Li-ion battery and DC power supply common ground

Thread Starter

ShockedDairy

Joined Mar 18, 2024
1
Hello, apologies for what is likely to be a simple question, but I've done some reading here and here but I'm still blurry on specifics. I'm currently working on a mobile pseudo telemetry system to put in my car that will track things like acceleration, velocity, all controlled by a Raspberry Pi Pico board. I'd like to have the setup be capable of being powered via a 3.7v lithium ion battery cell and a 5v wall charger (not at the same time).

I've got a toggle switch to change between the two sources, but do not know what I should do with the ground lines. Would it be ok to connect the negative leads of the battery and DC supply together, or would that cause problems? I'm aware that a direct short would lead to bad things happening, but my naive brain doesn't see a path where that would occur with the current setup.

Are you folks able to point me in the direction of how to deal with the ground lines of two independent power inputs? Thank you.
 

LowQCab

Joined Nov 6, 2012
4,196
I don't understand why You would want this system to be Battery-Powered,
but all of the Sensors, and the Negative or "Ground" in the Car should all
be tied together to reduce any tendencies to generate or amplify Electrical-Noise
which might interfere with the reliable operation of your Sensors or Micro-Controller.

Portability can easily equal severe-head-aches, and chasing your tail trying to debug it,
this should be a permanent-installation.
.
.
.
 

k1ng 1337

Joined Sep 11, 2020
986
Hi, page 19 from the Pico datasheet:

"The simplest way to safely add a second power source to Pico is to feed it into VSYS via another Schottky diode (see Figure 15). This will 'OR' the two voltages, allowing the higher of either the external voltage or VBUS to power VSYS, with the diodes preventing either supply from back-powering the other. For example a single Lithium-Ion cell* (cell voltage ~3.0V to 4.2V) will work well, as will 3xAA series cells (~3.0V to ~4.8V) and any other fixed supply in the range ~2.3V to 5.5V. The downside of this approach is that the second power supply will suffer a diode drop in the same way as VBUS does, and this may not be desirable from an efficiency perspective or if the source is already close to the lower range of input voltage allowed for the RT6150."

Untitled.png

Since you are not getting power from VBUS (the USB connection), you can simply OR your available power supplies using two diodes like this:

Untitled1.png

The advantage to this setup is if you remove one power supply, the device automatically switches to the other. It doesn't really matter how many power supplies you OR as long as they each have a diode and are less than 5.5V. In any case, all ground connections can be tied together because the potential difference between one ground connection (0V) and another ground connection (0V) should be 0V.
 

edmo117

Joined Sep 5, 2010
23
Why not fit a small Li- Ion charger board to your unit. They are VERY cheap with USB input sockets of any sort you like to use. Then the unit need only be powered from the battery. When needed, the 5v supply can be connected to the charger board to charge the battery and power the unit at the same time, no need to switch supplies.
 
Top