Maximum solar panels on Mecer hybrid

[Lindsay]

So I have the Mecer 3kVA 24V (Plus model) off grid inverter that can take up to 1500W of PV. Currently I have 1555W (~260 x 6) panels attached to it in 3 x parallel strings each containing 2 panels in series. As such the PV voltage is in the 60-70V range. 

My question is can I keep adding more strings of two panels in parallel without causing issues? Would there be any disadvantage, I know that if I have the inverter on on a sunny day and the load switches on and off often that the voltage can overshoot and occasionally push 30V to the batteries, would this make that situation worse?

The PV voltage would stay the same of course and my expectation is that it just would never charge with more than its rated 1500W of power. The advantage of adding more is of course is that it'd help on the very dark winter days where there's not enough sun to currently fully charge the batteries.

Thanks in advance.

Edited August 6, 2019 by Lindsay
More detail

[FranMilec]

The data sheet states max Voc to be 145V and Isc to be 60A. So in short, don't exceed 145V or 60A then you'll be fine.

[FranMilec]

You can confirm this with your supplier or installer.

[Lindsay]

18 minutes ago, FranMilec said:

The data sheet states max Voc to be 145V and Isc to be 60A. So in short, don't exceed 145V or 60A then you'll be fine.

Thanks, yes, Voc is 145V and PV MPPT range is 60-115V, so I guess changing from strings of 2 to strings of 3 would then allow me to add even more panels. 

But saying that does it mean that I could actually have 6 parallel strings of 3 in series, 18 x 260W panels total, that's a lot more than 1500W?

3 in series puts MPPT voltage at ~100V and the strings being parallel is 6 x 8A per string = 48A.

If your panels did exceed 60A wouldn't the controller just not use the amps above the charging maximum (as you can set this in software below 60A if you want to) or would it go into fault? 

(btw I installed my solar myself, and I'm not sure the supplier would know the exact workings better than the people on this forum!)

Edited August 6, 2019 by Lindsay
More detail

[FranMilec]

I never recommend to work on electricity if you're not a qualified electrician, it's also against the law, and against your insurance 

Having strings of 3 in series should be fine, paralleled up to 6 strings to stay below the max Isc.

1 hour ago, Lindsay said:

If your panels did exceed 60A wouldn't the controller just not use the amps above the charging maximum (as you can set this in software below 60A if you want to) or would it go into fault? 

No your inverter should trip, or could burn, never pass the max ratings.

 

1 hour ago, Lindsay said:

my expectation is that it just would never charge with more than its rated 1500W of power

On reputable brands they just clip in over power (wattage) conditions, but I'm not sure how Axpert would react.

[Lindsay]

31 minutes ago, FranMilec said:

I never recommend to work on electricity if you're not a qualified electrician, it's also against the law, and against your insurance 

Well I have a BTech in Electrical Engineering, I might be a but rusty, but I think I'm ok there

I've got all the dc breakers, fuses etc. 

I wasn't sure how these things would handle excess current, at the moment I'm not pushing more than 30 amps from the panels, so half the 60amps the MPPT can handle (even though I'm slightly over the max panel watts). I'm also limiting the battery charging to 40amps as I'd prefer to have thicker cables if I'm going to push anymore than that.

Thanks again for the info!

[Guest]

My two cents.

The two rules with MPPT's, all of the makes:
1) NEVER exceed the max rated volts - it will fry.
2) NEVER exceed the max rated amps - it will fry.

Over sizing in watts, within the above two rules, but not by too much, is perfectly acceptable IF you can use the power.

But note, the MPPT will work harder therefor hotter - in MY opinion.

[Dex_]

personally I wouldn't exceed any of the operating parameters...

i can't see how the unit can function with 3 times the rated PV input, i see how your numbers are adding up, but then why would it be rated to a max 1500W PV array?

Personally i would look at a separate mppt controller.

[Jaco De Jongh]

5 hours ago, FranMilec said:

The data sheet states max Voc to be 145V and Isc to be 60A. So in short, don't exceed 145V or 60A then you'll be fine.

 Nope this is not correct. That 60Amp rating is the max amps it can Output to the batteries (DC bus) . 24Volt x 60A = 1440 Watts. That is why you can only connect 1500 wh PV to it. (with a little more as described below) 

5 hours ago, Lindsay said:

Currently I have 1555W (~260 x 6) panels attached to it in 3 x parallel strings each containing 2 panels in series.

