Response from LTC factory applications

Cristian, it seems as if your questions have already been answered, but we’d still like to make some comments. In general, the greater number of batteries in series and greater capacity would require charge balancing. 3-4 series cells and up tends to be a general manufacturer borderline for cell balancing.  Farnell should be able to comment on this. Tenergy would likely say the same. And by the way, Tenergy does make 5-cell packs and can make one-off packs of pretty much any number of series cells. I’m not sure if your application is a low-volume consumer, private hobbyist, etc. Could you advise? We should be able to either suggest a pack for your usage (if you haven’t already chosen one) or get you in touch with the sales guys that could help you develop a one-off 5-cell pack. We have asked for this in the past for reference designs.

As you’ve already considered, if charging two distinct packs of 2 and 3-cells respectively, one could get away with two separate chargers without balancing. Since the batteries are stacked, you & Ryszard have correctly observed that the “top” charger for the upper cells of the stack would need to float in series from the “bottom” charger. You could reference that circuit’s GND from the VBAT+ of the lower circuit, but you would still need an isolated connection to the input side of each charger. It would be necessary to have a front-end isolated topology, such as a dual output winding flyback to take the wall adapter input. This would actually be helpful in terms of providing the lowest VIN possible for highest efficiency to each charger while providing the proper output voltage. For example, the 2-cell charger would only require ~10VIN, while the 3-cell charger would only require ~14VIN. But, your overall system efficiency will hinge on how well the front-end flyback is designed.

For your proposed approach with 5-cells all in one pack, you could certainly still use LTC4008. Again, Ryszard is correct in identifying the difference—or similarity, rather—between LTC4007 & LTC4008. However, we don’t recommend LTC4006/7 for anything beyond 2-4 series cell stacks; LTC4008 has a resistor adjustable output up to 6 cells and beyond. As I said, this approach requires cell balancing and since this is a step-down topology (buck), you’d always need VIN>VBAT, which is ~22VIN for headroom above the 21VBAT. LTC4009CUF-2#PBF LTC4009CUF-2#PBF & LTC4012 (recommended for higher current >4A) are also options, though you would need separate microcontroller connections for timer and thermal-qualified termination.

For Li-ion batteries, the temperature is very stable during the entire charge cycle, but temperature failsafes are required for charging either below or above the manufacturer recommended temperature range. Termination for Li-ion/poly/FePO4 chemistries are all the same: timer or current-threshold (C/x). Most times a C/x termination is sufficient as the remaining time spent charging the battery from ~90-95 up to 100% is almost as long as the first 95%.

Hi Cristian,

fo small quantity you can build your own pack using e.g. bq77910 protection/balancing IC:

http://www.ti.com/product/bq77910