Adding new PV's to older

[Solarson]

I have a small off-grid system and want to increase the feed-in to the batteries to use same-day when it's cloudy by upping the PV count. Doing that may generate too many amps for the current MPPT to pass through on sunny days, but I get enough power on those occasions.

The existing two solar panels are wired in parallel - if I add two more (similar wattage but different manufacturer):

1) Should I keep them all in parallel or go in series
2) When comparing the output of the old and new panels, do I use the voltage open circuit or VOC and short circuit current or ISC?


Current PV's: 235W Rated: 29.85v and 7.84amps and ISC: 37.11V and 8.58 amps
MPPT Charge Controller: 40A 12v/24v Tracer4215BN
Batteries: 4 x 110 amp
I will change the MPPT later in the year and upgrade the batteries when they are on the way out.

Thanks for your help on this.

[Sunking]

Why are you wiring the panels in parallel?

[Solarson]

Don't know why - the installer did it

[Sunking]

Don't know why - the installer did it

There was your first mistake. Your Installer does no know what he is doing. Well maybe he does, he got more money screwing you over with wasted materials and hardware. Whatever panels you add will have to be closely matched, or you are screwed. Your batteries will likely need replaced if they were sized correctly to begin with. What voltage do you have the batteries configure for.

Let me guess, 12 volts all in parallel for 12 volts @ 440 AH right? I hope not.

[Solarson]

4 batteries in parallel to produce max run time for the 12v appliances which the cabin runs on. Im now looking to bolster the batteries on cloudy days via more pv's to feed in more amps for the 12v system on the odd low-sun days.

[Mike90250]

Sometimes, massive parallel 12V systems work, and for some camps, it's fine. We can advise that it's not best practice, and when you add more than a 1Kw inverter, it's really time to move to a 24 or 48V system

[Mike90250]

Adding more panels, the rule of thumb is
in Parallel within 10% of Vmp
or
in Series, 10% of Imp

And it perfectly valid to add another Charge controller in parallel on the batteries.

[Solarson]

Hi Mike,

I'm definitely going to stick with less than 1kw inverter, I've heard about the risks for going higher when running 12v. Thanks for the hat tip on the possibility of a 2nd charge controller. Hadn't thought of that as an option.

If adding more panels in parallel, I know I need to keep the voltage similar to the original as current is additive but voltage will drop to the lowest in the linked panels. If VMP is not quoted, is it safe to choose within 10% of VOC in parallel?

[Solarson]

Mike

I just found the VMP rating on my existing 235w pv's is 29.85 - and a couple of new ones running 210w at 27.3 VMP, so I'm inside the 10% voltage differential and will go for them. Thanks again for your help!

[Sunking]

If adding more panels in parallel, I know I need to keep the voltage similar to the original as current is additive but voltage will drop to the lowest in the linked panels. If VMP is not quoted, is it safe to choose within 10% of VOC in parallel?

You want to avoid parallel panels as you are throwing away the advantages of your Controller. Your Controller has a VOC of 150 volts which means you want to series your panels up to 3 in series. You can only use 2 of your panels on that controller with 12 volt battery.

However that is the least of your problems. Your Tracer 4215BN is a 40 Amp MPPT controller. You are operating at 12 volts right? Have you looked at your controller specs? No you have not because if you did you would know the maximum input power is 520 watts @ 12 volts. Try reading the specs, page 18.

Right now you have 2-235 watt panels for a total of 470 watts. Your done, you cannot add anymore panels. My advice is fix what you have and wire the two panels you have in series and fix the mistake your installer made.

Only way you can add more panels is either buy another controller, or switch to 24 volts. That same controller at 24 volt battery is good to 1040 watts. If you want more power, buy another 40 amp controller and you do not have to worry about matching panels.

Easy Peasy.