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Need help designing a solar powered rechargeable device

RusselHolmes

Aug 14, 2022
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I have hit a wall and need some help. I am a newbie to this electronics stuff, need to be able to solar charge AA battery('s) probably over a 2 week period or so. they run 1 or 2 x 3Volt LED's and the power may get low enough to need a recharge while the LED's are running, the LED's power is controlled via a switch that may be on for up to several days. Any suggestions are appreciated and if this is posted in the wrong place please move it to where it needs to be as i'm new here as well.
 

John Canon

Jun 1, 2022
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Is this to light up a garden pathway?
They already have those units, some of them look like Gnomes or Mushrooms.
 

Bluejets

Oct 5, 2014
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Several days may be too much time between recharge.
Can you provide current specs of the LEDs , intended cell capacity and intended number of cells.
 

Alec_t

Jul 7, 2015
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The cell chemistry is also important. Are these NiMh?
 

RusselHolmes

Aug 14, 2022
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No these are not pathway lights of any sort. These lights are triggered to come on via a float system. Lights are 3V and 20 mA, 1 light could remain on for up to 7 days, the other light probably only a day until the system is reset. Batteries can be AAA or AA, battery type can be almost anything rechargeable within reason if they don't get a memory. Can't the extra flow of current while charging flow to the LED's and what's left over go to the batteries?
 

Bluejets

Oct 5, 2014
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Can't the extra flow of current while charging flow to the LED's and what's left over go to the batteries?
Do you intend to run the LEDs during the day also, if so, why...??

20mA (20) x 24 hours = 480mAh . You would need at least 3 cells in series to accomodate the LED voltage OR...
Use some type of boost converter.
Remember that say a nimh cell will have a certain capacity and only part of that is usable.
So a 900mAh would in theory run without charging for 2 days at absolute best.

For 7 days that would be 3,360mAh capacity (at least)

If you introduce solar charging, you will need to calculate your best daily sunlight average for your location and allow for rainy days.
In many locations an average of 4 hours sunlight per day is fairly safe guess.

Memory in the cells would be the least of your problems.
For 1 thing, it applies to nicad cells of which there are very few these days.
But most importantly, memory occurs when cells are not being cycled sufficiently through their voltage range.
As the solar works only during the day, suffice to say the night run would pull the cell down sufficiently.

So use a LiPo or lead acid.
Many types available. In fact a small (6Ah) sealed lead acid would probably overcome almost all of your design problems.
 
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RusselHolmes

Aug 14, 2022
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Do you intend to run the LEDs during the day also, if so, why...??

20mA (20) x 24 hours = 480mAh . You would need at least 3 cells in series to accomodate the LED voltage OR...
Use some type of boost converter.
Remember that say a nimh cell will have a certain capacity and only part of that is usable.
So a 900mAh would in theory run without charging for 2 days at absolute best.

For 7 days that would be 3,360mAh capacity (at least)

If you introduce solar charging, you will need to calculate your best daily sunlight average for your location and allow for rainy days.
In many locations an average of 4 hours sunlight per day is fairly safe guess.

Memory in the cells would be the least of your problems.
For 1 thing, it applies to nicad cells of which there are very few these days.
But most importantly, memory occurs when cells are not being cycled sufficiently through their voltage range.
As the solar works only during the day, suffice to say the night run would pull the cell down sufficiently.

So use a LiPo or lead acid.
Many types available. In fact a small (6Ah) sealed lead acid would probably overcome almost all of your design problems.
Yes the LED's would need to run 24 hours/day (its kinda a warning system), solar charging is required so the end user may only have to change the batteries every 2 or 3 years. I have run 1 LED well over 30 days straight on 2 AA alkaline batteries as a test (cheap walmart batteries too) I've seen alot of NiMh online that advertise 2800 and higher mAH. Will the charge controller not kick in when it detects a lower voltage to keep the lights lit with remaining charge going to the batteries?
 

