Need a solution: Why does the battery voltage drop for custom PCB?
A battery has an internal resistance (think of it as output resistance):







simulate this circuit &ndash; Schematic created using CircuitLab

The internal resistance that I've shown as Rint can be as high as around 1 Ohm (depending on the battery cell). Please note that the load current drops some voltage across the internal resistance.

When you measure the voltage of a battery, you place a few Mega-Ohms of resistance (i.e. internal resistance of the voltmeter) across the terminals of the battery. Thus only a few micro- or nano-Amperes flows through the internal resistance. This leads nearly no voltage drop across the internal resistance, so you measure nearly the actual voltage of the ideal battery.

But if the load draws higher currents then these currents will drop some voltage across the internal resistance. Thus the measured voltage will be lower.

For example, if the internal resistance is 0.5 Ohm (a fair value) and  the load current is 1A then the voltage drop will be 0.5VDC. Thus the measured voltage of a 1.8V battery will be 1.3VDC. So the load will be supplied from 1.3VDC.

The solution can be to use a voltage regulator depending on the voltage and current requirements.