Backup DC power supply for Fence lights with Li-ion battery and small PV panel

Thread Starter

marionlane

Joined Apr 28, 2024
8
Ok, so I am a former US Navy Electronics Tech (37 years ago) and I do remember most of my training but I work in IT / Cloud computing so this is not something I do every day. This is really just a hobby project so be patient with me.

1) I have purchased these fence lights and they work really well when they get full sun. However my backyard in certain areas doesn't always get enough sunlight and some lights will only work for a short period of time and then will turn off.
2) The lights have a nice battery, a good little PV panel and are very serviceable. A few screws and you can pop out the battery and access the control board as well.
3) My idea was to purchase this 300 watt DC power supply and have it do the following:
a. Provide power to the battery to charge in the event enough solar energy was not available.​
b. I expect that with DC power provided directly to charge the battery, it wouldn't take long to get a full charge.​
c. Have some way of measuring the current charge of the battery and if the current charge state was below a given threshold charge the battery fully and then open a relay to remove power.​
4) I don't know if I need some type of small charge controller or not I would expect that if I used one, the life of the system components would be extended.

I am using these connectors to wire in an IP68 connector to the fence light.

I will end up with about 22 of these small LED fence lights on one of the three legs of the 300 watt power supply. I have another leg of about 18 lights down the other fence line I am looking to install the same lights.

2024-04-28_09-17-59.png

Any help is appreciated.
 

Audioguru again

Joined Oct 21, 2019
6,782
A 3.7V lithium-ion battery will explode and catch on fire if it is charged incorrectly with a 12V power supply. The solar panels will also be destroyed with 12V.
Instead, a charger circuit designed to charge the single 3.7V Li-Ion cell must be used for each separate light.
The charger circuit might not work properly at night when the lights are turned on.
 

Tonyr1084

Joined Sep 24, 2015
7,991
Welcome to AAC.

My limited understanding of solar lights is that the solar battery charges the battery. When the light drops and the charger no longer charges the battery then the light comes on. If you were to parallel a 3.7V charger as @Audioguru again mentions then the batteries would be forever charging and the system would be thinking it's still daylight. The lights won't come on.

What I did was put low voltage (12V LED marker lamps) along my fence. They're not decorative and they are hardly noticeable until nightfall when they come on. They are controlled by a photocell that switches on at dusk. If you're going to be doing all that wiring AND have a constant power source then you're better off with a single sensor to turn the lights on at dusk, off at daybreak.
 

Thread Starter

marionlane

Joined Apr 28, 2024
8
A 3.7V lithium-ion battery will explode and catch on fire if it is charged incorrectly with a 12V power supply. The solar panels will also be destroyed with 12V.
Instead, a charger circuit designed to charge the single 3.7V Li-Ion cell must be used for each separate light.
The charger circuit might not work properly at night when the lights are turned on.
The 12 V power supply would obviously need to be transformed to be used effectively. I probably didn't do a good enough job explaining each detail. The point is to isolate the PV + Battery away from the pwr supply + charging control function + battery.

I would never need to charge the battery at night and likely not for a sustained period. The charging circuit would likely only need to be used for an hour or so prior to "average dusk" so 5pm - 6pm would be sufficient for those devices that need charging. Remember most may not need it specifically in the summer and a good portion of winter here in the South.
 

Thread Starter

marionlane

Joined Apr 28, 2024
8
Welcome to AAC.

My limited understanding of solar lights is that the solar battery charges the battery. When the light drops and the charger no longer charges the battery then the light comes on. If you were to parallel a 3.7V charger as @Audioguru again mentions then the batteries would be forever charging and the system would be thinking it's still daylight. The lights won't come on.

This is why I mentioned c. in my post. I would like a way to measure the charge and then deterministically decide when to charge.
c. Have some way of measuring the current charge of the battery and if the current charge state was below a given threshold charge the battery fully and then open a relay to remove power.

