Sizing a Charge Controller –

18 Posts
Aug 27, 2008 07:23 am
Sizing a Charge Controller –

I am designing a 3KW, ‘24V’ off-grid solar project.  The way I see it, to float the batteries at the proper voltage (28.8VDC) I calculate the current to be roughly 105 amps at peak. 
The planned battery bank will be 1150AH.
Is this calculation close? 
What size MPPT charge controller do I need? . .
Can I bridge them together in parallel if need be? . .
Still trying to wrap my head around this. . .
Thank you.
 
Aug 29, 2008 08:35 am
Re: Sizing a Charge Controller –

The size of the charge controller is determined by the total short circuit amperage (Isc.) rating of the PV array plus 156%.
3000 watts at 24 volts would be 125 amps, but the Isc. will be  higher than this.
Just as an example, lets assume the Isc. totals up to 135 amps. According to the NEC 125% more must be added to that 135 amps, in addition to the 125% that UL requires. 125% X 125% = 156%.
135 X 156% = 211 amps. Believe it or not! This would be the amperage used to calculate; Wire type and size, disconnects, fuses and or breakers, and charge controllers.

Assuming this same PV array in the example above...
@ 48 volts = 106 amps
@ 125 volts = 41 amps
This is the splendor of MPPT technology. You can utilize a higher PV array nominal voltage to charge a battery bank of a lower nominal voltage.
So to answer those questions, it depends on the nominal voltage of the PV array.
1105 amp hours @ 24 volts nominal battery bank.
1105 / 5 = 221 amp hours.
On a good day, a 3000 watt PV array @ 24 volts nominal should produce somewhere around 375 amp hours, conservatively.
3000 X 3 / 24
Of course your mileage may vary.
 
 

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