RCT Axpert 5K Misbehaving

[Henwilsch]

4 hours ago, Coulomb said:

SCCs are a bit tricky to test, because of the maximum power point tracking. I assume that this is a 145 V max PV type; I've never worked on a high PV voltage solar charger. These require serial commands and responses from the DSP to get them operating.

You didn't say if your 53 V power supply was current limited or not; it really needs to be current limited. Otherwise, the MPPT even if working properly will try to draw 60 or 80 amps from your power supply. I would initially limit the current to about 0.5 A, and try it at the power supply's maximum if all goes well.

Unfortunately, you really need a real battery (a small one should do, especially initially) for the SCC output. Otherwise, the charge current will have nowhere to go, and the output voltage will rise to silly levels, endangering other parts of the inverter. That's unless you have a special battery simulator test device, but those are unusual and expensive. That battery should have a DC rated fuse, because even a smallish battery is quite capable of blowing things up.

Using a resistor in series with a non current limited power supply is not ideal. As you have found, things get crazy hot very quickly. It might be possible with the right value resistor and a lot of heat sinking (like a bucket of water).

I've been lucky with my 145 V max SCC repairs; the only faults were fairly obvious like bad opto couplers preventing serial communication, and actually transistors on the control board at the other end of the serial comms cable. The actual switching part seems to be fairly robust; I don't seem to hear a lot about the MOSFETs failing.

Thanks for your explanation. I will rethink my test setup and maybe tests with solar panels.

[Henwilsch]

@Coulomb I tried to test the SCC again but no joy. I use a 53v power supply(not current limited) but to do current limiting I use 24v 21W globes. I used 2 in series witch should limit current to 0,8A. I got a small battery(5Ahr 52v) connected to the battery input. When I turn on the solar power supply a relay click and then the globes light up. I left it for a while but it just stay like that. If I measure at the solar input terminals I get a reading of -11V. I added more globes to give current limiting to 2.38A and same result. I removed the SCC and did some multi-meter test but I could not find any component that is short or could cause the problem.

The SCC looks good.

I would just like to know if it works. Any ideas what I can try or if there is something wrong?

Edited October 11, 2023 by Henwilsch

[Coulomb]

9 hours ago, Henwilsch said:

If I measure at the solar input terminals I get a reading of -11V.

That's telling, but I don't immediately know what. [ Edit: That should not be possible. The diodes and the fee-wheel diodes in the MOSFETs (inherent or explicit) should be combining to limit the negative PV voltage to some -0.8 V, perhaps a little over a volt if the current is very high (and it won't be, here). So I'm thinking that this measurement is spurious, or I'm misunderstanding you, or the power parts are blown. ]

I have to ask: are you sure that you have connected the PV power supply correctly? I guess you must because you heard the click soon after. Though it should take about 5 V above battery voltage for the power supply to come on:

D2, D20 and Q20 Vbe should add up to about 0.4 + 3.3 + 0.4 = 4.1 V before any sort of conduction should happen. So you should need more than a single volt difference between the PV power supply and the battery. So you might have to eventually get the battery voltage down to about 53 - 5 = 48 V, at least to start the SCC's power supply. Schematic is from this post.

So I would check this part of the circuit carefully. Once that is working, check that U3 is powering itself (via D10). Of course, your parts designators could be different, and even details of the circuit may differ, but I'd expect it to be pretty close. Check that there is 5 V for the micro chip, and there should be sensible voltages on the various power supply outputs.

Edited October 12, 2023 by Coulomb

[Henwilsch]

@Coulomb thank you for the explanation. I think that you maybe solved this. The battery and the solar input is just about the same voltage so it might be that the SCC does not power on and no control over the circuit is happening. I will try with higher PV input voltage and report back.

