AC Charging From excess solar

Roland W

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Oct 9, 2017
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Roland W, i've had a look through your videos and it looks like you are using relays to turn your chargers on/off rather than modulating based on available power?

Yes, I am using chargers with fixed power output, and remove the excess as it is available. It would be similar to divert into hot water tank using fixed resistant heaters.
It works well for me. I am cycling my pack almost every day. And using of the shelf chargers is extremely cost effective.
For me there is no point to modulate something to keep it steady around a zero line. I am going for the big chunks. You just need to use chargers which are sized correctly to the amount of excess you typically have. And using more and different chargers, gives you more levels.
 

Hamalot

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You just need to use chargers which are sized correctly to the amount of excess you typically have. And using more and different chargers, gives you more levels.

Unfortunately there is no typical excess on a good day I could have 3.5kw excess all day, on other days I might export nothing at all, or anything in between. I don't need perfect 0 export but if I just go for a 2kw charger then some days it'll never switch on, other days I'll be exporting energy I could be using.

Probably best to use several lower power ones to more finely track the amount of excess power.
I can post more details if you find this interesting.

Yes please, interested to hear more. I suppose I could go for 100w/200w/500w/1kw chargers and switch them on in combinations to make the power I need to the nearest 100w. It seems like there must be a more elegant solution though.
 

Roland W

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Unfortunately there is no typical excess on a good day I could have 3.5kw excess all day, on other days I might export nothing at all, or anything in between. I don't need perfect 0 export but if I just go for a 2kw charger then some days it'll never switch on, other days I'll be exporting energy I could be using.

Yes please, interested to hear more. I suppose I could go for 100w/200w/500w/1kw chargers and switch them on in combinations to make the power I need to the nearest 100w. It seems like there must be a more elegant solution though.

There are as well chargers with variable current output like this: https://www.aliexpress.com/item/400...earchweb0_0,searchweb201602_,searchweb201603_

If the potentiometer or rotary encoder which is dialing the current is replaced by a microcontroller output, it could be used to modulate the power output.

But this would require some hacking :)
 

ajw22

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I built my own power diverter so that excess solar is sent to my immersion heater through a PWM controller.

If you already have a water heater load that accurately adjusts power to just use the excess solar power, then the other loads (ie. chargers) don't need to adjust power accurately, because the water heater controller will slow down / step up to to fill the power gap.

I'd recommend using several 500W chargers. Makes programming easier, simple to see that's going on, and no need shuffle relays to get the desired load. Cheap and quick to find replacements, too.

What I found extremely important while building my system was to have a crystal clear plan on what you want the system to do.
Assuming the priorities are 1) not draw from the grid, and 2) charging batteries, last) heat water:
You're going to need to measure how much power you're EXporting and how much power the water HEater is using.
Whenever EX+HE goes above 650W, you turn on another one of your 500W chargers.
Whenever EX+HE goes below 50W, you turn off one of your 500W chargers.

You'll want to keep a certain power buffer, so that the devices aren't switched on/off too frequently.
I also keep a table with timestamps of when they last switched, total runtime, etc, and then activate the relay that has had sufficient cooldown time and the least runtime.

If you want to smartly use off-peak power, then you're going to want more data to make decisions: expected energy generation estimate, battery charge status, and water temperature. Then use off-peak power as required, but leave enough space in the battery / water tank to absorb potential excess energy generated during the day.

Zero experience with "Shelly", but I can send you some of my python code to query the IoTaWatt power meter and SmartPlug control script.
 

Hamalot

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Thanks both for the replies.

I agree that data is key and it's frustrated me for a long time that the only data I have about my power use is from me taking meter readings weekly for a year, the solar system had no data logging. I'm now logging data using aftermarket solution so will help going forward.
I also agree that I need a well thought out plan. Its expensive equipment to buy not knowing if it'll suit my needs.

That variable output charger might have been perfect if it had buttons instead of a potentiometer. I can hack buttons but no idea how to make a smart potentiometer. I may have to go down the non modulated route but would rather make the most of every few hundred watts that are being exported on overcast days, if possible. Also in the long run the immersion heater will go and the whole heating system replaced by an air source heat pump, run from the battery.

