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Have anyone seen those homemade portable cell phone chargers that are nothing more than 4 NiMh batteries in series and a switch?, they should give about 5 volts, which is the correct voltage but i am not sure if they are safe to use. Others use a 9V battery and a 5 volt regulator. Do cell phones have internal circuitry to limit the charging current or they rely on the charger to do that? Thanks in advance
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
Wikipedia
So the device plugged into a usb port must control it's current draw. Which means you can just have 4z Ni-Vd in series as a usb power source but you should put a fuse in series with the cells to protect the battery in case of short circuits.
I'd prefer 2x Lithium and an ldo regulator because here in Europe the use of environmentally unfriendly Cadmium has been phased out it is a voltage-regulated current-limited output
I've done it a few times. I just use paralleled 7805s or appropriate LDO regulators. Sulaiman nailed it; voltage regulated, current limited supply. 5V +-20mV @ 500mA - 4A. Certain devices won't charge with less than 500mA, and some devices want to draw up to 2.1A (usually tablets).
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
You can have a buck converter, sepic etc. but as a simple project an ldo linear regulator is simpler, and the voltage regulation and current limiting is worth the losses. (compared to a battery fire !)
The loss of power as volts x amps is a concern ... dispersing the heat from the regulator but the mA.h will be the same as the battery pack, and a LiPo pack has more mA.h per volume or weight than NiCd.
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Cell phone charging (homemade chargers)
