Matching Solar Panels, MPPT Charge controllers and Batteries

[Muhendis]

I have recently needed to carry out some repairs to one of my combiner boxes due to a faulty diode. At the same time I decided to review my setup which has morphed a bit over the years. No conflicts were discovered and all is still working optimally but the thought occurred to me to pass on my system matching knowledge to all. 

 

Starting at the solar panel end.

 

Firstly, the voltage output of the strings of solar panels must never exceed the charge controller input maximum voltage limit.

 

Secondly, the working voltage output of the strings needs to be within the range of the charge controllers MPPT range.

 

The label on the back of the solar panel includes a Pmax number in Watts. This is the maximum power the panel can give under direct, perpendicular to the panel (1Kw/M²), sunlight and at 25ºC. If you add all the panel's Pmax together this should be less than the manufacturers data for the maximum input power of the charge controller.

 

The reason for this limit is to restrict the current flow to below what the charge controller can handle when charging batteries. The batteries will take all they can get and more and the charge controller does not have the means to limit it. That's MPPT for you. The limiting factor is how much current the panels can deliver. A simple calculation reveal's all. If you have 6.4kw of panels and a 48v battery then applying Ohms Law the maximum current will be 6400 ÷ 48 = 133.33 Amps.

 

The charge controller must be able to pass this amount of current. However, in other posts it is noted that the solar panel outputs in Thailand will be somewhat less due to temperature derating. By assuming a very conservative 10% panel output derating the sum changes to 5760 ÷ 48 = 120Amps. From this, a charge controller capable of 120Amps output will fine. I use two at 60Amps each.

 

The capacity of the battery needs to be great enough that the maximum current does not exceed the maximum charge current rate. If the battery capacity is greater then they will simply take longer to fully charge so the batteries need to be sized according to the planned depth of discharge for the load.

 

In the case of my installation I targeted 30% depth of discharge which gives me full batteries by lunchtime'ish. The example of the total power of the panels and the charge controllers is from my own installation and  I have a 750Ah 48v Gell battery assembly.

Edited September 3, 2021 by Crossy
added some paragraph breaks and whitespace for readability :)

[Lantern]

There is a ton of data and info here                rpc.com.au

Edited September 3, 2021 by Lantern

[Muhendis]

On 9/3/2021 at 11:52 AM, Lantern said:

There is a ton of data and info here                rpc.com.au

Yes. I think I covered most things. This applies both to hybrid inverters with built in charge controllers and to the stand alone devices. It really is quite straight forward and I hope it proves to be of some use for all those DIY'ers on here. Note that some manufacturers may allow a bit of extra power into the charge controller but you need to check with them to make sure.

[Muhendis]

On 9/3/2021 at 11:52 AM, Lantern said:

There is a ton of data and info here                rpc.com.au

I note from your link you are a professional organisation. Any donations you may feel reluctant to make for my unsolicited consultancy will be gratefully received.