Mobile LiIon battery charger(?)

[alteh]

so, is it possible to make a mobile charger from some LiIon battery with greater cap, like ~3000mAh (could be lap-top battery or standard e.g. 18650), to charge smaller LiIon 3,7V cylindric batteries, like ~500mAh?
plz answer, because this will mean a lot to many ppl.

 

[itsmoked]

That can certainly be done.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alteh]

thnx so, it would probably need some simple electronics like in regular chargers, but with out adapter block, wright? and, could it fully charge that smaller battery even if the bigger one is not full (e.g. on 30% of it's cap.)?

 

[IRstuff]

Not necessarily quite that simple. You need either a battery with a higher voltage or some sort of switching power supply to ensure that the battery being charged sees a sufficiently large enough voltage to get charged, since you need to drive a certain level of charging current. Otherwise, you would barely get the battery being charged to about 30% of capacity, assuming that all the batteries have approximately the same voltage at the same level of charge. You should read:

http://batteryuniversity.com/learn/article/charging_lithium_ion_batteries

before you attempt and experiments.

Also, please refrain from using textspeak. While there is generally low risk of confusion, the engineering universe is already sufficiently inundated with acronyms and abbreviations.

TTFN
faq731-376

 

[itsmoked]

To continue. This would not be a trivial exercise since the "big battery" has to have it's output increased to charge the little batteries and THEN you need to run a safe charging regime for charging the smaller battery. In all cases you have to protect the big batter AND the small battery from over charging, over heating, and undercharging, or a fire can result. All the above has not included that you still have to charge the big battery safely too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alteh]

@IRstuff - ok, i get it, all. before i even registered to this forum, i have searched Google to find something that will solve this "problem". i came across that article too, and i have read it. but since i'm not an engineer, i didn't understand much of it.
yes i knew that i would need some electronics to make that charging possible. the battery that would be charged is already PCB protected, and it should be this one:

http://www.soshine.com.cn/html/Products557.htm

i was thinking to use this one as the charging battery:

http://www.alibaba.com/product-gs/505091201/7_4V_1300mAh_603465_Rechargeable_Lithium.html

as you can see, the bigger one has 7.4V - is it enough for charging the smaller one? if not, should i make parallel set up to increase the Voltage? (but, cap. will be 50%-, right?)

 

[alteh]

itsmoked, thanks. i referred your answer in my post above.
and, sorry for my "faulty" English - it's not my first language.

 

[IRstuff]

http://www.national.com/cat/index.cgi?i=i//78

TTFN
faq731-376

 

[alteh]

IRstuff, thanks man.

could this be helpful for battery to battery charging:

http://electronicdesign.com/article...ery-charger-using-a-single-trans.aspx?cpage=0

 

[IRstuff]

It's now time to suggest that you get a REAL EE to help you. This is not the forum for that. People here answer specific questions for practicing engineers.

The fact that you didn't recognize that the circuit is intended for lead-acid batteries is problematic. The fact that you didn't read the caveat in the linked commentary is problematic. Li-ion batteries are completely dissimilar to lead-acid batteries. The latter are tolerant of abuse, poor charging, and overcharging. Li-ion batteries tolerate none of that.

TTFN
faq731-376

 

[itsmoked]

I have to agree with IR. This task is fraught with hidden issues and needs a pro to pull it off.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[alteh]

yes, i suppose you are right. the only thing i wanted to know is: is it possible or not? so, since you say it is possible, now i can move on with this.
thanks guys.