Because we all know you can only expect 80% of the power of the connected PV array, you can safely oversize your array with 20% (some say 30%), but only if you remain below your max VOC rating. In your case adding 1 more string of 2 panels (260 Watt)  is pushing the boundaries a bit. Anything more that that will have no benefit because the MPPT cant use it and you stand the change of damaging your Inverter. 

NB: Doesn't matter what anybody says. Over-sizing your array remains a risk you must be willing to take. If the manual states 1500 watt  and you want to be save, remain within that limits. 

The spec sheet refers to the max "Charge" Currents for AC and Solar and for solar its 60A. (Not the Ics) 

[Coulomb]

23 hours ago, Lindsay said:

As such the PV voltage is in the 60-70V range. 

Sounds like you have 60 cell panels. If so, you could probably run your panels 3S, to reduce wiring.

 

18 hours ago, Jaco de Jongh said:

That 60Amp rating is the max amps it can Output to the batteries (DC bus) .

Exactly. 60 A x 25 V battery voltage = 1500 W [ edit: was 3000 W. Sigh. ]  So this is one of the models where they don't give a realistic maximum panel power rating for the inverter. You can interpret that either as 1) they just didn't give a figure, just echoed the maximum output value, or 2) that really is the limit; it can't safely take any more.

20 hours ago, Lindsay said:

 I'm also limiting the battery charging to 40amps

In that case, a current overload from an overshoot isn't likely to be too bad, providing your battery can cope with a voltage overshoot for a minute or less.

Quote

as I'd prefer to have thicker cables if I'm going to push anymore than that.

? Which cables? Surely your battery cables can handle more than 40 A, or you can't deliver much power to the loads.

The combined output of the panels won't see the 40 A, it will be less than half of that, due to the magic of MPPTs. 1500 W at 60 V is 1500/60 = 25 A. Even 4 mm² wire should be OK with that. As you have stated, the MPPT won't draw more than 1500 W from an array with much higher potential power. If you leave the charge current limit at 40 A, then the power will limit itself to not much more than 1000 W [ edit: was 2000 W; 48 V thinking. ]

The issue is control. When the MPPT wants more current, it will change the PWM ratio to cause more current to flow. It will be expecting about 1500 W panels or less, so if you have much more than that connected, much more will flow for the same change in PWM ratio. PWM ratios aren't infinitely variable, so the PV current could hunt around the maximum current level. But the average will still be very close to 1500 W or whatever you set with the maximum charge current, so the fans should be able to get rid of the heat just fine. So I can't see that "overclocking" the Solar Charge Controller is much of a risk. Especially with a 40 A maximum charge current limit. You could probably go to 50 A and still be quite safe. But that's just my opinion.

Edited August 7, 2019 by Coulomb

[Jaco De Jongh]

5 hours ago, Coulomb said:

Exactly. 60 A x 25 V battery voltage = 3000 W. So this is one of the models where they don't give a realistic maximum panel power rating for the inverter.

You had me thinking for a while now. 60 amps x 25 volt  = 1500 watts and that is in line with the max PV specs.

5 hours ago, Coulomb said:

If you leave the charge current limit at 40 A, then the power will limit itself to not much more than 2000 W.

Shouldn't that be  40 amps x 25 volt = 1000 Watt. 

[Coulomb]

44 minutes ago, Jaco de Jongh said:

40 amps x 25 volt = 1000 Watt.

Duh! Yes, of course. I've edited the original post. It's my 48 V thinking, I claim 

[Lindsay]

10 hours ago, Coulomb said:

Sounds like you have 60 cell panels. If so, you could probably run your panels 3S, to reduce wiring.

Yes, but I bought them in pairs at different times, so it was easier to stick to to pairs and also I have 4 that are closer to the house than the other 2, so for now I'll probably keep it this way unless I move them all to the same place.

10 hours ago, Coulomb said:

Exactly. 60 A x 25 V battery voltage = 3000 W. So this is one of the models where they don't give a realistic maximum panel power rating for the inverter. You can interpret that either as 1) they just didn't give a figure, just echoed the maximum output value, or 2) that really is the limit; it can't safely take any more.

40A x 25 = 1000W, although I've seen it overshoot when charging by a few amps and then correct itself. I've only recently been logging the output and the most it's reported the panels at is around 1200W. 

