Specifications

Smart Battery Charger Specification
SBS Implementers Forum -Page 27- Revision 1.1
Appendix B. Smart Battery System Safety Features
Safety is a primary design goal in the Smart Battery System specifications. The central concept behind the
Smart Battery specifications is locating the primary intelligence of the system inside the battery pack itself.
This enables the system to be much more accurate in measurement of battery parameters such as remaining
capacity and design voltage, and also allows the charging algorithm and parameters to be tuned to the
battery pack’s specific chemistry. By relying on the battery pack’s intelligence, a properly designed Smart
Battery system will safely charge and discharge any expected battery chemistry.
Protection against high rates of discharge is well understood by the battery industry and easily guarded
against in battery pack design through the use of safety devices such as fuses. However, protection from an
over zealous charger is more difficult requiring tight coupling between the charger and battery. The Smart
Battery system defines a standard set of messages as well as an independent means for the battery to
terminate charge. Correct operation of the charger is key to maintaining safe operation.
This section will review the safety features contained in the Smart Battery specifications. Particular
attention will be focused on the following operating modes: , “Wake-up” Charging “Controlled” Charging,
and Discharging.
“Wake-up” Charging
It is possible for a battery pack to be so depleted that its built-in intelligence will not have enough power to
operate. Therefore, the Smart Battery System specifications require a charger to apply some small amount
of charge (less than 100mA) to a battery when it is first connected so that it can receive enough power to
wake up and communicate. This “wake-up” charge may not begin if the Safety Signal indicates that no
“wake-up” charging is allowed. The "wake-up" charge will be terminated when the battery sends a critical
message to the charger, or the Safety Signal moves into a range in which “wake-up” charging is not
allowed, or the “wake-up” charge time-out period expires.
“Controlled” Charging
“Controlled” Charging (i.e., charging under battery control) can only be initiated when and if the battery
explicitly requests charging from the Smart Battery Charger device in the system AND the charger
independently determines it is safe to do so. Charging will be aborted at any time the battery or charger
detects an error condition. The battery requests charging when it sends two separate messages to the
charger indicating both its desired ChargingVoltage() and ChargingCurrent(); the charger will attempt to
supply the requested voltage and current if the Safety Signal indicates that “controlled” charging is
allowed.. Charging continues as long as new current and voltage messages are sent by the battery to the
charger (to prevent charger from timing out) and the Safety Signal value stays within a range that allows
“controlled” charging. Charging MUST be terminated immediately when any of the following conditions
occur:
• a critical message is received by the charger
• The charger receives a ChargingCurrent or ChargingVoltage message with a zero value.
• the Safety Signal moves out of a range that allows “controlled” charging.
• the charger doesn’t receive valid current and voltage messages within the time-out period.
Therefore, to begin charging, both the battery and charger must agree it is safe to do so. First, the battery
must explicitly request charging. If the battery detects some error condition (battery pack voltage too high
or low, temperature out of acceptable ranges, individual cell voltage shorted, etc.) it won’t request charging
and charging will never begin. Second, the charger will not begin charging if the Safety Signal indicates
that no “controlled” charging is allowed.
Furthermore, to continue charging, both the battery and charger must be satisfied it is still safe to do so.
First, the battery must continuously send messages to the charger indicating its requested charging current
and voltage. If any error condition occurs in the battery, the battery will send a message to the charger to
terminate charge. Or, the battery can simply stop sending messages to the charger and charging will stop
after the charger detects the battery is no longer sending out messages. (For the same reason, the charger
will stop charging if something happens to the communications channel between the battery and charger,