lower string voltage worse in low light?

[Will-UK]

Hi all
my off grid system I'm building for my cabin.

6 x 250W Intenergy mono panels. 38voc (planning to add another 6)
Outback FM80 Charge Controller.
32x 38AH gel battery wired to 48V (8 in parallel, four lots) = 300AH (14.5 kwh)
APC 3000 UPS (48V)

I'm trying to decide whether to have 3 PVs (114v) in each string or two PVs. (76v)

Am i right in saying a PV voltage closer to the battery voltage will increase efficiency?

And will I lose the charging power quicker in the evening with lower string voltage as it will get down nearer to the battery voltage quicker?

Also a question about the PWM % display on the Outback, Am I right in saying if it is at say 60%, it is disconnecting the PV 40% of the time? Therefore not being very efficient? (presumably the PWM figure on the display the higher the better?) Thankyou!

[lkruper]

Hi all
my off grid system I'm building for my cabin.

6 x 250W Intersun panels. 38voc (planning to add another 6)
Outback FM80 Charge Controller.
32x 38AH gel battery wired to 48V (8 in parallel, four lots) = 300AH (14.5 kwh)
APC 3000 UPS (48V)

I'm trying to decide whether to have 3 PVs (114v) in each string or two PVs. (76v)

Am i right in saying a PV voltage closer to the battery voltage will increase efficiency?

And will I lose the charging power quicker in the evening with lower string voltage as it will get down nearer to the battery voltage quicker?

Also a question about the PWM % display on the Outback, Am I right in saying if it is at say 60%, it is disconnecting the PV 40% of the time? Therefore not being very efficient? (presumably the PWM figure on the display the higher the better?) Thankyou!

I read about your charging problems in another thread and now I see you are off-grid. If the APC is not charging your batteries, and you don't need something to switch from grid power to battery power automatically, what is the APC doing for you?

[Amy@altE]

Those are nominal 20V panels. In order to have a high enough voltage to charge your 48V battery bank, you have to have strings of 3 in series for nominal 60V. The charge controller will then drop it down to the nominal 48V for the battery bank. You will not fully charge the 48V battery bank if you attempt to do strings of 2.

[Raul]

Hello Will
Your controler is the flex max 80? If so the mppt range is 60-120v . As already mentioned 2 strings of 3 panels will work wonders. That's exactly what my set up is with different controler for 48v.

[sensij]

Am i right in saying a PV voltage closer to the battery voltage will increase efficiency?

Yes, but the effect with MPPT controllers is not as significant as it is with PWM controllers. The tradeoff is that at lower voltage, you'll have higher conductor losses. As others have stated, strings of three would be the right choice for this setup.

And will I lose the charging power quicker in the evening with lower string voltage as it will get down nearer to the battery voltage quicker?

Lower sunlight reduces current, it does not reduce voltage. With either string configuration, the charging power available from the panels during bulk would be the same, as long as the voltage is in the MPPT range. Again, as others have pointed out, with a 2 panel string, you will probably fall out of the range.

Also a question about the PWM % display on the Outback, Am I right in saying if it is at say 60%, it is disconnecting the PV 40% of the time? Therefore not being very efficient? (presumably the PWM figure on the display the higher the better?) Thankyou!

In the absorb or float stages, your MPPT controller may operate in PWM mode instead of MPPT to hold the voltage constant. However, lower efficiency in those stages is not going hurt so much.

[inetdog]

...
Lower sunlight reduces current, it does not reduce voltage. With either string configuration, the charging power available from the panels during bulk would be the same, as long as the voltage is in the MPPT range. Again, as others have pointed out, with a 2 panel string, you will probably fall out of the range.
....

True but not complete.
Lower insolation will decrease both Vmp and Voc slightly. The effect is almost unnoticeable for Voc until you get to the light level where the internal leakage resistance of the panel is drawing down the apparent Voc measured at the terminals.
As for Vmp, if you have a fixed resistance load (or a PWM CC in Bulk mode) the lower light will cause the measured panel voltage to be below Vmp as the load attempts to draw a current which is higher than the Imp for that light level. At some point the current drawn will pull the panel voltage down to where the battery is not able to draw the current the CC is trying to achieve.

[sensij]

True but not complete.
Lower insolation will decrease both Vmp and Voc slightly. The effect is almost unnoticeable for Voc until you get to the light level where the internal leakage resistance of the panel is drawing down the apparent Voc measured at the terminals.

On that scale, lower insolation will also lead to lower cell temps, which tends to raise the voltage. Would the effects pretty much cancel out?

As for Vmp, if you have a fixed resistance load (or a PWM CC in Bulk mode) the lower light will cause the measured panel voltage to be below Vmp as the load attempts to draw a current which is higher than the Imp for that light level. At some point the current drawn will pull the panel voltage down to where the battery is not able to draw the current the CC is trying to achieve.

Yes... it wasn't clear that I was only referring to MPPT operation.

[Will-UK]

Thanks for replies. The Vmp of the panels is 30v

Today I have connected them in strings of three. (Two strings.. 1500w total)
Todays measured peak watts was 1400, while the washing machine was on. (Washing machine was consuming up to 1700 watts, mostly becuase of the heater but I then switched it to a cold wash as it was hammering the batteries.) Good test though.

I already have 8 panels so if I buy just one more I can have 3 x 3. (2250Wp)

Not got as far as mounting any yet

[Will-UK]

@lkruper
Repurposing an old computer UPS like that is a good way to get a very well built pure sine inverter, for very little money. Also it can be used to smooth out a backup generator. I am typing this now from my office which is running from my battery bank using the UPS to make my AC power.

[SunEagle]

Thanks for replies. The Vmp of the panels is 30v

Today I have connected them in strings of three. (Two strings.. 1500w total)
Todays measured peak watts was 1400, while the washing machine was on. (Washing machine was consuming up to 1700 watts, mostly becuase of the heater but I then switched it to a cold wash as it was hammering the batteries.) Good test though.

I already have 8 panels so if I buy just one more I can have 3 x 3. (2250Wp)

Not got as far as mounting any yet

Aren't you worried about a high wind taking those panels for a "trip" or do you have a solid attachment on both sides of each panel?