How to maintain current in battery chargers?

[Lord Loh.]

battery boost charge mode 0.3c

I have been reading all previous threads on Battery Charging circuits and was wondering that in a basic charger circuit which requires current to charge, how do I maintain the current,

If I place a current limiting resistor, shall not form a potential divider with the cell and give a low voltage drop across the charging cell?

I am assuming that I can consider the resistance of the cell large as I am connecting it parallel to the charging source. The current form the target cell shall oppose the current from the source cell. And as the target cell charges, it's voltage level shall incerease and so shall it's current delivering capacity. And it's "resistance" shall further increase. Is my assumtion correct?

So do I need to decrease the voltage output across the terminals of the source as the target battery charges?

And how much current would a NiCd cell need for fast charge(1/4 rated?) and trickle charge(1/10 rated?)?

Please guide me...

Thank you.

 

[IanP]

Re: Battery Chargers

There are several commonly used methods for charging Nickel batteries. They are all related to the length of the charge cycle which determines the recommended charge current. A slow charge (or low rate charge) consists of a relatively low charge current, typically 0.1C, applied for approximately 14 hours set by a timer. A quick charge applies a constant current of approximately 0.3C to the battery while a fast charge applies a constant current of 1C or higher. Both quick and fast charge cycles require that the charge current terminate when the battery becomes fully charged.

1C = battery capacity ..

Instead of a resistor you may consider constant current circuit .. see picture below ..
More details at:
**broken link removed**


Regards,
IanP

 

[Lord Loh.]

Re: Battery Chargers

...A slow charge (or low rate charge) consists of a relatively low charge current, typically 0.1C, applied for approximately 14 hours set by a timer....

Is the 14h standard? what if my starting voltage is not 0v? suppose I want to charge a battery from 1.1v, sould I still give it a 14h charging?

How do I decide the terminating voltage?

Thank you.

 

[IanP]

Re: Battery Chargers

For NiCad batteries 14h charging time is pretty much standard ..

A healthy battery will never have 0V and NiCad batteries shouldn't be charged before being discharged ..
Voltage of a discharged battery is not that important - you charge it with a constant current and do it over required period of time ..

Regards,
IanP

 

[Nimer]

Re: Battery Chargers

hi

you could use a buck boost converter also as a charger.

regaeds

 

[Lord Loh.]

Re: Battery Chargers

So what sould my terminating voltage be for a NiCd? I am currently thinking of 1.35v.

Or should I charge the battery for 14h without bothering about the terminating voltage.

My application requires a fast charge. I am supposed to be charging robots at a docking station. So Should I apply a 1C charging current for 2h (by timer) and then switch indefinitely in trickle mode thereafter?

What if the robot docked is not discharged fully? In this case is it not unwise to start charging using 1C for 2h?

On some site I read that NiCds give a constant 1.2v till they exhaust their power fully then the voltage drops abruptly. And charging is an endothermic process and the joule heating and the endothermic reaction cancle out each other and the battries are at room temperature once the battries are fully charged, they start joule heating. So it appears that temperature sensing is the best way of terminating the charging process. Have I got all this correct?

Please help.

Thank you.