li ion charging circuit??

[settra]

hello forum. so , hear is the deal. i want to make a li-ion battery, out of 4.2 cells. if i use 4 of those, i would have 16.8v . my question is this.
how are they being charged?? i know, they dont like to be over charged, and that when they are discharged , they are around 3.7 volts... so... would i just apply 4.2volt to charge it?? or 16.8 to charge them all together??

i might be missing allot of things on this, but any help would be appriciated... i also, need to charge them, as fast as possible... so, is there any kind of bottleneck on this??

 

[audioguru]

A Lithium battery is very dangerous when it is charged wrong. It will blow up or catch on fire!
1) You DO NOT apply 4.2V without limiting the current to the charging current rating of the battery.
2) If the voltage of a cell is less than 3.0V then its charging current must be reduced and its voltage must slowly rise or it should not be charged.
3) The voltage of EACH cell must not exceed 4.20V so you must use a "balanced" charging circuit to charge all the cells at the same time.
4) The charging current must be monitored and when it drops to a low value then the charging must stop.

Don't buy Lithium battery cells on e-bay because there are many fakes being sold. They are very much over-rated and will probably blow up if you try to charge them:

Why not buy a charger that is made to charge your battery? Radio controlled model airplanes, helicopters, cars and boats use Lithium batteries and hobby stores sell chargers made for them.

 

[settra]

i dont know... as i sed, i have JUST started looking into it...
i am "planing" to build an electic "bycicle", so i dont know how many volts i would need (depending on the motor, i would say 12 or 24), so i dont know, how i would find a specific charger for my application... (the way i though it, is , if i want say :12v, then i would go with as many 12v "cells" (containing smaller cells) , as i could fit on the frame..) so, nothing that charges the batteries, one-one would fit... it would have to be something bached....

1) , so, if each cell can be charged with 2A (etc), then if i had 4 cells, i would limit to 8A?? or , i would have to limit for each cell seperetly??
3) , what do you mean balanced?? just applying 16.8 wouldent work, becouse some cells would draw more than 4.2?? seems like it...
2-4 : in general it seems to me like the job of a micro-controller...

1,3,4 i can understand them.. but can you clarify, how i would do the 3d??

also, thanks allot for the heads up about the batteries

 

[audioguru]

I laugh when I see an e-bike crawling slowly along the road like its battery is almost dead. Maybe they drove it but forgot to charge its battery the night before.
A 12V motor uses double the amps of a 24V motor with the same power so the 12V speed controller will cost more but for the batteries to drive for the same time the 12V and 24V batteries will cost the same.
Electric cars use high voltages so that the Amps are lower.

1) You probably want to buy a battery already assembled with as many cells as you need. It will be stamped with a recommended charging current. But first you need to know how much current the motor uses and how long you will drive on one charge so that you can select the amp-hours rating of the battery.
3) A balanced charger monitors the voltage of each cell and prevents more than 4.20V on each cell because some cells charge faster than other cells.
2-4) Usually a battery charger IC is used because they have many safety features like temperature and pressure sensor inputs.

 

[NorthGuy]

Before charging lions, may be you can start from smaller animals, such as hamsters

 

[settra]

i was hoping, for something as custom as possible...
the controller is , "just" some transistors, that are being pwm'ed from a microcontroller, correct?

god , lead acid batteries are so much nicer

 

[audioguru]

You can make your own or buy a motor speed controller. It does not care what kind of battery you have.

 

[settra]

cool! thanks allot for the help! i will see what i can do !