Ok, this one should be so obvious, but I'm not seeing the implementation! I want to make a basic solar battery charger to charge 4 AA batteries, using an LM317T. The output gets a series resistor to limit current, then on to the batteries (in series). My solar panel is capable of delivering 9.5V on a sunny day, so all is good there. This is from a known working schematic from the net.

Now, if I want to add a resistor and LED to the output which will come ON when the 4 batteries reach near full charge, how would I go about that?? My assumption is that current flow will slow as the charge cycle nears its end, and the voltage will rise. I could probably just use a pot to dial in a 'charged vs. uncharged' state for the LED, but I wonder if there is an actual way this is done - maybe by just using the total 6V fully-charged state as the assumption of "LED ON"? Or a voltage divider to make the 6V = the LED turn on voltage?

Thanks for any ideas here, I don't have much time to experiment with this idea!

 

I hate to have to state the obvious, but we really need to see the schematic you found before commenting.

 

Sorry 'bout that, Wookie. Here's a link:
http://www.reuk.co.uk/Solar-Battery-Charger-With-LM317T.htm

They do have a page that uses Zeners to turn on an LED when the charge is finished, but it is for a 12V setup.
My charge voltage here would be ~6V (4 AA batteries).
I am interested in either using a voltage divider + LED to do the same, or to find a way to utilize a 4.7V zener (lowest value I have, that or a 5.1v).

So working the problem...I am not sure how the voltage and current would work with a semi-discharged battery on the output of an LM317. High current, low voltage, going to higher voltage/lower current as the charging cycle completes?
Simple, yet it is confounding me I mostly do audio electronics....thanks!

 

Well, if the solar cell is offering 9V, you have to subtract the Dropout voltage of the LM317T regulator, which is about 2.5V. This leave you with 6.5V, barely enough to charge your batteries in "full sun". If the solar voltage drops even 1V you will be wasting your time. You won't be charging anything as the regulator will not be on. I'd suggest using a solar pannel with 12V output or swapping the 317T for a regulator with a much smaller Dropout voltage. You did not mention what sort of current you could expect from the 9V solar pannel.

Sorry this does not really shed any light on your LED indicator.

 

The battery charging article you found is completey wrong.
A fully charged Ni-Cad or Ni-MH battery cell is not 1.2V, it is 1.4V to 1.5V.

 

The battery charging article you found is completey wrong.
A fully charged Ni-Cad or Ni-MH battery cell is not 1.2V, it is 1.4V to 1.5V.

+1 AudioGuru. That is an accurate assesment for a healthy NiCd or Ni-MH. I've seen some of my own batteries vary on the low end, close to 1.36V - 1.44V. But 1.2V is nearly discharged.

 

Does anybody use old Ni-Cads anymore? They are very poisonous and have a low capacity.
My cheap Chinese solar garden lights came with Ni-Cads that quickly failed and I replaced them with Japanese (Energizer) Ni-MH cells.
Mine are 1.55V when fully charged but the voltage depends on the temperature and the charging current.

 

We ought to keep on subject, but they are still selling too many devices with NiCd batteries in them, unfortunately their stockpile is not yet depleted.

 

they are still selling too many devices with NiCd batteries in them, unfortunately their stockpile is not yet depleted.

Maybe their government is telling them to get rid of the poisonous batteries. To sell them to us.
The cheap Chinese Ni-Cad cells rust away in a couple of months. Energizer Ni-MH cells last for many years.

 

OK, this circuit doesn't have an LED indicator (yet!), but is a pretty simple constant-current charger that stops charging at a pre-set voltage.

It's important that you understand this part first before going further.

The charge current flows through R4, which develops a voltage across R4.

150mA flowing through a 4.3 Ohm resistor develops ~.64 volts across itself, which is enough to cause the transistor Q1 to conduct, which keeps the output current constant until the terminal voltage (set by R2) is reached. 150mA is a somewhat arbitrary charge current; it's best to consult the datasheet for your particular batteries to determine the optimal charging rate.

The charger will maintain the batteries at the preset voltage indefinitely, as long as the input voltage is ~8v or more. If the input voltage drops below 7.7, the batteries will begin to discharge via R1/R2/R3.

 

thanks a bunch, Sgt. Wookie! I will use that circuit to see what I can come up with.
I think step 1 is to go outside to the garden shed, and test the panel I have (it is from one of those outside fountains - I thought it could do double duty to charge some AA batts. that we use often). The ones I have now are 1.2V NiMh, 2500mAh units. I will find out how much current the thing can provide and at what voltage, first.

May I assume the items in the right-hand block ("RBatt, Batt1", the R and C) are a model for a battery?

Thank you, this is getting interesting

 

Ok, basically I think the panel I'm messing with is not sufficient for what I want to do. With a 100 ohm load, the voltage drops to 4V, with a current of 40mA, of course. Lowering the resistance to 10 ohms, I get .5v/50mA.

Now, for charging batteries, I should have something like 10% of their mAh rating, correct? So I'd need a panel with an ampacity of over 4x what this one will provide...it's probably going to be cheeper to just buy a solar charger for AA batteries. I'd hoped to make one, just because I'm a DIY lover, but sometimes buying is better!
Thanks for the suggestions!

 

Nice description, Sgt. And, although we are helping him with an LED indicator, we ought to help him realize that the current design will be on the edge of non-functional quite often.
As soon as one cloud rolls over, the voltage will be too low to charge the battery and it will begin to discharge, defeating its intended purpose. The battery will likely be charging and discharging continuously.

I might be tempted to drop the voltage regulator and replace it with a diode, thus insuring a chargeable voltage. 9V isn't that bad when you need at least 6.5V...it's the current that we need to understand here a bit.

But I'm waiting for the LED non-the-less!!!


iONic

 

You could use a solar panel with a current rating of 10% of the battery mAh rating only if the sun is shining full blast all the time (including at night) like it does at noon in the middle of summer. But the sun is very bright only at noon and is also dull in the morning and afternoon and in the winter. The solar panel must point directly at the sun but then it needs to move.

 

Yes, the panel I have just doesn't seem able to do what I want. I'm now looking into cheap alternatives, 2 or 4 AA battery chargers. The ampacity of the panels is higher (over 150mA it seems). So you would get your charge within a day, while the ol' sun is shining, rather than over a FEW days!!

 

I would look for solar cells that offer both a higher voltage and current. Adding two 6V pannels will give you 12V, but the current will remain the same. Addin the pannels in parralel will double the current, but the Voltage will remain the same.
I always thought the best way for solar charging batteries would be to buy the smart charger and then use the power supply ratings to build a solar supply that would easily cover that range. Then the battery investment will not dwindle before their time. I was thinking about doing this with one of my chargers until I noticed the current requirements.
The voltage requirements were only 3.7V, but the current requirements were as much as 3A!
That's a lot of solar pannels, and bucks!


iONik

 

You can buy a plug-in "wall wart" type AA battery charger for $10-$15 at just about any department store. It would be pretty difficult to beat the low cost, efficiency and safety in a hobbyist-type project.