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Post Info TOPIC: Revised Connecting extra solar panel


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Revised Connecting extra solar panel


Can 2 solar panels  be fed into one regular already installed (labeling says regulator 180w at 12v ) & (max current 15a).

 I have 120w on van labelled ( Volt 17v, 120w,  Amps7.08 ) connected to battery via regulator.( if use W div V = Amps )( 120w div12v=10amps)

 DO YOU USE PANEL Voltage17v  or BATTERY VOLTAGE 12v [ if you use battery volt amps are higher.]

 Therefore I can have an 80watt panel based on above calculations :: ( 80w div 17v = 4.7amps) or (80v div 12v =6.6amps)

 Adding another panel would be in parallel to regulator .

 Am I working this out right or is there a fatal error appreciate any advice



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Hi jeepnudger smile

The figures on solar panels do not really work out and are for sales purposes. hmm

The important ones are the MPV ,voltage max power, which is your 17V and the MPA, current at maximum power, V which may be your 7.8 a. These are the maximum power Voltage and Current and if you multiply these they will give the max power. But you seldom get that, usually about 70-80% if flat on the roof. 

If you get about a 80+ panel with similar VMP it will work OK in parallel with the other one. But you should check the max allowed by your regulator as it may fail if overloaded, depending on the type. Some will protect themselves from occasional overload. Look at the information on the back and google the specs. 

 good luck jaahn 



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I have no idea what controller you have but in my situation with my MPPT 10amp controller it could use solar panels up to 145 watts according to the manual.

But it was reaching the 10amp limit OUTPUT with 120 watts of solar with only 4amps going in with panels in series.

I replaced it with a 20amp MPPT & under ideal conditions the output was about 11amps. So plenty of headroom.

I have gone overboard with really thick wiring to squeeze every last bit of power outlet of the panels.



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I'll assume you are using a PWM controller. Look at the Imp on the back of the panel, that is the most amps you can expect out of that panel besides the odd freak conditions like cloud edge effect and reflected light on light scattered cloud if you are near snow or water.
I think Jaahn as already covered most of this so just pass over the bits you have already read.
The advertised watts for the panel is measured in a lab with a special light box, the panel at 25*C and the light flashed for 5 secs. That information is marked as STC (standard test conditions) a the advertised panel output, say 100w, is the Vmp x the Imp, a 12v panel would probably have a Vmp of 17volts and an Imp of 5.88 amps, 17 x 5.88 = 100w. that 17v maximum power is only available if the panel is in full sun but it doesn't get above 25*C, not much chance of that happening in Australia. If you allow 70% of the advertised output you will be on about the right track, it is the voltage that drops as the panel gets hot, not the amps. If the battery is very low and pulling the panel voltage down to 11v, that amp output will remain the same, it will only reduce if the voltage goes above the Vmp of the panel at what ever temperature it is inside the panel.
To give you an idea, in full sun mid summer the panel temp can climb above 70*C so the Vmp will drop to 15v or even lower, yet the 5.88 amps will still be available if the battery voltage is less than say 14.4v, enough to allow the current to flow from the higher voltage to the lower voltage.

So armed with all that knowledge, the 180w panel with a Vmp of 17v would put out around 10.5 amps. The 120w panel divided by the 17vvmp would put out around 7 amps. If you wiring is excellent cable from the panel to the controller and from the controller to the battery, you can hope for around 17.5 amps from the panels to the battery when the sun is at the perfect angle to the sun. If you have a controller that can handle 20 amps you will be fine, if it only handles 15 amps you will loose a bit at that ideal sun angle time. I wouldn't use a 10 amp controller though, to over load was cause it to overheat and die.

T1 Terry

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I would have thought that the regulator specs were the output 180W, 12V 15A.
It is the regulated output that is required to determine if the regulator is suitable for your purpose.
But you are correct, the 180W plus 80W panels will give potentially 260W input which means the regulator will be outputting its maximum 15Amps at 12 Volts.
Q. Will 12V fully charge the battery?

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jegog wrote:

I would have thought that the regulator specs were the output 180W, 12V 15A.
It is the regulated output that is required to determine if the regulator is suitable for your purpose.
But you are correct, the 180W plus 80W panels will give potentially 260W input which means the regulator will be outputting its maximum 15Amps at 12 Volts.
Q. Will 12V fully charge the battery?


Hi jegog smile

No, 12V will not charge a battery. You need an extra voltage to push the current into the battery against the voltage and the resistance too. For lead acid batteries the usual voltage is about 14.2-14.4-14.6v to fully charge them. Look at what is on the battery or recommended by the manufacturer for cyclic use. 

So most quality regulators have several settings to cover the normal LA types. EG, max 14.2, 14.4, 14.6 V. Then an equalisation charge also for some types of 14.8V. They hold this charge for some time to get full charge into the battery. Then should drop back to float, say 13.8V if there is not much load. If you do not charge LA batteries high enough to get them fully charged regularly they will die early.  

Jaahn



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