Voltage drop solar controller to batteries

[SKMT]

What % voltage drop should I be aiming for when sizing the wire gauge between mppt controller and batteries?

400 watts of panels. About 8-10ft from controller to 12v bank.

Putting the maximum potential 32.5 amps into online wire calculator, says 2awg for 1% or 4awg for 2% drop. But am I ever really gonna see those numbers enough to be worth using 2awg?

It's a 1 time purchase so i will spend the extra for 2 awg if it makes sense. But I question whether that is unnecessary overkill?

[Bycrick]

Wire sizing calculators tell you the minimum size you can get away with. Every watt lost in the wiring is a watt that you’ve paid for but are throwing away. Why spend hundreds of dollars for high-output panels and a exotic regulator, then throw away the efficiency gains to save maybe $40 on wire?

[Nord Sal]

Originally Posted by SKMT

What % voltage drop should I be aiming for when sizing the wire gauge between mppt controller and batteries?

400 watts of panels. About 8-10ft from controller to 12v bank.

Putting the maximum potential 32.5 amps into online wire calculator, says 2awg for 1% or 4awg for 2% drop. But am I ever really gonna see those numbers enough to be worth using 2awg?

It's a 1 time purchase so i will spend the extra for 2 awg if it makes sense. But I question whether that is unnecessary overkill?

1% voltage drop at maximum rate (which will only occur a small fraction of the time) is overkill. AWG 8 will work with a 3% voltage drop for a 16 foot circuit (round trip) at max current. AWG 10 will work for 16 feet with a 5% loss. Most of the time you're current will be much less and the voltage drop will be less accordingly.

[smac999]

Normally 3% for power distribution and 10% for loads.

1% would be crazy. especially when solar will never see 100% current anyways.

[sailjumanji]

840 watts solar, LiFePO4 house battery. Rewired the boat to alleviate voltage drop and used 2 AWG between solar controller and common negative and positive bus bars. Then 4/0 to the LiFePO4 battery. Yeah, it was overkill, but wanted to eek out as much as possible from solar, and not have to install more panels/second controller.

To me, additional cost of the wire was lesser consideration. Upsizing and pulling wire thru limited circuit box openings, etc., on a fully-wired boat was a PITA that I never wanted to repeat. Also invested in tool to cut large diameter wire and another to make proper four-sided crimps. 4/0 cable is not easy to cut, nor crimp. Some good tips on Maine Sail's website.

[hlev00]

The other consideration is that you will seldom, if ever, see 400W. For example, I have 380W of Solbian flexible panels on my bimini. During May in the Bahamas my controller claimed to have a max power of 369W. More typically it is max of 280-300. And remember that is an instantaneous maximum. Most of the day it is less. You definitely want to be sized adequately, but if you consider typical is likely to be more like 300W, then using a 3% factor at 400W will get you 2% at 300W for the same size wire. Re-run the numbers based on your own assumptions (mounting position, latitude, typical max power, etc.) and targets.
Harry

[Snerf1]

Some really good comments

Basically you want to be as close and heavy as practible

Yes it may be overkill, and yes you'll never see max watts, but what is the true cost?

It's your time, money and energy dealing in the first place

The price differential between guages is peanuts compared to the other factors

Do it once, do it right, and forget about it

[bdgWesternMass]

Originally Posted by Snerf1

Some really good comments

Basically you want to be as close and heavy as practible

Yes it may be overkill, and yes you'll never see max watts, but what is the true cost?

It's your time, money and energy dealing in the first place

The price differential between guages is peanuts compared to the other factors

Do it once, do it right, and forget about it

Another reason to install heavier gauge is that it is a form of future proofing/resisting.

The only downsides that I see initial material cost and the possibility of a more difficult install.

The trend in electric transportation is bigger wires for the main bus.

[rslifkin]

It depends a bit on your solar controller. With my Victron controllers and the other Victron gear in the system, my controllers are sensing voltage at the batteries, not at the controller. So they compensate for any voltage drop in the controller wiring. I've only got about 2.5% voltage drop anyway as wired, but it has no real impact on charging performance for my setup.

[oleman]

I would use the largest wire gauge that is flexible enough to work with considering tools and space available. Connectors are a very important part of wiring a circuit.

Good luck with your vessel.

[mitiempo]

I would wire for a minimal voltage drop.

But the first thing I would do is install the solar controller closer to the batteries. 8-10 feet is too far in my opinion.

[captstu]

You can place the sensor wire the controller uses on the batteries rather than close to the sensor. If you do this, the controller raises the voltage at its end of the wires to adjust for voltage drop.

In the long run, providing wire gauge for the nominal condition is best, but voltage drop in the controller to battery leg is compensated by the controller if the sensor is placed correctly.

[mitiempo]

Originally Posted by captstu

You can place the sensor wire the controller uses on the batteries rather than close to the sensor. If you do this, the controller raises the voltage at its end of the wires to adjust for voltage drop.

In the long run, providing wire gauge for the nominal condition is best, but voltage drop in the controller to battery leg is compensated by the controller if the sensor is placed correctly.

Many controllers do not have a separate voltage sense wire.

[zstine]

Originally Posted by smac999

Normally 3% for power distribution and 10% for loads.

1% would be crazy. especially when solar will never see 100% current anyways.

My Panels regularly exceed their rated max watt value. I have a pair of 265W (530W total) Kyocera brand. In the tropic sun, It was not unusual to see 550-560W mid day per the Victron blue 100V/50A MPPT controller.

[stevo_002]

Originally Posted by captstu

You can place the sensor wire the controller uses on the batteries rather than close to the sensor. If you do this, the controller raises the voltage at its end of the wires to adjust for voltage drop.

In the long run, providing wire gauge for the nominal condition is best, but voltage drop in the controller to battery leg is compensated by the controller if the sensor is placed correctly.

The first sentence leaves me unsure if your intent. Good idea if I understood better.

Isn’t the sensor wire connected to the sensor? And the other end of that node is the controller.

Apart from any consequence . . . Where else does the battery voltage sensor go ??