Using solar micro inverters with batteries instead of panels

twrlp · May 3, 2023

fafrd said:
OK, so you are powering via battery and the measuring full-chain round-trip resistance by measuring DC current along with the voltage drop you get at the Microinverter input from that current, is that correct?

You've got it. I split the measurement into two parts - from the + mc4 connector to batt+ bus in between the batteries and main fuse; and from - mc4 to the negative battery bus. Sum of those values was 89.1mV at 9.47A, == 9.4mOhm. The remaining 50mOhm is my estimate of the batteries' internal resistance.

newbostonconst said:
Once I connected the IQ7x to the batteries connected to my hybrid inverter I got the grounding faults.

I temporarily* lost another m250 to "DC Resistance Low" today powering on the micro while my hybrid AIO inverter was charging from the grid. I guess it makes sense that the charger would open a non-isolated path between neutral/AC ground and DC negative (and probably AC hot and DC+, but the micro doesn't seem to check that.) @newbostonconst do you recall whether your hybrid was also charging when your IQ7x raised the fault?

The m250's only appear to perform this ground check at boot time so it's hopefully just a matter of sequencing the operations correctly. Right now I'm amusing myself charging 10A from the grid and sending that 10A right back via the m250…

* leaving the m250 connected to the Envoy for 45 minutes allowed me to clear the error on today's m250 as well as one of my original 2 down inverters. The third is still showing a red LED and isn't visible to the Envoy, I think it's likely dead dead.

kundip · May 3, 2023

agt said:
You've got it. I split the measurement into two parts - from the + mc4 connector to batt+ bus in between the batteries and main fuse; and from - mc4 to the negative battery bus. Sum of those values was 89.1mV at 9.47A, == 9.4mOhm. The remaining 50mOhm is my estimate of the batteries' internal resistance.

I temporarily* lost another m250 to "DC Resistance Low" today powering on the micro while my hybrid AIO inverter was charging from the grid. I guess it makes sense that the charger would open a non-isolated path between neutral/AC ground and DC negative (and probably AC hot and DC+, but the micro doesn't seem to check that.) @newbostonconst do you recall whether your hybrid was also charging when your IQ7x raised the fault?

The m250's only appear to perform this ground check at boot time so it's hopefully just a matter of sequencing the operations correctly. Right now I'm amusing myself charging 10A from the grid and sending that 10A right back via the m250…

* leaving the m250 connected to the Envoy for 45 minutes allowed me to clear the error on today's m250 as well as one of my original 2 down inverters. The third is still showing a red LED and isn't visible to the Envoy, I think it's likely dead dead.

IO7's do not require grounding.
As I understand it - any shorting on the DC side is put back through the AC earth cable.
"The microinverter itself has a Class II double-insulated rating, which includes ground fault protection (GFP). To support GFP, use only PV modules equipped with DC cables labeled PV Wire or PV Cable."
.

fafrd · May 4, 2023

agt said:
You've got it. I split the measurement into two parts - from the + mc4 connector to batt+ bus in between the batteries and main fuse; and from - mc4 to the negative battery bus. Sum of those values was 89.1mV at 9.47A, == 9.4mOhm. The remaining 50mOhm is my estimate of the batteries' internal resistance.

I temporarily* lost another m250 to "DC Resistance Low" today powering on the micro while my hybrid AIO inverter was charging from the grid. I guess it makes sense that the charger would open a non-isolated path between neutral/AC ground and DC negative (and probably AC hot and DC+, but the micro doesn't seem to check that.) @newbostonconst do you recall whether your hybrid was also charging when your IQ7x raised the fault?

The m250's only appear to perform this ground check at boot time so it's hopefully just a matter of sequencing the operations correctly. Right now I'm amusing myself charging 10A from the grid and sending that 10A right back via the m250…

* leaving the m250 connected to the Envoy for 45 minutes allowed me to clear the error on today's m250 as well as one of my original 2 down inverters. The third is still showing a red LED and isn't visible to the Envoy, I think it's likely dead dead.

