Figure 7 Interleaving connections and charge transfer performance. For full resolution click here .
However, as shown in the Figure 7 example, most of the balancing is accomplished simply by redistributing charge to or from the cells closest to those in need of balancing. A secondary side stack of 10 or more cells allows a weak cell, which would otherwise limit the run time of the whole stack, to recover over 90% of its “lost” capacity simply by running one balancer. Hence, with the LTC3300 interleaved topology, there is no need to move charge all the way from the top of the stack to the bottom – most of the balancing work is done by the local neighbor cells.
Safety Comes First
In addition to providing excellent electrical performance, the LTC3300 bidirectional active balancer provides numerous safety features to prevent mishaps during balancing and to maintain the highest possible reliability. Data integrity checks (CRC checking on all incoming and outgoing data, watchdog timer, data read back) guard against balancer response to unintended or erroneous commands. Programmable volt-second clamps ensure that current sensing faults during balancing do not result in runaway current conditions. Cell by cell over- and undervoltage checking, as well as secondary side overvoltage detection, prevent sudden battery wiring harness faults from causing damage to the balancing circuits during balancing.
These characteristics enable the LTC3300 to provide both high performance and reliable active balancing in series-connected battery systems. As the cells in such systems age or need replacing, it becomes increasingly important to compensate for the resulting mismatch in cell capacities without further compromising run time, charge time, or the lifetime of the battery pack. The LTC3300 was designed specifically to address this challenge, providing designers with a new level of safety and charge efficiency.
About the author: Samuel Nork is Director, Boston Design Center, Linear Technology Corporation