automatic switchover cicuit for when batteries go flat

[Thunderchild]

I'm going to set up a small solar power system (40 W of solar panel and a car battery with a 150 W inverter). now obviously I won't have enough power for non stop power. I'm planning on using the solar system to run the house lighting system only. What I thought of doing was setting it up so that the solar system supplied the power to the lights whilst the battery had power and once that was used it will automatically revert back to the grid.

Now bearing in mind that the inverter will not be located by the mains supply I need to devise a way of having automatic changeover from battery to mains, even if this means that once the battery is dead it stays on the mains until I "reboot" the solar system into supplying power

 

[3v0]

I have been thinking about this sort of thing.

If it is only lighting I would switch over to low voltage lighting (maybe LEDs) and skip the wasteful inverter. It would be much easier to switch the circuit from one source to another.

3v0

 

[colin55]

Simply have a relay driven by a transistor. Put a zener on the base that holds the relay closed when the voltage is above 9v. When the voltage drops below 9v, the relay drops out and the mains supply is passed to the inverter via double pole contacts.

 

[qrper]

for a really low tech fix, try this.

connect the output of the inverter to a continuous duty ac relay.

connect the loads you want to run on the inverter to the contacts that will be closed when the relay is turned on.

the other set of contacts to the mains. (be damn sure you install the required fuses and or circuit breakers)

when the battery runs down and the inverter can no longer supply power to your loads, the inverter will shut down.

when it does, the relay will drop out and the loads that were running on the inverter will now be operated from the mains.

when the solar panels get the battery back up, provided the inverter automatically resets, then the relay pulls in and you're back on inverter power.

remember safety first!

If you get the output of the inverter connected to the grid, you'll let the smoke out of the inverter.

and the relay you need will be a high quality industrial strength unit...nothing from radio shack!

 

[kchriste]

When the voltage drops below 9v, the relay drops out and the mains supply is passed to the inverter via double pole contacts.

You really shouldn't discharge a 12V lead acid battery below 11.0V. Discharging down to 9V is going to severely reduce the useful life of the battery. In this application, I wouldn't let the battery go below 12.0V if a lead acid battery was used and a load of 1/10th capacity is applied.

 

[Jules_Theone]

I've tried this using loads of batteries, a 150w inverter and a 12Amp solar charger / regulator. The regulator had a 'load' output which disconnects from the battery when it discharges to 11.8v. I used a 12v relay on that to switch between mains and inverter and because the regulator was doing the sensing it could switch back when the battery is charged up again. Using in the low batt alarm in the inverter meant that it wouldn't reset when the batteries are charged. The only major problem was you MUST ensure that the relay breaks contact with the inverter output before the mains is connected otherwise you will be buying a new inverter. Also fuse the DC inputs / outputs and AC inputs / outputs as I made the mistake of bypassing the fuse in the breaker enclosure and the 30Amp blew further up the line.

If you didn't want to buy a regulator (mine was £6) you would need to build a circuit with several volts histerisis so it doesn't switch back to batteries too soon.

 

[Thunderchild]

hm window comparator