Charger for 1.2v NiMH battery without contact

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neazoi

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Hi, I have an enclosure and a circuit inside, which is powered by a 1.2v NiMH battery inside the enclosure.
I need to find a way to charge the battery inside, without having to open the enclosure and remove it, or without having to fit a power jack to the enclosure. Size is very important in my application and must be kept as small as possible.

One way I am thinking is some kind of inductive charger.
Another way is some kind of solar charger, where I will provide the Light to a small panel or a phototransistor.
I like the second method if it can be done.
How about these bpw34 or similar phototransistors? How much output current can they output assuming a bright flashlight pointing into them?

What other ways/technology are there, to satisfy my application?
 

neazoi,
Unless I am missing something in your post, how do you expect to transfer power via a photo transistor? They are sensors, not power transfer devices.
All that's happening with a photo transistor is you are reducing the Collector to Emitter resistance to (sort of) zero. No power is transferred.
Your only real option, depending on your power needs, which you have not specified, is inductive coil transfer.
Where do you you expect to get the light from if you decide to go the solar charger route? If indoors, you will be surprised at just how much light you will
need to get any real power. A flashlight won't power anything except a LCD display. With no backlight.
When you arrive at the conclusion you need to go inductive coupling, plan on having the 2 coils as close as possible, as the power dissipates VERY quickly with distance.
Power loss=distance squared. Plan on having the coils within a few mm's of each other. Better yet would be a smaller coil that is the supply coil, going inside the receiving end coil. And no metal between them. More info, especially on power requirements would be helpful.
-Lisa
 
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    neazoi

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BPW34 is a photodiode not a transistor. So it could be used as a photo element. Perhaps you did already look into a datasheet and found the expectable photo current for 1000 lux (=overcast daylight). You'll need multiple devices series connected to reach 1.4 V for NiMh charging and still needs days or weeks to charge an AAA battery in bright sunlight... Effectively not a promising idea. Solar cells with larger area can work.
 
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    neazoi

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Thanks for the post, Also FWM.
There is another idea I once had, to have a coil inside the box then use as the charger a small motor placed near this coil. A small neodymium magnet would be attached to the moror. The rotating magnetic field would produce voltage on the coil. Since neodymium magnets are quite strong I think the power that would be produced would be enough to charge a AAA. The voltage would depend on the rotation speed. This will work as an "open AC generator".

In fact I had this idea for powering devices on the desk without having to have high frequencies involved at all. Why not?
 

well in that case some kind of solar panel, or as above..and just have the output of the panel slowly filling up an output cap....then when there is enough voltage on the cap, you switch in a smps charger, and shovel some charge into the battery..then repeat.
The stipulation of no connection with a wire or connector going into the enclosure is a tough limitation.
 

In the original post he described the it as a phototransistor. I stand by my conclusions, it would not be very efficient to use a photo cell especially if it is not at it's best at that photo DIODES wavelength.
A motor with neodymium magnets would indeed work , but be very tough to balance perfectly, making for a lot of noise and vibration. Still best using inductive coupling. That's why so many power supplys use transformers! : )
 
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    FvM

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    neazoi

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My solar garden lights use a 2.5V solar panel that is 3cm by 3cm all day long in the sunshine (not a flashlight) to charge a AAA Ni-MH cell so that 7mA is used by a voltage stepup circuit at night that drives an LED for 11 hours. An LED lights for only one or two hours following a couple of cloudy days.
 


I am not sure that the magnet-coil solution is worst, compared to the inductive coupling of high frequency signals.
Can we get enough current to charge a single AAA in reasonable time (20mA?) with the inductive coupling method, whereas at the same time limit the size of the pick up coil to something like the top half of a AA battery?

I also wonder if capacitive coupling can be used. Why use inductive coupling is a single plate can electrostatically transfer power (but how much?)
 

I need to find a way to charge the battery inside, without having to open the enclosure and remove it, or without having to fit a power jack to the enclosure
Click to expand...
is the reason you dont want to take_out/put_back the battery , or attach a connector, because you fear "contact sparking"?
If so, then the isoltera system is for you.
**broken link removed**

Regarding capacitive coupling, i think you would need some kind of connector to get the plate to plate distance low enough to give sufficient capacitance.

- - - Updated - - -

in fact, isotera can do it for you, just put one of their couplers on top of your enclosure, and slide the bus wires into the coupler.......totally contactless connection....no sparking, and you charge your battery nice.
The world has forgotten the brilliant isotera system...maye it is for you?
 

treez said:
is the reason you dont want to take_out/put_back the battery , or attach a connector, because you fear "contact sparking"?
Click to expand...

No it is not sparking, the voltages and currents are low anyway.
It is because it will be very difficult to remove the battery from the enclosure (a tube with two connectors at the ends) everytime I need to charge the battery.
Also splashproof feature is desired.
I might end up in using a connector and a rubber protector, but I need to investigate of other ways to di it as well.
 

Capacitive coupling is out of the question, if you knew the surface area, and plate proximity of a 1uF cap, you would understand what I mean.
If you want, Sacrifice a 1 or 10 uF cap, carefully take it apart with gloves on, and unroll the 2 plates separated by the dielectric. You will get the idea fast.

Are you adverse to the inductive coupling method? It really is the obvious solution, for complete isolation, best power transfer, and could easily be made completely waterproof.
My electric toothbrush is a perfect example of the functionality of this method. Use schottkey diodes in the secondary rectifier to minimize power loss. A single diode, half wave rectifier and cap works great for charging batteries, they don't care about ripple.
 


Thanks Lisa,
I think, after some investigation, I might end up in using an actual connector with waterproof rubber protection. The reason is that I can use external smart chargers this way, that would protect battery life and charge it faster. Implementing a smart charger inside the limited space of the enclosure of the device, would be very difficult...
 

ok, there is a certain connector that has like a compartment where you can squeeze in silicone gel to give extra waterproofing.
 

treez said:
ok, there is a certain connector that has like a compartment where you can squeeze in silicone gel to give extra waterproofing.
Click to expand...

Can you propose a chassis-mount connector with a tiny footprint to use?
The current is low, so anyone will do, but the tiniest the best.
I am thinking the charging connector from Nokia phones (2-pins with hole) but I do not know how it is called and if there is a chassis version.
 

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