While searching for a DIY battery charger, I landed on this website. http://www.apartmenttherapy.com/make-your-own-batterypowered-u-100359
So my question is, is it possible? I have a 9V rechargeable battery and I wish to use it again for our project.
If the answer is yes, can I directly attach the so-called 9V USB battery charger to my Relay Card? This card requires an input of 9-12V DC.
Thanks in advance!
P.S. I don't want to waste materials since I'm running out of funds for our project. T.T
While its possible I think it would just be a disappointment, since PP3 sized
9V batteries, even rechargables, have very little output current capability.
Most USB devices want the full 0.5A on recharge, some want more, and the battery
would just give out and the output voltage drop below 5V very quickly.
Furthermore the capacity of a PP3 rechargable is around 150mAh to 250mAh, whereas
most phones for example have a battery capacity of 1000mAh or so, ie a lot more.
2 quality AA rechargables can comfortably provide 1A with a capacity of 2500mAh,
put through a boost converter to 5V gives 0.4A at 1000mAh or so which is more plausible
as a charger. Google 'minty boost' for an example.
The 9V batteries aren't energy dense, they are designed for powering low power
devices needing 9V, typically radios.
The 9V batteries aren't energy dense, they are designed for powering low power
devices needing 9V, typically radios.
Not only is the energy density low for those 9v PP3 batteries, their price is typically 2 for $5 around here! It's just a unpractical path to take. Those batteries are useful for only very low average current usage, as in powering smoke alarms. Use AA batteries unless physical size is a critical requirement where you could move on up to Li-po battery.
I don't need much of a current so I think that alone would suffice. I am also thinking that the answer to my question is Yes, right?
If so, what should I buy to amplify the 5V output from USB to 9V? I have a LV7905 Voltage Regulator. Would that be enough? I'll just reverse the output and input terminal.
guuwey:
I have a LV7905 Voltage Regulator. Would that be enough? I'll just reverse the output and input terminal.
Are you being serious?
Are you trying to make a charger or a cigarette lighter?
Don't bother with that circuit for lots of reasons.
As already been explained to you, those 9V batteries don't have much "grunt", there are no capacitors on either the input or output of that regulator, there is no reverse polarity protection on the input to the regulator coming from the battery (it's very easy to accidentally touch the terminals on the battery the wrong way round), no diode on the output from the regulator to protect it from the possibilities of it being reverse fed, no heatsink (with a differential voltage of 4V and a current draw of say 250mA) that regulator will start heating up.
That circuit is, to put it nicely, rubbish and the person that thought of it an idiot, don't waste your time or money on it.
guuwey:
While searching for a DIY battery charger, I landed on this website. http://www.apartmenttherapy.com/make-your-own-batterypowered-u-100359So my question is, is it possible? I have a 9V rechargeable battery and I wish to use it again for our project.
Yes, but it's a terrible idea. 9V batteries are rubbish (and so is that circuit, it needs at least a capacitor on the output).
If you really want a battery power source with USB connector, use these two things instead:
guuwey:
If the answer is yes, can I directly attach the so-called 9V USB battery charger to my Relay Card? This card requires an input of 9-12V DC.
If the relay needs 9V then why don't you just connect the 9V battery to it?
Well because if this idea is possible and I can rely on this, I would be using it while doing test with my Relay. I'll just buy another one when I believe that everything works well..
But then everyone insist and highly discourage this idea, then I will not continue it. I'll just buy another battery that isn't rechargeable. I just want to make sure that this thing wouldn't get busted again.
Anyway, thank you for the answers. I'll find another alternative aside from those that you've posted. I had fun reading them. At least I've learned. 
Simpler way to understand the problem with 9V batteries:
ie. They do work, but there's a lot of useless air and packaging material inside them...