Solar batteries on the cheap

JackM

I have an ingenious (or maybe diabolical) design for adding emergency battery power to my existing solar system on the cheap, for use when the grid goes down. I would appreciate feedback from the community.

My goal is to be able to obtain at least some power from my existing grid-tied solar system in the event the grid goes down. In addition, it would be nice to have a small amount of batteries to power lights etc. at night when the grid is down. And the price has to be under $5000. 

My existing solar system is very vanilla. I have 18 standard 340W panels on my roof, each outfitted with an Enphase IQ 7+ microinverter. 

My plan is to purchase a "solar generator" such as the Bluetti EP500 and to connect this to the DC outputs of three of my existing panels, running them in series. Thus, the input to the Bluetti device would be 3 x 40 volts = 120 volts, and the current would be a bit under 10 amps, which is well within the threshold of the Bluetti device to handle. The Bluetti device includes a built-in MPPT.

Attached is a diagram of the connection that I am proposing.

I am only planning to connect the Bluetti solar generator to the panels when the grid goes down. But when the grid comes up again, it will likely take me a few minutes before I can get to the Bluetti solar generator to disconnect it. So, for a small period of time, both the Enphase microinverters and the Bluetti solar generator will be tied into the panels, fighting with each other for power.  

I’m wondering if you have any advice about whether this will work and whether it might damage my existing Enphase microinverters. I called Enphase and they said, of course, that this is "not a supported scenario". Frankly, I don't need them to support it, I just don't want it to damage anything.

And ... if anyone knows how to achieve my goals another way, without spending more than $5000, let me know!

Thanks in advance!

Photowhit

So micro inverters are connected on the back of your solar panels, so you would intend to disconnect the panels from the micro inverters then connect them in series to each other? your diagram doesn't represent that;


Heck with that expense I'd look at adding a 'real' battery back up for your home.

JackM

Hi, Photowhit. Looks like you are a frequent poster to this site! Thanks for your reply.

Yes, I propose to connect the three panels in series.  I thought the diagram represented that, but I admit that I am not a professional diagram drawer. And no, I don't propose to disconnect the micro inverters. When the grid is down, they would be powered off because they would not sense any voltage coming from the grid, so all the power would flow from the panels to the solar generator.

The price I've gotten from local installers for a "real" battery backup is $16,000 and up. I'm not willing to pay that much. The price for the Bluetti Pro EP500 is $4000. Other units could be had for even less, although they offer less battery storage. 

Ralph Day

That's not a realistic price for what you can DIY for a battery backup.
Golf cart batteries at under $100, 1kw inverter under $1000,, Charge controller under $1000, miscellaneous wire and such under $1000.  You're looking at under $4000 for the entire setup (panels at 50cents per watt).

Lots of help here too.

JackM

Here is an updated diagram, probably a lot clearer. The red connections would be the new connections that I would add. 

When the grid is down, I would throw the switch in the combiner box (not shown below) to isolate the AC side of the micro inverters from my house, and I would throw the transfer switch to isolate my house from the grid. I would plug the DC wires (shown in red) into the input of my "solar generator", the output of which would then be feeding into my house via the transfer switch.  

BB.

I would not do this series/parallel Enphase connections...

To know if this is "safe and reliable", you would need to ask Enphase about what would happen if added DC bias/offset voltages to the inputs of each Enphase Micro Inverter... Without knowledge of the front end--It is impossible to know how they would respond.

At best, it may work. Or it could cause issues with the Enphase because of the DC offset, to effectively a short circuit between the series solar panels and a non-isolated GT inverter and they AC output.

The Enphase inverters were (most likely) designed to assume that they only have a single XX Vmp by YY Imp solar panel connected to the input. To series/parallel connect into an array--Even if it "could work", Enphase would probably answer "no" to this configuration (liability, unknowns, etc.).

-Bill

solarvic

Why don,t you just charge your blue-eddy with a dc battery charger from the grid? 

JackM

Because my goal is to be able to obtain power from my existing grid-tied solar system in the event the grid goes down. When the grid goes down, Enphase micro inverters shut down and my solar system is useless. 

Thanks, everyone, for your responses. 

I think probably a safer alternative would be to outfit each panel with a 220 volt DPDT relay. When the grid is up, AC power is applied to the array, the relays are closed, and each inverter is connected to the panel. When the AC power is removed, the relays fall open, the series connection is made through the panels, and DC power is provided through the extra wires (in red) to flow down to the Bluetti solar generator device.

Yes?

Photowhit

I know you want to use what is already there, but I think there is no simple solution. Any connection you make on the roof are going to be out of the way things are normally done, string 3 panels together, even if there is no connection to your power bank, and you are likely in violation of current NEC code for voltage potential without disconnects on your roof. The reason Micro inverters have had an increase in popularity.

It might just be easier to buy 3 additional panels to feed your power bank unit in case the grid goes down. If you find someone who doesn't think they have 'gold', 1000 watts of panels should run $350-500.