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Can I charge my 1800mAh battery with 450mAh?

My 1800mAh battery is for my smartphone, and Im using a 9V niMH battery 450mAh to charge it. Basically, I used a regulator to make the 9v input to 5v output.

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  • \$\begingroup\$ Are you asking if a 9V 450mAh battery can fully charge your 5V 1800mAh cell phone battery? Or are you asking if charging is possible period? Or do you need help figuring out how to do the charging? \$\endgroup\$ – I. Wolfe Mar 20 '15 at 16:16
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In theory, yes, you probably can do it (sort of). However...

  1. You will lose a lot in the conversion, especially if your voltage regulator is linear rather than switching.
  2. Even with 100% conversion efficiency, you can only charge the 1800 mAh battery to a fraction of its capacity - assuming it's a 5 V cell (probably unlikely) the 450 mAh would only be able to charge it to just under half-full.
  3. I'm assuming that you're talking Li-Ion or Li-Pol batteries here. If that's the case, then be warned that they're very sensitive to bad charging (overcharging in particular), and are liable to overheat and burst into flames if not handled correctly. With the greatest of respect, from the tone of your question, I suspect that you don't really have sufficient knowledge and experience to do it properly.
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  • \$\begingroup\$ yeah :D im just a newbie doing some diy stuff.. and yeah, i know that it cant get my phone battery to fully charge based on the mAh difference (math stuff). but basing on what you said "in theory", i suppose this dont work in actual, cause when i tried to build my circuit, my phone didn't change to charging mode. \$\endgroup\$ – Knives77 Mar 24 '15 at 16:14
  • \$\begingroup\$ OK, if you're plugging it into the phone, then the phone should be handling the management of the battery charge. Your problem is probably that the source battery can't supply sufficient current to keep the output regulator at 5V. Don't forget that the phone's current drain isn't necessarily constant - it might be switching it on in pulses, and seeing how much the input voltage drops, to detect how much current it's able to draw from the supply. Have you got a good smoothing capacitor on the output of the regulator? Have you tried checking the voltage with a voltmeter? \$\endgroup\$ – David Smith Mar 26 '15 at 15:21
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The answer is yes, you can, but it doesn't seem all that useful since you can only charge your phone about 20% assuming that your charging setup is 80% efficient.

Edit: I am assuming that you are using a linear regulator. If you using a switch mode regulator, you could potentially get around a half of a charge on your cell phone battery.

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