Rookie here, help me design a small 24v system starting from the batteries.

[SunEagle]

Was curious to check the home load path for a backup system of 8 hours for a 'small' load for essentials.

For a 300W load at 24V, It would require a 12.5A (rounded 15A) of DC current. If my math is correct(300W*8H)/24V, that would require a 100Ah battery capacity.

It's recommended to charge the batteries at the current of 10% of its capacity? That's 100Ah*0,1= 10A charging current. (valid for LifePO4 too? )

Solar panel bit as far as I've understood it. Solar Array Current, Battery charging current +load current, 10A + 15A = 25A. So, apparently, it's said that Solar Array power is V*I, 24V*25A = 600W of solar.

Here's the bit It gets confusing for me.

If it's recommended to charge the battery with a current of 10% of its capacity which is 10A.

If I understand correctly, CC(charger) will always take solar W / Battery V = Charging current?? 600/24= at 25A?

Am I missing something or did I mess something up here? It sure looks contradictory to the recommended battery charge current. ?

As always, I appreciate the help.

That 10% is for Lead Acid type batteries. I am not sure how many charging amps the LifePO4 requires.

Your are correct in your math to determine the CC rating of being at least 25A. Now a 600 watt panel may be too much for a 100Ah Lead Acid battery but may be fine for the LifePO4 type. Each system requires a balance of charging amps and battery Ah rating. I would look into what those batteries like in charging amps and work from there.

Again just because the panel is rated a certain wattage does not mean it will produce that amount even in direct sunlight. But too many charging amps could hurt a battery. So you really need to understand what your panels & CC are doing or you can hurt the batteries.

[Daxo]

That 10% is for Lead Acid type batteries. I am not sure how many charging amps the LifePO4 requires.

Your are correct in your math to determine the CC rating of being at least 25A. Now a 600 watt panel may be too much for a 100Ah Lead Acid battery but may be fine for the LifePO4 type. Each system requires a balance of charging amps and battery Ah rating. I would look into what those batteries like in charging amps and work from there.

Again just because the panel is rated a certain wattage does not mean it will produce that amount even in direct sunlight. But too many charging amps could hurt a battery. So you really need to understand what your panels & CC are doing or you can hurt the batteries.

That makes a lot of sense. Thank You.

I've dug out info on the LifePo4 battery I'm considering buying. It seems that at a bare minimum anything below 0.5C is a must, yet, 0.2C is recommended.

With that said, looks like a 200Ah would be a much better fit overall.

[SunEagle]

That makes a lot of sense. Thank You.

I've dug out info on the LifePo4 battery I'm considering buying. It seems that at a bare minimum anything below 0.5C is a must, yet, 0.2C is recommended.

With that said, looks like a 200Ah would be a much better fit overall.

OK. But that 0.2C charge rate would require 40amps for a 200Ah system (200ah x .2 = 40) So make sure you have a CC and panels rated for that amount.

[Daxo]

OK. But that 0.2C charge rate would require 40amps for a 200Ah system (200ah x .2 = 40) So make sure you have a CC and panels rated for that amount.

Good advice, Appreciated!

Was thinking along that line, guess now I'll need to make the decision on what gear to start with.

[Daxo]

OK. But that 0.2C charge rate would require 40amps for a 200Ah system (200ah x .2 = 40) So make sure you have a CC and panels rated for that amount.

Btw, it's probably a silly question, but what happens if the solar panel delivers a bit more current(up to 5ish A) than the CC is rated for?

Does the CC ignore the available excess of current and provides only its rated A to batteries? Is this perhaps what is referred to when folks use the term, over paneling so there's as little loss of power in winter time compared to excess power at peak hours during summer?

Appreciated.

[SunEagle]

Btw, it's probably a silly question, but what happens if the solar panel delivers a bit more current(up to 5ish A) than the CC is rated for?

Does the CC ignore the available excess of current and provides only its rated A to batteries? Is this perhaps what is referred to when folks use the term, over paneling so there's as little loss of power in winter time compared to excess power at peak hours during summer?

Appreciated.

It depends on the CC. A quality one may just "clip" and ignore the excess current. A cheaper CC may burn out. Some people find a way to divert the excess amps but I think that is risky.

[Daxo]

It depends on the CC. A quality one may just "clip" and ignore the excess current. A cheaper CC may burn out. Some people find a way to divert the excess amps but I think that is risky.