What I did was put low voltage (12V LED marker lamps) along my fence. They're not decorative and they are hardly noticeable until nightfall when they come on. They are controlled by a photocell that switches on at dusk. If you're going to be doing all that wiring AND have a constant power source then you're better off with a single sensor to turn the lights on at dusk, off at daybreak.
These lights are both decorative and functional for ambient lighting around the perimeter of our yard. The power supply has a PV sensor so I can supply power only at night if that was the case. My main point was to let the tiny solar panels do their job when they can and only provide power assist when needed. I realize if I wanted, I could simply remove the battery, remove the panel connection, transform the 12 volts and call it a day. Not my goal here though.
 

Tonyr1084

Joined Sep 24, 2015
7,991
These lights are both decorative and functional for ambient lighting around the perimeter of our yard. The power supply has a PV sensor so I can supply power only at night if that was the case. My main point was to let the tiny solar panels do their job when they can and only provide power assist when needed. I realize if I wanted, I could simply remove the battery, remove the panel connection, transform the 12 volts and call it a day. Not my goal here though.
Understandably due to shadows you have lights that don't charge sufficiently. The only way I see around that problem is to hard wire them to a low voltage source. And a 12VDC source will be way too much for those little batteries inside the lights as well as too much power for the LED Lamps themselves. The only solution I can see is to hard wire them to a low voltage source. Your 12V source would be fine in this case, BUT you'll have to add constant current for each lamp so as to not burn them out. Likely they operate on not more than 30mA (0.03 amps).

LED's require two things: Sufficient voltage above their forward voltage rating (Vf) and a specific current. The voltage can (in theory) be anything above their Vf. But the current must be regulated to no higher than their max capability. So with a 12 volt source and, assuming a Vf of somewhere around 3 volts, with a max current rating of 30mA you would need a resistor just before each LED. If the assumption is 12VDC with 3Vf at 30mA you'd need a 300Ω resistor for each lamp. Two things you're doing in this case is running 12VDC to all the lamps, each lamp wired in parallel, not series, gutting the solar charging and battery capability and adding a resistor to each lamp. Polarity must be observed. If you're doing anything at all then this is the minimum I can see you doing.
 

Thread Starter

marionlane

Joined Apr 28, 2024
8
Understandably due to shadows you have lights that don't charge sufficiently. The only way I see around that problem is to hard wire them to a low voltage source. And a 12VDC source will be way too much for those little batteries inside the lights as well as too much power for the LED Lamps themselves. The only solution I can see is to hard wire them to a low voltage source. Your 12V source would be fine in this case, BUT you'll have to add constant current for each lamp so as to not burn them out. Likely they operate on not more than 30mA (0.03 amps).

LED's require two things: Sufficient voltage above their forward voltage rating (Vf) and a specific current. The voltage can (in theory) be anything above their Vf. But the current must be regulated to no higher than their max capability. So with a 12 volt source and, assuming a Vf of somewhere around 3 volts, with a max current rating of 30mA you would need a resistor just before each LED. If the assumption is 12VDC with 3Vf at 30mA you'd need a 300Ω resistor for each lamp. Two things you're doing in this case is running 12VDC to all the lamps, each lamp wired in parallel, not series, gutting the solar charging and battery capability and adding a resistor to each lamp. Polarity must be observed. If you're doing anything at all then this is the minimum I can see you doing.
Thanks Tonyr1084 for the recommendation.

I am leaning towards adding a small relay at each fence light that will remove the PV connection and allow a transformed 5 VDC supply from the 12V source to charge the battery from 5pm - 6pm on a timer at the source as well.
 

Tonyr1084

Joined Sep 24, 2015
7,991
Thanks Tonyr1084 for the recommendation.

I am leaning towards adding a small relay at each fence light that will remove the PV connection and allow a transformed 5 VDC supply from the 12V source to charge the battery from 5pm - 6pm on a timer at the source as well.
Better be sure of the correct voltage. If you have a single battery in the lamp it's likely a Lithium Metal Oxide type with a voltage of (if I remember correctly) 1.2 volts. 5 volts would blow it up. If you have two of them that's only 2.4 volts.

