I've a defective charger for a small LED torch light, its output is 4.2V / 500mA.

I may not find an alternative one, so I'm considering modifying a mobile phone charger as following:

  • The mobile phone charger has output of 5V / 1 or 2 Amps.
  • I put at the output three diodes: 1N4007, and two schottky diodes of about 0.1V voltage drop each: 0.65 + 0.2 = 0.85 V
  • So the output becomes: 5-.85 = 4.15V (edited )

Is this voltage adequate and suitable to charge the battery? Is it safe to use? Let me know your thoughts.

Thanks in advance

  • 1
    \$\begingroup\$ Add a schematic if possible. Charging Li+ improperly can yield spectacularly pyrotechnic events. \$\endgroup\$ Commented Jul 21, 2018 at 12:21
  • 1
    \$\begingroup\$ Likely unsafe. The forward volt drop of a diode depends on current and this means the output will rise about 4.2 volts when delivering low currents. Go buy one. \$\endgroup\$
    – Andy aka
    Commented Jul 21, 2018 at 12:35
  • \$\begingroup\$ Phone chargers are not chargers, they are constant-voltage power supplies. \$\endgroup\$ Commented Jul 23, 2018 at 2:58

1 Answer 1


The original charger had only 4.2V which leaves no room for a charger driver

This means the charger had the charging logic inside.

A diode is still conducting a small current even under it's forward voltage.

You better use a dedicated module like this one that can be modified for 500mA, using a diode will overcharge or even destroy your battery


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