I've checked a bit more about Victron SCC, and apparently, it's possible to set the max charging current limit. (something I was previously concerned about)

With that said,
as far as I can see this means I don't have to worry about the bifacial PV solar array total W potential (in relation to I=P/V ) as long as the SCC V and A ratings are respected. (ideally with a bit of headroom)

So in any given weather condition, If I use 2x 455W(theoretical) with an I potential of 35A I can set the SCC's max charging current limit at 20A and the SCC will allow only 20A for the battery charging.


For now,
I'm not certain if limiting the battery charging current also denies using the excess for load at the same time, but it does solve the bit with 'over paneling'. (If I've guessed correctly that this might be what folks refer to as over paneling?)



Btw, what tools I'll need to prepare my new battery for use, Voltmeter, and a DC charger? Any recommendations on good ones out there?

Doesn't have to be anything fancy.

Appreciated.

[Daxo]

It depends on the CC. A quality one may just "clip" and ignore the excess current. A cheaper CC may burn out. Some people find a way to divert the excess amps but I think that is risky.

Thought to share my progress on my first small solar build, I would really appreciate it if you could share your thoughts on the fuses, busbar, and wiring.

Would you do something differently? Thanks

Much obliged.

Battery - 4AWG/25mm2 - 150 ANL fuse
SCC - 10AWG/6mm2 - 30A ANL fuse
Inverter - 4AWG/25mm2 - 100A ANL fuse
Daxo'sBuild.png

[SunEagle]

Thought to share my progress on my first small solar build, I would really appreciate it if you could share your thoughts on the fuses, busbar, and wiring.

Would you do something differently? Thanks

Much obliged.

Battery - 4AWG/25mm2 - 150 ANL fuse
SCC - 10AWG/6mm2 - 30A ANL fuse
Inverter - 4AWG/25mm2 - 100A ANL fuse
Daxo'sBuild.png

Any bare terminals or connection needs to be covered or someone could get shocked. Also I am not sure how those components are wired that are mounted to the box. If it is going to take a few minutes to unscrew the box then IMO it should be rewired so every terminal and wire can be accessed.

[Daxo]

Any bare terminals or connection needs to be covered or someone could get shocked. Also I am not sure how those components are wired that are mounted to the box. If it is going to take a few minutes to unscrew the box then IMO it should be rewired so every terminal and wire can be accessed.

I'll definitely take that into consideration! Thanks.

I've used machine screws with nuts on the other side to secure the components to the box. But some wires do go through the walls so that is indeed at least a few minutes if I'd need to replace it etc.

[Daxo]

Any bare terminals or connection needs to be covered or someone could get shocked. Also I am not sure how those components are wired that are mounted to the box. If it is going to take a few minutes to unscrew the box then IMO it should be rewired so every terminal and wire can be accessed.

Update: still need to work out a few bits and pieces, but its getting there. I'll be replacing the battery wire with 2AWG instead, t fuse on main and evetually replace the negative busbar with a smartshunt, and make a cover for the fuses and relevant mounting studs.

Daxo'sBuild.png

[Daxo]

Success!!

I've tested the panel, and although installed vertically, flat, and far from ideal orientation it yielded 235W(70% rated power) at peak, storing 180Wh in the battery within one hour of the test run, shading included.

Once the shade was covering one-half of the panel, it continued to produce 132W. which is good to see meaning the panel is working as intended.

Next time, I'll orient the panel to the sun and see how close I can get to the rated 335W.

There are still a few bits and pieces coming in for the build upgrade/optimization but it's very rewarding to see the system powered up and running for the first time after about 3 months from knowing nothing to knowing very little about solar.

Good time as any, Special Thanks to SunEagle for taking the time to help me connect the dots after doing as much homework as possible for a newbie. Truly, a pivotal point, and invaluable help during my learning of the basics behind solar.

Much obliged!!

Power.png

[SunEagle]

Your welcome. Keep us informed if anything goes wrong

[J.P.M.]

Success!! Next time, I'll orient the panel to the sun and see how close I can get to the rated 335W.[

Without a pyranometer to measure irradiance input it'll be a rough estimate.
Depending mostly on irradiance level, and secondarily on cell temp., expect something between maybe 70% and 85% or so of the panel's STC rating.
Keep the panel covered with something opaque and white to minimize panel temperature effects until you measure the panel output.