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Need advice on design battery charger

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bhengsoh

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Currently, I'm designing a solar battery charger which can charge a battery using a LM317T as a current regulator.

But my problem is I need a "automatic switch" that can allow me to switch between two curcuit:

one is for charging the battery, the other one is for discharging through load (eg some LEDs).

I am still a beginner.

Can some experts give me some advice on this design?

The battery im using is two 1.2 V Ni-Cad, im assuming they will give me 3 V after fully charged, is it correct?

And how to design that "switch"?
 

Sceadwian

Banned
It will only be 3 volts for a very short period of time and drop quickly to 1.2 volts per cell or less when loaded. What is the open circuit voltage of the solar cell and the short circuit current? Also what is the capacity of the batteries?
 
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mytech

New Member
Why do you wanted to switch as soon as your batt. get fully charged??
Anyway you can use a simple voltage detector circuit, using zenor,resistor and transistor. Which the transistor will be turn to ON position once its reaches the threshold voltage of the zener which connected to your battery. You can adjust the voltage you want to be turn ON your load by using the resistor. Even you can put variable resistor also. And the loads you can keep it with the transistor.

But the problem is,
once the circuit reaches it desired voltage it will switch ON the loads. As soon as the loads starts working then your batt. looses the volatge and get back to charge condition. So it switches between the two modes at very short times, depends upon your load.
So my suggestion is , you can use one more detection circuit say after certain voltage only it should get back to charging mode, i mean after complete discharge of your battery. Then it will be fine i guess.
 

MikeMl

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bhengsoh

New Member
It will only be 3 volts for a very short period of time and drop quickly to 1.2 volts per cell or less when loaded. What is the open circuit voltage of the solar cell and the short circuit current? Also what is the capacity of the batteries?
I am using two 3 V solar panel with 1 W . My battery is 1600 mAh so current will be less than 160 mA.
 

Sceadwian

Banned
You should probably be using NiMH's not Nicads, and why bother with a switch at all? 3V 1watt is peak, the actual average you're going to get out of it is going to be much lower and should probably be safe for constant charging of the batteries, when the circuit is in use the solar cell will provide current directly to the circuit reducing the load on the batteries if there is a lot of sun. Is this for something like an LED lawn light?
 

bhengsoh

New Member
You should probably be using NiMH's not Nicads, and why bother with a switch at all? 3V 1watt is peak, the actual average you're going to get out of it is going to be much lower and should probably be safe for constant charging of the batteries, when the circuit is in use the solar cell will provide current directly to the circuit reducing the load on the batteries if there is a lot of sun. Is this for something like an LED lawn light?
Actually im trying to make a switch so that the battery won't over discharge when no sunlight.

Anyway ty for the replies.

If anyone have new ideas, feel free to share. ;)
 

Sceadwian

Banned
You should have said that first...
 

MikeMl

Well-Known Member
Most Helpful Member
So I should use a relay in my circuit?
No, presumably your load is a few mA. The output of a 555 can switch your load directly.
 

bhengsoh

New Member
Is there any alternative than using NE555?

It would be clear if someone can post a circuit design on this.

Ty in advance. :)
 

MikeMl

Well-Known Member
Most Helpful Member
Here is a simpler version using a TL431/LM431. It Cuts In at a higher voltage, and Cuts Out at a lower voltage, controlled by R3. The CutIn is settable over a range of 3-4V using the Pot. The quiescent current is very small.

In the simulation, I show what happens when the solar panel puts out less current than it takes to run the load, i.e. the load is off until the battery charges, then the load turns on but the solar current is not sufficient to supply it, which means the battery discharges until the load is turned off again. Obviously, if the solar current is more than the load requires, then the battery keeps charging and the load stays on.

Note that the solar panel is modelled as a current source, while the battery is modelled as a HUGE capacitor. The time axis in the simulation is arbitrary...
 

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