Best to follow the recommendations of the experts here. I don't count myself an expert. The only thing I'm an "Expert" at is at taking good advice. If you're doing ANY rewiring then the best approach is a single source of DC volts. An SLA battery (Sealed Lead Acid) battery of sufficient size charged by a solar panel with a PV sensor to turn the lights on at dusk along with all the wiring I mentioned in the previous post, gutting individual solar panels and individual batteries and opt for a single resistor in each lamp unit of the appropriate resistance. One advantage I can see is that all the lights will come on simultaneously. MY solar lamps come on at different times. Some are slightly more sensitive to light while others are less sensitive. Watch them long enough and you can learn which ones will turn on first and which will turn on last. Move them around so that they slowly ignite from left to right (or vice versa). More hassle than I care for.

I have some OLD solar lamps in my shop. I think I'm going to hard wire them and mount them on my deck. You have me thinking now. I've thought of that before and thus far haven't pursued it.

One other thought came to mind just now - you can get a buck converter to control the amount of current going out to your individual lights. You still have to modify the lamps and run dedicated low voltage wiring, but at least you're not putting your lamps at risk. And if your fence is flammable, your fence may thank you as well.

Do it right or don't do it.
 

AnalogKid

Joined Aug 1, 2013
11,196
Besides being way over-sized and expensive, the power supply linked in post #1 has an AC output. It will need rectification and regulation, more cost and complexity. Better to start with a much smaller, DC-output supply. Since you are going to run wires to each light, you could start with something small like a 5 V USB wall wart. That, plus one diode and a resistor should make a safe trickle-charger. Charging a lithium battery is not a simple thing, but a low enough trickle current should be safe.

While opening up and modifying a bunch of lights can be a fun home project, dual-charging multiple independent lithium batteries is not a trivial task. As the complexities mount up, when is it time to stop and ask if the effort is disproportionate to the result. As in - wrong lights - ?

If you are running wires to each light to keep the batteries topped off, why not just get lights that run on 5 V or 12 V directly, and put the power supply on a timer or light sensor? No solar cells, no batteries . . .

ak
 
Last edited:

Tonyr1084

Joined Sep 24, 2015
7,991
It's like walking around the block to the right just to get to your neighbor's house one door to the left. Overly complex is a waste. It's like Engineer Scott said to Admiral Kirk regarding the Excelsior (the newest class star ship) "The fancier the plumbing the easier it is to stop up the drain."

Keep it stupidly simple (KISS). It'll last longer, be easier to repair and be less of a drain on your wallet. Whatever voltage you start with, control the current to the LED's and all will be well.

I think I'm going to go in the shop and have a look at my solar lamps. See if I can totally gut the interior and just replace them with low voltage LED's designed to operate on 12VDC.

Peace bro.
 

Audioguru again

Joined Oct 21, 2019
6,782
The cheap fence lights have specs that say 3 light sources that are probably one red, one green and one blue. White is produced when the 3 colors are added together correctly.
It says 3.7V which is the average discharge voltage (4.2V at a full charge) of one Li-Ion battery cell.

It says 3.6W. That is wrong because then the battery and solar panel would be huge.
The battery cell is probably a common 18650 size at 2.6Ah. Then it can supply 2.6A/30 hours= 87mA.
 

Thread Starter

marionlane

Joined Apr 28, 2024
8
Besides being way over-sized and expensive, the power supply linked in post #1 has an AC output. It will need rectification and regulation, more cost and complexity. Better to start with a much smaller, DC-output supply. Since you are going to run wires to each light, you could start with something small like a 5 V USB wall wart. That, plus one diode and a resistor should make a safe trickle-charger. Charging a lithium battery is not a simple thing, but a low enough trickle current should be safe.

