My electronics knowledge is pretty good but I wasn't aware of the internal workings of charge controllers (MPPT and not), so I posed this question to a local guy who is an electronics wizard and he assures me that it's no problem at all to have multiple controllers in parallel.
One point I can set you straight on is, which is the crux of the issue is this:
Originally Posted by neelie
Will not the highest voltage on the bus be sensed and presumed to be battery voltage?
It comes down to the power output of the panels and therefore the drive from the controllers, and the input requirements of the battery banks.
Let's say you have a 80W panel, and 800Ah of severely drained battery (all at 12V). The full output of that panel through the controller might be something like 5A at around 14.4V give or take a bit depending on efficiency of the controller, but that sort of current
going into a discharged bank isn't going to drive the terminal voltage up to 14.4A, it's only going to get to about 12.9V, 13V, 13.2V or something like that. Your controller is going to be working overtime for some time to push the terminal voltage up to the full charge voltage of the bank (14.4V or thereabouts depending on battery type).
If you had a 160W panel going into the same controller, that's going to be more like 10A into the battery which is going to push the terminal voltage up higher, perhaps to 13.8V. It's only going to have to work for half as long to get the terminal voltage up to 14.4V.
Now go back to the 80W panel through the charger
, and put a second charger
in parallel. The second charger isn't going to see the terminal voltage at 14.4V when the two start working, it's only going to see the 13V or thereabouts which is where the first panel is going to drive it to, and in turn is going to push the voltage up a smudge more, perhaps to the same 13.8V mark that the 160W panel would have done.
All smart controllers have an initial stage where they try to push the voltage of a drained battery up as high and as fast as they can to get it up to the proper charge voltage. They don't know the total Ah capacity of the battery, they don't need to know, they just work on the fact that the higher the capacity and the more severe the drain the longer it will take to get there. In effect having the two controllers working in parallel approximately halves that time, in about the same way as having twice the wattage of panel (and hence amperage output of the controller) would do.
As someone else has pointed out, the subsequent stage charges often work on a time basis -- hold the voltage up to a level for 48 hours or so, and for that time it doesn't really matter how many controllers you have in parallel either because they will all be regulating their output to what the batteries require. If you put two smart controllers in parallel and they are running in trickle charge mode then effectively each will approximately halve their output based on the fluctuations they see in voltage on the output sensors.
So the answer to your question is: Yes they do, and no it doesn't matter.