Battery charger

[John80]

I have a few problems with the bq2002 battery charger. I can’t understand the datasheet right and I don’t have so much experience with charging circuits. I want to charge two 1.2V cells in series, like the NiMh.

The output of the bq2002 is an open drain one and I want to use a FET as the switching element. The charge power will come from a USB socket and I don’t want to have more than 300mA of charge current. Which must be the charge voltage of the batteries?

The charge power must be from a current limited or constant current source, according to the datasheet. How I will set the USB to be a constant current source with 300mA?

[neoone]

I did it like this:

I've used LM317 as a current source. I haven't test this circuit myself but a company I designed it for didn't complain so I think it's ok. You won't need some elements for example D1 and D8. As I remember values of elements R52, R54-56 are absolutely random and they need to be chosen according to thermistor (if you want to use one). Probably some of them are needless but it was designed to be universal. VCC_EXT is an external supply - in this case USB 5V. JH4 is battery + connector and JH5 is -.

[Zero999]

That won't work reliably from 5V.

The LM317 constant current source needs 3V of head room to regulate properly, plus another 0.7V for D8. It may work but you'll find the current will drop below the expected level as the battery charges. You might as well replace the LM317 with a transistor current source.

[neoone]

true, I don't remember what was the voltage but by mistake I assumed 5V be cause they used mini USB connector but there was no USB so the voltage could be higher. Some other current source for example a transistor with Zener diode (or maybe better simple Si diode like 1n4148) should work well.

[John80]

First of all thanks for your answers!

Probably I will eliminate the LM317 and I will put a current limiter IC with programmable output current. The problem is that I don’t know the charge voltage for the two NiMh cells in series.

A little bit of help about that?

[neoone]

and what do you need that voltage for ? You charge with constant current. Hmm ok you need this for voltage sense pin 3. And my circuit is for 2 cells and as you can see I've used 2 330k resistors for the divider. Check this application note for clues and you'll find that R63 on my schematic needs to be R62*(N-1) where N is a number of cells.

As for current source you can use some low voltage LDO like 1.2V or simply PNP transistor. I'll draw my vision and post it in a minute.


that should give about 300mA. You may add restor between gate and source of T2 which I forgot.

[John80]

and what do you need that voltage for ? You charge with constant current.

I am saying that because even if I charge with constant current I can not have a 30V source that will supply that current! Even with the small voltage drops into the FETs/BJTs, resistors and diodes, more than 15V will drop to the battery terminals. I haven't got so much experience with electronics but this is probably a big problem! Thats why I am asking for the voltage that the battery must have on their terminals during the charge.

[neoone]

OK, maybe I didn't understand what you wanted to say but it sounds like you need to read a bit about current sources. If you want a constant current of about 300mA from current source on 2 cells the voltage won't be 30V or 15V for sure. In fact it probably won't be even 5V and it doesn't mater if the current source is capable of supplying 30V, 100V or 1000V.

[meurico]

Hi,

someone can explain me what is doing the R60 in the first schematic? Please help me

Regards