> 5.1 | Effects of Application Power Supply Resistance

> The power supply resistance of a mobile application typically results from the following parameters:

> Table 3: | Typical resistance of a power supply line

> Battery | Cell resistance | 10...40m Ω
> Resistance of battery protection circuit | 80...100m Ω
> Application | Resistance of PCB and connectors | 20...100m Ω (depends on design)
> Total: | Minimum overall resistance | >110m Ω

> A transmit burst current of 2A causes a voltage drop of 0.22V across the power line. The undervoltage shutdown threshold of a Siemens GSM module is 3.2V. This means that when making a call at a battery voltage of VBATT+ 3.42V the GSM module reaches its shutdown threshold, due the internal resistance, and switches off.
> Figure 9 illustrates a measurement under realistic conditions, assuming battery discharge due to 10mA current consumption in idle mode and a 1-minute GSM call made every hour at PCL5 with a current profile 2A / 70mA, until the battery is empty.
> Figure 9: Voltage level of a typical 1000mAh Li battery discharged at 20°C
> In addition to voltage drops caused by the internal resistance the effect of the recovery voltage is significant (see Section 5.2). Both effects together will lead to voltage loss during operation. The lower the battery voltage, the greater is the voltage loss.

> Table 4: | Measured resistance and voltage drops of a typical 1000mAh battery

> Internal resistance @ 4.2V / 3.6V | 90m Ω / 100mΩ (to multiply by 2A)
> Recovery voltage @ 4.2V / 3.6V | 80mV / 210mV
> Voltage loss @ 4.2V / 3.6V | 260mV / 410mV

> In Table 4 you can see a voltage loss of 260 / 410mV resulting from the total of the recovery voltage and the voltage drops due to internal resistance. This means: At a voltage of 3.6V the module is close to its shutdown threshold, because a 1-minute call at PCL5 would discharge the battery down to 3.2V where the module switches off automatically. Note that at 3.6V a typical battery has only about 5% capacity left – the voltage is falling rapidly compared to the nominal voltage range as well under idle load (10mA). See Figure 9 for details.
> To sum it up: The available battery capacity depends on the recovery voltage and the power supply resistance. Therefore, design engineers are advised to choose a battery with a low recovery voltage and a low internal resistance. Also the power supply lines on the application PCB should be dimensioned for with low resistance.