Teensy 3.2 on battery (without usb port connected)
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No, not if USB and the battery are never connected at the same time.
Epyon
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You can actually leave the vusb trace like it is and connect the battery and USB at the same time. The battery is a floating power source. Because it doesn't share a common ground to the computer you don't have a potential difference and no circulating currents that could damage the Teensy. That's not the case when you use a power supply that potentially shares a common ground with the computer, e.g. though the mains, and where cutting the trace is recommended.
In most of my projects I power the Teensy with an isolated DC-DC converter to achieve the same isolation.
In most of my projects I power the Teensy with an isolated DC-DC converter to achieve the same isolation.
The battery is not "floating" if it is connected to the Teensy's ground pin and you plug in the USB cable.
Epyon
Well-known member
The definition of floating is: not sharing the same electrical reference to Earth. Therefore, a battery is by all means floating compared to a power source connected to Earth (e.g. USB). Sure you connect both grounds, but any current generated by the battery will flow back to the battery ground and never into the USB ground.
The reason why the cutting of the VUSB trace is recommended is for when you supply power by two Earth-connected power supplies, which can both have a different ground potential to Earth. This would result in current flowing from one ground to the other. If you use a battery, that ground potential is floating in relation to Earth and won't cause any current flow.
Or when you want to power the Teensy from Vin without causing your USB port, Teensy or the other supply to release the magic smoke. If he powers the Teensy at Vin with another source then plugs in the USB without cutting or disconnecting his second source he could easily kill something.
If the battery's minus shares a path with the USB it is no longer floating because it will be fixed to the USB negative.
For the original question, as long as your only powering the Teensy from 1 source(within voltage range) there is no issue there, you don't need to cut the trace.
Epyon
Well-known member
You can safely use two power supplies at once without cutting the VUSB trace if at least one of the supplies is floating (not connected to Earth), as is the case when you use a battery. Connecting the grounds together does not change the fact that the battery is still floating, because 'floating' is referenced to Earth and not to the common ground.
Of course this implies that the only point where the two grounds are tied is the Teensy. If the battery ground is somehow connected to the computer chassis, this could cause currents in the USB connection damaging the Teensy. However because batteries are mostly used for internal power, it seems very unlikely that this would be the case.
Almost all my projects are simultaneously powered by two power supplies: one isolated DC-DC converter (floating) and the USB connection when programming. By using a floating power supply I have never needed to cut the VUSB trace. In fact, this is the way those fancy 'USB isolators' work: they galvanically isolate both USB power and data (using a DC-DC converter and optocouplers respectively), thereby ensuring that the USB host is now floating compared to the USB slave and you can safely power the slave from both USB and internal power. Or when using a laptop, you can just unplug the mains making it floating.
Of course this implies that the only point where the two grounds are tied is the Teensy. If the battery ground is somehow connected to the computer chassis, this could cause currents in the USB connection damaging the Teensy. However because batteries are mostly used for internal power, it seems very unlikely that this would be the case.
Almost all my projects are simultaneously powered by two power supplies: one isolated DC-DC converter (floating) and the USB connection when programming. By using a floating power supply I have never needed to cut the VUSB trace. In fact, this is the way those fancy 'USB isolators' work: they galvanically isolate both USB power and data (using a DC-DC converter and optocouplers respectively), thereby ensuring that the USB host is now floating compared to the USB slave and you can safely power the slave from both USB and internal power. Or when using a laptop, you can just unplug the mains making it floating.
There is only 1 ground(0V) on the Teensy, if your powering it then your connected to that ground(floating or not). If you plug a second supply into the Teensy it will also be connected to that ground, if both supplies are connected to Vin and one is 5.2V and the other is 5.0V they will fight each other. If you connect 2 sources floating or not they will fight each other, causing increased power loss or magic smoke. Do not confuse it with the earth ground used to protect us humans from the bad things that can happen when AC voltage is allowed to energize the housing of your computer. I am referring to any 2 supplies that share the same 0V(ground) reference.
Here I will put it another way, you connect a AA and a 9V battery in parallel with each other and tell me how happy the 9V will be. They are both floating, but when you connect them they are not floating with respect to each other.
And I am well aware how galvanic isolation works, I test and design 750VDC inverters for trains and other large transit propulsion systems. We use double and triple isolation to prevent very bad things from happening. We do not 'tie' the 0V(grounds) of those isolated supplies together because that would defeat the purpose of isolating them.
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