10 hours ago, Coulomb said:

In that case, a current overload from an overshoot isn't likely to be too bad, providing your battery can cope with a voltage overshoot for a minute or less.

I think the battery bank is ok with that, I've seen brief voltages up to a bit over 30V when running an induction cooker on the inverter and everything still seems to be ok. I worry more about my direct DC appliances, a 24V fridge/freezer and then some other items such as 24V car USB chargers and some DC-DC buck converters. I think I already fried one 24V USB charger due to the voltages so I might rather replace these with DC-DC bucks that can handle up to 38V.

10 hours ago, Coulomb said:

? Which cables? Surely your battery cables can handle more than 40 A, or you can't deliver much power to the loads.

Yeah, I'm just being paranoid. I have 16mm cable (with 60amp fuse), so should be good with 60amps and the cable could probably handle more (although when I've looked online I've seen all sorts of varying tables on how much 16mm can handle). My loads, as I said, are mostly DC. But I do occasionally run the induction cooker, slow cooker etc usually less than 1000W. I've also got a low current circuit breaker on the AC because if I pull more than 60 amps from the batteries and the fuse trips, I hear that these inverters don't like being disconnected from the batteries when there is solar power present and it damages them (according to the supplier). Is this true?

10 hours ago, Coulomb said:

The combined output of the panels won't see the 40 A, it will be less than half of that, due to the magic of MPPTs. 1500 W at 60 V is 1500/60 = 25 A. Even 4 mm² wire should be OK with that. As you have stated, the MPPT won't draw more than 1500 W from an array with much higher potential power. If you leave the charge current limit at 40 A, then the power will limit itself to not much more than 1000 W [ edit: was 2000 W; 48 V thinking. ]

Yes, I currently have my 3 sets of panels each coming in on 4mm cable fused separately, then joined at the disconnect and 16mm from there.

10 hours ago, Coulomb said:

The issue is control. When the MPPT wants more current, it will change the PWM ratio to cause more current to flow. It will be expecting about 1500 W panels or less, so if you have much more than that connected, much more will flow for the same change in PWM ratio. PWM ratios aren't infinitely variable, so the PV current could hunt around the maximum current level. But the average will still be very close to 1500 W or whatever you set with the maximum charge current, so the fans should be able to get rid of the heat just fine. So I can't see that "overclocking" the Solar Charge Controller is much of a risk. Especially with a 40 A maximum charge current limit. You could probably go to 50 A and still be quite safe. But that's just my opinion.

Thanks Coulomb and others. For now I'll leave my panels at 1555W to be safe.

[Coulomb]

2 hours ago, Lindsay said:

I hear that these inverters don't like being disconnected from the batteries when there is solar power present and it damages them (according to the supplier). Is this true?

I've read reports to that effect. I certainly think that the battery being disconnected would be a problem, solar power present or not. But with solar power present, you have all the power of the panels, and none of the voltage limiting ability of the battery. So all that solar power would be unconstrained. It could well get ugly.

[Dex_]

19 hours ago, Jaco de Jongh said:

 Nope this is not correct. That 60Amp rating is the max amps it can Output to the batteries (DC bus) . 24Volt x 60A = 1440 Watts. That is why you can only connect 1500 wh PV to it. (with a little more as described below) 

Because we all know you can only expect 80% of the power of the connected PV array, you can safely oversize your array with 20% (some say 30%), but only if you remain below your max VOC rating. In your case adding 1 more string of 2 panels (260 Watt)  is pushing the boundaries a bit. Anything more that that will have no benefit because the MPPT cant use it and you stand the change of damaging your Inverter. 

NB: Doesn't matter what anybody says. Over-sizing your array remains a risk you must be willing to take. If the manual states 1500 watt  and you want to be save, remain within that limits. 

The spec sheet refers to the max "Charge" Currents for AC and Solar and for solar its 60A. (Not the Ics) 

thanks this makes a lot of sense now

[FranMilec]

On 2019/08/06 at 9:01 PM, Jaco de Jongh said:

 Nope this is not correct. That 60Amp rating is the max amps it can Output to the batteries (DC bus) . 24Volt x 60A = 1440 Watts. That is why you can only connect 1500 wh PV to it. (with a little more as described below) 

Correct, my bad.