Hi All,

 

Long time lurker, first time poster. I want to start by thanking all the contributors on this site. A fair chunk of what I know about solar/backup power has been learnt here.

 

I’m hoping someone can help with a problem I encountered while adding PV to my existing setup.

 

Some background. For about a year I’ve been running a Synapse 5.0K+ inverter (believe this is a rebranded Voltronic Axpert King) and Synapse 4.8kWh battery (believe this is a rebranded Pylontech) as a pure loadshedding back-up solution. Load is a dedicated plug circuit (completely isolated from rest of house) & lights (separate neutral in DB) that has relatively low consumption at around 500W idle and peaking around 1500W every once in a while. This has been running without problem for a year or so. Configured as USB.

 

I recently added solar. Intent is to use power as it is generated (cost saving) and have battery power available for loadshedding. Will change configuration to SUB.

I intent to add additional load to this at a later stage to increase saving (powered via contactor/relay that drops out during loadshedding to save battery as it is only 0.5C rated), but I will keep the load unchanged for the next 1-2 weeks as I don’t want to change too many variables at the same time. If understand correctly then, if PV production isn;t enough to power the connected load, the inverter will "blend" utility power on the DC bus up until the max 5kw rating of the inverter. If the load exceeds 5kW then the inverter will go into bypass. Intent is to connect loads so that this (bypass) doesn't happen in the first place.

 

According to the datasheet & label on side of inverter the MPPT specs are as follows:

• Min Solar Voltage             40Vdc
• Max Solar Voltage (Voc) 145Vdc
• MPPT Voltage Range       60-115Vdc
• Rated Current                    80A

 

Solar panel spec is as follows:

• 420W monocrystalline
• Maximum Power Voltage (Vmpp)              42.50Vdc
• Maximum Power Current (Impp)               9.89A
• Open Circuit Voltage (Voc)                            50.50Vdc
• Short Circuit Current (Isc)                             10.54A

 

Installation is 10 x panels in a 2S5P configuration. This should give me a theoretical:

• Total max power of 4200W (slightly over the 4000W rating, but well within the 15% over-rating rule of thumb)
• Open Circuit Voltage of 101Vdc (well under the rated max of 145Vdc)
• Max Power Voltage of 85Vdc (within the 60 – 115 Vdc range)
• Max Power Current of 49.45A (well under the 80A max)

 

Panels were installed over the weekend. With fuses in combiner box disconnected, I measure between 93Vdc and 95Vdc on each of the 5 strings. With fuses closed I read approximately 95Vdc combined on the outgoing DC breaker (with breaker still open/disconnected). This is all still as I expect it to be.

 

Before I connected the PV to the inverter I did a voltage check on all terminals (old habit – don’t want to blow anything).

 

What is very strange is that I measured approximately 21Vdc on the PV terminals without anything connected to it. Inverter was switched on at the time (AC and battery connected; feeding load in line mode).

I connected using Watchpower (BT) and there is shows 0Vdc on PV1 input voltage.

 

This doesn’t seem right to me. Surely there shouldn’t be voltage on a MPPT input while nothing is connected to it?

I don’t want to connect PV and then potentially allow the magic smoke to escape.

I've attached images for reference. Please let me know if any more info is required.

Any advice here would be appreciated!

2 hours ago, HandySmurf said:

Long time lurker, first time poster.

Welcome to the forum (as a poster).

2 hours ago, HandySmurf said:

I’ve been running a Synapse 5.0K+ inverter (believe this is a rebranded Voltronic Axpert King)

I'm not so sure. It may be a clone, which is very much not the same thing as rebranded (all legitimate Axperts are rebranded). But then I used to think that EASuns were clones (likely they were), but now seem to be rebranded and legitimate. So quite possibly Synergy inverters are the same (reformed cloners).

2 hours ago, HandySmurf said:

[ Panel specs ] * Max Power Current of 49.45A (well under the 80A max)

Actually, the 80 A is at the battery end; you'll get around 80 A from your ~80 V and 50 A (efficiency of the solar charger is likely about 98%, so let's ignore its losses for now). So you'll get roughly 80 A at 50 V from your 50 A and 80+ V. But 4200 is only 105% of the rated power of 4000 W (80 A x 50 V). My rule of thumb is that up to 120% of rated power is OK, and 110% is no problem at all.

2 hours ago, HandySmurf said:

What is very strange is that I measured approximately 21Vdc on the PV terminals without anything connected to it. Inverter was switched on at the time (AC and battery connected; feeding load in line mode).

It's fine that the inverter says it's zero when the multimeter says otherwise. Any reading under a certain voltage (46 V comes to mind), and they report zero, so as not to alarm the user. As for why there is any voltage there at all, it's part of the switch-on circuit:

Ignore D2 for a moment. See those diodes (D22 and D20) with the arrows pointing from the battery positive towards the PV+ input? Diodes conduct (with "conventional" current, positive to negative) in the direction of the arrows. So there is effectively those diodes and a 100 kΩ resistor (two 200k resistors in parallel is effectively 100k) in series with your multimeter to battery negative (= PV negative for your model). Your multimeter typically presents at least 10 MΩ impedance to the circuit. That's 10,000 kΩ, 100x greater than the 100k, so the 100k only drops 1% and can be ignored. The diodes drop about 0.6 V each, so that's 1.2 V. So you still have about 48 V dropped between D2 (we consider it again now) and your multimeter. Measuring 21 V across 10 MΩ means the multimeter is drawing about 2.1 μA, That means that if D2 has a leakage of around 2.1 μA, you'll measure that 21 V. 2.1 μA is a tiny bit on the high side (I would have wild guessed 1 μA), but it's nowhere near difficult to believe. That's the curse of modern multimeters; they can see very small leakage currents. An old school moving coil multimeter with a typical 50 μA full scale meter would barely register 2.1 μA.

