Grid tie and battery together

[josepr]

I’m not sure you can do what you want with your setup. In order to not export power to the grid you need some type of consumption meter and some way to turn off your ‘eBay’ grid tied (GT) inverter. Fortunately, you can connect your grid tied inverter with your solar charge controller (CC) if you are using 24v panels or if your string voltage is supported by both your CC and GT. Your just need a normally open relay (if possible, use a solid-state relay) {Relay need to be 250VAC {or your grid voltage} control and whatever DC voltage is your panels or string for the load side}. When power is available the panel or string will be connected to the GT, when power is out the panels with be connected to the CC. But in my opinion your best option is a small 2k hybrid inverter with a zero-grid option, if you do not want to export to the grid. Others DIY solutions will require a custom PLC or a raspberry pi with a consumption coil (amp meter). I strongly recommend solid state relays. For that try to limit yourself to a 24v battery because 48v solid-state relays are not cheap.

[Tired sparky]

Well I managed to figure out a way with help from a YouTuber by the name of Andy Kirby.

How is I did it was to run the power from the panels to a 40A DPDT 2NO 2NC 24 volt Relay. This is controlled by one of those cheap charge controllers that say MPPT but are really PWM. It has the normal three sets of terminals Solar/battery/load. Only I used the battery terminal to monitor the voltage and the load terminals operate the relay.

I have the load parameters set to 25.5 and 28 volts. This may change if I start using more load.

What happens is if the battery bank drops below 25.5 volts the cheap secondary CC senses the drop and the relay switches the incoming solar power to the main charge controller which will charge the batteries until 28 volts is reached. Once done the cheap CC switches the load off and the relay switches over to providing the Grid Tie Inverter with power which I call the default state. It is fully automatic and works like a charm!!

I get Grid Tie power most of the time with the switchover to battery charging when and as it needs it. So far absolutely no issues with this DIY cobbled together set up! Well pleased!!!!

[SunEagle]

Well I managed to figure out a way with help from a YouTuber by the name of Andy Kirby.

How is I did it was to run the power from the panels to a 40A DPDT 2NO 2NC 24 volt Relay. This is controlled by one of those cheap charge controllers that say MPPT but are really PWM. It has the normal three sets of terminals Solar/battery/load. Only I used the battery terminal to monitor the voltage and the load terminals operate the relay.

I have the load parameters set to 25.5 and 28 volts. This may change if I start using more load.

What happens is if the battery bank drops below 25.5 volts the cheap secondary CC senses the drop and the relay switches the incoming solar power to the main charge controller which will charge the batteries until 28 volts is reached. Once done the cheap CC switches the load off and the relay switches over to providing the Grid Tie Inverter with power which I call the default state. It is fully automatic and works like a charm!!

I get Grid Tie power most of the time with the switchover to battery charging when and as it needs it. So far absolutely no issues with this DIY cobbled together set up! Well pleased!!!!

Hmm. I give it a couple of months before that 24V relay fails. But if it works for you then have at it.

[Ampster]

Hmm. I give it a couple of months before that 24V relay fails. But if it works for you then have at it.

Is that a safety issue or are you just having a bad day? Most relays I know of have a MTBF significantly longer than 60 times. Relays have been around a long time and actually performed logic functions before computers. I am sure there are ways to abuse a relay but assuming it is matched to the voltage and amperage of the circuit it should last some time. In this case it was two poles at 40 Amps for each pole. What am I missing?

[SunEagle]

Is that a safety issue or are you just having a bad day? Most relays I know of have a MTBF significantly longer than 60 times. Relays have been around a long time and actually performed logic functions before computers. I am sure there are ways to abuse a relay but assuming it is matched to the voltage and amperage of the circuit it should last some time. In this case it was two poles at 40 Amps for each pole. What am I missing?

It all depends on the type of contacts used in the relay and the voltage it is interrupting. Some relays are not designed to interrupt DC voltage and tend to Arc causing the contacts to fail.

If that relay is a solid state type then the life span and cycle count go way up compared to a mechanical relay that is being abused.

IMO I think the OP is really not thinking this through but if it works for them then great.

[Ampster]

Well, if that happens, let's hope the OP gets back to us with feedback so that the readers can benefit. I would hope he would use a relay rated for DC and the Amperage of that circuit. At high Amps there are contractors that can handle hundreds of DC Amps.

[Tired sparky]

The relay is DC rated at 24 volts. Looking again the amperage has 40 in the title but in the info section 30A at 28 volts?? It has Two separate ‘gangs’ for switching two sets of loads, I’m using only one of the gangs.

Im UK based if anyone didn’t know. I got my relay from Amazon.co.uk but they are also on Amazon.com as I just checked. If you put...... taiss jqx 12f 2z .......in the search it comes up the same unit. $15 if it blows up so no biggie. It’s been running since I last posted maybe a month ago? Has about a dozen switch cycles so far.

Can you post post pics here?

[SunEagle]

The relay is DC rated at 24 volts. Looking again the amperage has 40 in the title but in the info section 30A at 28 volts?? It has Two separate ‘gangs’ for switching two sets of loads, I’m using only one of the gangs.

Im UK based if anyone didn’t know. I got my relay from Amazon.co.uk but they are also on Amazon.com as I just checked. If you put...... taiss jqx 12f 2z .......in the search it comes up the same unit. $15 if it blows up so no biggie. It’s been running since I last posted maybe a month ago? Has about a dozen switch cycles so far.

Can you post post pics here?

What voltage are you interrupting? Is that 24V the coil rating or the contact rating? Most of the high voltage relays have a DC rating of 300V. Since your are disconnecting a string of panels from the battery CC to the grid tie inverter I would think the voltage could be pretty high. But if you have selected a minimum number of panels and their Vmp is lower then the contact rating on the relay you should be ok.

[Tired sparky]

At the moment I’m only putting about 16 amps max through the relay. I plan to double my panels eventually but even then due to placement issues I doubt I will ever get much more than 20 amps at any one time?

[SunEagle]

Well, if that happens, let's hope the OP gets back to us with feedback so that the readers can benefit. I would hope he would use a relay rated for DC and the Amperage of that circuit. At high Amps there are contractors that can handle hundreds of DC Amps.

You are correct. There are quality contactors that can interrupted high DC voltage and are designed with arc shoots to minimize their affect. But based on what Tired Sparky is posting the relay is a cheap amazon unit that could easily fail and cause more then just smoke.

[SunEagle]

At the moment I’m only putting about 16 amps max through the relay. I plan to double my panels eventually but even then due to placement issues I doubt I will ever get much more than 20 amps at any one time?

It is not only the amps but the voltage that you are interrupting. What is the relay rated? The amp rating of contacts is when they are closed and touching each other. If you open them under load then an arc can be created which can cause damage to the relay.

[Tired sparky]

I have a 2S2P panel setup. Four 160 watt Panels with a Vmp I think of 18 volts? Under load I should be running something like 30 odd Volts through the relay?

[Tired sparky]

Just looked again at the relay specs, 24 volts coil and 30 amps @ 28 volts contacts.

[SunEagle]

I have a 2S2P panel setup. Four 160 watt Panels with a Vmp I think of 18 volts? Under load I should be running something like 30 odd Volts through the relay?

Then you are probably ok and the relay can handle that voltage during an interrupt. But again each time you open those contacts you can create an arc which will degrade the contacts over time. Please keep an eye on them.

[SunEagle]

Just looked again at the relay specs, 24 volts coil and 30 amps @ 28 volts contacts.

With only 28V you may exceed the arc rating and lose those contacts pretty quickly.