Automatic DC power switch

[feralgeometry]

Hi everyone,
I am trying to design a system to power a 24V DC load which can automatically switch between two DC power sources (choosing the one that is not off)
The way I would use it is to switch between a DC power supply connected to the grid and a bank of batteries.
Ideally the switch should be seamless, so that the load is not turned off at any time, but it's OK if this is not possible.

I know there are devices called "power transfer switches" that do this function for devices that run on AC.
But I can't find anything equivalent for DC.

Any ideas ?

[bcroe]

Hi everyone,
I am trying to design a system to power a 24V DC load which can automatically switch between two DC power sources (choosing the one that is not off)
The way I would use it is to switch between a DC power supply connected to the grid and a bank of batteries.
Ideally the switch should be seamless, so that the load is not turned off at any time, but it's OK if this is not possible.

I know there are devices called "power transfer switches" that do this function for devices that run on AC.
But I can't find anything equivalent for DC.

Any ideas ?

A couple of properly rated diodes would supply from which ever source had more voltage.
I have reduced losses with some controlled, rather large MOSFETs. Bruce Roe

[feralgeometry]

A couple of properly rated diodes would supply from which ever source had more voltage.
I have reduced losses with some controlled, rather large MOSFETs. Bruce Roe

I would rather play it safe and use an off-the-shelf product, if any exist.
I'm sure they do but I can't seem to be able to find any. Maybe everyone just builds their own, but I find that hard to believe.

[PNPmacnab]

Most anything anyone wants is made. You just have to come up with the right name. Some dual diodes come with screw terminals and others with 1/4 inch quick connects. That is just connecting three wires. Load will pull from whatever source has higher voltage. I assume grid source would always be higher keeping the battery bank fully charged. You didn't mention what current.

[feralgeometry]

Most anything anyone wants is made. You just have to come up with the right name. Some dual diodes come with screw terminals and others with 1/4 inch quick connects. That is just connecting three wires. Load will pull from whatever source has higher voltage. I assume grid source would always be higher keeping the battery bank fully charged. You didn't mention what current.

Peak current would be no more than 20A, normally less than that.

I also just realized that something based on just diodes might not actually work well for this, because batteries when fully charged have higher voltage than the nominal one.
Specifically I am using LFP batteries so a bank that's nominally 24V is actually 25.6V when fully charged and stays that way until almost completely discharged.
So the power would be drawn from the battery until the battery dies, and only then it would switch over to the grid PSU (which has a stable actual 24V output), which is the opposite of what I need.

Also you mention "keeping the battery fully charged". Note that the battery should not get charged by this system, only be there and provide power when grid power goes away.
Recharging the battery in my case is done elsewhere when the system is not in use.

[PNPmacnab]

The Li is fortunate because a lot of equipment has a wide range of acceptable voltage. If the AC source is a regulated supply, it could be set to 28V, an acceptable voltage from a lead battery source. The batteries would never be touched until a power failure.