Question about charge controller current...

[amwbox]

So I've been reading Morningstar's materials here, and I notice that the max current to the batteries is 45 and 60 amps respectively from those controllers.

But this is a theoretical maximum, right? Like if the panels were tilted and tracking the sun in the middle of July in the Southwestern Desert. For my purposes I'm likely going to wind up at a much lower current.

So, if I have a lot more potential wattage from the panels than the controller can turn into current for the battery, what happens with the excess? If the controller can make 45 amps, but there is 1000 watts worth of panels (1000W/12V=83.3A), what happens to the extra power? Is it wasted as heat? Or what?

Also, I don't think the usual flooded deep cycle batteries can accept 45 or 60 amps of current anyways, can they? Unless of course the bank is very large?

So...should I be sizing the controller on the basis of the total maximum power the panels can provide, or should I be sizing based on the actual current that the system can actually utilize? If I overbuild the panel wattage to account for reduced power in winter, what happens in summer when I've got gobs of extra power coming in? Can it damage the controller?

 

[akrvbob]

First, there is rarely a 12 volt panel, nearly all are at least 17 volt and today many are 36 volt and up. If you have a PWM controller all the voltage above 12 volt is simply lost, thrown away. If you have a MPPT controller it reduces the voltage and increases the amps so nothing is lost.

Where does it go if the battery can't take it? I have no idea, and I don't care! All I want is a full battery that lives a long life so I learn enough to make that happen and then I have a beer!

The answer to your question that will make that happen is to read your manual and see how many watts of panels it says the controller can handle and then stay below that. If it tells it to you in amps then divide the watts by actual volts of your panels and that is the amps. Get a controller that can handle that or a little more for a cushion.

The controller is designed to protect the batteries so most err on the side of giving the batteries too little amps and not too much. But each battery has a different preference for how many amps are fed to it in the bulk charge. Often, they want much more than the controller gives them. Unfortunately, we have little control over that. Sternwake can tell you much more about how to take control of it and get more life out of your battery.

Personally, learning all that and then doing all that cuts too much into my beer time, so I don't bother. I'd rather replace the batteries a year or two earlier.
Bob

P.S. the truth is I don't like beer, but it sure conveys my meaning well!

 

[highdesertranger]

I would match the controller to the panels or vice versa. but first you have to decide how much energy you need per day, then how much battery you need, then how much solar you need. highdesertranger

 

[amwbox]

I guess what's confusing me is that Morningstar states that their controller can handle 1200 watts at the nominal 24 volts from the solar panels...but in the same breath states that the same controller has a max current to the batteries of 45 amps.

As I understand MPPT controllers, they are related to a DC-DC converter, and step the voltage from the panels down to the appropriate charging voltage for the batteries. In my case its the nominal voltage from the panels, somewhere around 30 volts...which gets stepped down to around 14 volts for 12 volt charging purposes.

So, if the controller is taking in 1200 watts at 30 volts, and outputing 45 amps at 14 volts, then there is a bunch of missing power. Because 1200 watts stepped down to 14 volts would be around 85 amps.

I guess I should ignore their output amperage rating and just go by their input wattage/voltage rating?

As to sizing my loads and my battery bank, I'm in a weird situation because I'm trying to maximize winter rainy season power output...but I'm beginning to worry that when summer comes around the solar controller could be damaged by the panels...

And, Bob not liking beer has given this barley and hops swilling Portlander an extra grey hair.

 

[Optimistic Paranoid]

I think if you try to run too much input into one controller, it will fry.

Dig around on the Tech Support part of the Morningstar web site and you'll find info on running two or more Morningstar controllers in PARALLEL on the same battery bank.

Basically, you split your solar panels and hook half of them up to each controller.

Regards
John

 

[akrvbob]

I see the problem now. When you divide the 1200 watts, you don't divide it by 14, you divide it by the 24 volts of the panels, which equals 50 amps. So you have 50 amps coming into the controller from the panel where it cuts the voltage in half (which makes the amps only 25) but then it doubles the amps back to 50.

That's the magic of MPPT. It doesn't create power, it simply puts all the power your panel makes into the batteries. If you had a PWM controller it would only put in the 25 amps leftover after cutting the volts in half.

But a nominal 24 volt panel is probably actually 36 volt so it isn't even making 50 amps so it is safely under the 45 amp limit.

Bob

 

[amwbox]

I see the problem now. When you divide the 1200 watts, you don't divide it by 14, you divide it by the 24 volts of the panels, which equals 50 amps. So you have 50 amps coming into the controller from the panel where it cuts the voltage in half (which makes the amps only 25) but then it doubles the amps back to 50.

That's the magic of MPPT. It doesn't create power, it simply puts all the power your panel makes into the batteries. If you had a PWM controller it would only put in the 25 amps leftover after cutting the volts in half.

But a nominal 24 volt panel is probably actually 36 volt so it isn't even making 50 amps so it is safely under the 45 amp limit.

Bob

Ah.... That makes sense. Messing up my math again. :s

Thanks

 

[ccbreder]

What a shame to lose all that sun shine. You could build a bigger and then bigger battery bank until you capture all that sun shine. Or just wait for the next sunny day to charge the bank that matches your usage.