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Good afternoon

 

I am a newbie with solar, i have a farmhouse with no Eskom power and have now ventured into installing my own solar system.  So what i have currently

4XBosch Silver Calcium batteries

3000va Axpert inverter

2X255watt solar panels 

7kw generator for backup

 

Now for my problem, i installed the two pannels on my roof but with the fixed angle and some trees in front of the pannels some time of the day i am not getting enough power out of the panels.  the specs of the inverter says that the max watts of panels that i can connect is 500watt.  will it damage the inverter if i connect a total of 1000watts of pannels to the system? i will keep the 70volts but at peak the amperage will be more than wat the system spec. i am not looking to get more than the 500 watts, i just want to get 500 watts for a longer time of the day

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31 minutes ago, Leondk said:

specs of the inverter says that the max watts of panels that i can connect is 500watt

That sounds like the PWM model. I don't know these well enough, I do remember they made a 24V model with only 600W PV capability.

If it is a PWM controller, then yes, if you connect 1000W on there you will damage things. With an MPPT, you can oversize the array somewhat and get the required behaviour: 500W for a larger part of the day.

If its a 48V bank (you didn't say), 70V is rather low for the PV side.

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it is a 24V battery bank

the inverter is Axpert 3Kva-KS - 2400W 24V Pure Sine Wave Inverter. It says in the specs that it is MPPT charge controller.  what would the maximum pannel wattage be that i can put on the system

 

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6 minutes ago, Leondk said:

it is a 24V battery bank

the inverter is Axpert 3Kva-KS - 2400W 24V Pure Sine Wave Inverter. ( Maxmum Solar Charge Current: 50A)

 

I can't tell if it has an MPPT or a PWM solar charger. Internet is not being helpful and Voltronic ships so many variants it is hard to say.

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1 minute ago, plonkster said:

I can't tell if it has an MPPT or a PWM solar charger. Internet is not being helpful and Voltronic ships so many variants it is hard to say.

sorry, edited my reply.  it is a MPPT according to my manual i recieved

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5 minutes ago, Leondk said:

sorry, edited my reply.  it is a MPPT according to my manual i recieved

In theory you can always go a little bigger on the PV side (as long as your voltages remain in band) and the SCC will simply clip the power at the max. In practice I found that you should not oversize TOO much. I think 120% is a common array size (ie 20% oversized).

Story in that: Recently I installed an extra 600W of panels pointing north-west to catch the afternoon sun. The MPPT I wanted to use -- which I thought was properly repaired -- failed, so in it's place I installed a 100/15 BlueSolar (15 amps at roughly 28V, so around 400W). In other words, I had a 50% oversized array on it. That worked well MOST of the time... but I got over-current errors two or three times a day. I can only conclude that the MPPT can't always adjust fast enough, that for short periods an oversized array will overload a smaller controller, and therefore gross-oversizing is probably not a good idea.

So if it was me, I'd rather split the array and add another external mppt.

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@plonkster His model Axpert aka Axpert 3Kva-KS is MPPT indeed, but the limit as he stated is on 600W Solar and 70odd VDC.

The Axpert 3Kva-MKS or (PLUS) is capable of 115VDC and 1500W Solar.

@Leondk I agree with @plonkster on rather getting an aditional MPPT controller and then add your panels that side.

As stated by Mr.P, it would most likely survive the excess voltage and Wattage spikes, but only for so long.

PS: Welcome to PF and enjoy.

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I am not an Axpert expert but from what you have bought, as I have been there myself, buying stuff only to learn oeps ... the calcium batteries won't last long and 2 x 225 panels will barely charge them in a day if there are any loads powered.

Can you resent and start over? Or must it be made to work?

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35 minutes ago, Leondk said:

Unfortunately I need to make it work. The PV and the inverter I need to keep. Batteries is in need of being replaced so I can still play around with that. 

