newbie question

Re: newbie question

Depends on the size of your current array and the type of controller you have (PWM or MPPT).

Basically, if you add panels in parallel, the Vmp of the additional panel should be ~17.5 volts.

If you have a MPPT array or other configurations, the rough rule of thumb is to add panels in parallel if their Vmp's are within ~10% of each other; and add in series if their Imp's are within 10% of each other.

The more the miss-match in Vmp/Imp, the more loss of energy harvest you will have.

-Bill

Re: newbie question

Glossary

You can add panels in parallel until you meet the maximum input current/power of that charge controller...

Nominally, with a 90 AH battery bank and 13% rough maximum charging current:

• 90 AH * 0.13 rate of charge * 17.5 volt Vmp solar panel = 189 Watts

Depending on the design of your charge controller (and your wiring), you could add upwards of a total of 190 watts of solar panels cost effectively for the battery you have at this time.

However, you will need to look at fusing, wire size, and your charge controller specifics to make sure everything is OK. But with a 30 amp controller and ~11 amps of solar array--you should be OK.

You can go with a larger array (~2x larger), but unless you have a lot of loads during daylight hours--the battery will not accept that charging current for long (will recharge fairly quickly).

-Bill

Re: newbie question

PWM controllers are ~$100 or less... MPPT controllers are around $300-$400+.

There are (possibly) a few inexpensive MPPT controllers coming out of China/Overseas now--So that adds to the confusion--Plus many fly-by-night vendors that claim to sell MPPT controllers but they are really PWM.

If you have a 30 amp controller and did not pay north of $200-$300 for it--the controller is probably PWM.

For smaller systems/arrays, PWM controllers are frequently the correct choice (below 400 watt systems) as they are less costly and more efficient. And MPPT frequently are the better choice for larger system (over 800 watts or so) because they allow you to run higher voltage arrays (many larger MPPT controllers support Vmp~100 volts or larger) on smaller gauge wiring with the panels mounted much farther (10's to 100's of feet) from the charge controller/battery shed.

All About Charge Controllers
Read this page about power tracking controllers

Unfortunately, there are no Right and Wrong answers here--Just shades of gray and what makes the best choice for your specific needs.

-Bill

Re: newbie question

Generally, the recommended Voltage Maximum Power rating for a PWM controller on a 12 volt lead acid battery bank is around 17.5 to 18.0 volts (+/- a bit does not matter too much).

Panels with Vmp~<16 volts will not charge well on hot days with cool batteries.

Panels with Vmp>~19 volts will just waste power from the solar panel. Everything will still work, but just not the wattage rating of the solar panel/array.

Panels with Vmp>~20-24 volts or so; may be too high of input voltage for some PWM (and some MPPT) charge controllers (Voc--Voltage open circuit cold; as solar panels approach freezing and below, their Voc and Vmp voltages rise--Enough that it can damage some charge controllers if the panel ratings are too high--maybe approaching 30 volts and higher). The manual for the charge controller should say what the Vmax input voltage is.

-Bill