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LED that light up if the battery is charged Answered

I am making a solar charger for one Ni-Mh battery 3000ma.
My solar cell is 2V 300ma. I want there to be an LED that will light up, if the battery is fully charged, I went through alot of articles online, and here is a circuit that I think might work, but I'm not sure, so if you could tell if this is going to work or not, it would be great, and any modifications I can do. I'm not sure about the value of the diode. 


Have you found an answer for this one. I need to do the same


3 years ago

and what diode is that in the middle? tunnel diode?


3 years ago

Is there a way to mkae the led turn of when i unplug the charger ?

and could you list the product list please??

It looks to me like you've got too much voltage dropping for the LEDs to light up at all. 

If that's a NiMh it wil be fully charged at 1.5V or so, so I'd just try various coloured LEDs (with a low value resistor in series) across the battery until you find one which suits.  This is a bit below the normal Vf for LEDs but they do start to turn on slightly below the specified voltage.  It's worth a try.
A cleaner way would be with a brown-out detector IC but I doubt if you want to go to that level of sophistication.

Bear in mind that the 2V 300mA solar cell will give that under absolutely best-case conditions.  If the sun is slightly clouded or not dead above the cell the output will be less.  You have very little excess voltage to play with there and your blocking diode must be a germanium or schottky type.
Unless you're running a VERY low current circuit (or a higher current one VERY infrequently), I think you're going to run out of power.  I would suggest series doubling up on the solar cells. 

Right, thanks, I will see how it works, if it doesn't work out nicely with two, I will get more solar cells. I'll let you know if you want. Also you said that Ni-MH fully charged at 1.5V isn't it 1.2-1.3V? And just out of interest, when solar cell isn't producing it's maximum, is current, voltage or both are reduced?

Actually, the final voltage is dependent on the charge current, and will be more like 1.35 - 1.4V.  What happens is the fully charged voltage drops off quickly once the cell starts to discharge, then sits at around 1.2V for most of its capacity before dropping off at the end.
I've just been doing something along similar lines and was surprised how dramatically charge rate drops under less than ideal conditions. Both voltage and current decrease.

Just to summarise . . .  4 solar cells in series parallel configuration to the battery via a schottky diode (although could now use silicon as you have more voltage available).  Lose the 317 and the other diodes.  Put a LED and resistor across the battery, colour to be determined by experiment.
If those solar cells are 300mA, that's C/10 (1/10th the battery capacity) which you can charge at indefinitely.  In practice, you won't be getting anywhere near that for most of the time.

Alright, I never knew that. I am going to a hot country, were it is around 30 ish on a regular afternoon, and I will be spending most time outside. Ideally I would want to attach solar cells on my bag, and a couple of LEDs which will light up if I'm out at night time. I would've bought more solar cells straight away, but they are pretty expensive, I haven't found a place where I could get a bulk of them for much cheaper.

If you've got guaranteed sunshine then you should be OK.  (I'm in the UK where we don't have a climate - We just have weather #;¬) 
I've just bought one of THESE to play with as buying the whole unit may be cheaper than buying individual solar panels. 


7 years ago

Sorry, the way its set up you can not charge or light the LED.

Subtract the Red diode (0.56V) and the minimum LM317 (1.25V)
and you cannot equal the battery voltage.

Made some changes to your circuit, fixing polarities etc.
Had no idea what you were doing with the 317;-|
But you can add wiring now.


I was thinking that if the current goes down to some voltage, 317 will stop charging the battery, and will start powering the LED instead.

I know I'm not answering your direct question.

Without exception, fully charged means two things are true

1) the battery voltage has reached some critical value. This value is dependent on
a) the mfg specs
b) some fudge factor to accommodate depreciation over time in the ability of the battery to accept charge (I would make this settable via a trimpot)

AND (not or)

2) the charging current drops below a critical value.

That is, if you put a "bad" battery that will not draw power (for instance, an SLA battery that has outlived its useful life and gone to open circuit mode due to lead-sulfate acumulation), then only checking the charging current will produce an erroneous result.

What *I would do is use a threshold detector (et for the nominal battery voltage per mfg specs) to evaluate the voltage and a current sensing resistor and amplifier (followed by another threshold detector set for low detection) then run the results into a logic gate