If that's the case then a separate SCC will be your way forward. Being in an off grid situation your system needs to cater for 95% of your loads. The whole reason for going solar is to get away from the huge cost of running a gennie. The cost of a total solution (100% renewable energy) is prohibitive so you keep the gennie for the rare occasions when your load is too large or the weather is overcast. Where grid is available it would be prudent (IMHO) to supply one's baseload from solar and keep the heavy draw appliances supplied by Eskom. Look at the average house and the baseload is usually under 1kW 90% of the time. The cost to supply that with solar is minimal and your ROI is good since your kit is operating near capacity. 

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What Chris and Plonkster said, I agree, get a separate charge controller, add the max panels to it to power your daytime load.

However, being 24v, it is limiting. 48v systems can have bigger arrays using the same make and model controller.

Re. batteries, we need your evening loads and run times to help you spec the batteries properly, which in turn dictates the right controller and max array - to power it all.

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We keep skipping the most important part though. What's your loads like, and for how long? Do you even have the right amount/size of batteries, nevermind the quality? What do you want to power, a few lights, a cell phone charger and a laptop, or something much larger? I do understand how this works though: It's a farm house. Normally there would be NO electricity. So you're not in the position us city-dwellers are where we have to scale down and meet in the middle, for you it is a matter of picking the most essential appliances and leaving the rest in their former default position (they only work when the genny runs).

Working backwards: 500W of PV can theoretically deliver 2.5kwh on a good sunny day (if you can solve the shade problem). If you use 2kwh to power loads during the day, that leaves 2.5kwh to recharge the battery. At an estimated 80% round-trip efficiency (that's optimistic), that leaves 2kwh to be used at night, and it also leaves the batteries at just above 50% SoC, which is good.

So that's the capacity of the system right now. If your loads fit, solve the shade problem (perhaps by adding more PV). If the loads don't fit, you'll have to distinguish between datime and night-time load. Night time load will require adding more battery capacity. Daytime load can be solved with PV.

It all comes down to the numbers.

The cheapest BlueSolar MPPT (100/15, around R1300 ex vat) can do just under 15 amps at 24V, which I found is good up to 420 watt. I ran one of those on a 600W array for a few weeks, it got so hot you could smell it but never gave up! Definitely worth the money and an easy way to add a few extra panels.

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25 minutes ago, plonkster said:

We keep skipping the most important part though. What's your loads like, and for how long? Do you even have the right amount/size of batteries, nevermind the quality? What do you want to power, a few lights, a cell phone charger and a laptop, or something much larger? I do understand how this works though: It's a farm house. Normally there would be NO electricity. So you're not in the position us city-dwellers are where we have to scale down and meet in the middle, for you it is a matter of picking the most essential appliances and leaving the rest in their former default position (they only work when the genny runs).

Working backwards: 500W of PV can theoretically deliver 2.5kwh on a good sunny day (if you can solve the shade problem). If you use 2kwh to power loads during the day, that leaves 2.5kwh to recharge the battery. At an estimated 80% round-trip efficiency (that's optimistic), that leaves 2kwh to be used at night, and it also leaves the batteries at just above 50% SoC, which is good.

So that's the capacity of the system right now. If your loads fit, solve the shade problem (perhaps by adding more PV). If the loads don't fit, you'll have to distinguish between datime and night-time load. Night time load will require adding more battery capacity. Daytime load can be solved with PV.

It all comes down to the numbers.

The cheapest BlueSolar MPPT (100/15, around R1300 ex vat) can do just under 15 amps at 24V, which I found is good up to 420 watt. I ran one of those on a 600W array for a few weeks, it got so hot you could smell it but never gave up! Definitely worth the money and an easy way to add a few extra panels.

 

Hi Plonkster

 

I use about 4000wh per 24 hours.  Hot water and stove is gas.  i use the PV/Batteries for a pressure pump, A+ Fridge, some lights and a satellite internet dish.  I do not stay there permanently. (on averege every second weekend).  Because of my short stays i prefer to use the generator for topup power.  My goal is to have enough energy stored for the whole night and then i would like to run the whole day on the pannels. so i factor in 2 hours of generator power to partly charge the batteries early in the morning.  500watts of solar should be enough for my needs but that is my problem, the Axpert only alows me 600watts of pannels.  so 1 have 2 255 watt panels.   how do i now add a 100 watt pannel to the system? If i add it in serie the voltage will be to high.  in paralell i will have the two 255 pannels coming in at 70volts.  can i then add a 100watt at 35volts in paralell with the other string at a different voltage?

