential is synonymous with the size of a water pipe. Current is the water flowing through the pipe. Undersized cabling is like having ridges inside the pipe causing increased resistance to the flow of the water. If the voltage is high (big pipe) and the cables are adequately sized (smooth inner walls of the pipe), then the current flows with less resistance. That's why our alternators put out 14V (+/-) for a 12V system, so current will flow from the higher voltage alternator to the lower voltage battery.
If you have batts with 11.5V and 10V, you've really got 2 low batts. A fully charged battery in a steady state should read 12.6V. The 10V batt may have a bad cell or 2 cells shorted out causing the low voltage. Optimally, each cell should provide 2.1V. If one is dead, you'll get 10.5V at best. But let's look at the situation with these numbers.
If you put a voltmeter on each batts when separated, you'd see 11.5 and 10V. The instant you combine them, you'd start to see the 11.5
gradually decrease and the 10V may or may not slowly increase, depending on it's condition. The 11.5V decreases because the 10V batt places a load on it and the current flows from high voltage to low voltage.
If the 10V batt is your start batt, it's too low to start the engine. By placing the switch to "ALL", you allow the better 11.5V batt to assist in the process. But it must still flow "through" the high resistance of the 10V battery to get to the starter. It would do a better job of starting if the 11.5 could provide the start power without the 10V batt in the circuit, but few if any of us have redundant cabling to allow is to completely bypass the bad batt without recabling or use of jumper cables.