Automatic transfer switch with 12V system

[Calebu2]

Relatively new to this. Installed a 400W solar system on my shed to power lights and a fan for the summer. Using the renogy rover MPPT controller with a 12V lead acid battery to manage everything. No inverter. Battery is somewhat undersized but (a) probably wouldn't solve the issue if I got a larger one and (b) haven't decided on how I keep it safe with swings from 10 below in winter to 100+ in summer (that's my next next project)

I'm not getting enough power throughout the day to keep the fan running during the summer, so I want to augment the power with a backup connection to the grid that kicks in when the battery / panels are depleted.

I know I could do this if I had 110v and a regular transfer switch, but the only 110v I want in this system is a backup power source. But I haven't found anyone talking about a 12V system.

Is there an automatic switch that can turn on a 12V power supply to augment the load power when the system battery output drops below a certain level?

 

[ncsolarelectric]

Relatively new to this. Installed a 400W solar system on my shed to power lights and a fan for the summer. Using the renogy rover MPPT controller with a 12V lead acid battery to manage everything. No inverter. Battery is somewhat undersized but (a) probably wouldn't solve the issue if I got a larger one and (b) haven't decided on how I keep it safe with swings from 10 below in winter to 100+ in summer (that's my next next project)

I'm not getting enough power throughout the day to keep the fan running during the summer, so I want to augment the power with a backup connection to the grid that kicks in when the battery / panels are depleted.

I know I could do this if I had 110v and a regular transfer switch, but the only 110v I want in this system is a backup power source. But I haven't found anyone talking about a 12V system.

Is there an automatic switch that can turn on a 12V power supply to augment the load power when the system battery output drops below a certain level?

Use a DC voltage-controlled relay. Connect the input coil of the relay to your battery system. When it goes dead, the relay switches and connects the power supply. Be sure to put a blocking diode on the power supply. I could easily design the circuit, but I don't know where to buy one.

 

[Calebu2]

Use a DC voltage-controlled relay. Connect the input coil of the relay to your battery system. When it goes dead, the relay switches and connects the power supply. Be sure to put a blocking diode on the power supply. I could easily design the circuit, but I don't know where to buy one.

Makes sense. I assume that the output coil controls the ac to the psu transformer and that the output of that is connected in parallel with the battery output to load, but downstream from the blocking diode.

So that when the battery dies and the relay kicks in with the power, the new supply of power doesn't kick the relay off again, right?

Presumably, it would be safest to hook all of this up to the load on my mppt controller rather than the battery as that is better able to protect the battery from overdischarging.

 

[ncsolarelectric]

Yeah, it needs a more complex circuit to control when it turns on and when it turns off. I'm sure someone must make a programmable DC switch with input controls, based on some small microcontroller.

 

[Calebu2]

The renogy rover mppt controller has a load output that puts out 12v as long as the battery has enough juice in it and can be programmed to turn off if it drops below a threshold. That's what I currently have everything hooked up to, so would probably go:

Battery
+| |-
| |
Batt connectors
MPPT CONTROLLER
Load connectors
+| |
| |
Relay coil (parallel)
| |
Diode. |
| |
PSU 12 out (parallel)
| |
=== LOAD ===

Only gap i could see with this is if PSU gets damaged by 12V flowing across terminals when turned off, but I could solve that with a second diode on the positive terminal of the psu only allowing flow out of the PSU.

 

[John Frum]

legend {
    <-> { bi-directional current flow }
    -> { uni-directional current flow }
    <- { uni-directional current flow }
    | { busbar }
    scc { solar charge controller }
    vsr { voltage sensitive relay }
    lvd { low voltage disconnect }
}
positive
|<->fuse<->battery<->shunt<-->|
|<--fuse<-scc<----------------|
|<--fuse<-vsr<-ac2dc_charger<-|
|-->fuse->lvd->load---------->|
                       negative

 

[pollenface]

I have a 13.8v power supply that comes on when the battery bank hits 12v and it turns off once the SCC's produce more than 14.2v. I'm using the OVD/OVR settings on an old Epever tracer 2210AN as a voltage sensing switch (no solar connected to it) to control the power supply.