DC Switching Relay for Solar Application

A photocell- coupled inverting relay can do that for you. Explain hybrid to me, will you? Other factors may be in play also, like state of load of the batteries? A photocell is a good start but might need some added intelligence to make you happy afterwards.

By hybrid, I mean, that i use solar power for about 10 hours ( sunlight, and thereafter battery bank ), and for rest of 14 hours I use AC Mains supply routed through an Inverter to the same battery bank.

I have tried to make a simple line drawing of the Hybrid system and attached herewith your reference.

this might helphttp://www.civicsolar.com/resource/sizing-acdc-disconnect-solar-pv-systems

If I were you, the first thing I would do is to install a TF (Transfer Switch), be it manual or automatic. Make also sure to install a proper sized breaker between your inverter output so that contact between your mains and the inverter at least is secured (for what it is worth). One day, the inverter could by accident be parallel to the mains and that is smoke.

2000 Watts is not a lot, but maybe your load is also small, so it can work.

I never had 10 hours of sunny day light, but I also do not live next door to you.

The photocell and relay work but do not give you the safety that you get with a transfer switch.

stuff like thishttp://www.youtube.com/watch?v=WNrANzXsNRM

As a general rule, "grid tie" solar inverters already do this, they are required to, and the process by which it happens is typically regulated, because getting it wrong could mean killing a line worker. Are you sure that yours does not? Or are you somehow not using a grid tie solar inverter? If that's the case, then there is the problem.

Here in the US, that would be illegal.

OP has no Grid Tie inverter as I understand, but a standalone 48 VDC/220 AC. He is not transferring power into the grid. In case he tries, his inverter will blow. The question is not very clear: I assume he wants to use the grid only when when no sun and when his bank is low. For the rest of the time he wants to operate his inverter, (always) isolated from the grid. Pls correct me if I am wrong.

@dvmdsc is absolutely correct. Mine is not a Grid-Tired Inverter, and we in Pakistan, cannot give power back to Grid. Secondly, I agree that dvmdsc has very correctly grasped my intention.

My intention is to use the solar power for maximum time in a 24 hours cycle, and use AC Mains only for the remaining needed time.

Inverter output has a AC circuit breaker. Further I have DC fuses on the cables between (a) PV panels to charge controller, (b) Charge Controller to batteries, and (c) batteries and Inverter.

Have sunlight for 5 to 7 hours, as my PV panels are more than power usage, so through battery bank I am able to remain on Solar power for 9 to 11 hours. Then for the rest I use AC Mains. Now I be increasing the panels, and thus I be able to harvest and store power for about 16 to 18 hours of use. At the moment I manually switch On/Off the AC Mains of inverter, but would like this to be automatic.

As I suggested in a an earlier post, a photocell can help you. Since you make and interrupt the mains from your load and inverter, a relay is somewhat simple and not very safe. When e.g. the relay "sticks" you can have a short circuit with fire hazard.

A transfer switch has 2 electromechanical relays that are mechanically coupled, just to avoid this situation.

A simple way to make a photocell switch is a small switching night light ( $2.00 @ walmart) and make a small black box around the sensor with an adjustable hole in it to adjust the light intensity to switch.(I once used a plastic housing of a DC adaptor)

The little bulb you need to keep (the circuit might not switch a relay coil only) and over this bulb, you can connect a 220 Volt relay to control a bigger one.

You need to play with it, since clouds can make the switch active too. A timer in combination with it helps, but also your battery cannot be discharged too deep.

You see: We just started and found already 3 parameters to work out some logic sequence. Once you automate more, you will want to make it more perfect.

Just do it the safe way.

It was a bit hard to read your diagram, but I will offer some possibilities for what I perceive to be your requirement.

1. A time clock controlled changeover relay will do the job if you can average out your sun hours, but this won't cover you for overcast days or for abnormal drain which could leave the battery in a too low Soc.

2. If you are using an MPPT type solar controller, then a changeover relay with coil voltage the same as the solar panel Voc and with the coil connected across the panel output would only be energised when there is sufficient insolation, but this won't keep you on battery power once the sun goes down and again doesn't protect the battery from over discharge.

3. A voltage comparator with large hysteresis controlling a changeover relay so that the system only goes to mains when the battery voltage falls to a set level for a set time. This is probably your best option as it relies only on battery capacity to supply the loads, will not false switch on motor start ups etc. will not reconnect the battery until it has regained sufficient charge, and will likely give you the best economy from the system.

There looked to be a battery charger included in your sketch, so you would have to have a means of isolating that from the system so that it doesn't recharge the batteries from the mains prior to the solar doing the same.

The means of isolation could be manual or another comparator that turns the charger on only when the battery voltage falls to a set value.

@ bwilko

" 3. A voltage comparator with large hysteresis controlling a changeover relay so that the system only goes to mains when the battery voltage falls to a set level for a set time. This is probably your best option as it relies only on battery capacity to supply the loads, will not false switch on motor start ups etc. will not reconnect the battery until it has regained sufficient charge, and will likely give you the best economy from the system. "

Your suggestion is very good, it was thought by us earlier, but then rejected for the reason that at night time, it will be switching battery charging On / Off . Allow me to explain my view : say at 7pm, batteries are full, the changeover relay, will cut the AC mains supply , now the load will be on batteries. Say in 90 minutes the battery voltage drop to our predefined lower limit, then the relay will switch On the charging. It will charge till battery voltages reach upper defined limit, and then it will again cut the power.

So all night this On/Off to battery charging through AC mains will continue.

You should have also read the last bit of my post where I made the point of having to prevent the battery charging, and how to go about it.

