Charging & balancing 2 LiIon cells in series?

[Dave92F1]

Is there a simple way to charge 2 LiIon cells (that are in series) while balancing them at the same time?

This is for a system for autonomous recovery of model rockets with a steered parachute based on GPS navigation. (Lots of details at http://nerdfever.com/?page_id=695).

I've got a 2S battery pack that runs it. I see there are lots of chips that will charge a single LiIon cell, or 2 cells in series (without balancing them).

But I haven't been able to figure out how to charge both cells interdependently (so they'll be balanced) from a single supply (+5v USB supply).

Right now I charge them on the workbench using 2 separate power supplies, but I'd like to be able to charge them just by plugging in a USB cable.

Any advice?

 

[(*steve*)]

You can place shunt regulator across each of the cells. This should be set to the "fully charged" voltage of the cell (or just a tad under) so that as the batteries get to full charge, they stop charging.

OK, that works for batteries in series.

In parallel you can do the same thing, but place a diode in series with each cell so that a fully charged cell doesn't rapidly discharge into its discharged neighbour.

If you're wanting to use USB, then series is out (unless you go to extremes). The lack of significant extra voltage means that regulation of the charge current is going to be harder than (say) using an LM317. If you simply use a resistor (along with a shunt regulator to prevent overcharge) then you're not going to get very fast charging, especially considering USB's current limit.

 

[GonzoEngineer]

Check Linear Technologies. They make some great charger circuits and have a lot of example circuits for multiple cell charging.

I did a design a few years back using the LT1512. Used the schematic right off the Data Sheet, and it worked perfectly! (4 cells.)

 

[(*steve*)]

The LT1512 does not address the balancing issue.

 

[GonzoEngineer]

The LT1512 does not address the balancing issue.

I stand corrected! All I can remember is that I had 4 cells in series, and everything worked OK. The system is still working today.

Guess I need to some more homework.....never to old to learn.