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I have a baby monitor that accepts a 3.6v 900mA battery and is charged via USB mini. The USB port broke off and so I was unable to charge or power the device.

The USB pins were too small to solder back on.

Instead, I removed the battery, soldered the 5v (+) USB wire to the positive pin, and the ground (-) to the negative pin.

The USB charger puts out 5v and 1A. Is this safe? The thing works fine. What issues could it cause?

I'm not trained or educated at all in electronics. I just like to dabble in fixing things and restoring old electrical motors.

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    \$\begingroup\$ You soldered the 5V USB wires to which pins? Which positive and ground? The battery's, or the USB charge port's? Big difference there. \$\endgroup\$
    – Bort
    Commented Feb 7, 2018 at 18:33
  • \$\begingroup\$ The battery's positive and ground. The USB charge ports were too small to solder. \$\endgroup\$ Commented Feb 7, 2018 at 20:38
  • \$\begingroup\$ May or may not work. Can you reduce the voltage to 3.6-ish volts somehow, LDO, zener or DC/DC? \$\endgroup\$
    – winny
    Commented Feb 7, 2018 at 21:54
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    \$\begingroup\$ I should have clarified...it absolutely works. I just don't know the repercussions of having it on for long periods of time. Will it eventually fry? \$\endgroup\$ Commented Feb 8, 2018 at 0:37
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    \$\begingroup\$ The 3.6V battery probably is around 4.1V charged. If you are really worried you could add one or two diode drops to the 5V supply. \$\endgroup\$
    – Wesley Lee
    Commented Feb 8, 2018 at 15:46

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It very well could fry. It depends...

If the battery output was put through a voltage regulator before the rest of the board, then your 5V will be converted anyway. If not, then you risk damaging lesser voltage components. Sometimes they fail slowly.

Try following the traces from the battery input pin and see where it goes.

There is also the concern of the board expecting a battery on those pins, which is now a USB supply. If anything, it should shut down the board, because 5V from that battery would mean the battery is failing in a dangerous fashion. Since you say it works, then I would reckon there is no issue there.

Because the standard USB power input port is gone, there is no risk of the bard trying to charge a battery on the battery pins.

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