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AAA solar batteries acting weird? Answered

I recently got a bunch of solar garden lights intended for a different purpose of course ;)
During the modding stage I made a simple wireless charger for one of the lights an it worked as intended.
However, while testing the light and how long it works I got a bit confused.
The battery states 300mAh (NiCd), which is fine for a single LED solar light and a small cell.
The light uses between 6 and 10mA, depending on the current color.
With my dumb calculations I would say a fully charged battery should last around 30 hours - 24 at least.
But after 8 to 9 hours the light goes off and the battery is down to a low level.
Being dirt cheap I tried the same again with a good 1000mA NiMh battery, which otherwise does a good job in other applications.
The 1000mA battery lasted 3 days, which again is way short of the calculated target.

At this stage I suspect the Joule thief used in the solar light is the culprit as my own circuits don't show this problem...
Would it be worth to get the oscilloscope out to check the actual spikes produced by the circuit?
And how would I go with properly calculating the consumption?
Being a color changing LED (circuit inside the LED) does not make it much easier :(


Josehf Murchison

Best Answer 3 years ago

Believe it or not it is working about right.

Although the battery is 300 mAh, and the LED uses 10 mA, the circuit driving the LED uses some of the power and the LED will stop working long before the battery gets to 0 volts or 0 amp hours.

You are doing good to get between 8 to 10 hours of light from your LED.

Nothing works at 100% efficiency and the calculations you are using are 100% efficiency.

In generating systems 50 to 60% efficiency is average some expensive systems reach 80% efficiency, and solar is a generating system.

If you look at the back of your microwave oven on the specs label, it will tell you input 1500 watts, output 1000 watts. 1/3 of the power is used just to run the microwave oven.

Take 1/3 of the time off for the power used to run your circuit and take 1/3 off the time off for when the battery won't run the circuit and you get 8 to 10 hours with a 300 mAh battery.


3 years ago

Add what Josehf very correctly said and add the fact that summer months charge the NiCd batteries longer then they are discharged... They (the NiCd) remember and there after will only deliver the short discharge rate irrespective of the batt label.

Also consider a joule thief is not designed to be efficient but only to suck the low voltage power left in a battery.

Josehf Murchisoniceng

Answer 3 years ago

Hey iceng did I ever tell you some months we only get 50 hours of sunlight where I live.


3 years ago

5mm leds use 20ma but can go up to 25ma max, so put a meter into the circuit with the light on to check current

Its possible that the charging circuit isnt charging the batteries enough during the daytime, so a partially charged batt will not last the theoretical duration, even more so if they are somewhat old.