At this point, are the only successful results on the thread using Enphase microinverters?

I can’t recall whether NCSolar tested his Hoymiles Microinverters with battery power or not (but since he would have been powering through his current-limiter boards, not really relevant to what we are discussing anyway).

I just received all my DCDC boosters, PWM controllers and 0.05 Ohm power resistors so I hope to join the party soon. I’m out of town for the rest of this month so I won’t be able to start testing my NEP Microinverters until this summer,

I’m hoping my NEP Microinverters are not sophisticated enough to force me to jump through as many hoops as your M250s are forcing you to jump through…

twrlp · May 4, 2023

fafrd said:
At this point, are the only successful results on the thread using Enphase microinverters?

I think @kundip's got me beat in terms of success & scale!

fafrd said:
I’m hoping my NEP Microinverters are not sophisticated enough to force me to jump through as many hoops as your M250s are forcing you to jump through…

Agreed. I predict we'll see plenty of used m215/m250's on the market as people update systems, but a $25 micro that's complicated and unreliable isn't such a bargain.

Safe travels!

fafrd · May 4, 2023

agt said:
cannot find any way to delete this…

agt said:
Before building out my planned 4-6 inverters I plan to try the PWM and DC/DC approaches and see if they'll work sans resistor

If I’m lucky, you’ll have some new results to share before my return

.

One difference between our setups is that you don’t seem to care much about excessive current ripple prematurely degrading the input capacitors of your microinvetters, while I only have a few spare NEP Microinverters and would like to keep current ripple within spec if possible.

Powered direct from battery as you are doing current ripple will be a non-issue, so the only potential hiccups are that power cannot be controlled (maximum Microinverter output) and any hoops needed to get the Microinverter to lock-on properly upon Bootup.

Powered by DCDC booster is my preferred approach to programming / limiting power output and shenanigans at Bootup should be a non-issue so the only real potential hiccup is excessive current ripple when responding to the 60Hz variations of the Microinverters MPPT (the second less critical issue being poor efficiency).

The more fundamental issue with the cheapo DCDC boosters is that if they are as finnicky to program / adjust as kundip and others have experienced and as as unreliable after programming as he and others have reported, they won’t represent a viable option.

Powered by PWM should deliver close to the best efficiency possible with an adapter board between battery and Microinverter and offer the easiest-to-control option if dynamic control of power output is a priority. They seem highly unlikely to suffer from the finnikiess and/or instability of the cheapo DCDC boosters but represent the absolute worst-case in terms of current ripple (by design).

So I’m only going to put effort into filtering PWM output if I give up on the cheapo DCDC boosters and decide I need t find a way to deliver throttled / controlled output from battery.

Your opinion on whether the cheapo DCDC boosters are reliable-enough to represent a viable alternative will be very interesting (and I already outlined for kundip my suggestion to complete adjustment of the pots using DC-only loads, not while being loaded by a dynamic Microinverter).

kundip · May 4, 2023

fafrd said:
If I’m lucky, you’ll have some new results to share before my return .

One difference between our setups is that you don’t seem to care much about excessive current ripple prematurely degrading the input capacitors of your microinvetters, while I only have a few spare NEP Microinverters and would like to keep current ripple within spec if possible.

Powered direct from battery as you are doing current ripple will be a non-issue, so the only potential hiccups are that power cannot be controlled (maximum Microinverter output) and any hoops needed to get the Microinverter to lock-on properly upon Bootup.

Powered by DCDC booster is my preferred approach to programming / limiting power output and shenanigans at Bootup should be a non-issue so the only real potential hiccup is excessive current ripple when responding to the 60Hz variations of the Microinverters MPPT (the second less critical issue being poor efficiency).

The more fundamental issue with the cheapo DCDC boosters is that if they are as finnicky to program / adjust as kundip and others have experienced and as as unreliable after programming as he and others have reported, they won’t represent a viable option.

