Getting solar power into my truck

[DaveInDenver]

Yes you are. The voltage lost is dissipated as heat to the atmosphere, not easily detectable at these levels but there.

Power is lost, which is measurable. There won't be any lost voltage but there is current lost through resistance (dissipated as heat primarily). There is an apparent loss, in other words the voltage drops but the equations will balance when you account for everything including the losses into the real world beyond the solar circuit.

 

[DaveInDenver]

Is current really "lost", or just impeded ?

True. The energy is conserved and some is converted. Perhaps "lost" in the conventional electrical sense. Potential remains constant across the I^2 x r loss. So I guess it would look like a voltage loss (drop) then. But the current did the work.

Is this a hypothetical "Where is your observation point in Ohm's Law" question? Current, voltage, power and resistance (impedance) have this terribly incestuous relationship.

 

[Gs WK2]

Are we talking DC? If so, impedance should not even be discussed. The dynamics of a solar panel is not an AC dynamic.

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[DaveInDenver]

Are we talking DC? If so, impedance should not even be discussed. The dynamics of a solar panel is not an AC dynamic.

Explicit expression of the I-V curve for the diode in the PV would be done with the small signal AC model, as would be the battery, when subjected to PWM, which would be required to stay in regulation. None of this is a static large signal DC analysis. That's the fundamental point here. It's not a purely linear freshmen circuit analysis constant DC voltage on a fixed resistor Kirchhoff problem.

But I think he was just using "impede" casually to mean a resistance to flow of electrons, not necessarily the strict complex characteristic.

Is current really "lost", or just flow impeded ?

Just thinking about this more, voltage is potential energy required to move charge, e.g. one volt is one joule of energy that moved one coulomb of charge. So voltage is lost when work is done. Current only flows because there's a force to move it.

 

[Gs WK2]

@DaveInDenver thank you for the response. This is helpful info.

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[Rando]

OMG! Not this discussion again.

Of course there is a voltage drop on the wires going from the solar panel to the charger controller, as there is in all conductors. Ohms law still applies and thus the voltage drop across the wire is equal to the resistance of the wire times the current (V = IR). With a 50W solar panel and MPPT you are looking at ~3A at the absolute most (Isc), so with 100' (200' round trip) of 12 AWG you would be looking at 0.3 ohms or about 0.9V drop (3A* 0.3 ohms) which is 2.7W dissipated in the wires (P = IV = 3*0.9). This is about a 5% drop in overall power production, so nothing to worry about.

 

[Rando]

Power is lost, which is measurable. There won't be any lost voltage but there is current lost through resistance (dissipated as heat primarily). There is an apparent loss, in other words the voltage drops but the equations will balance when you account for everything including the losses into the real world beyond the solar circuit.

Don't forget the wire is just a low resistance resistor, when current flows through it there is a difference in potential at the two ends of the wire (ie a voltage drop) that is proportional to the resistance times the current (Ohms law). So yes there is power dissipated in the wire AND there is a real voltage drop across the wire. The fact that this driven by a solar panel doesn't change the basic physics.

Alternatively you can think of this in terms of power - the power dissipated in a resistive element is equal to voltage drop across the element squared divided by the resistance (Joule Heating P = V^2/R), so by definition, if there is power dissipated in the wire there MUST be a voltage drop across the wire.

PS Sorry to the OP for taking this tangent, but there appear to be some basic misunderstanding about electricity and circuits that are just going to make things more confusing.

 

[DaveInDenver]

Don't forget the wire is just a low resistance resistor, when current flows through it there is a difference in potential at the two ends of the wire (ie a voltage drop) that is proportional to the resistance times the current (Ohms law). So yes there is power dissipated in the wire AND there is a real voltage drop across the wire. The fact that this driven by a solar panel doesn't change the basic physics.

Alternatively you can think of this in terms of power - the power dissipated in a resistive element is equal to voltage drop across the element squared divided by the resistance (Joule Heating P = V^2/R), so by definition, if there is power dissipated in the wire there MUST be a voltage drop across the wire.

