# solar (walmart) light with ac charging

#### dsamis

Joined Feb 9, 2018
15
I'm a novice and wanted to hack a $15 solar/battery led porch light that walmart sells to be able to charge and run off AC in case there's not enough sun or bad exposure for the panel (i'm finding it only gets 2 hours worth of usable charge on some sides of my place). They use a 3.7V lithium batt and obviously the solar is 12V. So what I was planning and started to do was hook up security camera 12V dc pigtail jacks to the wires than run from the panel to the whole board unit and run cheap security camera cables (4 pack of 100' for$20) to an AC adapter with 4 way splitter. I figure that way the board should have a charge controller that stops the charging feed from the panel (or ac) once the battery is full. I was going to put a manual switch and/or timer on the AC adapter so I could control that at will or on a schedule. My question is what I need to do so that in the rare instance that the battery is full and the solar panel is still outputting some charge so that it doesn't back feed upto and damage the AC adapter which I assume aren't designed for reverse flow. Some type of zener diode I'm assuming? And where do you think is the best place to locate that? My thought was right between the AC adapter and splitter so that they could possibly charge/balance other units and just prevent the damage to the AC adapter.

light is 500 lumens, I'm assuming about 5W max
the security camera ac adapters i got are 2A

As always thanks for any assistance and advice.

#### KeithWalker

Joined Jul 10, 2017
1,062
A diagram or sketch of what you are proposing to make would help me understand it much better.
Regards,
Keith

#### Audioguru again

Joined Oct 21, 2019
1,602
A 3.7V lithium battery is charged to 4.2V maximum.
Your 12V is much too powerful. The solar panel is probably only 4V and will burn up with 12V fed to it. The lithium battery will explode then catch on fire.
Why not remove the useless battery and solar panel and power the light from an AC/DC adapter producing 3.7V-4V?

#### dsamis

Joined Feb 9, 2018
15
A 3.7V lithium battery is charged to 4.2V maximum.
Your 12V is much too powerful. The solar panel is probably only 4V and will burn up with 12V fed to it. The lithium battery will explode then catch on fire.
Why not remove the useless battery and solar panel and power the light from an AC/DC adapter producing 3.7V-4V?
I checked before, but I will check again just to be double sure, the solar panel is outputting 12V. I don't want to remove the solar or battery because I like the idea of some free energy and the battery backup is a nice feature, keeps me from having to setup a UPS for the whole system. Last thing I want is no security lighting if all the power goes out. If I wanted to just do straight AC I would have just bought an AC light.

#### Audioguru again

Joined Oct 21, 2019
1,602
Why would they use a 12V solar panel to charge a 4.2V battery? Why didn't they use a 4V or 4.2V solar panel?
Or why didn't they use three 4V battery cells in series charged with 12V?

The street lights in my city are reliable. They rarely fail and if they do then we all cheer.

#### dsamis

Joined Feb 9, 2018
15
I checked before, but I will check again just to be double sure, the solar panel is outputting 12V. I don't want to remove the solar or battery because I like the idea of some free energy and the battery backup is a nice feature, keeps me from having to setup a UPS for the whole system. Last thing I want is no security lighting if all the power goes out. If I wanted to just do straight AC I would have just bought an AC light.
I'm sorry you're right, I must have been confusing this model with a different one I looked at, maybe the larger street light one. Or maybe I'm just getting senile. The solar panel on this porch light one is indeed 4.2V. So I guess I need to get a 4.2V power supply instead of the 12V. But I still have the question about the "back flow" protection.

There ain't no street light in my backyard

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#### Audioguru again

Joined Oct 21, 2019
1,602
At night sometimes my backyard gets a skunk or a raccoon. One time there was a possum (didelphidae marsupial).
They do no harm and are not a security risk.

A Lithium battery needs a charger circuit designed for it to avoid an explosion and fire. Maybe the designer of your light used a solar panel with a low output current and was lucky. If you attempt to charge the lithium battery with a power supply then it will be a serious hazard.

#### dsamis

Joined Feb 9, 2018
15
Believe me I'm not worried about the wildlife (I have a nice complete opossum skeleton from where one decided to lie down and take a nap and never got back up). I'm more concerned about my crackhead neighbor who has on several occasions threatened me with lines like "You're dead". I'm assuming the solar light has a charger circuit on the main board with the motion sensor, how else would it know when to stop charging? I don't see why if I hook up the 4V power supply to the solar panel line (I'm not connecting it directly to the battery) it wouldn't treat it any differently than as if the sun was still shining. Do I need a zener diode to stop the solar panel from sending voltage to the AC?