Bluejets

Oct 5, 2014
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I'd be rather suspicious of any claims of a nimh being 2800mAh.
That aside, what you could do if it is a warning LED is either pulse it on at say a 60/40 ratio at full 20mA via say a cmos555 timer or use an off the shelf flashing LED.
Yes, you can charge the cells while discharging at the same time, take a gork at your motor vehicle that does exactly that.

Yup, I can run an LED for 30 days also but not at the current and voltages you quoted originally.
 

RusselHolmes

Aug 14, 2022
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EBL 20-Pack AA Rechargeable Batteries Ni-MH 2800mAh High Capacity AA Battery​

So these batteries on amazon probably aren't 2800 mAh. Hmmm come to think of it for my test i was using an off the shelf flashing LED. Anyhow the voltage to the LED does not need to remain at a constant 3 volts, voltages can drop somewhat as long as the LED remains lit and visible for the period. I have attached a photo of the unit if that helps
 

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Bluejets

Oct 5, 2014
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Those appear to be high brightness LEDs but without a data sheet no one would know.
Leds require a certain voltage to work along with needing to have current limiting to the design spec.
You can run LEDs at a much lower current than the maximum advertised without a noticable visual difference.
The above current limiting can be achieved several ways and it depends on the LEDs used as to which is best/cheapest/simplist.

Flashing the LEDs on and off will result in much less power usage over a given time and the amount depends on the frequency of on to off time.
This also can be achieved in several ways.

If the project starts to get more involved (i.e. sensors for whatever) it would become more appropriate to use a small microcontroller such as say an Arduino Nano or Promini but at this stage I think your discrete devices will suffice. The micros can be put to sleep when required and so current draw is then about 2 knobs of stuff all.(microAmps)

For your future reference, if you are working with batteries/cells then batteryuniversity web site fully explains the expected capacity etc. etc.
It's not all what you read on the label or, for that matter, what Amazon or other similar sites tend to shove in ones face.

https://batteryuniversity.com/
 
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RusselHolmes

Aug 14, 2022
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Aug 14, 2022
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Those appear to be high brightness LEDs but without a data sheet no one would know.
Leds require a certain voltage to work along with needing to have current limiting to the design spec.
You can run LEDs at a much lower current than the maximum advertised without a noticable visual difference.
The above current limiting can be achieved several ways and it depends on the LEDs used as to which is best/cheapest/simplist.

Flashing the LEDs on and off will result in much less power usage over a given time and the amount depends on the frequency of on to off time.
This also can be achieved in several ways.

If the project starts to get more involved (i.e. sensors for whatever) it would become more appropriate to use a small microcontroller such as say an Arduino Nano or Promini but at this stage I think your discrete devices will suffice. The micros can be put to sleep when required and so current draw is then about 2 knobs of stuff all.(microAmps)

For your future reference, if you are working with batteries/cells then batteryuniversity web site fully explains the expected capacity etc. etc.
It's not all what you read on the label or, for that matter, what Amazon or other similar sites tend to shove in ones face.

https://batteryuniversity.com/
I do not think they are a high brightness but wouldn't guarantee it. Those either came off amazon or aliexpress and are running off of 2 x 1.5 volt batteries in series (C-cells), not much for data provided besides 3 volt and 20 mAH, which was all i needed at the time. Now i'm wanting to downsize the batteries to AA and have them recharge via solar power, one light can be on for maybe 6 days the other maybe 2 days with both at the same time (24 hrs/day) (i over estimate when i say several days), the third light is simply hooked up to the test button so the user can make sure the batteries are still ok, the batteries will have probably at least 2 weeks to recharge for the cycle to start again. Just wondering mostly if solar charging will work good enough to get the batteries back up to power for when needed and if the batteries can be charged while the lights are on or the charger sends power to the lights and whats left over goes to the batteries. If any of that makes sense...lol
 
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