Specifications

Smart Battery Charger Specification
SBS Implementers Forum -Page 33- Revision 1.1
Appendix E. Accuracy Requirements
This specification places minimum accuracy requirements for ChargingCurrent() and ChargingVoltage() on
all Smart Battery Chargers to ensure basic operability between SBS system components. The body of the
specification is purposely kept generic with regard to cell chemistry, pack voltage, pack capacity in keeping
with the scope defined in Section 1.1. This appendix will discuss requirements for higher accuracy than
specified in sections 5.11, 5.12, 5.21 and 5.22.
There are basically two alternatives for optimizing the charger output as required by the particular system
or cell chemistry. The first is to improve the accuracy of the charger output in response to the values in the
ChargingCurrent() and ChargingVoltage() functions. This follows the traditional route of requiring the
charger to include precision reference, gain and control circuits, which add cost to the charger. The second
alternative is to re-use the battery’s precision circuitry and take advantage of the battery electronics close
physical proximity to the cells to tune the charger output in a closed loop fashion over the SMBus. This
second method is encouraged because it reduces system cost, and enables the system to be less dependent
on the cell chemistry and pack configuration.
Charging with an Accurate Charger
Increased charger output accuracy may be required on some systems to maximize stored charge while
ensuring safety, as is dictated by aspects of particular cell chemistries. Lithium Ion cell chemistry is a case
in point where tight tolerances on minimum charging voltage in the constant voltage phase allow more
charge to be stored in a cell, thus increasing capacity. At the same time, the maximum charge voltage must
be limited to maintain safe and reliable operation.
Lithium Ion cell manufacturers frequently recommend charging with +/-100mV per cell accuracy, or +/-
50mV per cell accuracy. If the charger alone is responsible for maintaining the accuracy of its output, then
it will need to regulate with much more accurately than the +1%/-9% tolerance on ChargingVoltage() that
the specification requires. In such cases, the ChargingVoltage() accuracy should meet both the +1%/-9%
minimum accuracy and the cell manufacturer’s recommendations. For example, charger with a
programmatic maximum of 17.0V would be required to have +1%/-3.7% accuracy to meet the +/-100mV
per cell recommendation on a 4 series cell pack, and +1/-2.3% for a 3 series cell pack. The same 17.0V
charger would need +1%/-1.3% to meet the +/-50mV recommendation on a 4 cell pack and +/-0.8% on a 3
cell pack.
Charging with and Accurate System
Among the many benefits of Smart Battery Chargers and the Smart Battery model in general is the low
system cost that results from not duplicating costly measurement and control components. A Smart Battery
already contains A/D conversion and other measurement electronics of sufficient accuracy to provide state
of charge, remaining capacity and other information for that battery. It is not necessary to duplicate this
measurement capability at the charger, nor is required to exceed or even meet the level of accuracy internal
to the Smart Battery, because the battery already contains a sophisticated measurement system and a
method of communicating with the Smart Battery Charger (SMBus).
The Smart Battery has everything necessary to ‘close the loop’ around the Smart Battery Charger and
therefore control the accuracy of the charging voltage and current. For this reason, the Smart Battery
Charger is not expected to be as accurate or precise as the Smart Battery. Even the accuracy of the Safety
Signal inputs is regarded as that necessary only for safe charging, not so accurate as to optimize charging.
Under normal conditions the Smart Battery electronics are in the best physical location to measure internal
voltages, currents and temperatures to optimize charging.
For these reasons, the data provided by the Smart Battery to the Smart Battery Charger is expected to be
more accurate and of higher resolution than the Smart Battery Charger may be able to provide. If the