Double Power Supplies: Battery or DC Power Supply?

[Pagedown]

If I had a battery as backup, and a DC power supply connected to a load.

And say the battery is 4.5V and the DC power supply is 5V, is the current now drained from the battery or the power supply?

I am doing a project where it needs battery for portability sometimes but can have the power supply indoors.

 

[NascentOxygen]

It depends how you have them connected. Because you don't mention otherwise, perhaps you intend us to picture the battery being simply connected directly across the terminals of the power supply? That is rarely a suitable arrangement. It is not a good way to keep the battery charged (assuming it's rechargeable), because almost never will the voltage needed to charge the battery be precisely that also needed by your load. Additionally, the battery will not have a long life by constantly being charged like that. The normal arrangement is for the power supply to power the load, unless the voltage falls to a low value, in which case power is drawn from the back-up battery.

 

[Pagedown]

so i need a relay or switch in between to switch between the supplies?

 

[NascentOxygen]

A switch is simplest, and foolproof, and you can see which is in use. If the load is low powered, you might be able to arrange a jack, so that when you plug in the power supply plug it mechanically disconnects the battery.

Alternatively, a simple diode isolator may suffice. One disadvantage is you lose 0.7 volts from both your PS and battery; there are other drawbacks, too. Switching is automatic: just make sure the PSU delivers at least 1 volt more than the battery, so the PSU will be predominate.

PSU +-------Diode --------,------------Load
                          |
                          |
Batt +------Diode --------'

 

[Pagedown]

Thanks, Nas. The information u provided is very useful. I may go for a dc relay, if psu connected, it disconnects the batt.