PS Sorry to the OP for taking this tangent, but there appear to be some basic misunderstanding about electricity and circuits that are just going to make things more confusing.

Yup, see post 37. I was thinking of it backwards physically. Joule's Law states power is proportional to current square times resistance, but the rate of electron flow is uniform around the whole series circuit so it's not strictly lost like the electrical energy converted to heat was.

 

[Rando]

Sorry, I missed #37, you are right on there.

The root issue here appears to dwh's idea that "voltages are the same everywhere in a circuit", which is clearly not the case. I think this is confusing voltages and currents - in a steady state DC circuit, the current is the same everywhere in a closed loop, but not the voltage.

By Kirchhoff's voltage law the sum of the voltages across all elements in a closed circuit adds up to zero, which tells you that if you add more resistance to the wires which lead to a higher voltage drop in the wires, either the battery voltage has to decrease or the solar panel voltage has to increase. In the case of a PWM charge controller in bulk charge (ie the battery is directly connected to the panel) with a large battery, then it is the later that happens - the panel voltage increases by the same amount as the voltage drop in the wires and the current decreases based on the solar panel I-V curve.

 

[Gs WK2]

Thank you Rando for challenging dwh's insistence on voltage being the same everywhere and refusing to follow basic circuit logic.

 

[Photobug]

Not to distract from this thread but..................

The solar cables I ordered are on the slow boat from Amazon and the 3 days concert I wanted this setup for starts tomorrow, so I just improvised with what I had, which was a spool of 14/2 automotive wire. I made 25 foot wire set that has MC-4s at the end. The other end is wired directly into my Victron Smartsolar 100/20.

Reading the instructions it says the voltage from the solar panel must equal Vbat+5 volts. After that it only needs Vbat +1v to continue. Since my panels max potential is 17.4 volts, I don't think my panel will ever kick the MPPT charger into action. Based on my previous usage at a concert I don't see needing solar but want to be able to play with this setup. Is there anything I can do to get this panel to start the charger?

 

[Rando]

It will work just fine - before the MPPT kicks in it is not drawing (hardly) any current, so there is no voltage drop on the wires. Secondly, the panel will be producing a voltage closer to Voc, which is more like 18 or 19V, so lots of overhead between that and battery voltage.

 

[Photobug]

I am going to keep this thread going although this a thread shift. I have my solar working now. I want to now wire my system right.

Currently I have a battery in a compartment in my truck bed. Originally I had two sets of fused wires coming off the battery studs. One goes to a 7 amp charger and the other goes to a cigarette lighter outlet just above this area.

I have since added a wire going to the back part of the truck where I have a dual USB outlet and another cigarette lighter outlet.

I have two more wires coming off the battery feeding two Anderson power poles.

All circuits have inline fuses and it is working okay, although I know the 5 wires coming off a battery lug is not ideal. My solar has a 25 section of 14/2 auto wire that plugs into my solar panel and then wires straight into my Victron Smart Solar charger, from then out to an Anderson Plug. I plug this into a Anderson Plug to my battery. Right now I have not used the Load Out part of the Victron Charger.

I was thinking to clean this up. Keep the wire to the battery charger going to the battery and having one 10/3 wire going to a distribution blocks for positive and negative, then attach all wiring run through these distribution blocks. I have two runs coming off the battery now to Anderson Plug. One goes to a Powerwex bulkhead fitting and the other goes to a pigtail. I would leave the run from the battery to the bulkhead powerwerx plug. I would move the pigtail to the distribution blocks. I could plug the pigtail from the distribution block into the Anderson plug to run everything off the battery. When I am using the Victron solar charger I can run the power from the battery through the solar charger to take advantage of the Victron's features.

I think I have it planned just looking for feedback.

 

[shade]

Blue Sea Systems makes a variety of fuse blocks that would work well.

ST Blade - Blue Sea Systems

Compact ATO® / ATC® fuse block consolidates branch circuits and eliminates the tangle of in-line fuses for electronics and other appliances.

www.bluesea.com

Adding a circuit breaker between the fuse block and battery makes it easy to de-energize the fuse block when you're working on it.