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Outdoor Wi-Fi light bulb setup Answered

Hello everyone,

With the social distancing going on right now, I wanted to take a crack at a project that has been rolling around in my head for a while.

Short story is I would like to build a solar charged, battery operated, power supply to run a color changing Wi-Fi light bulb in a lantern housing that would then hang outside on a lamp post.

I will freely admit I have pretty much no idea what I am doing.
I know I need to make sure that the Amp-hour are at least 50% more than what I need. I know there could be a way to use an inverter to get the voltage from DC from the batteries to AC for the light bulb base. And I know there are a lot of numbers and equations that I am unsure of how to use.

And yes, I am aware that there are many different kinds of solar powered garden lamps out there. But none of them are as powerful, nor have the color options, I am interested in .

So here is my idea.
1 - 7-10 watt WiFi color light bulb that screws into a standard E26 socket and runs at 110 volts.
So far I think I have figured out that this bulb would run at 0.0909 amps, or 100 mA.
If I did the math right, that brings me to needing a 6 Ah battery to run it for 24 hour.

My stumbling block is I do not know how to size an inverter to correctly go from the DC of a battery to the AC needed by the bulb. Nor what size of battery I would need to run a 7-10 watt bulb at 110 volts for 24 hours including the additional energy to compensate for the inverter.
I cannot imagine I would need a battery as large as a car battery, but I also have no idea.

Any assistance you could provide would be greatly appreciated.


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12 months ago

I see 2 paths here:
First path (Simple but wastefull): Go as you described:
- Solar panel charges your battery (Thru some sort of a charge controller please)
- Inverter feeds from th battery
- 110V bulp is connected to inverter
Problems i see here:
- An inverter is normally built for MUCH higher wattage as your simple 1-10W and may even shutdown as it thinks there is no load attached. Dont know your inverter...
- As the inverter is built for much higher power, it will be used at the very low end of its spec and therefore be very wastefull. If i take a DC/DC-Converter for comparison, i would say, go with 80% waste in inverter and 20% in bulp. So 4times the power of your bulp as waste in inverter... Adjust your battery capacity accordingly.
Not ideal, but simple...

Second path (Efficient but more complicated): Go 12V!
- Most LED-bulps run 12V internally. if yours does as well, why boost your battery of 12VDC to 110V AC which is then converted back to 12V DC? Feed the 12V of the battery directly to the bulps interior 12V-rail
- Yes: You would need to open the bulp and understand its inner workings (Which may be a problem as you wrote and i understand this completely).
- Your bulp MAY run on lower voltages... Have you tried feeding it 12V DC directly on the socket? Sometimes (Depending on the type of AC-DC-Converter in there) this works.
- If you want a constant light-output, you may need a small SEPIC DC/DC-Converter like https://www.aliexpress.com/item/32825968259.html set to 12V

If i have more ideas, ill post here again :)

As far as your battery goes: Why not test it?
- charge the battery fully
- Connect your inverter (Assuming you have it already) to your battery and add the bulp as load to the inverter.
- Measure the current from battery to inverter
- Measure the voltage of the battery while at the inverter.
- Calculate its power-draw: power = voltage * current
- Add like 20% to that for compensating a more lossy conversion for lower battery-voltage
- Wh-Capacity needed = power * time * "fear-factor"= power * 24h * 1.5
(In case your inverter draws lets say 3 Ampere at 12V for for your 10W bulp: 3A * 12V = 36W.) the fear-factor is simply how much bigger you want your battery than needed to just work barely in case not all works perfect.
36W * 24h * 1.5 = 1296 Wh.)
- Convert this back to Ah: Wh-Capacity / nominal voltage
(in our example, a 12V batpack is around 11V nominal. So 1296Wh / 11V = 117.8Ah at a 12V-pack.) Quite big...
If we go with path 2 and a typical efficiency of a DC/DC-Converter in the range of 10W (Around 80-90%) we see how this works out:
10W bulp / Efficiency of DC/DC = powerdaw at battery
10W / 0.8 = 12.5W on battery.
10W * 24h * 1.5 = 360Wh
360Wh / 11V = 32.7Ah battery at "12V" Still quite a big pack but manageable.

Now to the maybe overlooked part: Solar.
Solar is impressingly inefficient for small-panels and big batteries... Know those USB-Powerbanks with solar? Yeah... They take about 3-4 days (!!) of 8h direct sun to charge fully...
Lets say you go with the small 360Wh-Battery. You want it to recharge from 20% to 100% over a day of sunshine. Thats around 290Wh of charge. Lets assume your Solar chargecontroller is 100% efficient. It isnt, but they normally are quite high... (You HAVE a good charge-controller, dont you??). So 290Wh over lets say 6h direct sunshine:
290Wh / 6h = 48Watt. You need a 50W panel to charge the small battery over the course of a normal day like i know them in europe (Clouds and stuff).
With the big battery (1296Wh) it boild down to 172W so maybe a 200W is a standard panel you may get? If i look online, those things are quite big and expensive...

And by the way: Cudos for leaving your "comfort zone" and tackling an interesting project :)

Seeing the battery-size and Solar-requirements above, you may be intrigued to add another thing to your project: A Dark-Sensor!
Why having the bulp lit at day where nobody sees a puny 10W against the propably quadrillions of watts of the sun?
And the best thing: you already HAVE a sensor... Your Solar panel! :)
Simplest i can immagine: Add a 12V relais to your schematic and let the coil be fed from the solar panel. If the panel stops giving power, this means: "Darkness" and the relais falls to its normal state. In this normal state, the Battery gets connected to the inverter or the DC/DC-Converter and the whole thing is starting and your bulp is illuminated.
Of we say, your average night with darkness is 10h out of 24h, you cut your battery-size in 10/24 or in other words down to 41% which leaves you with 50Ah or 14Ah respectively. Now we are talking good numbers!
The 14Ah is within usable specs for like https://hobbyking.com/en_us/turnigy-high-capacity-...
But if you have a car abbtery around, you may take this aswell instead of buying a new one. Too big of a battery is of no issue here...