Hi All,

I am trying to design a custom lithium ion battery charging circuit. I thought about using a simple zener diode regulator circuit. I’ll be monitoring the voltage with R6 and R7 and cutting off the charging via Q1 when the battery reaches full charge. The circuit will work fine if the battery is close to full charge already. The problem will arise when the battery is very low. At that point it will try to draw more than the maximum current this circuit can source (~150 mA) at which point the zener will stop regulating.

My question is can I limit the current by pulsing Q1? I think the answer is no in this case? Is that correct? If not, what would you recommend to limit the current?

Thanks,
John

 

You would be better off using pwm on Q1 to charge it, using a series resistor to monitor the current.

 

Hi Dave,

So you mean that I can limit the current by pulsing Q1? I thought maybe the battery would try to draw too much current during each on time.

Thanks,
John

 

It would be safer, and probably cheaper in the long run, to use a dedicated Li-ion charger IC or module for the job. They are pretty picky about charging conditions.

 

Li-ion batteries can explode and catch fire if improperly charged.
Follow Albert's suggestion and use a dedicated charger designed for Li-ion batteries.
Unless, of course, you are in the running for this year's Darwin Award.

 

Hi all-

Thanks for your replies. For my own understanding... could I pulse Q1 to limit the current going to the battery or would it immediately draw too much current during the on time?

Thanks,
John

 

If you pulse Q1 then you get PWM. It will draw lots of current during the on time, and no current during the off time. The average current is determined by the proportion of on time to off time. Providing everything can cope with the maximum current - the power supply, battery, etc. - then that would work.

 

Got it. Thanks. In that case, I think the answer is no. It makes me wonder how the packaged IC battery charging chips work. The internal block diagram looks like they are just pulsing the output. Here is the block diagram from the ST Micro STBC08PMR chip.

 

I think that chip number may be wrong, I can't find a datasheet for it. Such chips generally use an inductor which averages out the PWM current for the battery.

 

I may have copied it wrong. Here it is... http://www.st.com/content/ccc/resou...df/jcr:content/translations/en.CD00132950.pdf

Maybe there is more to the internal circuit than they have listed in the block diagram. It doesn't call for an external inductor.

 

In that case it is using the mosfets in a linear mode, not using PWM.

 

That makes sense. Thanks for helping with my understanding.