Small scale solar battery storage considered - a DIY project

[MeldrewsMate]

I've had solar PV on my roof for 6 years now; very pleased with it, but had to repair it last week by replacing one of the 16 micro-inverters, one under each solar panel. I had bought 2 of these as spares and, after having the defective inverter replaced, now have 2 spare again. This got me thinking, and the question/opportunity is this:-

How can I rig up these two inverters to a battery bank so that the batteries charge via solar during the day, and power the fridge and some lights/radio/TV at night?

This has turned out to be not such a straightforward problem to solve, for the following reasons.

1. I have rigged-up 3 lead acid batteries in series to give 36v dc - this is perfect as a substitute for a single solar panel, so no problem there.
2. I can charge these batteries via an old electric bike charger, so again no problem.
3. I can attach both spare inverters (each has about 200 W output, and will auto-sync to the mains frequency) to the battery bank, that's not where the problem lies.
The problem is that, once powered-up, these inverters will run as hard as they can until the batteries are exhausted, and if I don't use that power immediately they will export it freely to the grid...and what's worse, I won't even get paid the feed-in tarrif payment for it!

I need a method of limiting or controlling the power they produce so that they only feed into my electrical system.
Any ideas how I can achieve this? I'm not yet willing to shell out thousands for a commercial battery product such as Tesla Powerwall, but presumably they have a simple way to achieve the same.

MM

 

[lostinthelight]

Is there any type of induction control clipped onto the mains cable that when it senses your using power it brings the inverters on...something like the control for turning the immersion solar switch on

 

[big-all]

a fridge freezer perhaps 120w on a perhaps 33% cycle over 24hrs so an average 40wx24= so perhaps 1kw at 14p
is it worth the trouble for perhaps £1 a week ?? and thats assuming you can generate 80 ah/960w at 12v or 27ah320w at 36v per day ??
ok looking at my calculations somthing seems wrong
not shure what

 

[MeldrewsMate]

Is there any type of induction control clipped onto the mains cable that when it senses your using power it brings the inverters on...something like the control for turning the immersion solar switch on

If you mean a current transformer then yes, there is one, and it is used by a Solar iBoost (similar to Immersun).
Is there a reason for your question? A suggestion as to how it can be used?

...is it worth the trouble for perhaps £1 a week ?

You're right, big-all, not worth it for £1 a week...but the 'big-ger' picture is that I can scale it up once the concept is proven. It also helps to keep my brain ticking over, so it's worth it for more than its monetary value.

MM

 

[lostinthelight]

If you mean a current transformer then yes, there is one, and it is used by a Solar iBoost (similar to Immersun).
Is there a reason for your question? A suggestion as to how it can be used?

I raise the subject as I thought you could look into the possibilities if such bluetooth operated switch would switch your inverters on much in the same way as the immersun works ,if one actually exists that is?

 

[MeldrewsMate]

I'm off to investigate how to control/limit the output from the batteries, that is to try and get the batteries to simulate the varying outputs from solar PV panels. If I disconnect the inverter's output from the mains, even for just a second, it will take up to 5 minutes to reconnect and re-synchronise frequency with the mains, and that would defeat the main purpose of the project.
However I need to establish how quickly the inverter would start to generate once the battery output is connected (its 'wake-up' time). In this way I can use a current transformer (CT) to read the mains input, and a microcontroller (Arduino/Raspberry Pi, etc) to match the battery output to the load, such that the net input from the mains is zero. There is a whole community out there using microcontrollers in a similar way, usually to replicate or improve on the Solar iBoost/ Immersun system....I just need to change the coding a little to suit my purpose. Once working and proven I can add to it bit by bit, eventually meeting most of my electrical needs by net charging of batteries by day, and discharging by night.

At least that's my aim when there's enough sunshine.

MM