Powered by PWM should deliver close to the best efficiency possible with an adapter board between battery and Microinverter and offer the easiest-to-control option if dynamic control of power output is a priority. They seem highly unlikely to suffer from the finnikiess and/or instability of the cheapo DCDC boosters but represent the absolute worst-case in terms of current ripple (by design).

So I’m only going to put effort into filtering PWM output if I give up on the cheapo DCDC boosters and decide I need t find a way to deliver throttled / controlled output from battery.

Your opinion on whether the cheapo DCDC boosters are reliable-enough to represent a viable alternative will be very interesting (and I already outlined for kundip my suggestion to complete adjustment of the pots using DC-only loads, not while being loaded by a dynamic Microinverter).

Yes I need to test Boost Converter using DC only loads.
You guys have probably said this a number of times but I was just too dumb.
I have been using a old 24VDC to 240VAC inverter with 240V globes on other side. to TRY & set load.
In hindsight this has been a mistake.
I will get a string of 24V DC globes and add as many as I need to get the right amount draw setting if I can.

fafrd · May 4, 2023

kundip said:
Yes I need to test Boost Converter using DC only loads.
You guys have probably said this a number of times but I was just too dumb.
I have been using a old 24VDC to 240VAC inverter with 240V globes on other side. to TRY & set load.
In hindsight this has been a mistake.

Yes.

kundip said:
I will get a string of 24V DC globes and add as many as I need to get the right amount draw setting if I can.

You can use DC bulbs or alternatively, you can use a multimeter to first set maximum voltage output / pot (no load) and then to set maximum current output / pot (either shorting positive to negative through a multimeter set to measure current or alternatively using a resistor as a load and measuring voltage across it to measure current).

For example, if you are aiming for 250W / 95% efficiency = 263W @ 35VDC, you would first adjust the voltage pot until you read 35VDC with no load or a high-valued resistance (1kOhm or more) as a load.

263W / 35VDC = 7.5A, so that is the current-limit you are aiming for.

R=V/I so the deal resistance to test 7.5A @ 35VDC would be 4-2/3 Ohms. But with a resistance that large, power will exceed 100W within the resistor itself - too much for the 100W power limit of a typical power resistor.

100W / 35VDC = 2.85 Ohms, so the power resistor you use should be 2 Ohms or 2.5 Ohms at most. Let’s assume you have a 2.0 Ohm 100W power resistor.

With an 2 Ohm resistance as a load and assuming the current limit is set too low - let’s say 1A, you will measure only 2.0V across the resistor.

As you start increasing the current pot, current will increase along with measured voltage across the resistance.

At 2.0A of current measured voltage will increase to 4.0V and at 4A of current the voltage will increase to 8.0V.

When you have adjusted the current-limit pot until you read 7.5A x 2.0 Ohms = 15.0VDC, you will know the current limit has now been set to 7.5A and you are done.

With a DCDC converter set for a maximum of 35VDC and 7.5ADC, you can now connect to your Microinverter to measure how much output power it delivers.

Let’s say, for example, you only measure 225W of AC power and want to increase by 11.1%.

You need to disconnect from the Microinverter, reconnect the 2.0 Ohm resistor and increase the current limit pot until you read 16.7VDC instead of 15.0VDC.

When you reconnect to the Microinverter, you should see that AC power output has increased from 225W to 250W (assuming your Microinverter can actually deliver that full level of power).

Using a power resistor is a more reliable way to adjust the current pot than using a DC lightbulb, but either can work.

Just measure resistance of the power resistor or lightbulb in advance, set voltage limit pot first with no load, and then set current limit pot second using voltage you measure across the power resistor or DC lightbulb rather than relying on the wattage rating of the lightbulb.

The resistance of a power resistor is the same whether the current being passed through it is small or large. I have no reason to believe that is not the case with a DC lightbulb as well but I am much less certain and if the resistance does change between when you measure it and when you put current through it, this technique breaks own.

