# make a USB cell phone battery from a 9-volt battery

#### blue82vette

Jun 14, 2010
9
I'm making a USB cell phone battery from a 9-volt battery. I neeed 2 parts. Please tell me what wattage for these two parts and if not too much trouble, please tell me the formula and give me the steps on how you figured it.

1- 5.1v zener diode / I need to know what watt value I'll need [1, 5, 10 watts]
1- 330-ohm resistor / I also need to know the wattage for this resister.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
OK, I will answer your specific questions. However, it's not the approach I would take.

Assume:
* Rs is the series resistor (330 ohm)
* Vu is the unregulated input voltage (9v)
* Vz is the zener voltage (5.1v)
* Rl s the load resistance (R then lowercase L)
* Il is the load current (upper case i then lowercase L)

The maximum current through the Zener is: Iz - (Vu - Vz)/Rs

The maximum Zener dissipation Pz = Iz * Vz

Under regulation (where Vl = Vz), the resistor dissipation Pz(reg) = (Vu - Vz)^2/Rs

The maximum dissipation of the resistor (where Rl = 0) is Pz(max) = Vu^2/Rs

The maximum current you can draw and remain under regulation is Il(maxreg) = (Vu - Vz)/Rs

The short circuit current is Il(max) = Vu/Rs

HOWEVER If the result of the calculation is that you need a 1.5W device, then (for safety) you should get a device rated at least 50% to 100% more. There are also considerations about ambient temperature and de-rating of components, but the 50% to 100% factor should allow for that in most "normal" circumstances (at least for components that are not typically fitted with a heatsink).

#### blue82vette

Jun 14, 2010
9
Thank you Steve,

I appreciate your input. I just found the directions on UTube. If you've got a better suggestion, I would appreciate your personal imput on this.

While your at it, I'd like to make a portable solar charger for my iPhone3G , as well. I'll be looking forward to your reply.

All the best,

Last edited:

#### blue82vette

Jun 14, 2010
9
Hi Steve,

Please tell me how I can send you or another Moderator a private message.

Thanks,

#### florinanghel

Jun 14, 2010
55
Go to his profile, click on "Send Message" (below his username and rank), then on "Send a private message to (*steve*)".

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
If your power source is a solar panel you can simply use a zener diode without any series resistor.

The zener should be rated for higher power than the solar panel -- it may get quite hot.

I'm not sure what the iphone is going to do when the current available from the panel is insufficient and the output voltage falls. It probably won't break anything, but I don't really know.

#### florinanghel

Jun 14, 2010
55
I'm not sure what the iphone is going to do when the current available from the panel is insufficient and the output voltage falls. It probably won't break anything, but I don't really know.

Could it... discharge?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
Possibly, but I think it's quite doubtful.

#### blue82vette

Jun 14, 2010
9
steve, did you get my private message yesterday?

#### blue82vette

Jun 14, 2010
9
Steve,

I've purchased a schematic of the Acer 2700 from the UK, so I can get the voltage of each of the battery pins. I'll let you know as soon as I get it and find what I need.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,510
As I said in the private message, the more complex solution is not something you would get going (at least guaranteed to get going) quickly.

A longer term solution is a DC-DC converter (or buck regulator) powered from one of the various higher voltage sources.

The main issue with the battery pack from the notebook is charging it. Typically the charging circuitry is in the laptop, so you have to retain that (presuming it works). Another issue is preventing excessive discharge, which is something that kills lithium cells very quickly.

It's probably better to post your questions here as I have limited time and there are others who will be pleased to help.

#### blue82vette

Jun 14, 2010
9
Steve, Thanks for your time/help. I sent you a private message before reading this last post. I'll do as you say after your reply to it if you will. thanks

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