While opening up and modifying a bunch of lights can be a fun home project, dual-charging multiple independent lithium batteries is not a trivial task. As the complexities mount up, when is it time to stop and ask if the effort is disproportionate to the result. As in - wrong lights - ?

If you are running wires to each light to keep the batteries topped off, why not just get lights that run on 5 V or 12 V directly, and put the power supply on a timer or light sensor? No solar cells, no batteries . . .

ak
I guess the reason I am using the lights is because that is what I have. I could go buy new lights and also have a landscape lighting contractor install them as well but what's the fun (and learning) in doing that?
 

Jon Chandler

Joined Jun 12, 2008
1,095
Before deciding how to proceed, you need to know what batteries are in these lights. People have assumed Li-Ion but they would well be NiMH. These have totally different charging requirements.
 

Tonyr1084

Joined Sep 24, 2015
7,991
As I said, you've inspired me to move forward with MY solar lights. This is just the first, and I'm not done with it.
So far I start with this:
LED 1.png
Obviously it's old. It still works but it isn't all that attractive. I'll be painting them black. That will cover the solar panel - but that's no problem.
LED 2.png
Three screws and it comes open. Remove the battery as well.
LED 3.png
Cut the leads from the solar panel and remove the electronics. They won't be used. Maybe one day - - - .
LED 4.png
All this stuff has to get cut out. I used a Dremel and a friction wheel.
LED 5.png
This octagon is from a redwood fence cut to fit the inner ring inside (not shown - sorry).
LED 6.png
Bore a 3/4" hole for the black rubber grommet.
LED 7.png
Install the grommet.
LED 8.png
Push the LED module into the grommet. It will snap in and hold tight.
LED 9.png
Turn it over and push it into the inner ring (OK, finally shown)
LED 10.png
You're ready to wire it. Black is negative, white is positive. The LED will shine down on the glass and there's a reflector in the middle at the bottom.

The next step is to run wires to each unit. These LED's have built in current regulation, probably just a resistor. At 12 volts they're plenty bright. And they can be purchased as "LED Marker lights" from Amazon in different colors. I have a small power supply that can push 3 amps. I'm assuming each LED doesn't draw more than 30mA. There will be 8 of them on a string.

Gotta go - my cat is pestering me and loading my face with fur.
 

Tonyr1084

Joined Sep 24, 2015
7,991
Found this for current draw.
Screenshot 2024-04-29 at 3.51.35 PM.png
Half amp each and using 8 of them means I need a power supply capable of a minimum of 4 amps. I could add a series resistor to drop their current and get away with my 3 amp PS. 1 watt at 12 volts means they draw 83mA each. Eight of them means they'll draw 667mA. I know - their numbers don't make sense to me either. Maybe tonight I'll test the current on one and come up with a hard number. It appears there are three high intensity elements. That they draw more than 30mA is no surprise. Testing will ring out the truth.
 

Thread Starter

marionlane

Joined Apr 28, 2024
8
Found this for current draw.
View attachment 321208
Half amp each and using 8 of them means I need a power supply capable of a minimum of 4 amps. I could add a series resistor to drop their current and get away with my 3 amp PS. 1 watt at 12 volts means they draw 83mA each. Eight of them means they'll draw 667mA. I know - their numbers don't make sense to me either. Maybe tonight I'll test the current on one and come up with a hard number. It appears there are three high intensity elements. That they draw more than 30mA is no surprise. Testing will ring out the truth.
I like it. I am doing some LED strips with diffusers around the deck and pavilion. I am setting up some Alexa routines since the power supply is Alexa enabled. I'll send some pics once I get back on the project this weekend.
 

Audioguru again

Joined Oct 21, 2019
6,782
The "common" lights in post #16 use a single AA or AAA Ni-MH 1.25V rechargeable battery cell. It uses a "solar garden light" IC with an inductor to boost the voltage and limit current for one LED. The IC also switches between lighting the LED in darkness and unlighting but charging the battery from the low power 2V solar panel in sunlight.
 

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