# Lithum 9v Charger Using 5v

#### ajm113

Joined Feb 19, 2011
174
Hello all!

I'm designing a circuit for my gf as a second xmas gift, and don't worry her first gift was a \$200 purse, so don't think I'm cranking out cheesy gifts. ... Anyways on topic, I'm designing a circuit which will charge a 9v Lithum battery using 5v. Now the reason why I wont use 12v, is that I want the user to be able just to plug this charger into their computer's USB port or connect it to a cell phone charger with a USB port. Now a number of things pop into mind designing this.

My Idea:
Simply write a charger code for my favorite PIC the 12F675, which will control a NPN that has a input of 5v and the output will have a diode to keep the current from going backwards. The problem with this is that there isn't enough juice to make the battery go back to 9v. So I must use a MAXIM IC to turn the 5v into 10v. Then use the ADC pin on the pic to read the voltage on the battery to see if it's fully charged or not.

Is this a good way of accomplishing this? So I avoid over charging, tickle charging, and etc? Or... Should I just buy a IC that does it all, but will have to use the MAXIM IC to boost the voltage?

#### SgtWookie

Joined Jul 17, 2007
22,230
Which MAXIM IC are you talking about? The 7660 capacitive boost unit? The S suffix models are only good for about 20mA tops. It would take quite a while to charge the battery at that rate.

You need a boost circuit using an inductor. You should limit the peak current draw to <=200mA, as many laptops are limited to ~250mA-300mA. It would be silly to charge one from a laptop, but you never know about people.

#### Audioguru

Joined Dec 20, 2007
11,248
Nobody makes a Lithium 9V rechargeable battery.
Each cell is nominally 3.7V so two produce only 7.4V and three produce 11.1V.

Energizer has a non-rechargeable Lithium 9V battery that is actually 7.1V to 8.5V depending on the amount of current.

#### ajm113

Joined Feb 19, 2011
174
Which MAXIM IC are you talking about? The 7660 capacitive boost unit? The S suffix models are only good for about 20mA tops. It would take quite a while to charge the battery at that rate.

You need a boost circuit using an inductor. You should limit the peak current draw to <=200mA, as many laptops are limited to ~250mA-300mA. It would be silly to charge one from a laptop, but you never know about people.
I was actually thinking of the MAX756, such as the minty boost circuits you see flying over the net.

@Audioguru:
Woops, I must have read to fast when I saw that 9v. =/ Well I don't suppose if getting something like a 2.5 - 3.0v headset phone battery and being able to use the MAX756 to boost the volts on the output to charge your phone/device, so all you would have to do is use the 5v coming off the USB connection to charge it then?

#### ajm113

Joined Feb 19, 2011
174
Maybe if I use a capacitor bank of 20400uF and have them charge up from 5v to 12 - 15v? That would be a better alternative? The goal is to beat 800mah.

#### SgtWookie

Joined Jul 17, 2007
22,230