My advice is to invest a few $$$s is a 2.0 or 2.5 Ohm 100 W power resistor…

kundip · May 4, 2023

This may be of interest.
I am not sure if it would work in our case:

https://www.ev-power.com.au/product/rel-prechg180/

kundip · May 5, 2023

fafrd said:
Yes.

You can use DC bulbs or alternatively, you can use a multimeter to first set maximum voltage output / pot (no load) and then to set maximum current output / pot (either shorting positive to negative through a multimeter set to measure current or alternatively using a resistor as a load and measuring voltage across it to measure current).

For example, if you are aiming for 250W / 95% efficiency = 263W @ 35VDC, you would first adjust the voltage pot until you read 35VDC with no load or a high-valued resistance (1kOhm or more) as a load.

263W / 35VDC = 7.5A, so that is the current-limit you are aiming for.

R=V/I so the deal resistance to test 7.5A @ 35VDC would be 4-2/3 Ohms. But with a resistance that large, power will exceed 100W within the resistor itself - too much for the 100W power limit of a typical power resistor.

100W / 35VDC = 2.85 Ohms, so the power resistor you use should be 2 Ohms or 2.5 Ohms at most. Let’s assume you have a 2.0 Ohm 100W power resistor.

With an 2 Ohm resistance as a load and assuming the current limit is set too low - let’s say 1A, you will measure only 2.0V across the resistor.

As you start increasing the current pot, current will increase along with measured voltage across the resistance.

At 2.0A of current measured voltage will increase to 4.0V and at 4A of current the voltage will increase to 8.0V.

When you have adjusted the current-limit pot until you read 7.5A x 2.0 Ohms = 15.0VDC, you will know the current limit has now been set to 7.5A and you are done.

With a DCDC converter set for a maximum of 35VDC and 7.5ADC, you can now connect to your Microinverter to measure how much output power it delivers.

Let’s say, for example, you only measure 225W of AC power and want to increase by 11.1%.

You need to disconnect from the Microinverter, reconnect the 2.0 Ohm resistor and increase the current limit pot until you read 16.7VDC instead of 15.0VDC.

When you reconnect to the Microinverter, you should see that AC power output has increased from 225W to 250W (assuming your Microinverter can actually deliver that full level of power).

Using a power resistor is a more reliable way to adjust the current pot than using a DC lightbulb, but either can work.

Just measure resistance of the power resistor or lightbulb in advance, set voltage limit pot first with no load, and then set current limit pot second using voltage you measure across the power resistor or DC lightbulb rather than relying on the wattage rating of the lightbulb.

The resistance of a power resistor is the same whether the current being passed through it is small or large. I have no reason to believe that is not the case with a DC lightbulb as well but I am much less certain and if the resistance does change between when you measure it and when you put current through it, this technique breaks own.

My advice is to invest a few $$$s is a 2.0 or 2.5 Ohm 100 W power resistor…

Hello farfrd
Thanks so much.
I will give this a crack.

JeepHammer · May 5, 2023

chopwood said:
Yes confirming that I'm trying to timeshift. And yes a powerwall or other similiar product would do exactly what I want. I just don't want to spend $15,000 especially when I have a number of the micro inverters already on the roof and hooked into the grid.

So in summary, yes, connect the battery to the input side of the microinverter. leave the output side connect to 240V as it currently is.

Interesting point about batteries not being current limited. I would have expected that the inverter would handle a panel that produced more current than the inverter was rated at. Interesting idea about current limiting the battery.

Several inverters on the market let you time shift. Some states are regulating when you can use your own batteries.

This is where a cheaper hybrid inverter with batteries comes in handy, just disconnect the grid and let the backup function use your batteries. Flip back when the sun comes up. No timer involved, so you have to do it manually...

fafrd · May 5, 2023

kundip said:
Hello farfrd
Thanks so much.
I will give this a crack.

Let us know how it goes. Whether you use a power resistor or a DC lightbulb, it’s important to measure the resistance using your multimeter of the calculations will be off.

