When connecting two battery grounds together, which battery specifications can tell us the limits?
Figure 1. There are two current-loops in the circuit: control and power.

Notice that provided the transistor's ratings are not exceeded that no current flows from the power circuit back into the control circuit.

The control circuit turns on the transistor and allows current to flow from V2 through the R3 - Q1 loop.

In this circuit the base of Q1 will be held at 0.7 V no matter what voltage V1 is (provided it's above a volt or two) and independent of V2.


  ... because of the high electric fields from the 200v or higher, seemed they could affect a small battery just like it can affect a small component , and i thought that just like any other smaller components, batteries would also have some limit as to which voltage they can be connected to in this configuration. 








simulate this circuit &ndash; Schematic created using CircuitLab

Figure 2. A transistor consists of two P-N diode junctions.

From this it should be a bit clearer that V2's potential is dropped across D1 so V1 isn't affected by V2. It's just like connecting two different voltage batteries with a diode in reverse-biased configuration.