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Open Roads Forum  >  Tech Issues

 > Two Charge Controllers - Dumb idea?

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cruz-in

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Posted: 03/09/12 10:43am Link  |  Print  |  Notify Moderator

hi All,

I own 2 Morningstar Sunsaver MPPT-15 Charge Controllers with temp senosrs (got them new about $100 on ebay). I plan to install 400 Watts of solar panels on my roof. My current system design is to in essence have two completely seperate and identical systems. Each with 200 Watts feeding an MPPT-15 and charging a Battery. I will then have each Battery connected into the camper with a switch. Allowing me to chose either batttery. So when using one, the other is just sitting there being charged.

Looking for why the is a dumb idea and I need to simply purchase one MPPT controller (my panels have a Voc of 70 volts and VpMAX of 50 volts)with enuff capacity to support 400 watts. .

smkettner

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Posted: 03/09/12 10:52am Link  |  Print  |  Notify Moderator

I would use the two you have. I don't think they need to be on separate batteries. You might have a reason to choose the batteries separately but generally I would run them in parallel.

If it has an equalize function you may need to turn it off or plan accordingly as they would cycle independently.


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cruz-in

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Posted: 03/09/12 10:59am Link  |  Print  |  Notify Moderator

smkettner wrote:

I would use the two you have. I don't think they need to be on separate batteries. You might have a reason to choose the batteries separately but generally I would run them in parallel.

If it has an equalize function you may need to turn it off or plan accordingly as they would cycle independently.


thought about that but the manual says:

"Do not wire multiple SunSaver MPPT load outputs together in parallel to power DC loads with a current draw greater than 15A. Equal current sharing cannot be guaranteed and an over-load condition will likely occur on one or more controllers."

So I am kinda stuck with two parallel and seperate systems...

smkettner

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Posted: 03/09/12 11:18am Link  |  Print  |  Notify Moderator

You are not powering loads directly. As long as a battery is in the circuit you are fine.

BFL13

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Posted: 03/09/12 11:36am Link  |  Print  |  Notify Moderator

Peukert says to save AHs while drawing down the batteries by using both battery banks at once rather than draw one down and then draw the other down.

charging them doesn't matter if you charge both together at 30a or each separately at 15a it will take the same gen time (or solar time I suppose.

the issue of using both chargers at once where they do not share the amps equally is just from possibly being at different voltages and from perhaps having different wiring. If the battery bank will accept 60amps, and both chargers are at the same voltage, they will each do 15 amps and do 30amps total.

If one is at a lower voltage, then as the battery rises in its voltage, the lower voltage charger will do fewer amps until it stops, while the other continues providing amps until it too has its voltage matched by the battery voltage.

But how could that 15amp charger ever do more than 15amps even if the battery will accept 60amps? A regular battery charger is current limited, so that's that. A solar controller may or may not pass through more amps than it is rated for, depending on its design. My ASC non-MPPT "16a" one lets the amps through. I have seen 19amps from it on the Trimetric. Perhaps it would heat up and eat itself if that went on too long, don't know. (that 19 was a brief cloud effect thing)

So if the solar controllers are "current limited" IMO there is no risk in using both on the one battery bank and taking advantage of Peukert. You can still split the bank for equalizing one while running the rig from the other every so often as required.

It would depend on the scenario with some numbers if there would be any risk from having one controller run at over its amps long enough to melt it if the controller is not current limited.


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cruz-in

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Posted: 03/09/12 11:44am Link  |  Print  |  Notify Moderator

smkettner wrote:

You are not powering loads directly. As long as a battery is in the circuit you are fine.


Ooh that is a good point. I may have been reading this incorrectly. As the MPPT-15 has a load output and a seperate battery connect, the statement "Do not wire multiple SunSaver MPPT load outputs together in parallel ..." may be referring to the load output vice the battery connections. I will give Morningstar a call and check...

cruz-in

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Posted: 03/09/12 12:13pm Link  |  Print  |  Notify Moderator

Just got word from morningstar tech support. I can have as many charge controllers charging the battery as i want. Just cannot tie the load outputs from the charge controllers together.

Gale Hawkins

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Posted: 03/10/12 12:29am Link  |  Print  |  Notify Moderator

smkettner wrote:

You are not powering loads directly. As long as a battery is in the circuit you are fine.


My thought too.

cruz-in

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Posted: 03/10/12 05:12am Link  |  Print  |  Notify Moderator

Thanks all for your time and info...your responses prompted me to chat with morningstar and get confirmation that I can have both power the same battery bank....sure simplifies my install.....thannks

wa8yxm

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Posted: 03/10/12 09:44am Link  |  Print  |  Notify Moderator

Thanks to whomever posted the word/name Peukert. I needed that (Seriously).

If you put two charge systems on the same battery Best case is they both work, worst case is one of them sits there and does nothing. I'd put ammeters in both leads and see what happens.


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