47
on the actual terminal voltage minus the current times the estimated battery impedance. The impedance is a
variable with respect to the actual SOC but especially with temperature. Figure 24 shows the effect on DCR (direct
current resistance) by temperature and SOC. Notice there is very little effect on DCR by SOC, but a large effect
from temperature.
Figure 24 – AMP20 cell 60s, 20A DCR measurements wrt SOC at various temperatures
* Data for other A123 cells can be found in Appendix A, starting on page 61.
Integrated Circuits
Integrated circuit cell monitors can work within the Battery Management System to offer complete, scalable
design for use in packs of varying sizes. The integrated circuit monitor could be connected to each series cell in a
string and would report data measurements to a controller via an internal communication bus. Measurements
taken on an individual cell level would allow measurement close to each individual cell, resulting in improved
accuracy. In addition to monitoring functions, integrated circuit controllers offer cell balancing, protection, SOC
calculation and State of Health (SOH) estimation. Some of them are fully programmable using custom firmware
while others are programmed from the factory and no firmware development is required. Most of them are user-
configurable to suit a variety of applications, cell types and pack sizes.
For example, manufacturers such as Texas Instruments (TI) offer Analog Front Ends (AFEs) that may be suitable for
a variety of applications. AFEs integrate a digital communication interface (such as I
2
C or SPI) to allow a BMS to
monitor cell voltages and temperatures, enable cell balancing, enter different power modes, set current protection
levels and blanking delay times. Certain Seiko Electronics ICs provide safety protection for various fault conditions
such as short circuits and cell overvoltage. Maxim and TI offer SOC monitors that are designed to work with A123
cells.
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
40.0
0
10
20
30
40
50
60
70
80
90
100
Re
sist
an
ce
(m
O
hm
s)
%SOC
60 Second Discharge DCR - 20Amp Discharge @ All Temperatures Except
-30 (10Amp)
-20'C DCR
-10'C DCR
0'C DCR
10'C DCR
23'C DCR
40'C DCR
60 Second 20 Amp Discharge (DCR) @ All Temperatures Except -30 (10Amp)