Good luck.

fafrd · May 6, 2023

For those considering to use DCDC boosters to power their Microinverters from battery, these are not identical to the cheapo DCDC boosters from China, but they are similar: https://www.elecrow.com/1032v-to-1235v-150w-dcdc-booster-p-672.html

They include a pretty complete specification including these:

Conversion efficiency: 94% (measured at Input 16V, output 19V 2.5A)
Output Ripple: 2% (MAX) 20M-bandwidth

2% ripple is low enough that it is unlikely to cause any concern with premature Microinverter input capacitor degradation, so if this spec is representative of the amount of current and/or voltage ripple coming off of the cheapo Chinese boosters, it is likely a non-issue.

At 250W of output power, 2% current ripple represents 0.2A @ 25VDC…

cs1234 · May 6, 2023

Those things kind of break the bank though @fafrd at $5 each!

fafrd · May 6, 2023

cs1234 said:
Those things kind of break the bank though @fafrd at $5 each!

For sure. I picked up three of the 400W units from China for $3 each but just might have to upgrade

.

Seriously, though, while efficiency is not the highest-priority, low efficiency does translate to additional heat generation and accelerated deterioration.

I’m going to characterize my cheapo Chinese boosters: https://www.amazon.com/Constant-Cur...=&hvlocphy=9032082&hvtargid=pla-1018863265672

to see if I only get the same ~80% efficiency others have reported on YouTube.

If so, I might try one of these Elecrow 150W models to see if they actually deliver the 94% efficiency claimed…

twrlp · May 7, 2023

agt said:
I temporarily* lost another m250 to "DC Resistance Low" today powering on the micro while my hybrid AIO inverter was charging from the grid.

Further testing confirms that charging during m250 bootup will consistently trigger the DC Resistance error. I bet a fully isolated charger would be fine, but my AIO and it sounds like most chargers don’t isolate DC from AC— so I’ll just ensure the two activities don’t overlap.

In other news I’ve found that some m250’s will boot and MPPT lock with no added resistance, and that pairing one of these with a micro normally needing Rboot allows both to boot and begin producing power. All seem to run the same firmware release and I haven’t identified any external differences, perhaps board revisions or components with differing tolerances.

Two inverters in parallel do seem to fight over MPPT for a minute or so until one finally prevails, at which point the other also locks ont. Some way to stagger startup may be needed.

kundip · May 7, 2023

agt said:
Further testing confirms that charging during m250 bootup will consistently trigger the DC Resistance error. I bet a fully isolated charger would be fine, but my AIO and it sounds like most chargers don’t isolate DC from AC— so I’ll just ensure the two activities don’t overlap.

In other news I’ve found that some m250’s will boot and MPPT lock with no added resistance, and that pairing one of these with a micro normally needing Rboot allows both to boot and begin producing power. All seem to run the same firmware release and I haven’t identified any external differences, perhaps board revisions or components with differing tolerances.

Two inverters in parallel do seem to fight over MPPT for a minute or so until one finally prevails, at which point the other also locks ont. Some way to stagger startup may be needed.

Very Interesting. Thanks.

AntronX · May 9, 2023

Here are 2 of my IQ7 and IQ7+ running on 2 Tesla Model S modules in series. I start them in test mode by connecting AC power and DC within 3 seconds, so they start generating without 5 min delay. Start up via 2 ohm resistor then I jumper them out. There must be at least 0.1 ohm in all those test leads. I plan to add 10A DC fuses and arduino controlled relays to sequence the startup properly as well as monitor grid backfeed and turn inverters on/off to maintain zero backfeed. Using them to dump excess energy from my offgrid solar.

kundip · May 9, 2023

AntronX said:
Here are 2 of my IQ7 and IQ7+ running on 2 Tesla Model S modules in series. I start them in test mode by connecting AC power and DC within 3 seconds, so they start generating without 5 min delay. Start up via 2 ohm resistor then I jumper them out. There must be at least 0.1 ohm in all those test leads. I plan to add 10A DC fuses and arduino controlled relays to sequence the startup properly as well as monitor grid backfeed and turn inverters on/off to maintain zero backfeed. Using them to dump excess energy from my offgrid solar.View attachment 148544

Fantastic.
Great work - I love it!

fafrd · May 9, 2023

AntronX said:
Here are 2 of my IQ7 and IQ7+ running on 2 Tesla Model S modules in series.

