19 replies to this topic

[rando]

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.

[ntsqd]

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.

[rando]

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. 

[ntsqd]

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.

[rando]

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.

[deezlgeezr]

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!

[Helmut]

Hi there, I know its a long time, but I am seeing the same burn-spots on the vinyl of my camper, as the OP experienced. 2017Grandby has #12 coming through the roof to the ZAMP-controller and from there #10 to the batteries. Has anyone replaced the wiring from the roof? and can tell me what difficulties and problems I will run into?

[bfh4n]

I don't think much of the ordinary type of insulated crimp connectors, whether on lugs or splices. I've seen a lot of them that were not good connections. When I was re-doing the OEM wiring in my FWC, I found some that just pulled right off the wires.

The uninsulated type of crimp connector, used with the proper crimper, makes a much better connection. The key is the correct crimper. This is how all automotive wiring is done. They can be insulated, after crimping, with heat-shrink tubing. Adhesive-filled heat shrink can also seal out moisture and prevent corrosion. Tape might be adequate insulation, if there is no chance of abrasion (and low voltage).

You can get uninsulated crimp connectors and a decent multi-size crimper at places like Lowe's or Home Depot. For good crimping, one jaw of the crimper must be shaped like the letter B, while the other jaw is U-shaped. This rolls the sides of the connector around the wire strands and forms them into two tight bundles.

Uninsulated crimp butt splices exist, but aren't common. You can make your own by cutting the crimp part off of a lug or slip-on connector of a larger size. It has to be large enough to accomodate both wires. Insert the wires, one from each end, and crimp. This approach has the advantage that the strands of both wires are in contact with each other. No current flows through the connector. Don't forget to put the heat-shrink tubing on one of the wires first.

Some folks advocate crimping (uninsulated) and then soldering, for a good high-current splice or where moisture could be a problem. I've had good luck with this approach. For smaller wires, if you are good at soldering, you don't really need the crimp connector at all. Just twist the wires together and solder them. You can twist them better if you bring them together side-by-side, rather than end-to-end. (Having the bare strands prertty long makes them easier to twist tightly. You can cut them shorter after you solder them.) Then you can insulate with heat-shrink or by screwing on a wire nut. (I wouldn't use wire nuts for connections in automotive or camper environments, where there will be lots of vibration, but they are fine for insulating soldered connections.) Soldering alone is probably not the best approach for really big (and high-current) wires.

- Bernard

Edited by bfh4n, 26 May 2020 - 02:42 AM.

[craig333]

Even after buying a good crimper I've still had a lot of failures. Now I'm trying the no crimp adhesive and solder filled butt splices. So far it seems to work well.

[Fishyhead]

The quality of the crimp connectors themselves have a lot to do with how well the crimp holds.  I recently ran out of 3M Scotchlok butt connectors and bought some generic ones from the autoparts store and they just didn't work with my ratcheting crimpers.  Upon inspection, the metal insert was just too thin to work with the crimper so I had to get out my old non-ratcheting one and smash the heck out of them.  I ordered a new batch of connectors and all is right with the world again.  I now have what I hope is a lifetime supply of Thomas & Betts and 3M connectors, disconnects, and terminals.