Hello
I have a led run by a D size rechargeable battery the led light for five hours.
Looking for a circuit that when first battery go flat the second take over and light the led until daylight come when led turn off automatic.
mrel

 

You should post your existing circuit.

Why don't you simply use both batteries in parallel or series? That will greatly simplify your circuit.

 

I suggest using the circuitry from a cheap solar lamp, and using 2 D cells in parallel if one is not giving you enough time. There's really no reason to switch from one to another, just use them both together.

 

Following on wayneh's idea, AudioGuru posted a schematic from a cheap solar powered garden lamp a year or two ago:

From thread: http://forum.allaboutcircuits.com/showthread.php?t=9571
...the first reply.

Also, keep in mind that rechargeable "D" size batteries frequently are simply "AA" batteries with a much larger case on them. Compare the mAH ratings between your D cells and various AA rechargeables.

 

I suggest using the circuitry from a cheap solar lamp, and using 2 D cells in parallel if one is not giving you enough time. There's really no reason to switch from one to another, just use them both together.

I already have two battery connect in parallel.
But I was think if there circuit to switch to fresh battery the led will be more bright.
mrel

 

I already have two battery connect in parallel.
But I was think if there circuit to switch to fresh battery the led will be more bright.
mrel

The it doesn't matter how many D Cells you put in parallel, the LED brightness is dependent on more than just the batteries, but also the series limiting resistor and the LED's specifications. Adding another D Cell in series without changing the series current limiting resistor will likely blow the LED. Lowering the Series Current limiting resistor will make it brighter, but for less time and will eventually kill the LED's prematurely. Adding more circuitry to monitor the voltage and switch to new batteries will add more drain on the batteries and thus may require still more batteries. If the device is charged with solar energy, then the best thing to do is make sure it is getting as much sunlight as possible.

 

I already have two battery connect in parallel.

What's the rest of your circuit? The solar lamps produce a higher voltage from a battery, so that only one cell is needed to provide ~3v for the LED. This circuitry allows capturing most of the energy from a battery while the LED remains lit. There really isn't a better strategy, otherwise they'd use it in all those lamps. 2 D cells, even if they're really AAs in big cans, should give many hours of light from a single LED.

 

I already using circuit similar to the solar garden light to recharge the battery.
Looking for a circuit that sense when battery go low as .96 volts the circuit will switch to second(backup battery) to keep the light bright since the battery fresh.
mrel

 

Look up "Joule thief". It's a neat little circuit.

Hate that 1N4004 in AudioGuru's circuit. With only a couple of volts to play with, that 0.6V drop is costing plenty. A Schottky diode would be a lot more satisfying.

 

There is not only a simplicity advantage but an electrical advantage as well to having batteries in parallel instead of switching them in one at a time; that is the battery's own internal parasitic resistance.

All batteries have internal resistance. As they get discharged, the resistance increases. The higher the internal resistance, the more power is wasted on the internal resistance instead of being transferred to the load.

So, if you have two batteries connected in parallel, they effectively have 1/2 the internal resistance that a single battery at the same state of charge would have, so only half as much energy would be wasted. This makes two batteries in parallel powering a load actually last longer than having a single battery powering the load, then switching to another battery.

Also, if you had such a circuit to power the load by one battery until exhaustion, then switch to the other battery, that circuit would also drain power from the batteries.

0.98v is not enough to use MOSFETs. Relays, even if you could find one to operate from such low voltages, would be out of the question as they are very power-hungry. Transistors would drop at least 1/10 volt from collector to emitter, along with taking 5% to 10% of the current away from the batteries just to turn the transistors on.

I suggest that what you really need is a larger solar panel that can charge your batteries more effectively.

 

There is not only a simplicity advantage but an electrical advantage as well to having batteries in parallel instead of switching them in one at a time; that is the battery's own internal parasitic resistance.

All batteries have internal resistance. As they get discharged, the resistance increases. The higher the internal resistance, the more power is wasted on the internal resistance instead of being transferred to the load.

So, if you have two batteries connected in parallel, they effectively have 1/2 the internal resistance that a single battery at the same state of charge would have, so only half as much energy would be wasted. This makes two batteries in parallel powering a load actually last longer than having a single battery powering the load, then switching to another battery.

Also, if you had such a circuit to power the load by one battery until exhaustion, then switch to the other battery, that circuit would also drain power from the batteries.

0.98v is not enough to use MOSFETs. Relays, even if you could find one to operate from such low voltages, would be out of the question as they are very power-hungry. Transistors would drop at least 1/10 volt from collector to emitter, along with taking 5% to 10% of the current away from the batteries just to turn the transistors on.

I suggest that what you really need is a larger solar panel that can charge your batteries more effectively.

What about using separate battery to power the circuit that will do the switch of low battery to the second battery that full charge.?
Where would i find a circuit like that?
mrel

 

Look into comparator circuits using the LM339 quad comparator, which you can easily find.

There must be something you're not telling us. For one thing, when I put a freshly charged AA battery into my solar lamps, they'll stay lit all night for 3-4 nights even in the winter, until the lack of sunlight limits them. So two Ds, even if they are really AAs, should be more than enough. But battery failure is the main reason these lamps lose performance over time. So maybe you just need a better battery.

Once you install a better battery, the limiting factor is light. If they're not in direct sun all day, there isn't enough energy captured to light an LED all night, no matter how large the battery is. In fact performance will go DOWN with increased battery capacity because charging efficiency is highest when a battery is ~80% charged. A too-big battery that has discharged over a long night will never get up to that efficient charging level during the day.

 

What sort of LED is it, a 1W Luxeon Rebel??