Switching between two 5v/3a USB power supplies, with little to no delay

Thread Starter

Rowa

Joined Dec 20, 2019
4
Hello,

I am currently building a raspberry pi 4 system that will be placed inside a vehicle. What i want is that whenever the car is on the raspberry will receive power from the car charge port using a 5v/3a charger. If the car gets switched off i want it to switch to the backup power bank that also supplies 5v/3a without turning off the raspberry so it needs to switch almost instantly. My question is is this possible to do to keep the voltage at 5v and at 3amps?

I am a noobie when it comes to circuits so any help would be appreciated

Kind regards,
 

hexreader

Joined Apr 16, 2011
387
Why not supply the power bank from the car supply?

Leave the Pi permanently powered from power bank...

EDIT: Or if lighter socket has permanent power (many do not) buy a good quality Cigar-lighter USB converter
 
Last edited:

Thread Starter

Rowa

Joined Dec 20, 2019
4
Why not supply the power bank from the car supply?

Leave the Pi permanently powered from power bank...
I've tried, this but the power bank cannot handle being charged and charging another device at the same time. I've looked into finding a power bank that can do this but no luck so far. If you know of one that can do that while supplying 3A let me know
 

hexreader

Joined Apr 16, 2011
387
Would need to be a hefty power bank to supply 3A for any significant time. Have you compared AmpHour rating against required running time?
 

Thread Starter

Rowa

Joined Dec 20, 2019
4
The powerbank i've found can do this for about 6h before running out of power. What i've tried is to have the powerbank charging while also powering the pi but the problem is as soon as i disconnect the powerbank from charging it will turn off the pi. That is a problem since my requirement is that the pi always stays on
 

bassbindevil

Joined Jan 23, 2014
52
Build a power bank that runs at 8.4V nominal (2S) (get a 2S PCM and something like an SY6912 charger module (set for 2s)). Then, an LDO (low drop-out) buck converter that can deliver 5V/3A from an input voltage ranging from 6V to 15V. Feed the buck converter though a couple of suitable isolation rectifiers.
The LDO thing might be a problem, unless you don't need to use the full capacity of the battery pack. So, another approach would be a 3S battery pack, with a 3S PCM, SY6912 (set to 3S), and a boost converter to step up the car 12V to whatever is enough for the SY6912 to work with. Finally, isolation rectifiers and a 5V/3A buck converter.
 

dendad

Joined Feb 20, 2016
3,261
Your car has a pretty decent power bank. The 12V battery.
The easiest thing to do is have a high efficiency switch mode regulator run off the car battery itself. Forget another power bank.
If you wanted to spend extra money, just buy the largest 12V battery that can physically fit into the car.
 

dendad

Joined Feb 20, 2016
3,261
An automotive battery doesn’t like continuous discharge. Ever leave an interior light on by mistake?
As long as the battery voltage does not drop too far, it does not matter to have continuous discharge. Leaving the light on is not the problem in itself. Fully discharging the battery is.
Most cars now do have a continuous load. The remote entry system is always on, for example. No doubt, more of the car computer will be on too.
As the TS wants the Pi to be on all the time, adding a second smaller battery makes no sense. It will discharge long before the car battery.
Some under voltage discharge protection could be a good idea to protect the car battery. Over discharge can damage it.
 

dendad

Joined Feb 20, 2016
3,261
But I think, unless the Pi needs to be on all the time, a better way to go is one of the automatic safe power down options for the Pi.
And make the file system read only. Have backup data stored on a USB stick if needed. The RasPis have a reputation for killing SD cards.
 

ArakelTheDragon

Joined Nov 18, 2016
1,327
We are already discussing a similar question in another thread. You don't need to switch, like the previous post suggested. Just charge the battery bank and use it at the same time.
 
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