Dual batt banks or parallel bank?

solarix

My standard off-grid system is a Magnum 4448PAE inverter with a Midnite classic 200, 8 Trojan L16RE-B batteries and 12 PV panels. Not big enough for the current well-to-do (utility connected) customer so am doing a double system except only 20 panels.
Question for the forum is: Would you combine these together with 2 strings of batteries in parallel (inverters and controllers both stack) or would you split into two systems with one string of batteries for each inverter and controller? It takes the same amount of breakers, and wire etc. either way.

Cariboocoot

Re: Dual batt banks or parallel bank?

If the AC side needs to be coupled so does the DC side. That's the key.

inetdog

Re: Dual batt banks or parallel bank?

I would make up a single battery bank, using low voltage batteries as necessary to avoid paralleling, and attach both CCs and both inverters to it. With their own individual fuses, of course.

Were you thinking of stacking the inverters or just wiring them independently to different groups of loads? For stacking I am pretty sure that they are happiest with a single common battery bank for best performance.

Just remember that the one thing that cannot be shared is the panels. Each CC must have its own group of panels, particularly if they are MPPT type and do not intercommunicate.

vtmaps

Re: Dual batt banks or parallel bank?

inetdog wrote: »

Each CC must have its own group of panels, particularly if they are MPPT type and do not intercommunicate.

I think I know what you're getting at, but I'm having trouble with the use of 'particularly'.

Are you trying to say that PWM controllers should have their own array, and that non-communicating MPPT controllers must have their own array?

--vtMaps

solarix

Re: Dual batt banks or parallel bank?

no, no not sharing the arrays. Each charge controller will have its own 10 panels. Also seems to me the batteries will be happier if they are two separate banks that get individually managed. I'll set the inverters up in voltage controlled mode so that when below 48.8V the inverter charger will kick on and charge from the grid until over 52.6 volts leaving room to fully utilize the solar charging.

Cariboocoot

Re: Dual batt banks or parallel bank?

vtmaps wrote: »

I think I know what you're getting at, but I'm having trouble with the use of 'particularly'.

Are you trying to say that PWM controllers should have their own array, and that non-communicating MPPT controllers must have their own array?

--vtMaps

I don't know what inetdog is trying to say but MPPT controllers, communicating or not, should each have their own array. Otherwise at the very least they will have trouble selecting proper power point due to the connection of other controller(s). PWM's being basically a switch don't care so much.

Cariboocoot

Re: Dual batt banks or parallel bank?

solarix wrote: »

no, no not sharing the arrays. Each charge controller will have its own 10 panels. Also seems to me the batteries will be happier if they are two separate banks that get individually managed. I'll set the inverters up in voltage controlled mode so that when below 48.8V the inverter charger will kick on and charge from the grid until over 52.6 volts leaving room to fully utilize the solar charging.

You haven't addressed the issue of whether or not the AC side of the inverters will be coupled. If they are then the DC side must be as well.

This would not be the same as a system where some loads are on one inverter and some on another.

vtmaps

Re: Dual batt banks or parallel bank?

Cariboocoot wrote: »

I don't know what inetdog is trying to say but MPPT controllers, communicating or not, should each have their own array.

I think the bit about 'communicating' is because some MPPT controllers are stackable and you may have more than one controller per array. example: the Midnite Kid in its "bully" mode.

--vtMaps

inetdog

Re: Dual batt banks or parallel bank?

vtmaps wrote: »

I think the bit about 'communicating' is because some MPPT controllers are stackable and you may have more than one controller per array. example: the Midnite Kid in its "bully" mode.

--vtMaps

Exactly. Instead of competing MPPT algorithms each making load current changes and expecting the resulting voltage to be dependent only on their action, the two could agree to let one do the voltage sweep and the other pull the load current directed by the "boss". This is different from agreeing on the charging current to a common battery bank. (A much more common feature in linkable CCs.)

But does Bully mode actually do this?

vtmaps

Re: Dual batt banks or parallel bank?

inetdog wrote: »

But does Bully mode actually do this?

Yes, from what I read.
Mario's (a Midnite engineer) comment here: http://midniteforum.com/index.php?topic=1710.msg15351#msg15351

-- vtMaps

solarix

Re: Dual batt banks or parallel bank?

The inverters will be stacked - meaning tied together so the AC outputs are synchronized and work together, but this doesn't mean that their batteries have to be common. I think the less parallelism the batteries have the better - and one set of 8 on each inverter is no parallelism at all. Each inverter pulls what it needs from its bank and if it runs low its charger should turn on (or if the inverters are slaved, does the charging act together as well as the output).

inetdog

Re: Dual batt banks or parallel bank?

If the inverters are slaved (in parallel, not to get 240V from two 120V inverters), then they will probably draw each draw similar current from their battery banks. But the inverters have no direct control over the chargers. So the two CCs will start charging at about the same rate and the same time, without any interconnection, as the battery voltages decrease similarly.
If the inverter parallel scheme uses one inverter only for low loads, to minimize the tare load of the other inverter by essentially turning it off) then the two battery banks will not be loaded at all equally and you may even get a situation where one inverter stops because of its low voltage cutoff and the other inverter may or may not take over and pick up the small load.
This is one of the reasons that a single battery bank is more robust and easier to manage.
You could have both CCs working together to make up the current drain of only one of the two inverters.

Cariboocoot

Re: Dual batt banks or parallel bank?

Ask the question, ignore the answer. Okay; not my money wasted.

solarix

Re: Dual batt banks or parallel bank?

Finally read the inverter manual. Definitive answer is to run one batt pack shared by both inverters as the parallel inverters are controlled as master/slave where the master does most of the work and will imbalance the batteries if on separate batts.

vtmaps

Re: Dual batt banks or parallel bank?

solarix wrote: »

Finally read the inverter manual. Definitive answer is ...

Definitive answer was the very first reply to this thread. --vtMaps

zoneblue

Re: Dual batt banks or parallel bank?

Solirix: Er, thanks Coot. you were right.

Coot: No worries, my pleasure.