What is the voltage of each Tesla module? What is the DC voltage being delivered by both modules to power the IQ7/IQ7+?

AntronX said:
I start them in test mode by connecting AC power and DC within 3 seconds, so they start generating without 5 min delay.

‘Test mode’ is your own test mode (ie: with jumpers to 2 Ohm resistance connected) or a mode built into the Tesla modules or a mode built into the IQ7/IQ7+ Microinverters?

AntronX said:
Start up via 2 ohm resistor then I jumper them out.

You are doing this manually with jumper cables? 2 ohm resistance is present when AC power is connected and then you remove jumper cables connecting that 2-ohm resistance after power generation begins following a ~3 second delay?

AntronX said:
There must be at least 0.1 ohm in all those test leads.

I see all the power resistors - are those all 0.1 Ohm?

What are ‘all those test leads’? A simple circuit diagram showing the circuit from battery to Microinverters both configured for Bootup/Test mode as well as running mode would be great - I don’t understand which wires / leads you’ve had to add 0.1 Ohms of resistance to make things work…

AntronX said:
I plan to add 10A DC fuses and arduino controlled relays to sequence the startup properly as well as monitor grid backfeed and turn inverters on/off to maintain zero backfeed. Using them to dump excess energy from my offgrid solar.View attachment 148544

This is exactly what I’m aiming to do (with my 24VDC LiFePO4 battery, 1kW of DC-coupled off-grid solar and controlled by PLC rather than arduino.

I understand you have found a way to speed up the response time of your IQ7 / IQ7+ relays but I don’t understand whether you Ae controling export through AC relays at the output end or DC relays at the input end.

A bit more detail on your plan / circuit would be very helpful (as well as a bit more detail on what testing / failures you experienced to determine that you need 2 Ohms in the DC path at Bootup and 0.1 Ohms on various other wires / leads during running condition…

AntronX · May 9, 2023

I ran Tesla modules from 43.2 to 47.1V today (21.6 - 23.55V each, modules are 6S74P Li-ion). Voltage delivered to each IQ7 was probably 1 - 1.5V less due to thin DC wires used as well as a bunch of crocodile clip wires in line, this is where I estimate 0.1 ohm effective resistance between battery and inverters. I can measure exactly tomorrow. IQ7 micros have built in test mode where if you apply grid and PV within 3 seconds they skip the 5 minute wait time and begin feeding power to grid as soon as they boot up. I had 2 ohm resistors in series with each IQ7 during bootup and bypassed those resistors once AC power began to flow out to grid. Previous posters in this thread used 4 ohm resistors to avoid getting low DC impedance error in the micros which bricks the inverters until they can be reset using Envoy. I think I already lost one inverter to this impedance error in the past. Everything is controlled manually for now.

fafrd · May 9, 2023

AntronX said:
I ran Tesla modules from 43.2 to 47.1V today. Voltage delivered to each IQ7 was probably 1 - 1.5V less due to thin DC wires used as well as a bunch of crocodile clip wires in line, this is where I estimate 0.1 effective resistance between battery and inverters. I can measure exactly tomorrow. IQ7 micros have built in test mode where if you apply grid and PV within 3 seconds they skip the 5 minute wait time and begin feeding power to grid as soon as they boot up. I had 2 ohm resistors in series with each IQ7 during bootup and bypassed those resistors once AC power began to flow out to grid. Everything is controlled manually for now.

Great, thanks.

