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I have RGB solar spot lights in my yard that come in a set of 3 parts each, which connect together to form the system.

  1. Solar Panel rated at 6 V / 4 W output.
  2. Battery Charge Controller, this one has a 18650 battery pack with 2 batteries wired in parallel for 5000 mAh, it has 8205A and DW01A chips for battery protection. The output from the batteries only turns on when there's no input from the solar panel (great for landscape lights). This does nothing but output DC power to the next piece when there's no solar input.
  3. Four spotlights chained together, each one a PCB with an unbranded MCU and an antenna to control it with an RF remote, and an RGB led. These only take in DC power and each light handles the color changes on its own.

Each part of the system has a waterproof 2-pin connector, so they can be disconnected and replaced. These need 6 to 8 hours to get enough charge to light them up for a good 6 hours.

There are only two spots in my yard that get sunlight for that long, both are in far away corners of my house. The lights I have there will get enough power to stay on the longest, while the others will last 1 to 2 hours.

As an experiment, I ran a long outdoor-rated wire to where the most sunlight is from a darker area. I wired two solar panels in parallel and connected two of the battery packs to that wire. As expected, it works: both lights get power and charge. The battery pack with an additional 10 ft extension cable is running shorter than the one without. Only connecting one battery pack, as expected, leads to a longer runtime.

With this in mind I'm considering two options:

  1. Get 8 or 12 W panels for each set of lights. This would at least provide more juice so the batteries can charge faster but it wouldn't solve the issue in the areas that get less sunlight would still have some lights on and some off at different times of the day unless I run long extensions for each panel.

  2. Get a 12 V 50 W solar panel which would be placed on the sunny side, then run that wire from the shadier area. This wire would now have 12 V. I can then add an additional waterproof box near each set with a regulator to drop the voltage down to 6 volts and feed the four sets of lights on each side of the yard from one panel. I'm aware I will need a wire that is waterproof and rated for that voltage and wattage and there will be a voltage drop since it's about 90 feet from front to back.

Are either of these ideas good?

I'm a programmer and I do embedded circuits (self-taught) but I've never dealt with solar panels or battery chargers.

I would prefer option #2 using a 12 V 50 W panel but I'm not sure if there are risks I haven't considered.

If it's a 50 W solar panel and the load is only 4 W, will that damage the panel, or will the panel just generate the amount of power that the load draws and the rest will be heat on the panel?

EDIT: Here's a little diagram of what I have vs what I'm thinking of doing I hope this makes sense:

diagram

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  • \$\begingroup\$ Welcome to SE.EE, this question would really benefit from diagrams to make it easier to understand. You can edit your question to include these \$\endgroup\$
    – LordTeddy
    Nov 9, 2023 at 17:32
  • \$\begingroup\$ if you start a question with "excuse the long post", then maybe make the post shorter? I must admit that I'm not sure what your preference for the shape and source of these devices have to do with the technical problem at hand, so I'm playing a bit of an editor for you here. \$\endgroup\$ Nov 9, 2023 at 17:39
  • \$\begingroup\$ I'll put one together, thank you for the recommendation! \$\endgroup\$
    – Nero F Rox
    Nov 9, 2023 at 17:39

1 Answer 1

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Cable Considerations

Low voltage landscape lighting cable is common and would easily allow you to place the solar panel in a better location. This would probably be less expensive than purchasing separate panels for each set of lights.

I'm aware I will need a wire that is waterproof and rated for that voltage and wattage and there will be a voltage drop since it's about 90 feet from front to back.

Most landscape lighting cable (that I've seen) is 16 to 12 AWG and rated for ~150 volts. It's readily available at home improvement/DIY shops. Voltage drop as you mentioned will be of primary concern. Using 180 ft 12 AWG at 12 V 4.2 A (maximums)*, the voltage drop would be about 2.4 V, giving you just 9.6 V at the end of the run. Not great.

I would consider using voltage regulators that can handle 24 V input, and use two lower-wattage panels in series, which would reduce the voltage drop.

Routing/burying the cable to avoid, for example, an enthusiastic gardener's spade, will probably be more work than having separate solar panels, but placing panels in a sunlit location is, in my opinion, a worthwhile goal.

Power Calculations

If it's a 50 W solar panel and the load is only 4 W, will that damage the panel, or will the panel just generate the amount of power that the load draws and the rest will be heat on the panel?

The 50 W rating is the maximum (and perhaps overly optimistic) power that the panel can produce. Photovoltaic panels generally produce a wide range of voltages depending on current draw and amount of sunlight. If the specification is accurate (and 12 volts is produced at max power \$V_{MP}\$), then with full sun it may be able to supply up to ~4 A.

The system will draw only what it needs from the panel(s). (More info if this is unfamiliar.) Even though the panel is capable of 50 W, the load determines the amount of power consumed.

I don't have enough information about your charge controllers or batteries. The ICs appear to be for battery charge/discharge protection, so I'll assume they will limit charge current to an acceptable level. I'll assume LiFePO4 or similar lithium batteries (instead of lead acid) based on the form factor you described.

According to Alex Beale's solar panel charge time calculator, your existing 4W panels would require ~7 hours of sun to fully charge the batteries. (Values used: 3.6 V, 5 Ah, LiFePO4, 100% DoD, 4 W panel, PWM type). Connected to the bigger panel (drawing 10 W each), this drops to about 3 hours**.

Recommendation

Since each battery can realistically only be charged at 10 W maximum, I would consider using two 12 V 20 W panels in series (or one 24 V 40 W panel; it's just how the cells are arranged). Provided that you have voltage regulators to buck 24 V to 6 V (whatever is acceptable to the charge controllers), you will have less power wasted in the long wire run. At 24 V 40 W (1.7 A), the voltage drop is only about 1 V.

If the 12 V 50 W panel is all you can find, it'll still work okay. If the chargers only draw a cumulative 40 W, current would be 3.3 A, making the voltage drop about 1.9 V. Not ideal, but tolerable. Just don't skimp on wire gauge!

* Based on the panel maximum power, not the load.
** Limiting the charge current to 2.5 A for a 5 Ah battery.

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  • \$\begingroup\$ Thank you so much, everything you've pointed out I've considered which is extremely reassuring to hear, the wires will be along the fence over a mulch bed and i already dug a little channel for it and it's out of the way so that will work, i'm using 16 AWG landscape wire as well but i can switch to 12 AWG. I thought of the 12 V panel only because the battery charger only requires 6 V, I have some variable output voltage regulators in hand and I can drop from as low as 7 V down to 6 V but I will look into a 24 V panel as well. \$\endgroup\$
    – Nero F Rox
    Nov 10, 2023 at 20:00
  • \$\begingroup\$ The link to the current draw was a HUGE help, I'm aware that a circuit only draws what it needs but I wasn't sure if solar panels require different considerations. Thank you for your feedback! \$\endgroup\$
    – Nero F Rox
    Nov 10, 2023 at 20:02
  • \$\begingroup\$ If you've already purchased 16 AWG you may want to more strongly consider 24 V with the less significant voltage drop. As with any system, you find yourself doing cost-benefit analysis to determine which combination of components will be practical and affordable. Good luck! \$\endgroup\$
    – JYelton
    Nov 10, 2023 at 23:25

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