Seite 19 - Boards & Solution/ECE Magazine July 2013

19 July 2013
EMBEDDED DESIGN
76% to 80%. As a result of this measurement
inaccuracy, the operating range must be re-
stricted by a guard-band to ensure that the op-
erating limits are not exceeded. In this example,
the operating range must be restricted to the
measured range of 22% to 78% instead of
20% to 80%. If the pack is expected to maintain
the same range, a BMS with this accuracy will
require additional battery capacity to com-
pensate for the guard-band restrictions. Con-
sidering the 60% usable SOC, the battery must
be oversized by >7%. Oversize penalty equals
[(0.6/0.56)-1]) to compensate for cell meas-
urement inaccuracy of ±10mV. For an HEV
using a 5kWh pack costing $3000 ($600/kWh),
this translates to an additional $214. This ar-
gument can be extended to highlight the guard-
band penalty for various cell measurement er-
rors, and its dependence on the SOC range. As
shown in figure 3, a system with only 1mV of
measurement error requires less than 1% over-
sizing, even when the pack is restricted to an
SOC range of 25% to 75% (50% usable SOC).
Although most lithium cells are generally well
matched when first acquired, the SOCs of a
long string of cells will diverge over time and
charge cycling. This is due to small variations
in cell characteristics and localized operating
conditions, which can lead to small differences
in self-discharge and load current. To avoid
operating any cell beyond its SOC range, as
the SOCs diverge, the total operating range
will be slowly constricted by the most unbal-
anced cells. To address this, nearly all battery
management systems include cell balancing.
With passive balancing, cells with higher SOC
are discharged to normalize the SOC for all
cells. This is a low cost, simple balancing
method. However, it has significant limitations.
Passive balancing only operates by removing
charge. It wastes energy, as a function of the
amount of imbalance, and it generates signifi-
cant heat. This means that the balancing cur-
rent must be kept relatively small, typically
5% or less of the cell capacity. As a result, pas-
sive balancing is primarily limited to operation
offline and it requires significant time to com-
plete. Passive balancing becomes less effective
as the variations in SOC increase, and over
time, SOC variation will increase due to di-
verging cell capacity.
Cells lose capacity as they age, a process that
can differ from cell to cell due to a number of
factors, such as gradients in pack temperature,
and variations in cell manufacturing. With
differences in capacity, cells will more readily
become imbalanced. Allowing just one cell to
operate beyond the SOC limit will simply ex-
acerbate this problem by premature cell aging.
Relying solely on passive balancing becomes
increasingly difficult, as capacities diverge. To
address the limitations of passive balancing,
new battery management systems are imple-
menting active balancing.
With active balancing, charge is moved between
cells, rather than being dissipated with passive
balancing. Active balancing can operate both
during the charge and discharge cycles. When
charging the pack, the active balancer can
move charge from the weaker cell to the
Figure 1. Simplified battery discharge curve Figure 2. Guard-band requirements for ±10mV cell measurement error
Figure 3. Dependence of guard-band on cell measurement error