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I’m making a Lithium Ion charger circuit and successfully used a Texas Instruments bq2057c to charge 2 Lithium Ion cells in parallel (4.2V). Now I want to do the same with the cells in series (8.4V). I read that when charging multiple cells in series you have to do cell balancing to take into account the difference in charge speed between the 2 cells to avoid gradually reducing the battery’s capacity.

I bought the bq2057c in the first place because in the datasheet it said that it could charge 1 or 2 cells in series. Indeed, you can set the threshold level to 8.4V and the IC will charge the cells to 8.4V. The IC doesn’t balance the cells though so it might very well be that one cell is at 4.1V and the other is at 4.3V which in the long run will ruin the battery’s capacity. I don’t even know why anyone would want to charge 2 cells and not do cell balancing and therefore why this feature of charging to 8.4V is there. Maybe there is something I’m missing. Is it common to use multiple cells in series and not use cell balancing?

I could use an IC that has cell balancing, or I could use 2 single cell ICs and use them to charge my cells individually. Which one is best, and what is the most common? What I don’t like about the cell balancing ICs (e.g. bq29330) is that they always come in packages with a high pin count. I can’t seem to find one with low pin count. Do they exist?

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  • \$\begingroup\$ Some chemistries have shown very little drift and are okay to charge without balancing for several cycles (a hundred or more), but they tend to be expensive and have less capacity than others. So they aren't commonly used. \$\endgroup\$
    – Arsenal
    Commented Oct 13, 2015 at 17:55
  • \$\begingroup\$ Agree with Arsenal, have read many experts opinion that 3 or less lithium cells in series not as important to have balancing as experience has shown them that they just tend to stay in balance. This is only true for cells that start out from the same manufacturer and are of the same age and cycle count. Two very dissimilar batteries put in series would likely need balancing. Having said that, I would still watch for imbalance occurring by measuring voltages after charging. \$\endgroup\$
    – Filek
    Commented Oct 14, 2015 at 6:54

2 Answers 2

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It's likely that balancing will be reasonably OK over a moderate number of cycles. You can measure the two cells occasionally and if there is a significant imbalance, discharge the higher voltage cell "enough".

A simplistic method that will work well enough is to place a 4.2xV clamp regulator across each cell. I say 4.2X rather than 4.2V as ideally it will be just above what the regulator sees as Vmax/2.

The regulator needs to be reasonably precise. A cheap and easy version is a TL431 shunt regulator diode with a Series resistor load in the Cathode to V+ lead and two resistors setting the voltage (Vref to ground and Vref to V+).
Setting the trip voltage to 4.25V at 25 degrees C should be "good enough".

TL431 Datasheet
Figs 21 27 30 relate to this application.
A TL431 by itself has limited sink current.
Fig 30 shows how to easily and cheaply extend the current capability.
If doing this, use of the 0.5% part is recommended plus 1% (or better) voltage setting resistors.

http://www.baronerosso.it/modellismo_articoli/show/622/lipo-balancer.html

An implementation of the above concept.

From here

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And again ...

Here is a simple method that should be effective.

From here
Note that this loads the battery at all times when powered on.

enter image description here

A slightly tidier implementation - but probably no more effective. Allows higher rebalancing current due to transistor buffer.

From here

enter image description here

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Your charger seems to work fine- the total pack voltage is 8.4V as advertised.

A simple way to balance two Li-On batteries is to use this balancing chip: http://www.ti.com/lit/ds/symlink/bq29200.pdf. It has just 10 pins.

TI claims that the maximum voltage imbalance is kept below 30 mV.

enter image description here

Is it common to use multiple cells in series and not use cell balancing?

Always balance battery/ultracap strings to maximize energy capability and lifetime.

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