Adequate Panel Voltage for 24v Bank?

mryimmers · October 2015

Hi All,
I have been running my small system for almost 2 years now, 2X 255 watt panels to 12Volts into 4 Trojan 105RE, wired series / parallel which ran my Suresine 300 watt inverter. The system works great except for in the winter the inverter kicked out on high voltage due to temp compensation when the CC goes into ABSORB and the battery bank got below about -13C.(Suresine 300 is limited to 15.5 volts). I recently got an Exeltech XP250 to replace the Suresine as they have a higher cut-off voltage (16.5v on a 12v, 33v on a 24v). I got a 24v to eliminate the parallel strings. I have been running my panels in parallel, and it seems to be charging okay, these are the panel specs: Pmax 255w, Vmp 30.5v, Imp 8.35A, Voc 37.7v, Isc 8.87A. Should this be adequate to keep batteries charged, or should I put panels in series to boost the voltage up? My system is very lightly used and has never had any trouble keeping the batteries charged up.

Thanks

inetdog · October 2015

You left the CC details out.

30V Vmp is too high for a 12V system using a PWM CC, but OK for an MPPT CC. If you have a PWM CC, definitely do not put the panels in series as that would cut your available power even more. You do not need to put them in series with an MPPT CC as the voltage is quite high enough.

The 105REs are six volts each?

mryimmers · October 2015

CC is Rogue 3048 MPPT, in my sig, sorry

mryimmers · October 2015

Yes, Trojans 105REs are 6volts

inetdog · October 2015

mryimmers said:

CC is Rogue 3048 MPPT, in my sig, sorry

Then you do not need to put panels in series, but if you wanted to minimize wiring cost and avoid a combiner you could put them in series, up to the point that the sum of the Voc values in cold weather was still within the input range of the Rogue.

vtmaps · October 2015

inetdog, he has switched to a 24 volt system and is asking if an input voltage of 30.5 Vmp is high enough.

mryimmers, you should put the panels in series. The MPPT controller needs an input voltage that is several volts higher than its output voltage. When your batteries are cold (-13°C) they will need at least 31 volts. Of course when it is cold the Vmp of your panels also increases a bit, but not enough for the MPPT controller to have any headroom or voltage drop of its own.

--vtMaps

mryimmers · October 2015

Everything is in place already, and I have a combiner box, so I will just leave things as is then, it seems to be working ok. I was just wondering as the panel voltages I've been seeing is usually only a few volts over the battery voltage since I changed to 24volt. I have had as high as 40 volts in the dead of winter, or 80 volts in series, the Rogue 3048 max is 100v. Thanks!!

mryimmers · October 2015

vtmaps, haha, I see you answered me the same time I replied to inetdog, so you think I should series them? It would be within the limit for the Rogue. Thanks

Mountain Don · October 2015

Voc is 37.7. So 37.7 x 2 (in series) = 75.4. If we apply the 1.25 x 1.25 rule that gives .... 118 volts. The Rogue specs are for a max of 100 volts. So depending on where this is, how cold a winter, panels in series could be a problem with that CC on a cold bright morning. What do ya' think? Anyone know if the Rogue safely shuts itself off?

vtmaps · October 2015

Mountain Don said:

Voc is 37.7. So 37.7 x 2 (in series) = 75.4. If we apply the 1.25 x 1.25 rule that gives .... 118 volts. The Rogue specs are for a max of 100 volts.

For cold Voc why do you multiply by 1.25 twice? I design with just a single 1.25 multiplier for Voc.

--vtMaps

mryimmers · October 2015

Would the 1.25 rule not only be applied 1x? That would be 94.25v.? When I was buying all the parts for my system, I talked to Marc, the Kingpin at Rogue and allowing for temps down to -40C he said I would still be okay. I have run them in series and the highest voltages that were recorded were 80v in the dead of winter. I paralleled the panels because I had all the necessary parts and I like to play with things, and I thought it would be "easier" on the CC when I had a 12v system. In the header of this post it says I am a solar expert, HAHA, I certainly am not!

Mountain Don · October 2015

vtmaps said:

For cold Voc why do you multiply by 1.25 twice? I design with just a single 1.25 multiplier for Voc.

--vtMaps

OOOOps! I made a mistake. Multiply once for voltage. Multiply twice for Amperage rating for wire.

Very sorry about my carelessness.

inetdog · October 2015

Mountain Don said:

vtmaps said:

For cold Voc why do you multiply by 1.25 twice? I design with just a single 1.25 multiplier for Voc.

--vtMaps

OOOOps! I made a mistake. Multiply once for voltage. Multiply twice for Amperage rating for wire.

Very sorry about my carelessness.

Your advice was not totally out of line, just wrong in this context.
If the specification for Voc is not immediately available you can approximate it as 1.25 times the Vmp voltage. So you will sometimes use that as your first factor of 1.25.
Then you can either use the known temperature coefficient of Voc and worst case temperature to calculate maximum voltage or you can get a conservative (except for Alaska!) multiplier of 1.25.

You are right in your recognition that the Imp and Isc figures are first multiplied by 1.25 to allow for cloud or snow effects increasing the output above the STC numbers. And the second 1.25 there (in the NEC) is because the current will be continuous (full value for three or more hours.)

vtmaps · October 2015

inetdog said:
You are right in your recognition that the Imp and Isc figures are first multiplied by 1.25 to allow for cloud or snow effects increasing the output above the STC numbers. And the second 1.25 there (in the NEC) is because the current will be continuous (full value for three or more hours.)

And the second 1.25 multiplier (for current) is not needed if your circuit breakers are designed to carry their rated current continuously.

--vtMaps

mryimmers · October 2015

Ok, thanks guys!!! I think I'm going to series them next time I'm out there, thanks again!

inetdog · October 2015

vtmaps said:

inetdog said:
You are right in your recognition that the Imp and Isc figures are first multiplied by 1.25 to allow for cloud or snow effects increasing the output above the STC numbers. And the second 1.25 there (in the NEC) is because the current will be continuous (full value for three or more hours.)
And the second 1.25 multiplier (for current) is not needed if your circuit breakers are designed to carry their rated current continuously.

--vtMaps

More properly the breaker and the enclosure it is mounted in have to be tested together to allow 100% rated operation.

Adequate Panel Voltage for 24v Bank?

Comments

Categories