Battery-management IC tackles high-current, multicell-pack, high-discharge/low-charge applications

Bill Schweber, Editor, EE Times' Planet Analog -- EDN, July 18, 2011

If your experience with battery management centers primarily on lower-current applications where the battery is fixed in place and charged within the device it supports, then the world of high-power motor applications will be a new world. In these situations, a multicell pack (typically between 4 and 10 lithium-chemistry cells) supports applications with high current drain when in use, highly intermittent use cycles, physical separation between battery and motor (several feet in some cases); and physically distinct discharge/charge settings—often with a product that has no internal MCU.

In the high-power situations, management of the battery pack requires special attention to cell balancing and other special characteristics of the situation. To meet these needs, the Texas Instruments bq77910 targets 4- to 10-cell lithium-pack applications such as power tools, e-bikes, and garden tools, and two of these devices can be stacked to manage 11- to 20-cell packs. The IC checks and monitors individual cell voltages, and directly drives two low-side, N-channel power MOSFETs (lower cost than high-side or P-channel devices), interrupting the current flow if any fault is detected.

The fully autonomous, standalone battery-management device does not require a microcontroller. The user can set it up in advance for the preferred parameters (fault thresholds, time delays) for the battery chemistry (such as Li-ion, LiCO2, LiMn₂O₄, or LiFePO₄), using a one-time, upfront loading of an internal EEPROM.

Note that monitoring of individual cells in a multicell pack is critical: if one cell is underperforming, for whatever reason, the overall pack may still read that it is "topped off" as mismatched cells cannot be detected from external contacts, and cell imbalance can accumulate over time, see Figure below. The bq77910 uses internal mostly analog circuitry to implement an algorithm to monitor, manage, and balance the cells.


In one example from a TI employee's power-tool battery pack, the pack measured around 19V, within the acceptable range for the six-cell pack, but five cells were a little over 4000 mV while the sixth cell was at -536 mV, and physically burst, since it had been driven into reversal from repeated over-discharging; the pack had only lasted 100 use cycles, see Photo.


Quiescent current is 50 µA typically, and 2.5 µA in shutdown, critical to minimizing drain in these applications where there may be long storage or idle periods.

Package, price, and availability: To ease setup and familiarity with the bq77910, Texas Instruments provides an evaluation module with GUI. The bq77910 is available now, and is housed in a 38-lead TSSOP package; it is priced at $2.70 each in 1000-piece lots.

For more information, go to http://www.ti.com/bq77910-pr; and there is a video at http://www.ti.com/batterymgmtvideo-pr.

This article originally appeared on EE Times.