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# Charging a battery using a smaller voltage source?

lightwate
I've been wondering about this topic for a while now. Is it possible to charge a battery, let's say a typical 3.7V cell phone battery with an input voltage lower than 3.7?

I think that if your input voltage is lower, the current will reverse and you'll end up discharging the battery. If that's the case, I assume that solar chargers (since there's no guarantee that they always meet the required voltage) use some sort of diode to prevent discharging. But during the time when the photovoltaic cells don't meet the required voltage, then the battery isn't charging at all, right?

Let's say your PV cells only produce an output of 1V, is it possible to store that energy first, and when you have enough then you can charge your battery?

Can anyone enlighten me on this topic?
jetgirltaxi
 lightwate wrote: I've been wondering about this topic for a while now. Is it possible to charge a battery, let's say a typical 3.7V cell phone battery with an input voltage lower than 3.7? I think that if your input voltage is lower, the current will reverse and you'll end up discharging the battery. If that's the case, I assume that solar chargers (since there's no guarantee that they always meet the required voltage) use some sort of diode to prevent discharging. But during the time when the photovoltaic cells don't meet the required voltage, then the battery isn't charging at all, right? Let's say your PV cells only produce an output of 1V, is it possible to store that energy first, and when you have enough then you can charge your battery? Can anyone enlighten me on this topic?

Voltage is another word for potential energy. The difference in potential energy makes current flow. You can make an analogy between electricity and water. Let's say you had two buckets with a pipe connecting them, and you lifted one of them higher than the other (greater voltage). The water will flow until the level (voltage) in the lower bucket is sufficient to resist any additional flow. Therefore you are correct in stating that the current will flow from the higher-potential source (the 3.7 volt battery) to the lower-potential source (the PV panel)l. Solar chargers DO have blocking diodes to prevent the battery from discharging across the PV junction when its voltage is lower than the battery.

The only thing that can actually "store" electricity is a capacitor. You can make a circuit with a capacitor and transistor (and blocking diode) which will allow the capacitor to charge (slowly) from the PV panel, then "switch" it so that it discharges (rapidly) through the battery on a periodic basis. This is analogous to pushing a boulder up a hill that has a shallow and steep side. You push it up the shallow side, which requires less energy (voltage) for a longer period. It rolls down the steep side, which releases more energy (voltage) for a shorter period. The amount of WORK put into is it the same you get out of it, you can't get something for nothing. Most DC-DC voltage converters do this.
lightwate
 Quote: The only thing that can actually "store" electricity is a capacitor. You can make a circuit with a capacitor and transistor (and blocking diode) which will allow the capacitor to charge (slowly) from the PV panel, then "switch" it so that it discharges (rapidly) through the battery on a periodic basis. This is analogous to pushing a boulder up a hill that has a shallow and steep side. You push it up the shallow side, which requires less energy (voltage) for a longer period. It rolls down the steep side, which releases more energy (voltage) for a shorter period. The amount of WORK put into is it the same you get out of it, you can't get something for nothing. Most DC-DC voltage converters do this.

I see, that clears up a lot of things. By the way, can the same thing be done using a transformer?
ocalhoun
 lightwate wrote: By the way, can the same thing be done using a transformer?

Only in AC circuits.
Transformers work by induction, which requires moving magnetic fields. In a DC circuit, the field is stable, not moving, so transformers won't work.

To use a transformer in this situation, you'd have to go: DC battery --> convert to AC --> run through step-up transformer --> convert back to DC --> higher voltage DC battery.
...You'd lose a lot of power in the conversion process.

If you're trying to go from low voltage, high current AC to high voltage, low current AC, transformers are ideal though.
Dennise
Alternately, one could use a boost switching regulator.

Such a regulator uses a high speed switching circuit that uses the decaying magnetic field of a previously stored magnetic field in a inductor to add a regulated voltage to the input voltage.

Thus: Vout > Vin.
chatrack
Hi,

It can be done by, using a "joule thief " circuit.
This circuit need only 0.7 Volt to start working,
It oscillates using a transistor and boost voltage
with a toroid transformer.

Out put of the toroid transformer can be used
to charge a battery to a continent high voltage.

a Good and useful circuit to reuse the "useless" battery

johans
 lightwate wrote: I've been wondering about this topic for a while now. Is it possible to charge a battery, let's say a typical 3.7V cell phone battery with an input voltage lower than 3.7? I think that if your input voltage is lower, the current will reverse and you'll end up discharging the battery. If that's the case, I assume that solar chargers (since there's no guarantee that they always meet the required voltage) use some sort of diode to prevent discharging. But during the time when the photovoltaic cells don't meet the required voltage, then the battery isn't charging at all, right? Let's say your PV cells only produce an output of 1V, is it possible to store that energy first, and when you have enough then you can charge your battery? Can anyone enlighten me on this topic?

this depends on the input..
Bikerman
No.
Another phrase for voltage is 'potential difference' and it better explains the concept. Current will always try to flow from higher to lower voltage, regardless of amperage.You could never fully charge a 4 volt batters with a 3.4 volt source/charge for example....
ocalhoun
 Bikerman wrote: No. Another phrase for voltage is 'potential difference' and it better explains the concept. Current will always try to flow from higher to lower voltage, regardless of amperage.You could never fully charge a 4 volt batters with a 3.4 volt source/charge for example....

Even if the total potential energy of the 3.4 volt battery is higher?

If nothing else, my earlier:
 Quote: DC battery --> convert to AC --> run through step-up transformer --> convert back to DC --> higher voltage DC battery.

Would do it, as long as the 3.4 volt battery had enough power in it to charge the 4 volt plus all the wasted energy from those conversions.
The radar I used to work on converted a 480 volt difference to a 32,000 volt difference, after all.

Besides, wouldn't there be easier ways to do it?

Say...
1: charge up capacitor A to 3.4 volts
2: charge up capacitor B to 3.4 volts
3: connect them together in series to produce 6.8 volts for a short time, charging the 4 volt battery
4: when discharged, disconnect and repeat

And if you did that kind of thing often, you could use transistors to control the whole process and automate it.
Bikerman
Oh sure, but what you are doing is stepping-up the voltage, so my point holds. You can certainly step-up the voltage if you have enough overall energy.
The idea of charging capacitors in parallel and then discharging them in series should work, so I guess that is a way........
I'll refine the point I was making then.....you are only going to get current flow from higher voltage to lower, so your source needs to be ramped in some way so that it produces a higher voltage than the battery to be charged.,..
ocalhoun
It's not often one get's a 'oh yeah, you're right' from Bikerman!

So I'll enjoy it, even if it's not easy to recognize.
Bikerman
Yes, it was a little ungracious when I read it back...
Yep YOU'RE RIGHT
Ramesh_subramani
i need an interfacing circuit to charge a high voltage battery ranging approximately 12V using a variable voltage from a micro hydro turbine..
can anyone suggest any method or any circuits for that..??