I read your post again.

If you choose a relay that monitors your panels, you need one with a threshold window: adjustable and comparator - controlled and with a timer.

@dvmdsc :

Hi, I like your photo-cell idea. And I plan the following, please guide will it work for me or not ?

I put a photo-cell on the roof top, adjust its exposure to light ( as you did through a plastic box with holes for light ). I connect this Photo-Cell to a Switching AC Relay of 220V, to work for upto 30Amps current. This Relay or Automatic Switch be connected to AC Mains Input to Battery Charger. At time sun-light is sufficiently available the circuit for battery charging through AC mains be open, and no charging is done through AC Mains. As sun goes down, this relay/switch closes the circuit, and batteries start getting power from AC mains.

Do I need some resistances or any other such things too to make this work ?

As you note this system is not connected to any PV panels, etc.

In this scenario, as I understand well,

your load (house or whatsoever) is always on the inverter and NOT on the mains (GRID)

This saves you a Transfer Switch. Just make sure that the inverter, batteries, sun and panels are dimensioned big enough. That your charger and/or panels (through regulator) do not over charge your batteries.

For this part, you are good to go.

For the light box- the black box comes handy because you can direct your photocell in a certain position. Small clouds however can make it switch at unwanted moments, because the cell thinks it is night/or/day. To be good, a kind of hysteresis (timer) helps a lot. If the cell activates a relay with adjustable timer you are good to start. Otherwise that charger is going to switch on and off under these conditions.

If you want to see my light box, I can send a picture later. I have it on a ball (camera) positioning pad to adjust it easily. You do not need to put it on the roof. Here it is close to my garage door and scares some thieves away too when the light is on.

Good luck

@dvmdsc

Yes , if possible and convienent please send me your light box picture.

Some problems that I see here:-

1. The batteries will now fully charge overnight. The panels will then provide no charge advantage to the batteries in the morning, which would appear to be in contrast to your original requirement.

2. Any sporadic cloud cover will cause the photo cell to re-engage the battery charger on an ad-hoc basis whether the charge is needed or not, further defeating the original purpose of the solar.

3. Your proposal still hasn't addressed the requirement to isolate the inverter from the mains and connect it to the batteries.

4. I think you need to employ someone who understands the requirements.

All can be categorized in "extra intelligence". The OP has a small solar array. We have no info about the capacity and nature (deep cycle or not) of the battery bank (except it is 48 Volts in voltage).

1. He can control this, or with a voltage relay( with window) or by just adjusting his charger that the batteries remain hungry - undercharge. (KISS principle)

2. A timer should take care of this or at least smooth it. If the clouds remain, the charger is supposed to work, to load the batteries

3. The inverter is NEVER connected to the mains (OP meant GRID by means) Only the charger will be connected (switched by Photocell)

If OP wants to connect the mains (house) with the grid, that inverter must be put OUT OF PLAY.

4. As I stated before. When the load matches the production, this can be working. I see a problem when his production exceeds his supply from solar + charger.

A kick in switch might be a next step, only to engage the charger when the sun shines, but the batteries drain due to excessive load.

Far too complicated........

Hello,

In my solar system I use an OutBack MX60 MPPT charge controller. This controller has an AUX output which gives about 12vDC at 200ma and the user can program how it works. I have mine set up with a relay which operates at 12vDC 150ma and this relay can handle up to 30Amps at 120vAC. I have my chargers connected to this relay and I programmed the OutBack to disconnect the chargers at 13.5vDC and then to reconnect the chargers at 12.2vDC The charge controller is set to slightly higher than 13.5vDC so during the day time when the sun is shining the chargers are disconnected from the grid. During cloudy days or night time the batteries run the load until reaching 12.2vDC and then the chargers are reconnected. You can chose a lower voltage and run on the batteries longer, its up to the user.

Does your charge controller have such an output? If so you could use it to switch power on and off into your inverter. My inverter is a pure inverter without a charger, an EXELTECH, and my chargers are separate, IOTA´s.

Best Regards

John Van Allen

Some years ago, I built what you need, but for slightly different reasons. I built a charger controller that monitored the voltage on the battery and turned on the mains to the charger via a relay or mains electronic relay, when the battery voltage dropped below a fixed level.....

This could also be due for instance to nightfall or a cloud for example, or just that the batteries are too depleted in your case....

I always use 12.6 volts as a minimum, but assuming that the battery will never be left discharged under say 12.6 volts for longer than 24 hours, an even lower voltage could be set. Say 12.0 volts for example, at which the charger is then switched on.

It did not even need any form of computer.....I will try and find the circuit if you wish.....

It also turned off the charger as soon as a certain voltage on the battery was achieved.....I used 13.2 volts to prevent gassing and cell aging, but you could use a higher voltage if required

If used in parallel with the Solar charger, with good sized blocking diodes (which should already be installed by the way), I see no problem.

I would suggest that a time clock could be used on the mains to stop charging say after midnight (you pick the time!) so that at sunrise, the batteries are not fully charged from the mains at your expense, but can take a charge from the Sun for free.

An earlier or later time may be better, or even make that point at stopping a mains charge, PIC controlled, also easy to achieve....

There are many possibilities....

Best of luck!!

@Andy Germany : Yes , if you could find that circuit, please post it.

Thanks

Omar

Send me a private email address via the CR4 email.

I have found the original circuit I based my charger on, but do not forget it was for only 12 volts and it only switches a relay not a solid state version, and worst of all, its not in English. But someone good in electronics can change it to work at 48 volts very simply...the principal remains..