D Cell Battery Charger for Smartphones

5/2022

Smartphones typically charge through a USB connection. The standard charging voltage is 5.0VDC with various "fast charging" technologies used by various suppliers of smartphones to speed up the charging process. Portable Li-ion recharging modules are readily available. While charging currents vary depending on the charging technology, a typical current value at 5V is ~ 400mA and will vary during the charging cycle.

Is it possible to charge a typical smartphone with alkaline cell batteries? To determine the feasibility, I measured the voltage requirements for representative smartphones: Samsung Galaxy S4 (2600 mAHr capacity) and Galaxy S9 (3000 mAHr capacity) using a variable voltage power supply and a DVM as shown below.





A USB cable with a female connector was cut in half and the 2 power charging wires (pins 1 and 4) were used to power the smartphones connected to this USB plug. Starting at 5.0 V, the voltage was reduced until the charging current suddenly dropped to zero which in both cases was about 4.7VDC. The test voltage was then increased to about 5.5V which is slightly overvoltage but likely quite safe as there is some circuit protection built into the phone itself. However, higher voltage is likely risky. Using fresh D alkaline dry cells, at least 4 batteries in series is required providing ~ 6.0V initially. To drop the voltage down to a safer value, the simplest solution with minimum circuit complexity is to insert a Si rectifier diode in series with the D cells. Since the phone charging current is quite high, a suitable power diode should be rated at 1A or higher such as the 1N5402 used here, rated at 3A. At 500mA current, the diode forward voltage drop is ~ 0.7V which reduces the 4D cell voltage to an effective initial charging voltage of 5.3V which is safe.

4*Vcell - 0.7 = Vcharging

where Vcell is the voltage of each D cell at any instant during the discharge cycle (1.5V initially).

The image below shows the configuration used. (A more complex voltage regulator circuit or a Zener diode is possible, but will consume more of the D cell power as it is charging the phone). Now since the 4D cell battery pack is only effective in charging these smartphones as the supply voltage drops from 5.3 to the cutoff voltage of 4.7V, how much charging time does this provide? While modern D cells quote "capacities" of ~ 12000 mAHr, this capacity isn't all available for this type of smartphone charging because the alkaline cell voltage drops as the battery is drained. Assuming a ~ constant 0.7V drop across the Si diode as the batteries discharge, the charging will stop when the battery pack voltage reaches 5.4V (or 1.35V per D cell). Looking at a typical D cell discharge curve for 500mA current (Energizer E95 D cell) show this takes ~ 3 hours! However, at this stage, the Si diode can be bypassed with, for example, a simple SPST toggle switch, (since the total battery voltage is now in the safe zone of 5.4V). In this second phase, the total battery voltage of 5.4V (which is now the charging voltage) can drop to 4.7V before cutoff (or about 1.2V per D cell). This will extend the charging for an additional 3 hours which will be sufficient for a complete charging cycle.

Therefore using this simple circuit, about 1 smartphone charge cycle is available with fresh D cells dropping from 1.5 to 1.2 V during discharge.

Using 8 or 12 dry cells in series with a 5V regulator circuit, will provide greater charging capacity with added circuit complexity.



Energizer D Cell Data Sheet