I have a battery powered (CR2032) project that could be also fed with 3.3V from a USB/Serial connector if the battery is left in the device. Is this a safety issue?

 

Yes, it's an issue. That will eventually overcharge a battery that isn't meant to be recharged. It may not be terribly unsafe, but it will destroy the battery. A blocking diode would solve the problem but you don't have much voltage to work with. A switch of some kind might be a better solution.

 

Not advisable. Here's data for CR2032 from Energizer; it states maximum allowable charging current is 1μA.


That being said, I have seen LED flashlights with a crank generator to charge them and the battery has always been a lithium coin battery. When the batteries no longer charged with the crank, I've successfully recharged the batteries with a low current but larger than 1μA.

 

Yes, it's an issue. That will eventually overcharge a battery that isn't meant to be recharged. It may not be terribly unsafe, but it will destroy the battery. A blocking diode would solve the problem but you don't have much voltage to work with. A switch of some kind might be a better solution.

Not worried about destroying the battery as it is documented that the battery should be removed before using connecting external power.

That said, some fool will not read the documentation (or follow it). I just don't want the battery to explode.

 

You could use an ideal diode such as this to prevent the battery from being charged

 

You could use an ideal diode such as this to prevent the battery from being charged

Thanks, did not know of that part.

 

Use a switched jack for the external power that would remove the battery from the circuit.

 

That's what I alluded to in #2, but he mentioned a USB feed in the same breath with 3.3V and confused me. I'm not sure about what source he wants to use. A traditional, cylindrical power jack with a switch is easy to find.