A few questions:

1. Can the SCC work stand alone or must it be plugged in to the inverter?

2. What is the 4 wire cable that  plugs into the control board used for?

3. What is the 2 wire cable that plugs into the inverter main board used for?

4. Does the SCC have its own PSU to power the CPU or does it rely on the inverter to power it?

[BritishRacingGreen]

8 hours ago, Coulomb said:

That's telling, but I don't immediately know what. [ Edit: That should not be possible. The diodes and the fee-wheel diodes in the MOSFETs (inherent or explicit) should be combining to limit the negative PV voltage to some -0.8 V, perhaps a little over a volt if the current is very high (and it won't be, here). So I'm thinking that this measurement is spurious, or I'm misunderstanding you, or the power parts are blown. ]

I have to ask: are you sure that you have connected the PV power supply correctly? I guess you must because you heard the click soon after. Though it should take about 5 V above battery voltage for the power supply to come on:

D2, D20 and Q20 Vbe should add up to about 0.4 + 3.3 + 0.4 = 4.1 V before any sort of conduction should happen. So you should need more than a single volt difference between the PV power supply and the battery. So you might have to eventually get the battery voltage down to about 53 - 5 = 48 V, at least to start the SCC's power supply. Schematic is from this post.

So I would check this part of the circuit carefully. Once that is working, check that U3 is powering itself (via D10). Of course, your parts designators could be different, and even details of the circuit may differ, but I'd expect it to be pretty close. Check that there is 5 V for the micro chip, and there should be sensible voltages on the various power supply outputs.

Thanks @Coulomb that is some great resources on the MPPT/SCC you have. I have somehow missed out on it.

Some observations from me :

The SMPS is good old Voltronics style built around the UC3843. The initial bootstrap power comes from the battery and steered by Q12 and D15 on to pin 7. And as you state that requires the PV  VOC to be  higher than BAT+  to turn it on. Once the SMPS delivers then the VCC comes alive and  powers the UC3843 power pin 7  via steering diode  D15 (not shown on the schematic snippet). From there one the SMPS is no more dependant on the bootstrap path. 

So this means that the open circuit voltage of PV input must be higher than the battery voltage by some margin as you calculated. So we probably need a up to  56V Voc depending on a fully charged battery.  However a fully charge battery does not require anything from pv, so if the SCC does not switch on at that stage, its fine. When the battery starts discharging its voltage will drop and SCC starts. These conditions of course are only applicable on cold start where the bootstrap will be required. 

Edited October 12, 2023 by BritishRacingGreen

[BritishRacingGreen]

5 minutes ago, BritishRacingGreen said:

Thanks @Coulomb that is some great resources on the MPPT/SCC you have. I have somehow missed out on it.

Some observations from me :

The SMPS is good old Voltronics style built around the UC3843. The initial bootstrap power comes from the battery and steered by Q12 and D15 on to pin 7. And as you state that requires the PV  VOC to be  higher than BAT+  to turn it on. Once the SMPS delivers then the VCC comes alive and  powers the UC3843 power pin 7  via steering diode  D15 (not shown on the schematic snippet). From there one the SMPS is no more dependant on the bootstrap path. 

So this means that the open circuit voltage of PV input must be higher than the battery voltage by some margin as you calculated. So we probably need a up to  56V Voc depending on a fully charged battery.  However a fully charge battery does not require anything from pv, so if the SCC does not switch on at that stage, its fine. When the battery starts discharging its voltage will drop and SCC starts. These conditions of course are only applicable on cold start where the bootstrap will be required. 

So if this is all correct as I am seeing it, a cold start of SCC might be delayed for some lengthy time when we have a high battery charge bulk voltage on a fully charged battery. 

[Henwilsch]

Call me a Idiot and I would agree. Found the problem, I was connecting the PV reverse polarity. I will not go into details how this happen, but it happened and I am totally embarrassed. Every thing calmed down after I connected it the right way round. @BritishRacingGreen yes nothing happen regarding solar charging when the battery was 52V and the PV at 53V. I didn't even get a PV voltage registering on the display. I then increased the PV voltage to 65V and SCC came alive and after a few seconds started to charge the battery.

I am just totally amazed at how hardy the SCC is and that it took my abuse with a smile and protected itself!

Thanks guys for all you help.

[BritishRacingGreen]

4 minutes ago, Henwilsch said:

Call me a Idiot and I would agree. Found the problem, I was connecting the PV reverse polarity. I will not go into details how this happen, but it happened and I am totally embarrassed. Every thing calmed down after I connected it the right way round. @BritishRacingGreen yes nothing happen regarding solar charging when the battery was 52V and the PV at 53V. I didn't even get a PV voltage registering on the display. I then increased the PV voltage to 65V and SCC came alive and after a few seconds started to charge the battery.

