Supplement battery charging (solar) with 15v landscape lighting input?

Question

I have a 150w low voltage (DC) landscape lighting system (15v) but I'm only using maybe 50w of it. I also have a 400w solar array connected to a 40a MPPT controller that keeps a 50ah LiFePO4 battery charged...unless it's winter with long nights and subsequent cloudy days.

I'm curious if it would be feasible to run a leg from my landscape lighting setup to the solar array for supplemental charging? I have it on a dusk/dawn timer so the thought is that it would charge the battery overnight and if it's cloudy the following day the battery will have enough power stored to last until the next charging (given that it's cloudy and the solar panels can't charge).

My charging voltage is 14.4v. Is this pretty straight forward or a no-no? I would need to also have a diode so that the power from the battery doesn't flow back the other way.

Equipment brands/models:

• Renogy Rover Li 40a MPPT solar charge controller (https://www.renogy.com/rover-li-40-amp-mppt-solar-charge-controller/)
• Volt 150w transformer https://www.voltlighting.com/150w-slim-line-led-transformer
• Weize 50ah LiFePO4 battery https://www.amazon.com/gp/product/B0BGS1KTVZ/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1

Answer 1

My charging voltage is 14.4v. Is this pretty straight forward or a no-no?

Depends how you feel about lithium battery fires. Lithium batteries absolutely require proper charge management.

The better way is to feed power into the solar charge controller, and let it figure out how to appopriately charge the battery. You may need to step it up to a voltage range appropriate for the solar panels and if necessary rectify it and smooth it. I would also include diodes to prevent reverse current - that way the solar will overpower the utility charging and cause it to suspend.

The ideal voltage would be lower than the solar panel output yet high enough for the charge controller to work. That way the solar will overpower the utility.

However, this largely defeats the purpose of having solar

Because every morning at sunrise, the battery pack will be full, charged from utility power which you pay for. The solar will do nothing except replace any daytime usage.

The better thing would be to have it manually connectible (or switched) so you could turn it on during the rare times solar can't cover demand. It would also help to have some information about when that is, such as a battery state-of-charge indicator somewhere appropriate.

Or address the deficiencies in the solar performance.

If the array is too small for winter conditions, enlarge it.

And let's talk about panel tilt. If you have a grid-tied solar system, you want to tilt it for the summer sun, because those days are longest and that is when power is most valuable (A/C systems going nuts). However, in an off-grid system the better aiming point is Winter solstice - it's OK to be sub-optimal for summer because summer charge days are long and many.

However, the ideal tilt is straight vertical. This will be bad in summer and not ideal in winter, however snow won't pile on it. Now "fresh snow" does not disable the panels - it actually improves them due to reflected light from all that snow. The south facing quickly melts any blown-and-stuck snow.

Answer 2

Your proposal is logical and has working potential. The LiFePO4 battery pack will have protection built-in against anything too crazy going on.

You could safely just use a diode and potentially run the landscape "transformer" into the ground if you don't care about it, or it's cheap/free, or you really really need the solar capacity. To do it "right", my main concern would be how to limit the current taken from the landscape "transformer" so that it's not pegged at 100% for long stretches of time, which would probably shorten it's life considerably (80%/max is typically recommended).

You could accomplish that with a small DC-DC buck+boost converter with built-in constant current or at least current limiting. With that inline SMPS installed, you can feed it 10-30v from anywhere, and output 14.4v at <=5A, thus pulling no more than 125W from your "transformer" when the lights are on. That adds 5kWh/day of capacity to your solar setup.

Now, whether it's worth feeding grid into your solar depends on how you use it. You will lose about 10-20% of the input energy due to in-efficiencies within the chain: AC-DC, DC-DC, MPPT (maybe), and the LiFePO4 battery pack's internal SMPS. So, you're robbing Peter to pay Paul and spilling some of the booty in the process, but again, maybe you have important things on the solar and it's almost enough as-is, in which case this expansion would push it over the edge into a high beneficial or comforting availability.

Answer 3

If the battery charge controller has an input for DC then it might be worth considering and if that input matches the voltage from the lighting. Connecting DC direct to batteries is a bad idea or even two charge controllers, so if the battery charge controller doesn't support it you would need to get another one.