Path of energy?

Arkansasoffgrid

A question asked by a friend recently.
In his example question on a 4500 W array, it's 12 noon and the battery bank is 100% charged, If you turn on the air conditioner that using 850 W does that power still come through the battery bank? Or does the charge controller divert it from the panels to the inverter?

BB.

The solar array+solar charge controller supply the "average power". The battery bank "buffers" the voltage around the "float voltage".

So, if you put a oscilloscope on the battery bus cable and display the current--You should see lots of current flow--Probably a 120 Hz "sine squared" ripple current (from the AC inverter DC power input). And you may see some longer term (seconds to minutes) variation in DC current.

As long as the battery bus voltage is ~float voltage, you are not micro-cycling the battery bank--It is, more or less, acting like a giant capacitor and "buffering" current. The solar array/charge controller output their average DC current (or with a PCM controller, On/Off/On/Off current pulses) and the battery will take a pulse of current (from the solar charger) and supply a pulse of current (to the AC inverter) when the AC inverter DC consumption exceeds the solar array/charge controller output during those few milliseconds.

If the battery bank voltage falls below ~12.7 volts (for a 12 volt battery bank), then the battery is actually being discharged or possibly "micro-cycled"--Not a good thing to do in the middle of a sunny day (cycling wears out the battery over time).

That is one reason (micro cycling) why we recommend larger lead acid battery banks even if you only run the loads during sunny days--The battery is still buffering the current flow in the system and "holding" the battery bus average voltage.

The battery bank is behaving, sort of, like a flywheel in this operational mode. Does that sort of make sense?

-Bill

inetdog

Another way of looking at it is the old water analogy.
Imagine your batteries are a bathtub and your inverter is connected to the drain. Also attached at the same place in the tub as the drain is a pipe from your water tap.

When you turn on the tap while water is running out the drain, does the drain water come from the tub or the tap? The answer is that for all practical purposes it does not matter. The level in the tub will change in the same way whatever the path of the water is.

Now with the inverter and the CC both connected to the battery post, can you see any way that the CC can control which way the electrons will go when they reach the terminal?

ggunn

The simple (perhaps simplistic) answer is that with an off grid system you are always running off the batteries, and the sole reason for the PV is to charge the batteries. From a practical standpoint electrons are indistinguishable from one another and the AC does not know or care where they are coming from.