I am just totally amazed at how hardy the SCC is and that it took my abuse with a smile and protected itself!

Thanks guys for all you help.

I just love it !  Don't feel embarrassed  , you are man enough to state the bleedingly obvious mistake here , 10/10 for that .  But you must admit , you learned out of it , and it paved the way for people like myself to know and learn a bit more about the SCC.

I am glad you  came good.

[BritishRacingGreen]

11 hours ago, Henwilsch said:

@Coulomb thank you for the explanation. I think that you maybe solved this. The battery and the solar input is just about the same voltage so it might be that the SCC does not power on and no control over the circuit is happening. I will try with higher PV input voltage and report back.

A few questions:

1. Can the SCC work stand alone or must it be plugged in to the inverter?

2. What is the 4 wire cable that  plugs into the control board used for?

3. What is the 2 wire cable that plugs into the inverter main board used for?

4. Does the SCC have its own PSU to power the CPU or does it rely on the inverter to power it?

Let me attempt to answer this , then @Coulomb can verify/correct :

1. While the SCC is a standalone intelligent product , it must be 'instructed' by means of a high level serial protocol command set in order to start and perform its duties within a set of parameters , eg. max output voltage , max power , enable/disable  etc.

2. this is the serial communication interface  between the inverter and SCC as in required in (1) . The protocol is probably bi-directional , as the SCC must supply the inverter with measurement values as well. 

3. that's the battery feed from the inverter to the SCC in order to power the SCC  circuits.

4. Yes it has , it needs to produce two isolated gate driver supplies , as well as a VCC supply to power its own CPU as well as its own SMPS. But the source of this power supply is dependent on the inverter's battery supply.

Edited October 12, 2023 by BritishRacingGreen

[Henwilsch]

@BritishRacingGreen thanks for the explanation. It is good to know.

[Coulomb]

On 2023/10/13 at 3:05 AM, BritishRacingGreen said:

1. While the SCC is a standalone intelligent product , it must be 'instructed' by means of a high level serial protocol command set in order to start and perform its duties within a set of parameters , eg. max output voltage , max power , enable/disable  etc.

Yes, exactly.

On 2023/10/13 at 3:05 AM, BritishRacingGreen said:

2. this is the serial communication interface  between the inverter and SCC as in required in (1) . The protocol is probably bi-directional , as the SCC must supply the inverter with measurement values as well. 

Spot on again. This is serial communications at a blistering (not) 2400 bps. It's not RS-232, although it's nearly "TTL RS232". The protocol is fairly simple, consisting of about 4 commands and their responses.

On 2023/10/13 at 3:05 AM, BritishRacingGreen said:

3. that's the battery feed from the inverter to the SCC in order to power the SCC  circuits.

Nope! You were doing so well, too. It's simply the output of an opto-isolator (the transistor part, not the LED part), which will go low resistance when the SCC detects PV power and wants to wake the main inverter if necessary. So you can have the main power switch of the inverter off (meaning you don't want power at the AC output), but it will wake for battery charging, and then sleep again at night if the power switch is off. Really the switch is a "AC out" switch, rather than a "total power on or off" switch.

Battery power for the solar charger comes via other means. On some models, there is a pair of tall (~100 mm) posts that connect to battery positive after the fuse and battery negative. When the solar charger is side by side with the main board, there is just a pair of wires, from poor memory.

On 2023/10/13 at 3:05 AM, BritishRacingGreen said:

4. Yes it has , it needs to produce two isolated gate driver supplies , as well as a VCC supply to power its own CPU as well as its own SMPS. But the source of this power supply is dependent on the inverter's battery supply.

Correct again. All the power comes from the battery (for a long time I incorrectly believed that it came from the panels), but the panel voltage has to be about 5 V higher than battery voltage for the power supply to kick on. That way, the SCC only has an insignificant drain at night.

Edited October 14, 2023 by Coulomb
Added paragraph on how battery power gets to the solar charger.