BQ2000 from USB power

All of our charges are step down chargers so I am assuming you are tring to charge a single Li-Ion cell or two to three Nickel cells.

for the Li-ion look at the bq2419x family.

For the Nickel cell, how many cells in series and what is their capacitance?

Charles,

I'm trying to make a single circuit that can charge one Li-ion cell or 3 Ni cells.  And I'm not sure of the capacitance as they're the off-the-shelf AA Ni batteries.

Additionally, how do you hook up the circuit to run off of USB power?

Thanks

One really can't charge 3 Nickel cells off of the USB port, with a buck charger.

A nickel cell prior to termination will be 1.45to 1.55 depending on NiCd or NiMH.  One need a constant current source to charge and terminate the charge.  To maintain a constant current the voltage per cell is set as low as 1.7V/cell.  This requires 5.1V at the output of the converter to assure constant current throughout the charge.  The converter probably needs 1V of overhead.  thus you need a boost converter prior to the buck charger.  We do not have a buck-boost charger.

What about limiting it to 2 Nickel cells.  Can a circuit be run from the USB to charge either one Li-ion or 2 Ni cells?

Yes, there is enough voltage to charge two cells, but if one does not know the capacity of the cell and if it is too large, the termination signal may not be noticed.

I assume you will have some detection that will know which chemistry is inserted so the charge can be biased correclty.

I was under the impression that the BQ2000 had the ability to detect battery chemistry.  That was the whole point of using it.  I thought it automatically monitored the voltage/current of the charging battery to determine not only the chemistry, but also figure out when it was done charging (which would solve the unknown capacity issue).

No, it does not detect chemistry.  The host or an extra contact that indicates chemistry inserted would the bias the feedback to get the desired results.

"The bq2000 chooses the proper battery chemistry (either nickel or lithium) and proceeds with the optimal charging and termination algorithms" - this is quoted from the first page of your datasheet.  Are you telling me that it can't do this?

The design will probably work for two cell Nickel and 1 cell Li-Ion, but not for 3 cell nickel and 1 cell Li-Ion.

Let me explain.

The charge profile is constant current (CC) followed by constant (CV). 

If setup for a nickel charger, the MCV would be set between 1.7V and 2.0V per cell (3.4 to4V for 2 cells and 5.1V to 6V for three cells).  The nickel pack would charge and just before termination the each cell has ~1.5V (below the 1.7 to 2V setting) which means the charger is still in CC mode.  Termination occurs when the cell is full and rises in temperature causing the impedance of the cell to reduce and thus the voltage to dip.  After ~3mV dip in voltage the charge terminates.

If set up for a Li-Ion cell, the MCV would be set to 4.2V and the charger would startoff in CC mode charging.  The charger assumes it is a nickel cell looking for a dip in voltage to terminate.  Since the Li cell continues to charge to 4.2V and once there the IC switches to a Li-Ion termination and monitors the current until it tapers to around 14% of fast charge and then terminates.

If setup for a single cell Li-Ion at MCV of 4.2V, then this would properly charge a two cell Nickel pack since it would terminate prior to hitting the 4.2V threshold.

For a three cell nickel pack or single cell Li-Ion, the bias would have to be changed according to the chemistry by some external chemistry detection.

So setting the MCV to 4.2V

2 Ni or 1 Li cell would work for my purposes.  How would you hook it up from USB power to charge these batteries?

I may not be understanding your question.  A USB mini cable could be used from the USB type A jack to the charger.

The charge takes a DC input source that should be larger than the MCV setting so it can step down the voltage.

a 5V input would work for a single cell Li-Ion.

This part does not enumberate with the USB port and would not be USB compliant.

I am looking to use the BQ2000 to charge 2 Ni or 1 Li cell.  I am planning to provide it power in the form of 5 V, 500 mA (possibly 1A).  How would you build the circuit knowing that you will have a 5V input?