Proposed off-grid design attached - would love your input

[max2k]

sensij thanks for that explanation, you're right - a disconnect may be more to the point here. I see what you're saying here.

max2k the CC can take 100v input, which is well above the 75v max of those two panels in series. Good point you make on the battery capacity, draw could even exceed 15A if the inverter plus the CC's 15A load output is being used for lighting. Is there a general rule of thumb for max battery input/output as a percentage of total Ah capacity?

I swear I saw on some page 75V although the name of CC hinted it could take 100V. Of course you'll be fine with your 2 panels even at cold temperatures: 82V at -15C.
The recommended max charging current for Trojan deep cycle FLA is 10-13% of their C20 value. Continuous max discharge current better be around the same level. Batteries will survive spike of higher discharge current values at the expense of reduced output voltage and reduced discharge capacity. If your CC has 15A charging current then min capacity should be 15/.13 = 115 Ah or 150Ah for 10% level.

[tombarton]

sensij thanks for that explanation, you're right - a disconnect may be more to the point here. I see what you're saying here.

max2k the CC can take 100v input, which is well above the 75v max of those two panels in series. Good point you make on the battery capacity, draw could even exceed 15A if the inverter plus the CC's 15A load output is being used for lighting. Is there a general rule of thumb for max battery input/output as a percentage of total Ah capacity?

[max2k]

@max: The CC in the layout diagram is actually up to the task. I was asking if we could step down by 25V. I'm not sure how much more 'coming up with a design' the above picture is!

unless I'm missing something- your CC doesn't have high enough input voltage to tolerate 2 panels in series but single panel voltage is not enough to charge 24V batteries. It also has too high charge current for batteries used and your inverter has too high current draw for your batteries. Even single one of these problems enough to either kill your batteries prematurely or prevent system from working. I think you need to use different controller with higher input voltage and deep cycle batteries with at least 150Ah capacity due to current levels involved.

[sensij]

@ mike and @ sensij: yep, I guess on the safe side I'll keep 'em in series for now. The risk is if one malfunctions, we're out of power. As opposed to parallel, where we'd just get half the current. May need to swap the panels out at some stage for true 24VDC, for the sake of reliability.

sensij do you think I should have a smaller-rated breaker on the panel side? You're correct: the main purpose of this is to isolate the panels easily. But, if I should fuse them, then I will. Advice appreciated.

@max: The CC in the layout diagram is actually up to the task. I was asking if we could step down by 25V. I'm not sure how much more 'coming up with a design' the above picture is!

The panels have bypass diodes, so if you were to lose a cell somehow, the 1/3rd of the panel containing that cell would be bypassed, and the series string is otherwise intact.

No reason to drop the 20 A to anything smaller. Even if you short circuit the DC+ and DC- at that point, the current is limited to the panel's Isc, which is no risk to the wiring and doesn't hurt the panel. In other words, overcurrent protection is not needed there at all, so a simple disconnect would be fine instead of a breaker.

[tombarton]

@ mike and @ sensij: yep, I guess on the safe side I'll keep 'em in series for now. The risk is if one malfunctions, we're out of power. As opposed to parallel, where we'd just get half the current. May need to swap the panels out at some stage for true 24VDC, for the sake of reliability.

sensij do you think I should have a smaller-rated breaker on the panel side? You're correct: the main purpose of this is to isolate the panels easily. But, if I should fuse them, then I will. Advice appreciated.

@max: The CC in the layout diagram is actually up to the task. I was asking if we could step down by 25V. I'm not sure how much more 'coming up with a design' the above picture is!

[sensij]

bummer. You are at 71Voc on a warm morning, If you get some chilly mornings, you Will be over the 75V limit. Voc happens first thing in the morning, as the panels wake up the controller, and before the controller boots enough to load the panels down, you fry it.

Peak Voc isn't quite first thing in the morning, and by the time you have enough irradiance to get there, cell temp will be starting to rise, and the controller should be active. That said, it is a convenient fiction that keeps designs on the safe side of the limit, and clearly, 71 Voc @ STC on a 75 V max input CC is a bad combination when temps approaching freezing are realistic.

[max2k]

Nope, 29V panels cannot charge a 24v battery that needs 30V to be charged. Charge controllers need some voltage overhead to work, and MPPT needs at least 1.5x the battery voltage

my bad, then CC seems not to be suitable for the task. I think OP needs to come up with entire design first and only then start buying stuff or it's getting expensive real quick.

[Mike90250]

I think OP can still connect panels in parallel to get under CC input voltage limit......

Nope, 29V panels cannot charge a 24v battery that needs 30V to be charged. Charge controllers need some voltage overhead to work, and MPPT needs at least 1.5x the battery voltage

[sensij]

I think OP can still connect panels in parallel to get under CC input voltage limit.

Yeah, but then the voltage is too low to sustain the battery... those panels won't get the battery to 28 V in summer heat (without sacrificing a lot of power to move up off the mpp). Probably cheaper to replace the charge controller at this point... the schematic already shows the 100/15 which would be OK here.

The 20 A breaker between the panels and CC isn't adding any safety, only convenience for disconnecting the panels from the CC.

[max2k]

bummer. You are at 71Voc on a warm morning, If you get some chilly mornings, you Will be over the 75V limit. Voc happens first thing in the morning, as the panels wake up the controller, and before the controller boots enough to load the panels down, you fry it.

Also, research the Load output of the controller, 16A may be over it's limit

But I'm not sure what sort of advice I can offer now.

I think OP can still connect panels in parallel to get under CC input voltage limit. I think he is short on batteries if those are deep cycle FLA: 92Ah can reliably sustain about 10A charge/discharge currents and his CC (if it doesn't have charge current limiting feature) will dump 15A there. His inverter will also draw 14.6A with spikes up to 29A.

[tombarton]

Aha, gotcha - so that's why VOC is still important. Thanks for the explanation of how it works in a practical sense. We get hot weather in Summer (over 104F), and in Winter we just stayed there and a bucket of water froze over outside!

Re the output, I'm planning on having 15A max load connected to the controller - the 16A is the closest DC breaker I can find, which will allow isolating of the lighting if required. Each light circuit is individually fused too with blades (because I'm a 'be prepared' type!)

[Mike90250]

bummer. You are at 71Voc on a warm morning, If you get some chilly mornings, you Will be over the 75V limit. Voc happens first thing in the morning, as the panels wake up the controller, and before the controller boots enough to load the panels down, you fry it.

Also, research the Load output of the controller, 16A may be over it's limit

But I'm not sure what sort of advice I can offer now.

[tombarton]

Hi all, I've been lurking for some time. Setting up a system for infrequent visits to a bush property in Victoria, Australia. Power requirements are low: about 14-20Ah per day @ 24v. Mostly lights, some tool charging & music. 2/3 days of autonomy. Unobstructed north-facing roof gaining an average of 1.1 kwhm2 in Winter to 6.6 kwhm2 in Summer.

I did what the usual beginner did - went arse-about and bought the panels first!

Turns out they're not true 24VDC panels, so though I'd originally intended on wiring them in parallel, advice has been to wire them in series to achieve high-enough charging voltage. We want 24VDC storage as there are some longer cable runs required. I thought I'd still be under the 75V max of the Victron 75/15 with them in series anyway (doubling VMPP, not VOC) but it appears I am mistaken, and that you double VOC in series (even if this is the load-free voltage measurement)?


Design layout attached. Would love to hear your input on any aspects of the system at all.


Tom solar-setup-layout.jpg