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Solar butt connection gone bad

solar electrical butt connect

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#11 rando

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Posted 07 August 2019 - 02:20 PM

If you have a PWM controller, then the voltage the solar panel is operating at is set by the battery (ie the panel voltage is pulled down to the 14V or what ever the battery is charging at).   As long as the voltage drop in the wires is less than the solar panel voltage minus the charge voltage (~17 - 14 = ~3V) then the voltage drop in the wires has no effect on the power to the battery.   So in this case you can sustain ~ 15% voltage drop in the wire before it impacts the power to the battery.

 

With an MPPT controller, you will see the voltage loss as a power loss as the controller converts this excess voltage to increased charge current.   However the power loss is still only proportional to the voltage drop - so a 3% drop in voltage will lead to a 3% drop in charge current and a 3% drop in power to the battery (not a 15% drop).    Also this is only under peak power production, under any other condition the loss is smaller.

 

So yes, there is a loss with MPPT, but my point was that the 3% guide line is an arbitrary figure and not a safety issue.  I would argue that rewiring inside the headliner and behind the lift panel to get a 3% increase in efficiency is probably not worth the trouble.   If you were worried about this, it would be better to rewire the panels in series and drop the current by a factor of 3.


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#12 ntsqd

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Posted 08 August 2019 - 01:43 AM

Voltage drop is a low Ohm resistor in the circuit that dissipates some power as heat. It matters not what the charge controller is, or the battery voltage, or the panel voltage. That power lost in the panel to controller wiring is gone before it reaches the controller. The controller can do lots of cool things, but it can't make up that lost power.

 

Not sure what I did this morning, it WAS pre breakfast, but taking my example numbers,

17 VDC Panel Voltage,

3% Voltage Drop,

100 watts solar panel.

I get 16.49 VDC @ 3% VD;

17.00 - 16.49 = 0.51 VDC voltage drop;

100/17 =  5.88 Amps;

.51 * 5.88A = 3 watts due lost to the voltage drop. Nothing can make that up, it is gone as heat.

 

It may not be worth the trouble to re-wire, but it should be understood to exist.


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#13 rando

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Posted 08 August 2019 - 02:29 AM

Agreed, the loss in the wires is real.   However, my point was two fold:

 

1. With a PWM controller the loss doesn't actually decrease the amount of power going to the battery.  As counter intuitive as it may seem, as long as the loss isn't huge it is actually made up by the panel producing more power. 

 

When a PWM charge controller is in bulk mode (which is most of the charge) then the battery is just connected directly to the solar panels, and it is the battery that sets the voltage it sees at it's terminals, which let's say is 13.7V .  In the case above with a 100W panel with a Vmp of 17v, the battery will get ~ 100W/17V = 5.9A.   However the battery has pulled to solar panel down to 13.7V, so it is only producing 13.7*5.9 = 81W.   If you now include the 3% or 0.5V voltage drop, the battery is still pulling the voltage it sees down to 13.7V, but now the solar panel has to produce 14.2V in order to get 13.7V to the battery, so the panel is now producing 14.2*5.9 = 84W but the battery still only gets 81W.  Weird, huh?

 

2. These losses are small, so as long as the currents are within the safe operating range of your wire (and connections!), upgrading the wire is unlikely to be worth the cost/effort compared to adding more/bigger panels or rewiring in series. 


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#14 ntsqd

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Posted 08 August 2019 - 12:55 PM

If it is a 100w panel and it is actually outputting 100w then when pulled down to 13.7VDC it should be outputting 7.3A, not 5.9A.


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#15 rando

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Posted 09 August 2019 - 01:05 AM

A 100W panel only produces a 100 watts when it is operating under full sun, and at the specified Vmp (Voltage maximum power) which is typically around 17 -19V.     If you pull the voltage down to 13.7V (or what ever your battery voltage is) you will no longer get the full rated output.  This is why MPPT charge controllers, which allow the panel to operate at its Vmp, can produce extra power compared to PWM charge controllers that force the panel to operate at the battery voltage.     


Edited by rando, 09 August 2019 - 01:06 AM.

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#16 deezlgeezr

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Posted 19 August 2019 - 06:41 PM

To the OP: Haven't seen anyone comment so I will: That's a great preventive maintenance find of yours; it really pays off to pay attention to your equipment and in your case it seems to have REALLY paid off. Great find!! Good luck with the fix!


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