cell phone and i-pod charger from lithium battery

[roltex_rohit123]

I have just rebuild my laptop battery and the cells that i replaced with new ones still hold a good charge. laptop used to give me 20 min backup when i took out the cells. I want to utilise the cells in a backup charger circuit for my cell phone and I-pod. i measured the voltage in the cell charger and it was about 6 volts. i want to use 2 lithium cells in parallel. they give about 4.1-4.2 volts, when charged. how can i proceed for the circuit. I need a higher voltage 6 volts at 300mA. The cells I suppose might be holding about 0.5 amp charge each. I have plenty of these cells for use. i can charge my cell when power is not available and then recharge both cells and backup battery pack when power is available. which IC's and what components can i use?

 

[roltex_rohit123]

no one to help?

 

[Dragon Tamer]

Charging batteries with batteries is a difficult task, especially with a lithium cell. If you're not careful, the lithium cells could easily overheat and even catch fire. But since I didn't want to post to deter you, I think that what you need is some kind of buck converter. The ICL7660 is a pretty good one. Your iPod will charge at 5V @ 300-500mA, I have no idea what your cell phone would charge at.

 

[roltex_rohit123]

Charging batteries with batteries is a difficult task, especially with a lithium cell. If you're not careful, the lithium cells could easily overheat and even catch fire. But since I didn't want to post to deter you, I think that what you need is some kind of buck converter. The ICL7660 is a pretty good one. Your iPod will charge at 5V @ 300-500mA, I have no idea what your cell phone would charge at.

I want to use a switch mode boost charger. actually the batteries are meant to be connected parallely. the charger wont be in operation continuously. i think by buck charger you are expecting to connecting the batteries in series, but this is not the case. i want to connect them in parallel. I wanted a switch circuit design help. would you?

 

[roltex_rohit123]

Charging batteries with batteries is a difficult task, especially with a lithium cell. If you're not careful, the lithium cells could easily overheat and even catch fire. But since I didn't want to post to deter you, I think that what you need is some kind of buck converter. The ICL7660 is a pretty good one. Your iPod will charge at 5V @ 300-500mA, I have no idea what your cell phone would charge at.

I want to use a switch mode boost charger. actually the batteries are meant to be connected parallely. the charger wont be in operation continuously. i think by buck charger you are expecting to connecting the batteries in series, but this is not the case. i want to connect them in parallel. I wanted a switch mode circuit design help. would you?

 

[Sceadwian]

So shop around for a switch mode boost converter? There are plenty of schematics available on the web, try looking for some of those first come up with a few of your own ideas first then we have something to help with. You should have no problems finding dedicated IC's for boosting from/to those voltage levels, or even discrete circuits.

Keep in mind it's not safe to charge lithium batteries in parallel, they need to be charged individually, lithium's low internal resistance and the slight variation between cells will cause one to overcharge (possibly fatally) while the other one will be under charged which will lead to even more wasted power as the stronger of the two cells tries to balance the weaker one. NiMH's and NiCads were much easier to ballance because they could handle a slight trickle charge all the time which would do the balancing, such a scheme does not work for lithium's because the batteries will gradually be destroyed if they're trickle charged the lithium metal actually plates out onto the anode or cathode (I forget which)

 

[roltex_rohit123]

So shop around for a switch mode boost converter? There are plenty of schematics available on the web, try looking for some of those first come up with a few of your own ideas first then we have something to help with. You should have no problems finding dedicated IC's for boosting from/to those voltage levels, or even discrete circuits.

Keep in mind it's not safe to charge lithium batteries in parallel, they need to be charged individually, lithium's low internal resistance and the slight variation between cells will cause one to overcharge (possibly fatally) while the other one will be under charged which will lead to even more wasted power as the stronger of the two cells tries to balance the weaker one. NiMH's and NiCads were much easier to ballance because they could handle a slight trickle charge all the time which would do the balancing, such a scheme does not work for lithium's because the batteries will gradually be destroyed if they're trickle charged the lithium metal actually plates out onto the anode or cathode (I forget which)

I have many options for shopping and they are even cheaper. but i want to make one for myself, which would give me an experience and satisfaction of designing my own circuit. I have couple of IC's sorted for this purpose. hope you could help me in selecting the IC.

TPS61220
LTC3458L
LTC3423
LTC3499. The LTC 3499 seems to be promising.

 

[Sceadwian]

void .....

 

[roltex_rohit123]

why void? i have decided to use only a single cell. no parallels no series.. hope you could help me now..

 

[Sceadwian]

It was void because I wrote something that was a mistake, and they took away the ability to delete a post so I had to fill it in with something. I don't know what the actual current draw of an Ipod charging a low battery is but there's even an example for a lithium ion to 5 volt circuit in the PDF for the LTC3499, it only puts out 350ma though, I'm not sure what's limiting it's current, I don't know if a larger inductor would help or if you just start losing regulation if you draw more than 350ma, I'm not too well versed in the idiosyncrasies of switch mode supplies

 

[roltex_rohit123]

300ma in a charger is not bad. i've just bought a spice 5151 cellphone and its charging current is just 500ma. so 300ma is just going to take a longer time thats it. coz in my older nokia i had a 800ma charger and used to sometimes charge it with other chargers 350ma. then it used to take an hour longer to charge. i could leave it on charger the whole night with no fear of battery harm like from my 800ma charger.
now if i plan to use the LTC3499 then how can i proceed for designing SMPS for the charger using this IC?

 

[mneary]

How long will your host battery be able to charge the phone? You're using a single cell from a pack that's already near end of life?

 

[roltex_rohit123]

the battery can still hold much charge. i have surplus of such batteries. i have even new ones, that can cahrge the phone atleast 3 times fully. so i dont have shortage of the battery packs.