C40 or TS45 -- max. voltage question

**khotton** · December 3rd, 2007, 7:00 PST

I am spec-ing a 48V system and was initially planning to use the Morningstar TriStar-45. However, I now have a concern. In the TriStar manual is states that maximum operating voltage is 68VDC. This seem too low to me for a 48V PV array. Many standard "12V" panels have a peak power voltage of 17.5V (and "24V" panels are 35V) and this results in a operating voltage of 4*17.5 = 70VDC (i.e. greater than the TS-45 allowed maximum of 68VDC). If this is the case, how can the TS-45 be consider a viable 48V system charge-controller?

With this in mind, I wonder what the allowable maximum operation voltage for the C40 charge-controller is? Both the TS-45 and C40 list the same maximum open-circuit voltage of 125VDC. Do any of the experts on this forum know what the C40 charge-controller's maximum operating input voltage is (hopefully over 68VDC)?

Thanks,
Kevin

**icarus** · December 3rd, 2007, 9:12 PST

According to the xantrex site:
"Max. PV Open Circuit Array Voltage 55 VDC"

On the other hand, the Morningstar tech spec sheet says: •" Max. solar voltage (Voc) 125V "

Am I missing something?

Icarus

**khotton** · December 3rd, 2007, 9:37 PST

I think you are refering to the C35 or C60 models. The C40 is the only one rated for 48V operation and the Xantrex manual references to the C40 does state 125V (Voc).

Kevin

**crewzer** · December 3rd, 2007, 9:37 PST

Kevin,

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.

HTH,
Jim / crewzer