So you are operating off of a 48VDC battery and your IQ7 / IQ7+ Microinverters integrate a ‘test mode’ which allows you to connect AC and as long as DC is connected within 3 seconds and is connected through a 2 Ohm resistance, it will begin producing output much more quickly / immediately that the ~5 Minutes otherwise required for the full POST / grid signal characterization, correct?

You don’t have any added inline resistance but are estimating ~0.1 Ohm of wiring + contact resistance.

How did you determine a 2 Ohm resistance was required for the first ~3 seconds?

Have you tried and failed with 1 Ohm?

Have you tried and failed with 0 Ohms (of added resistance, meaning wiring + contact resistance only)?

The effective resistance slope of most solar panels is ~1 Ohm, so I’m trying to understand whether 2 Ohms is required or merely boots successfully.

If you could boot successfully with only 0.2 Ohms of inline resistance, would you bother with the complexity of bypassing a boot resistance to get back the ~20W / ~7% in lost efficiency?

AntronX · May 9, 2023

fafrd said:
So you are operating off of a 48VDC battery and your IQ7 / IQ7+ Microinverters integrate a ‘test mode’ which allows you to connect AC and as long as DC is connected within 3 seconds and is connected through a 2 Ohm resistance, it will begin producing output much more quickly / immediately that the ~5 Minutes otherwise required for the full POST / grid signal characterization, correct?

Correct except the bold part. 2 ohm resistor is not required for IQ7 to skip 5 min wait.

fafrd said:
How did you determine a 2 Ohm resistance was required for the first ~3 seconds?

Half cell 60 cell solar panel I use is rated 33V 9.5A at MPP so 33/9.5=3.5 ohms. 2 ohms is closest value I had on hand. A bit stiffer than my solar panel but should be fine.

fafrd said:
Have you tried and failed with 1 Ohm? Have you tried and failed with 0 Ohms (of added resistance, meaning wiring + contact resistance only)?

Did not try 1 ohm. Tried and ran another IQ7 without resistor at bootup and it ran for multiple days until one day it refused to generate power and kept blinking red light, if I recall.

fafrd said:
If you could boot successfully with only 0.2 Ohms of inline resistance, would you bother with the complexity of bypassing a boot resistance to get back the ~20W / ~7% in lost efficiency?

I would not bother. But I don't have Envoy and don't want to risk losing more inverters for now. In the future I plan to install them with solar panels as designed.

twrlp · May 30, 2023

agt said:
Further testing confirms that charging during m250 bootup will consistently trigger the DC Resistance error.

This turns out to be an issue also when the AIO’s inverter is operating - discovered when I forgot to turn on the input breaker after adjusting that wiring.

I’ve started putting together a microcontroller to handle startup and shutdown sequencing: an esp32c3 running Micropython measures volts/amps through the m250’s along with temperature at a few points, all as input to a simple state machine ensuring set limits aren’t exceeded. I’ll post code once it’s stabilized.

I’ve also smoked a few v/a sensors discovering that two parallel m250’s give a pretty sizable inductive kick when suddenly shut off. Adding a few TVS diodes, including one 36V chain across the battery inputs, seems to have fixed the issue. Further incentive to pull the old scope out of storage to visualize…

Pic of current layout attached. A proper enclosure will probably happen next month.

zalamandern232 · Aug 16, 2023

wow, it took 3hours to read this topic... its been a while ago since i posted here... however, its hard to get into the technical details with so much text... To contribute, i tried to draw agt's setup.. And i might also start to experiment with my microinverter. I use 48VDC BB with a NTC-4.0 as a inrush current limiter, but didnt have any u-controller to SSR over,measuring V/A-meter over shunt and, diode chain... but maybe i need to try again but with NTC-10 (16A) combined with a varistor and fuse to protect the u-inverter..

svetz · Aug 16, 2023

zalamandern232 said:
wow, it took 3hours to read this topic...

You might also be interested in @ncsolarelectric's experiments, or this Enphase theory thread.

Using solar micro inverters with batteries instead of panels

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