So, I have 2 pwm questions (hopefully not stupid questions, 2nd one feels logical-ish) :
What would happen if you supplied an ac charger with a pwm input? (this would be an easy solution for me to implement if it would work)
Or
Surely a solar charge controller is perfect for taking a constantly changing power source and providing a nice supply to the battery. Would it be able to handle a pwm dc input?
 

Hamalot

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And I hadn't heard of IoTaWatt before, it looks similar to the shelly Em but with way more CT capacity, I only have space for 2!
 

ajw22

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What would happen if you supplied an ac charger with a pwm input? (this would be an easy solution for me to implement if it would work)

Haven't tried it, but I think at best, it will continue to charge at the previous rate. Or not work at all. More likely is that something inside or your PWM components will break.

The charger will almost certainly have 2 stages in it:
1. Convert AC to a safer and easier to use DC (wobbly, but around 60V?) using a transformer and rectifier and big capacitors.
2. Regulate the 60V down to the required battery voltage and limit charge current.

The issue with PWM on AC is that Stage1 will just compensate by drawing a higher current whenever it can, and Stage2 will just continue to see the normal 60VDC. It's very possible that components in Stage1 will not withstand the repeated high current.


Again, haven't tried it, but if you're willing to open up and modify a charger... many will have potentiometers inside to change the charge voltage and current. You could tap into that. Perhaps with a digital potentiometer to adjust the current, or PWM the voltage pot to make it think the battery is full. Would be an interesting hack, but don't think it's worth the time.
 

ajw22

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An even more ghetto hack, I'm almost ashamed to suggest:
Take a 10A lab power supply, set voltage knob to the max battery voltage (hot glue, so it can't move). Attach a small servo motor to the current knob with a pulley...
 

Roland W

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I agree that data is key and it's frustrated me for a long time that the only data I have about my power use is from me taking meter readings weekly for a year, the solar system had no data logging. I'm now logging data using aftermarket solution so will help going forward.
I also agree that I need a well thought out plan. Its expensive equipment to buy not knowing if it'll suit my needs.
Can I ask you in which world you are living in, 3ph 230/400V of 1ph split 120/240V?

I didn't know anything about my power usage as well, until I have got myself the own smart meter which I am reading out via Modbus and logging all 3 phases power averages every 6 minutes. Making a device like that is quiet inexpensive. A small microcontroller, SD-card module and Modbus module plus the meter...
 
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daromer

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I use SDM630 devices and read data couple of times per seconds. Its sent to MQTT and then parsed for diverting power if needed :)


And if you only need to read 1 meter i recommend running Tasmota though it limits every 10 seconds.
 

Hamalot

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I use SDM630 devices and read data couple of times per seconds. Its sent to MQTT and then parsed for diverting power if needed :)


And if you only need to read 1 meter i recommend running Tasmota though it limits every 10 seconds.
Thanks, but I have the monitoring (although I may require something faster later on) and know when I'm exporting, I need to know how to modulate the battery charging based on that available power.
 

Hamalot

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An even more ghetto hack, I'm almost ashamed to suggest:
Take a 10A lab power supply, set voltage knob to the max battery voltage (hot glue, so it can't move). Attach a small servo motor to the current knob with a pulley...
My first attempt at my power diverter was to replace the potentiometer on a pwm controller with a smart dimmer, when that didn't work I actually considered this approach.... For about 10 seconds 😁. Then I found a digital version that I could hack the buttons of.
 

Roland W

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My first attempt at my power diverter was to replace the potentiometer on a pwm controller with a smart dimmer, when that didn't work I actually considered this approach.... For about 10 seconds 😁. Then I found a digital version that I could hack the buttons of.

Need to use a digital potentiometer: THB 38.63 5%OFF | PCS Great IT X9C103S Digital Potentiometer Board Module DC3V-5V for Arduino

Like this, your MC is virtually turning the knob and changing resistance in the chip. They exist in different resistance values, so first you would need to have the charger and take a look what value of potentiometer is built in. There is a couple of videos on youtube on that topic if you are interested :)
If one day I can afford to upgrade my powerwall, I will definitly try it.
 
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