Michael McCandless's posts

I'm designing a small (600W using 3 200W panels in series) off-grid system.

Current plan is the Apollo T80 charge controller, Outback VFX 3648 inverter/charger/AC transfer switch, and a single string of batteries @ 48 V.  I plan to use a Midnite Solar E Panel to hold breakers & tie it all together.

I actually have grid access, so I'll tie the grid into the Outback's AC input.

I plan to attach smallish loads to the Outback AC output, such that I shouldn't need to pull AC from the grid very often (only after several days of rain, maybe).

So ... here's my question: how does the charge controller (T80) and the inverter's charger "agree" on who should do the charging, when?  What if they both try to charge the batteries at the same time (and perhaps disagree on whether it's time for bulk vs absorb vs float)?  How can I configure the T80 and inverter to not step on each other?

Thanks!

Yes I do have a technical background, though more in computers and less so in electronics.  And I too love to optimize

I already feel addicted and I haven't bought anything yet!

Aha!  You are solarjohn   I love your blog and I'd already read all about how your system upgrade to use an AC transfer switch.  I think that's a great way to go.

So, if my charge controller is in "bulk" stage while the inverter has a load, I think this is as efficient as can be (the panels are being fully utilized).

If my charge controller is in "absorb" stage (constant voltage) while the inverter has a load, I think this should also be as efficient as can be because the charge controller will hold the voltage constant, thus delivering as much current as the inverter plus batteries need, combined.  In this case I'm actually wasting the panels, in that they could deliver more amps, but this is only because I haven't attached a large enough load.

Finally, if charge controller is in "float" stage while the inverter has a load, I think this may be OK because float stage is constant voltage.  The inverter will pull the current it needs from the charge controller.

OK I think it will "just work"   Thanks John.

That helps, thanks.  It's good to hear your setup seems to do the right thing.

But, how did your charger controller "know" to kick into bulk charging as soon as the compressor kicked in?  Is it just because the voltage on the batteries drops when they are providing so many amps?

What if instead of a compressor, it was a somewhat lighter load (say a small TV or something)?  Would the voltage on the batteries then drop less, and maybe the charge controller wouldn't "know" to kick in and provide current?

I'm trying to understand the mechanism by which the whole system manages to fully utilize the panels...

Also: I wonder if this is a reason to use "matched" inverter & charge controller.  EG if I get an Outback inverter & charge controller, they do communicate with one another (I think?) and so perhaps the inverter would tell the charge controller to provide more current?

I have a fundamental confusion/question on how a charge controller and an
inverter interact (I'm very new to this).

Say I have a PV array charging a battery bank through a charge
controller (eg Apollo T80).  And, I also have an inverter (eg Outback
VFX 3524) attached to the same battery back.

The T80 wants to go through 3-stage charging to charge the batteries.

Yet, at the same time the inverter possibly wants to draw lots of
current (say if I suddenly turn on an AC load).

How do these two "communicate" to do the most efficient thing?

EG, say it's a sunny day and the batteries are mostly charged so they
are in float right now, meaning they are not fully using the sun since
the panels could put out much more current.

Then, I suddenly turn on a vacuum cleaner on the AC output of the
inverter.  This will draw alot of current from the batteries.  Will
the charge controller "realize" this and up the current to the max
possible from the panels?  Or, will they remain in float charge, thus
wasting (not tapping) the possible current from the panels?  How is this
normally managed?