Re: C40 or TS45 -- max. voltage question
PWM controllers like the Morningstar TriStar and the Xantrex C40 limit the bulk-stage array voltage to that of the battery bank. In fact, it’s essentially the battery itself that sets the array voltage. If the batter bank’s end-of-bulk stage target voltage is, say, 58 V, then the array will operate at that voltage (plus a bit more for losses in the wiring and inside the controller), irrespective of Vmp specs.
Accordingly, a 68 V operating voltage spec is actually a fairly high voltage limit for a system with a PWM controller and a "48 V" battery bank. In fact, the only time you’d likely see such a value would be when equalizing very cold batteries. Instead, the real challenge is to configure a “48 V” array that can deliver ~60 V on a hot summer day.
It’s likely that the actual array voltage will operate higher than the battery voltage when the controller is functioning in current limit mode (absorb, float, or most of EQ). In this case, the array voltage will be the PWM duty-cycle weighted average of the battery voltage and the array’s Voc.
For example, if the PWM duty cycle is 50%, the battery voltage is 58 V, and the array Voc is 70 V, then the array’s operating voltage would be (58 V x 50%) + (70 V x 50%) = 64 V. But, since the Voc is below the 125 V limit, this isn’t a problem, although this is when the controllers warm up a bit.
The 125 Voc limit shared by the TriStar and the C40 is sufficient, in my view, for most “48 V” PV applications. Allowing for the NEC’s Table 690-7 125% temperature correction factor for ambient temperatures down to -40 (C or F), this means that the array’s STC Voc should not exceed 100 V. This in turn means that the array’s STC Vmp should not exceed ~80 V, as Vmp is typically ~80% of Voc. However, target absorption and EQ voltages might be difficult to achieve in the summer if you live in a particularly hot area.
So, as long your planned array voltage spec is between 70 V and 80 V STC, then I believe either controller should generally work satisfactorily, although achieving the right window of minimum hot weather Vmp and maximum cold weather Voc can be a delicate dance for a “48 V” system.
Jim / crewzer
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