#### dsamis

Joined Feb 9, 2018
15
A diagram or sketch of what you are proposing to make would help me understand it much better.
Regards,
Keith
Here's a rough diagram, forgive my poor penmanship

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#### Audioguru again

Joined Oct 21, 2019
1,602
You must Never Ever connect AC power to a DC circuit. You need 4VDC.
Then at night the new 4VDC fries the solar panel instead of lighting the light and charging the battery. The solar panel needs a diode on series to block the new 4VDC, but the light circuit might already have the diode to prevent the battery from frying the solar panel at night.
So I think the new 4VDC should connect to the battery.

#### dsamis

Joined Feb 9, 2018
15
You must Never Ever connect AC power to a DC circuit. You need 4VDC.
Then at night the new 4VDC fries the solar panel instead of lighting the light and charging the battery. The solar panel needs a diode on series to block the new 4VDC, but the light circuit might already have the diode to prevent the battery from frying the solar panel at night.
So I think the new 4VDC should connect to the battery.
Yes I'm not too smart to use straight AC, by AC adapter I meant an AC-DC adapter. I looked at a similar model (where I can see the circuit board easily without dissembling the entire thing) and I do see what I assume is a diode D1 for the panel. I had never considered that the panel could fry out from a voltage feed the same as what it produces. Looks like there is a full charging circuit with chip controller and large thru hole capacitor next to battery output. Wouldn't connecting the AC-DC adapter to the battery directly also overcharge and fry it also? Don't you think I would need a a diode for the AC-DC adapter so that doesn't get fried too? I guess I could upload a pic of this board. But like I said getting the board out of the model I want to use is not easy or something I really want to do, thus why I was going to use snap on splice connectors on the wires.

#### Audioguru again

Joined Oct 21, 2019
1,602
You said the Light uses a 3.7V Lithium battery. Look it up, it is 4.2V when fully charged. You did not mention a charging circuit so I assumed that the solar panel produces only 4.0V (plus 0.7V for a diode) so it does not overcharge the battery, then the AC/DC adapter can also be 4.0V.

#### dsamis

Joined Feb 9, 2018
15
You said the Light uses a 3.7V Lithium battery. Look it up, it is 4.2V when fully charged. You did not mention a charging circuit so I assumed that the solar panel produces only 4.0V (plus 0.7V for a diode) so it does not overcharge the battery, then the AC/DC adapter can also be 4.0V.
I'm not questioning the battery or panel voltage, though I did mistakenly remember it originally as 12V for whatever reason. I did check again as I said before, and the panel is running @ 4V, so obviously I would use a 4V AC-DC adapter. But for whatever reason you keep skirting my 2 main questions at this point, do I need a diode for the AC-DC adapter, and whether plugging that into the battery directly (behind the charging circuit) would fry the battery. I don't know enough engineering to see/understand how anyone would make a solar battery circuit without a charging circuit to know and stop the charging, but my limited basic understanding of batteries was continued charging feed would damage them once full.

#### Audioguru again

Joined Oct 21, 2019
1,602
If the AC/DC adapter is a simple one then it already has diodes.
Since at first you did not say there was a battery charger circuit then I assumed that the battery with a voltage rating of 4.2V will stop charging when it reaches the 4.0V of the solar panel or AC/DC adapter.

#### dsamis

Joined Feb 9, 2018
15
If the AC/DC adapter is a simple one then it already has diodes.
Since at first you did not say there was a battery charger circuit then I assumed that the battery with a voltage rating of 4.2V will stop charging when it reaches the 4.0V of the solar panel or AC/DC adapter.
Forgive my ignorance about batteries, but why would it stop charging at 4V if there is no charge controller? Wouldn't the 4V just keep feeding into it eventually overloading its storage capacity? or is that ok and only when the voltage is too high does battery damage occur.

#### Audioguru again

Joined Oct 21, 2019
1,602
www.batteryuniversity.com talks about all the different kinds of batteries.
When an ordinary lithium rechargeable cell reaches its max allowed voltage of 4.2V then the charging current automatically reduces but the charger must be disconnected.
They do not say and I never tried charging to a lower voltage. I assume the current will reduce when it becomes fully charged to that lower voltage.
A Lithium battery cell will explode and catch on fire if it is charged to a voltage higher than 4.2V.