To put your mind at rest, you could put a 1 MΩ resistor across the PV input, and see that the voltage drops to around 2 V. 1 MΩ is considered a very high value of resistance; it's almost open circuit. Few resistors are that high in resistance. The exception slightly ironically are the sense resistors used to measure things like battery and PV voltage; these are typically strings of 3-4 resistors of about 1 MΩ in series. They are a pain because being so high in resistance, they are susceptible to moisture ingress, which changes their resistance significantly.

In summary (pardon my maxing technical; it's what I do), this is not a problem.

BTW, that circuit is what turns on the solar charger when the PV voltage is at least about 5 V higher than the battery voltage. One of the diodes is actually a zener diode, which breaks down (in a controlled, safe way) with about 3.3 V of what is normally called reverse bias (when diodes normally don't conduct). That turns on a power supply that converts 50 V from the battery to some 15 V for an oscillator circuit, and lower voltages again for other parts of the solar charger. It also turns on a relay that connects the output of the solar charger to the battery. So that way the solar charger doesn't draw much current from the battery (especially when there isn't a multimeter across the PV input 🙃) until the solar charger can do some good.

 

Edited July 3, 20233 yr by Coulomb

Thanks for the detailed reply @Coulomb - I did a diploma in electronic engineering about 20 years ago (with very little application since) so could at least follow most

Strange and frustrating that they spec the MPPT current on the output side and not input. At least now the rated power makes more sense.

Reading my OP now I realise that I wasn't very clear on where I'm reading this voltage. It is actually between the chassis GND (there is only one earth as someone on this forum previously put it  ) and the PV+ that I'm measuring -21Vdc. I'm also measuring that exact same voltage -21Vdc between GND and PV-.

3 hours ago, HandySmurf said:

I realise that I wasn't very clear on where I'm reading this voltage. It is actually between the chassis GND (there is only one earth as someone on this forum previously put it  ) and the PV+ that I'm measuring -21Vdc.

Oh. Well the solar charger connects directly to the battery on your model (mine too). The battery is transformer isolated from ground and anything to do with the bus, mains, etc. Except for the aforementioned 1M battery vilrage sense resistors, and these lead to a slight current to ground, and ground connects to the neutral output of the inverter (or it should!). The DSP (main processor) is referenced to neutral AC out. So in this case, that's where the leakage current is coming from, that your multimeter (on volts range) is reporting as a voltage. So D2 may not be so leaky after all. All that blather... Oh well it was a fun puzzle, even if I was "solving" the wrong one. 

Edit : Also, who knows what other leakage there is from battery to ground, etc. So measuring where you did doesn't make much sense, at least to me. Hence my wrong assumption. This isn't meant to sound pompous, or to criticise your electronics knowledge. 

Edited July 3, 20233 yr by Coulomb

Thanks @Coulomb. I'll go ahead with PV connection then.

There is something quite confusing to me regarding the Synapse 5.0K+. Two specs are given regarding the solar panel input to the inverter: 

1. 60 - 115 VDC, (asking for parallel connections) or

2. 120 - 450 VDC (where serial connections could work, like 6x 48 V panels is series)

I found this double spec on https://bhgpower.co.za/product/synapse-5-0k-offgrid-inverter/

Are two modes available on the inverter? I would prefer the higher voltage configuration because of lower current for the same wattage. 

14 hours ago, Hennie M said:

Are two modes available on the inverter?

No, there are many models, and each model has one or the other. Most of the recent models have the higher voltage solar chargers, except for the Axpert King 1. There is even an Axpert King II with the higher voltage solar charger. 

The higher voltage PV is certainly more convenient. It's a pity that the high PV voltage Axpert models don't include earth fault detection to make them safe, and legal in Australia. 

Edited December 16, 20232 yr by Coulomb

20 hours ago, Hennie M said:

There is something quite confusing to me regarding the Synapse 5.0K+. Two specs are given regarding the solar panel input to the inverter: 

1. 60 - 115 VDC, (asking for parallel connections) or

2. 120 - 450 VDC (where serial connections could work, like 6x 48 V panels is series)

I found this double spec on https://bhgpower.co.za/product/synapse-5-0k-offgrid-inverter/

Are two modes available on the inverter? I would prefer the higher voltage configuration because of lower current for the same wattage. 

Better to confirm with another seller. Another mistake is the obvious float voltage. I just wonder is there nobody reading the published specs? 

Thanks @Coulomb, we have 500 W solar panels available in our country, so this should work I think: 

• 1 x Axpert MKS PF0.8 5 kVA or 1 x Synapse 5.0K+ 
• 4000 W nominal PV, 500 W panels, wired 2S4P

Edited December 16, 20232 yr by Hennie M

3 hours ago, Hennie M said:

this should work I think: 

• 1 x Axpert MKS PF0.8 5 kVA or 1 x Synapse 5.0K+ 
• 4000 W nominal PV, 500 W panels, wired 2S4P

A 500 W panel could be configured many different ways; many smaller cells, or fewer large cells.

2S4P should be fine as long as Voc is less than about 67 V. I suspect that most 500 W panels would fit this criterion.

On the orientation of solar panels: 

Despite the basics that when in the Southern Hemisphere, the best facing is Northwards and there is an optimal tilt angle depending on the season, my question is the following: Is there any difference in mounting the solar panel in portrait or landscape orientation regarding the amount of energy to be gathered in a full sunny day? 

I know this is important regarding tubed solar geyser panels for an obvious reason.