Switch power between batteries and cable

Thread Starter

yaxlei

Joined Jul 17, 2020
2
Hello !

I have an all in one pc project with raspberry pi and a screen with its controller

the RPI uses 5V 1A max
the controller uses 12V 4A max
I use voltage step down to convert 12V in 5V
I have battery capable of delivring 5A (wrong mAh on aliexpress) as well as a power supply with the same caracteristics

How can i build a system that :
- when the power supply is connected, it supplies the RPi with the screen+controller; otherwise if the power supply is not connected the battery power my system.
- When the power supply is connected and the battery is not charged, the power supply charges the battery.


Have a good day

(sry for bad english,i'm trying my best)
 

Tonyr1084

Joined Sep 24, 2015
5,855
the RPI uses 5V 1A max
the controller uses 12V 4A max
You have two devices; one runs on 5 volts the other runs on 12 volts.

I have battery capable of delivring 5A (wrong mAh on aliexpress) as well as a power supply with the same caracteristics
Your power supply will need to be more than 12 volts. Your battery will need a proper charger to both monitor the voltage AND the current that is being used to charge the battery.

How can i build a system that :
- when the power supply is connected, it supplies the RPi with the screen+controller; otherwise if the power supply is not connected the battery power my system.
As in the above response, you need the proper charger for your battery. It won't work well as a stand alone power supply to run the equipment. Without a battery on the line it won't know the state of charge and will probably do nothing. WITH a battery on the line, the state of charge will be indeterminate. The charger won't know how much current to deliver to charge the battery AND the equipment won't work properly if there's not enough amperage.

- When the power supply is connected and the battery is not charged, the power supply charges the battery.
The charger (the Correct charger) will charge the battery if the equipment is not running. And it would be best if the equipment was disconnected.

To do what you want you'd probably be better off with a Sealed Lead Acid (SLA) battery and a charger capable of putting out 15 volts. Then you'd need two regulators, one at 12 volts and one at 5 volts. In that case you can use a charger that has sufficient current delivery capabilities. For the 12 volts I'd recommend a buck/boost converter. Your battery will likely fall below 12 volts during operation, and your controller may malfunction.

That's my 2¢.
 

Tonyr1084

Joined Sep 24, 2015
5,855
Quick followup on the 15 volt charger: A smart charger would be advised. To charge an SLA battery it should be charged up to around 14.4 volts (depending on the health of the battery). Then it needs to settle down around 13.8 volts to maintain a proper charge. A fully charged and healthy SLA should be resting at 12.6 volts when not being used.
 

schmitt trigger

Joined Jul 12, 2010
422
I second Tony's suggestion, a smart controller is the very best way to go.
If you find that too difficult to implement, a regulated voltage of 13.8 volts will also do the trick, but the battery will charge slower and not fully.
But many commercial products follow this approach, so it is a perfectly sensible approach.
 
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