Classic rating

[asdex]

Hi, can you tell me if the current rating of the classic depends on the incoming from the pv array or the outgoing to the batteries? Therefore is it best to increase PV voltage or increase battery bank voltage to keep away from the maxium rated current?
Thanks,

[Vic]

Hi asdex,

Am not certain just what you are asking ...

Have you seen this Classic Rating Chart(?):
http://www.midnitesolar.com/images/classicFrontPage/graphs.php

Am certain that you know that as you increase the battery voltage from 12,  to 24 V,  and from 24 to 48 V battery voltages,  the maximum POWER from a single Classic will almost double.

And also in the above-linked chart,  you will notice that as the PV input voltage is increased,  the amount of output current from the Classic (or really any MPPT Charge Controller) will be reduced,  due to some decrease in efficiency.

Does this respond,  in any way,  to what you were asking?    FWIW.   Vic

[Rybren]

The maximum current rating for most charge controllers is, I believe,  based on the output of the CC.  For example , a 30A CC will push a max of 30A to the batteries. Is this what you were asking ?

[asdex]

I mean if my panels were set up to deliver 96 amps at 24 volts, I could change the wiring configuration to deliver 48 amps at 48 volts.
Or if the full charging current to a 24 volt battery bank is 96 amps, I could change the battery system to a 48 volt bank and the charging current would be 48 amps.
Which option is related to the Classic full load current?
From the graph, it looks like it’s the input current that needs to be kept under 96 amps so higher the PV array voltage the better.
So does that mean I can have an input current at 96 amps from a 48 volt PV system charging a 12 volt battery bank at a much higher current than 96 amps?
Thanks,

[Vic]

Hi asdex,

As Rybren noted,  CCs are usually rated for Output current.

You choose a battery voltage,  configure your PV modules to allow the battery to be efficiently charged,  and let the Input current fall where it will.

Here is the Classic String Sizing Tool:
http://www.midnitesolar.com/sizingTool/displaySizing.php

You will need to choose a string configuration -- number of modules per string and number of strings -- that will efficiently charge your battery.

The Classic has an output current capability of 96 Amps,  only on 12 V battery systems.

You are correct to wonder about increasing your battery voltage,  as you appear to have four parallel strings of batteries.  This will result in charge/discharge imbalances,  in time,  that will shorten battery life,  without constant vigilance.

From your Signature,   it is not clear that all of your PV modules are able to work well together.  What are the Vmp and Imp ratings of each of the different PVs that you have?  How are the strings of these PVs currently configured?

Thanks for any added information.     Vic

[asdex]

Hi, thanks for the info.
My batteries are getting old so I plan to buy a new set and feed them with the new 6 x 320 watt panels. So from what you say, I think I'll have series connected batteries at 24 volts. (4 x 6 volt batteries) From the calculator I can connect my panels in 3 strings of 2 panels and the current in will be max 26 amps and out to the batteries will be 66.7 amps.
I'll have to check on cable size for 26 amps as I have 15 meters to the controller. I already have the cable so if it's too small I may have to go to 48 volts.
My cable is 16 mm sq which is about 5 AWG
Thanks,

[asdex]

I have checked the voltage drop at 20 meters and 73 volts which is the VMP and current at 26 amps.
Would the below figures be an acceptable voltage drop?
Thanks,

Voltage drop: 0.80
Voltage drop percentage: 1.10%
Voltage at the end: 72.2

[boB]

It is best NOT to think about current from the PV array...

When thinking about PV current, it tends to make things way more complicated that it needs to be,
especially for MPPT because the current from the PV depends on the voltage of the PV and the
power.

Instead, take the power of the PV array and divide this by your battery voltage which is pretty
much constant.  That will give you controller output current to the battery.  Then things
will be more of an apples vs. apples scenario.

For instance, if you have a 1000 watt PV array and a 12V battery,  1000W / 12V = 83 and 1/3 amps.

Now, if you keep the same amount of power but double the output voltage, 1000W / 24V = 41 amps or so.

48V would be about 20 amps, etc.   This assumes 100% efficiency and all that which hardly happens but
it is still a better way to think about this.

boB

[asdex]

Ok so my pv array is 1920 watt divided by 24 volts (battery bank) that's 80 amps. Is that what I need to use in my voltage drop calculation for cable between pv array and controller?
Thanks,

[vtmaps]

From the graph, it looks like it’s the input current that needs to be kept under 96 amps so higher the PV array voltage the better.
So does that mean I can have an input current at 96 amps from a 48 volt PV system charging a 12 volt battery bank at a much higher current than 96 amps?

No.  The 96 amp rating is the output current to the batteries... it has nothing to do with the input current from the array.

You want to configure your array so that the string voltage (Vmp) is a few volts above the maximum charging voltage.  For example, if your battery is a 24 volt battery and your equalization voltage is 31 volts, you want your array Vmp to be a few volts above 31...  A dozen volts is good because it gives the controller some headroom to do a MPPT sweep.

As the input voltages increase, the maximum output current, efficiency, and power handling capacity of the Classic decreases.

--vtMaps

[asdex]

I see thanks. To get the voltage low from the panels I can connect them all in parallel and the voltage is 36.91 volts but the current is 52 amps. What figures would I use to calculate voltage drop in the cable between panels and controller?
Also, if it's best to have a low voltage but obviously above battery voltage are most pv panels connected in parallel?
Thanks,

[vtmaps]

I see thanks. To get the voltage low from the panels I can connect them all in parallel and the voltage is 36.91 volts but the current is 52 amps. What figures would I use to calculate voltage drop in the cable between panels and controller?

Use the Vmp and The Imp.  I don't think all 6 in parallel is a good idea on a 24 volt system for two reasons... I think the Vmp of 36.9 volts is workable, but is a bit low for a 24 volt battery.  Also with 15 meters you would need #2 cable to get your voltage drop down to 2%. 

If you put the panels in three strings of 2 panels per string, you will only need #8 cable. 

--vtMaps

[asdex]

Thanks, 3 strings of 2 does look ok but the voltage is 73.82 volts which hopefully isn't too high to be in the poor efficiency range. I'll checked drop with the Vpm and Rated PV Array Current (couldn't see Ipm) and it's:
Voltage drop: 1.07
Voltage drop percentage: 1.45%
Voltage at the end: 72.75

Is this acceptable?
Cheers,

[vtmaps]

Thanks, 3 strings of 2 does look ok but the voltage is 73.82 volts which hopefully isn't too high to be in the poor efficiency range.

74 volts on a 24 volt system is a good design.  74 volts on a 12 volts system is unnecessarily inefficient.

Voltage drop: 1.07

Is this acceptable?

Very acceptable   

--vtMaps

[asdex]

Thanks very much for that info. I'll be able to carry on getting the system together.
Cheers,