12v MPPT and 24v MPPT vs 24v PWM Test

CA Traveler wrote:
Agree but some caution is in order. BFL13 used 40' of various larger cables and some were aluminum/copper and he measured 29V on a prior test. The PWM results are appreciated and certainly adding to our knowledge but drawing generalized conclusions may/may not be appropriate.

My panel to controller wiring zoo has a line loss of 1% when amps are in the 7 to 8 amp range. 29.3v at panel and 29.0v at controller is typical. That's with the 24v panel.

With my PWM (which has to be to a 24v battery with the 24v panel), the amps on the path are the same at near the panel's Isc of 8.3

But if you have 12v panels to the same wattage and use PWM, now you need fatter wire on the panel to controller path where now you will have about twice the amps and the voltage will be about half.

I tried it on a voltage drop calculator. With #8 at 40ft distance (wire pair) 24v, 8 amps drop is 1.67% but change that to 12v, 16 amps and drop is 6.67%

Note the higher percentage is also due to the drop being a bigger proportion of the lower voltage.

With #4 instead of #8 and the 12v,16 amps the drop is 2.67%

So a key to the whole business is that with the higher 24v you can have the same actual voltage drop but it is a smaller percentage.

Sometimes it is the absolute value that matters not the percentage. On the controller to battery path you want the controller to "see" the actual battery voltage for hitting the set point. If you want it to start controlling when battery hits 14.6 and your drop is 0.4, then it will start controlling when the battery is only at 14.2.

You don't care how much that 0.4 is in percentage, you want 14.6 instead of 14.2

So you have to be careful with percentages--always ask, eg, "10% of what?"