System not charging batteries as expected

[namguy]

Hi all, New to forum and solar

Thanks for reading my post, i’m having problem to get my batteries fully charge – 69 to 73% is all and this is a 3 week old system, maybe i’m missing something or my setup is not correct

I've even did not draw / use any power for 2 day's from the system to see if that would help but only got it to 75% charge

The electrician helping with installation confirmed all connections is correct, but could not assist further

During the day there is nothing drawing power, put inverter off during daytime, sytem is curentlly only used for 7x house lights during night – all bulbs change to LED 11/18 W - the idea is to slowly add

My system:

6x 300W panels Series/Parallel connect 24V – spec’s Vmpp 18V, Impp 16.67A, Isc 17.83A, Voc 22V

6x 200AH 12V 25C Gel Batteries, Series /Parallel 24V

SAKO SC2450 PWM 50A / 24V Charge controller – DC out 15A - Auto settings Bat low – 21.1V, Low back – 23.9V, Bat high – 28.9V, Bat high back - 27.2V, Bat float – 27.0V, Bat Vast – 27.8V, max current 50Amp

SNK 3000W pure sine inverter

I suspect my charge controller maybe not be up to the job - i was under the impression it was a MPPT controller until i check the web site of the Manufacturer and found out it was a PWM controller

I live in Namibia, 5/6 hours direct sun on panels, any input will be appreciated

Thank you,

[DeepBass9]

How do you know They are 75% charged, is that coming from the inverter? Have they not gone to float and the inverter reads it as not fully charged?

[namguy]

Hi,

I get the readings from the charge controller and inverter after a full day of charging and up until now never pass 75%, isn't it suppose to charge to 100% at least in a full day of sun with nothing drawing power or is my expectations to high or is there something out with my setup maybe?

[DeepBass9]

Inverter and charge controller? How are they measurin nthe state of charge. Voltage is a poor indicator of soc if that is what is being used.

[namguy]

Both use volt readings

[___]

@namguy, you are not perhaps from Namibia? :-)

Edit: Oh nevermind, I see you mentioned it. Welcome. I'm a born Namibian too.

[___]

Okay, as the others have said. Can't use voltage. But lets look at the numbers.

You have a 24V 600Ah battery bank, and 1800Wp of PV. First, I am intrigued by the numbers, it is odd to find 36-cell panels rated at 300W. Usually the largest panels you get that are "12V" (they have 36 cells and make around 20V open circuit) are around 150W. So that is odd already... but lets say it us correct. Then because you have a PWM controller, you lose 30% of that, so you have a 1200Wp array. That can charge at 50 amps (probably closer to 45) at peak, which is under 10% of the battery bank capacity. In other words, your PV array is a little undersized for the battery.

This doesn't mean the batteries aren't being charged well enough. They might be, but if you use voltage to "measure" that, with such a low charge current, chances are it will always under-report.

The first thing -- as we always say here on PF -- get a Victron BMV-702, so you can properly monitor SoC.

The other thing you can do, is put an Ampere meter in line (many PWM controllers don't have current sensors and cannot tell you this info). If the batteries properly lift up to absorption voltage (28.8V usually), and the current tapers down to under 1% of the battery capacity (that's below 6 ampere) while it's at 28.8V, then that is an indication that it does fill up towards the end of the day.

You may want to switch to MPPT as well. That would unleash the full 1800Wp, 65 to 75 ampere (depending on voltage), which is closer to the 10%-15% (of the battery capacity) you want for recharging.

Also, I'd like to see a photo of the label on the back of those panels...

[namguy]

Hi Plonkster,

Thank you for the reply, having more clarity on it now, understand that I need a proper meter for monitoring soc and ampere - will get on that asap.

You also confirm that PWM is part of my problem, already talk to supplier to replace with MPPT

Will keep you posted

Attached is label of the panels

Once again thank you

[namguy]

11 hours ago, namguy said:

Hi Plonkster,

Thank you for the reply, having more clarity on it now, understand that I need a proper meter for monitoring soc and ampere - will get on that asap.

You also confirm that PWM is part of my problem, already talk to supplier to replace with MPPT

Will keep you posted

Attached is label of the panels

Once again thank you

One more question - what should the specifications of the MPPT controller be? Should I keep to 50A or go higher?

Thank you

[___]

For interest sake, can you count the number of cells (ie black dots, I see it is a mono panel) in those panels? It might be a 72-cell module, but with two internal strings of 36-cells each, in other words it's like two 150W panels under one glass sheet :-)

The MPPT should be sized to accommodate the maximum output current, so if you have a 1800Wp array, then during the afternoon peak the MPPT will swap out some volts for amps (it's a buck converter), so on the input side it will run at 36V and 50A, but on the output side it might run as low as 23.5V (when batteries very low) and 75 ampere! MPPTs are marketed by their output current, so you want something larger than 50A.

Since you will rarely get the peak value (only under ideal conditions in full midday sun with temperature at 25 degrees), and since the batteries will probably be above 26V while charging (1800/26 = 70 ampere), I'd aim for around 70 amps. Rather than one big mppt, I'd go for two smaller 35-40amp units. The Victron BlueSolar 150/35 is affordable (compared to the competition) and two of those will be perfect. You could also look at the 100/50 model, since you have a 24V system. The 100/x MPPTs can only go up to 24V, while the 150/x models can go up to 48V.

Note: the 150 and 100 in the model numbers is the peak voltage, ie if you take the Voc voltage on the back of the panel (22V in your case) and multiply with the panels in series (44V), then that voltage must be less than the voltage the controller can handle. So the 150/35 can handle up to 150V on the input and charge at 35A on the output, while the 100/50 can handle 100V on the input and charge at 50amps (but no higher than a 24V bank).

[ghatikar]

2 hours ago, namguy said:

One more question - what should the specifications of the MPPT controller be? Should I keep to 50A or go higher?

Thank you

The optimum charging rate for a 600 Ah battery bank is minimum 30 amps ( C20) or even 60 AMPS (C120) for most of the GELS. Keeping cushion for loads in future You can get either 80 Amps Victron or Midnight solar  ( 80 Amps very few models) or to keep budget lower get a 40 /45 amp Victron, Morningstar, EPEVER and add one more in future with additional bank 

 

what is the make of the battery or datasheet 

[namguy]

Hi,

Thank you plonkster and ghatikar for the information and clarity, I'll hopefully get this setup sorted this week 

I'll keep you posted 

Namguy