When charge controllers fight

Re: When charge controllers fight

I'd be lying if I said this makes sense to me.

For those who don't know: AC coupling does not involve a second charge controller; it back-feeds the battery-based inverter.
If you are using a Sunny Island it will do a fine job of regulating charging via this method; it's made for it (especially if using a Sunny Boy GTI).
There's no reason why you should want to change the charge profile from time to time on any given set of batteries; stick with the Voltage/time that works.

Otherwise if you charge batteries from two sources that do have different Voltage settings (such as when using PWM controller with MPPT where the PWM does not have precise setting controls) the one set to a lower Voltage will drop current trying to keep the Voltage low as the other tries to bring it up. In some cases where the charger capacities are near equal it will hold the battery bank in Absorb at the lower Voltage while the time runs out, which means the battery bank ends up getting two Absorb cycles; one at the lower Voltage and one at the higher. This results in too much Absorb time and an excess loss of water so it is not recommended.

Before anyone starts to argue, yes I have actually seen this with my own system while running the built-in charger from the generator (and playing with the numbers; remember I do this stuff so you don't have to). This is why we usually only do this to Bulk charge and let the solar do the Absorb/Float stages.

Your 1 hour of 2.5 Volts per cell every 14 Days is an Equalization cycle, not an Absorb cycle.

If the Morningstar is connected to a separate battery bank and you are using an inverter off that bank which is AC coupled to the Sunny Island the settings on the Morningstar are irrelevant; it has no way of knowing the Voltage in the Sunny Island's bank. The AC coupling eliminates the DC connection and simply provides an AC source for the SI to use much like hooking it up to a generator; its own charging program will function however it is programmed.

Re: When charge controllers fight

Dave Sparks wrote: »

I am missing something aren't I?

Me too.
The mention of AC coupling got me confused: "1kW will be connected through AC coupling to the Sunny Island".
In that case the SI becomes the second charge controller, as though it's being fed from a gen, right?
This begs the question of how the 1kW is turning into AC to back-feed the SI.
He could certainly use more than 2.8 kW of panel on 900 Amp hours of 48 Volt battery.
By why 1kW AC coupled? That implies the use of a GT inverter, a very small one or perhaps some micro-inverters. This would supplement the SI's 5kW capacity while the sun is up, but really will only add about 50% of the panel that battery bank needs for a good charge.
All sort of sounds out of balance to me. Instead of buying extra GT inverter(s) to do this I'd just get another controller and put the new panels on it.
If he actually needs more AC capacity (hard to imagine) add another off-grid inverter to the same battery. But there still wouldn't be enough panel for that forklift battery.

It'd help if we knew what the problem was that he's trying to overcome. There's more than one way, as they say.