Storage wise i have the 4 Bosch Silver Calcium batteries at 105 AH each.  that will give me 105x4x12x50%=2520watt hours at full charge that should be sufficient.  is that calculation correct?

 

 

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1 minute ago, Leondk said:

so 1 have 2 255 watt panels.   how do i now add a 100 watt pannel to the system? If i add it in serie the voltage will be to high.  in paralell i will have the two 255 pannels coming in at 70volts.  can i then add a 100watt at 35volts in paralell with the other string at a different voltage?

If you put panels in series they should preferably be the same, so adding a 100W module in series isn't an option anyway. In addition to that, your 255W modules are probably 60-cell modules (judging by the 70V voltage), while 100W modules will have 36 cells.

The voltage rules out adding another 255W module in series.

If you add two more 255W modules in parallel, I think you risk damage to the SCC. They can handle an oversized array, but I think 100% oversizing is looking for trouble. I trust a Victron MPPT to run at 100% all day but I don't trust the Voltronic one to do that, and if the Victron unit threw over-current errors, the Voltronic with it's known tendency to adjust slowly and over-volt is not even an option.

That means adding another MPPT. There is no way around this. It gives you the added flexibility that those modules don't have to point in the same direction and don't have to be the same size as the existing modules.

In my experience, you can pretty reliably generate 4kwh per day with 900Wp of PV. I have stats for that going back 2 years.

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9 minutes ago, superdiy said:

This inverter is thus not an MPPT model.

Two ways to test. Put a volt meter on the PV side while it's in operation. If it's MPPT, input voltage will be over 50. If it's PWM, it should pull down to battery voltage. Second way: put an ampere meter on the output (between inverter and battery). Remove all loads from inverter so you are only charging and turn it on. If it's an MPPT, it will make 480/24 =20A (to make the math easy) on the output, in full sun. If not, given that the panels are sitting at 70V, it will make less than 10A.

There is also the trick where you hold an AM radio that's off-tune next to the wire, and it should make a buzzing sound, but I think that test is to test between PWM and the switching types.

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15 hours ago, superdiy said:

KS = PWM

MKS = MPPT

@superdiy I tried doing some reading this side, and could not find that? Then again I will be the first to admit that the manual and spec sheets for them all up to the 5Kva never mention MPPT in the manuals, but do mention PWM on the charge current. :blink:

If we use the @plonkster method, then both indicate MPPT?

I know it doesn't take much to confuse me, but I can categorically state that now I am.

So just asking.

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31 minutes ago, Riaanh said:

If we use the @plonkster method, then both indicate MPPT?

It's the youtuber method :-) At least Jukian Ilett and another guy called Adam frequently test CCs from china to check if there is a gain in amps (more amps out than in) and if the product of volts*amps are more or less the same.

I have a feeling it's possible that some KS models now come with MPPT. Perhaps the branding rules don't work like we think it does? I'm really just saying, if it gets to the point where you cannot even trust the manual, the only thing left to do is actually measure it.

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16 hours ago, superdiy said:

KS = PWM

MKS = MPPT

 

This inverter is thus not an MPPT model.

 

26 minutes ago, Riaanh said:

@superdiy I tried doing some reading this side, and could not find that? Then again I will be the first to admit that the manual and spec sheets for them all up to the 5Kva never mention MPPT in the manuals, but do mention PWM on the charge current. :blink:

If we use the @plonkster method, then both indicate MPPT?

I know it doesn't take much to confuse me, but I can categorically state that now I am.

So just asking.

If you go and look at Voltronic's site  you will see that the KS model is PWM model (by omission) and the MKS model has a MPPT.

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