Wire Size

Re: Wire Size

If I understand you correctly...

You need to mount the Charge Controller within a few feet of the battery bank... Roughly, you probably want a maximum of 0.05 to 0.10 volts of drop between the charge controller and the battery bank.

For a 12 volt battery bank, each 0.1 volt drop represents (very roughly) a 10% loss in charging energy at the battery bank...

Make the long wire runs between the solar panels and the charge controller.

-Bill

Re: Wire Size

The current for 360 watts of solar panels, 0.77 typical derating, and 29 volts charging:

• 360 watts * 0.77 * 1/29 volts charging = 9.6 amps typical maximum current

Using a voltage drop calculator, 9.6 amps, 60' wire run, start with 6 awg wire:

• 0.55 volt drop @ 60 feet (one way run) and 9.6 amps

Typically, a 24 volt flooded cell battery bank will charge at 29 volts and float at 27.4 volts... For a 0.55 volt drop, the charge controller will be charging your battery bank at ~28.45 volts using 6 awg wire... Roughly equivalent to 14.2 volts for a 12 volt battery bank.

It certainly will charge the bank, but at a slower rate. You have the option of cranking the charging voltage up to 29.55 volts, but as the charge current tapers off, the voltage drop falls, and you are charging the bank closer to 0.55 volts higher than you really want...

So, you are left with a couple options (60' between battery bank and charge controller)--Leave the charge setting at 29 volts and charge slower--Or up the voltage and charge the batteries at a higher voltage as they approach full charge.

Neither is a great solution--but they both sort of work.

If you ever add more solar panels to your existing 360 watt array, the voltage drop is only going to get higher (note, the 5 awg wire will be around 0.44 volt drop for a 360 watt array).

Also, the charge controller needs to know the temperature of the battery bank. Hot batteries need a lower charging voltage and cold batteries need a higher charging voltage.

Either the temperature sensor in the charge controller itself needs to be in the same room/temperature conditions as the battery bank, or you need to install a remote battery temperature sensor that bolts/glues to a battery post.

Installing a charge controller in a temperature controlled room without a RBTS is not a good thing either.

Also, the charge controller should be fine in the same room as the batteries (if you have wide temperature swings, you may need to insulate the room/battery box anyway).

Lastly, you asked about efficiency... Remembering:

• Power=Voltage*Current = V^2 / R = I^2 / R

If you run the battery voltage at 29 volts @ 9.6 amps the long distance vs running the solar array with Vmp=58 volts @ 4.8 amps (for example)--The difference is:

• Ratio of power loss = 4.8a^2 / 9.6a^2 = 1/4

Running the "long" wire run at elevated voltage will give you only 1/4 the power loss (assuming same awg wire) as running at the lower battery voltage.

So, yes, there is an efficiency factor too involved in moving the charge controller from the office to the shed.

In reality--much of it is a savings in Copper Wiring costs if you run the solar array at a higher Vmp voltage (~60-100 VDC typically--depending on a bunch of stuff).

-Bill