The zinc-air battery chemistry has proven itself to be a leading technology in meeting the power needs of dismounted soldiers. However, along with advancements in zinc-air chemistry and cell development, improvements in the electronics associated with the batteries have the potential to bring zinc-air technology to a higher performance level – mitigating the growing logistical and tactical burden placed on today’ s high-tech soldiers. Equipping zinc-air batteries with state-of-charge indication (SOCI) functionality is one way of significantly improving current zinc-air offerings. This paper presents the development process of a next generation zinc-air battery with SOCI. The state-of-charge functionality is based on a previously-developed SOCI application-specific integrated circuit (ASIC).

1. Zinc-Air Battery with State-of-Charge Indicator
Jason Pecor, Bruce Strangfeld
Silicon Logic Engineering, Inc.
7 South Dewey, Eau Claire, WI 54701
Abstract: The zinc-air battery chemistry has proven itself
to be a leading technology in meeting the power needs of
dismounted soldiers. However, along with advancements
in zinc-air chemistry and cell development, improvements
in the electronics associated with the batteries have the
potential to bring zinc-air technology to a higher
performance level – mitigating the growing logistical and
tactical burden placed on today’s high-tech soldiers.
Equipping zinc-air batteries with state-of-charge indication
(SOCI) functionality is one way of significantly improving
current zinc-air offerings.
This paper presents the development process of a next
generation zinc-air battery with SOCI. The state-of-charge
functionality is based on a previously-developed SOCI
application-specific integrated circuit (ASIC).
Keywords: State of Charge; Battery Management; ZnAir Batteries; Battery Electronics; Primary Batteries
enhancements to the solution was the addition of a fan
speed controller.
Fan Speed Controller: Atmospheric oxygen is required by
zinc-air batteries. This air is supplied by a small DC electric
fan which is powered by one of the battery strings.
Typically, the fan operates at 100% anytime the battery is
connected to a load. However, to improve fan efficiency,
design features were added to regulate fan speed and
conserve battery capacity at discharge rates under 500 mA.
The fan controller was implemented using a pulse width
modulated (PWM) signal to drive three parallel transistors
which intermittently power the fan based on the PWM duty
cycle. A capacitor circuit was used to ensure a consistent
DC voltage across the fan. Table 1 represents fan voltage
and PWM duty cycle for given discharge rates:
Table 1. Fan Voltage and PWM Duty Cycle
Introduction
Equipping zinc-air batteries with state-of-charge indication
(SOCI) functionality is one way of significantly improving
current zinc-air offerings. SOCI would allow the soldier to
make better decisions about the current state of a battery
and allow for more efficient and thorough use of each
individual battery.
Silicon Logic Engineering (SLE) has developed an
application-specific integrated circuit (ASIC) targeted for
implementation into primary military batteries. This ASIC
was initially developed to address the need for a state-ofcharge solution in the BA-5590 LiSO2 and BA-5390
LiMnO2 batteries. This device, and the associated SOCI
solution architecture, was enhanced to meet the unique
requirements of the Zinc-Air chemistry.
Discharge
Rate (mA)
< 50
= 50
> 50 to < 500
> 500
Fan
Voltage (V)
Off / 0
6
1
6 to 12
12
PWM
Duty Cycle (%)
0
15.9
16 to 100
100
Notes: 1. Linear ramp
Since fans may exhibit poorer performance in cold
operating conditions, the PWM is not used at -10 °C and
below. Depending on the discharge rate, the fan is fully on
or off at these temperatures.
As shown in Figure 1, whenever the fan is turned on, it is
driven with 12 volts for approximately 100 ms to ensure a
reliable start.
At the end of the project, 25 BA-8150 Zinc-Air batteries
with SOCI were successfully delivered to U.S Army
CERDEC.
Zinc-Air SOCI Electronics
SOCI ASIC: As mentioned previously, the core SOCI
functionality is provided by the SLE SOCI ASIC. This
device implements all of the coulomb-counting,
voltage/temperature measurement, and calculation
algorithm features necessary to perform accurate state-ofcharge determination. However, the zinc-air platform
required additional ASIC functionality and enhancements
to the baseline solution architecture. One of the primary
Figure 1. Fan Driver Start-Up

2. The solution also included a method to calibrate fan
voltage. This would have been useful in a manufacturing
environment where several fan types could be used, each
with potentially different impedances. During fan
calibration, the voltage across the fan would be measured
by the SOCI ASIC and allow calibration of the PWM duty
cycle. Though potentially useful, the latest version of the
SOCI does not include this functionality in order to reduce
overall solution cost.
for 20 seconds. Tolerance on the voltage cutoff
measurement is typically +/- 0.015 volts at room
temperature and +/- 0.050 volts over the full operating
temperature range.
In preparation for designing the fan driver circuitry, four
sample fans were characterized.
These fans were
representative of those commonly used in zinc-air batteries.
The following information was extracted from the
characterization data:
Fuse Protection: Given the inherently safe nature of the
zinc-air chemistry, no fuses or thermal cutouts are
incorporated into the BA-8150 electronics.
• Once started, the fans remain running down to 3 volts but
require a minimum of 7.3 volts to reliably start.
• The fans require slightly more current in open air
conditions then with the fan totally blocked-off
(mimicking the effects of static pressure from a partially
blocked environment – such as a battery case).
• Current is reduced slightly after a nominal run-in period.
This is likely from a reduction in friction once the
bearings have broken-in.
From this data, Simulation Program with Integrated Circuit
Emphasis (SPICE) models were developed for the fan and
used to evaluate potential driver circuits. The resulting
circuit, shown in Figure 2., is very robust and will drive a
higher power fan – up to 100 mA. This offers cost
reduction opportunities by allowing removal of one of the
three drivers if higher impedance 70 mA fans are used.
BATT
POS
FAN
FAN_NEG
While uncommon and inconsistent, voltage delay can also
occur in zinc-air batteries. To mitigate this potential risk,
the SOCI employs a sixty second delay before performing
the first voltage measurement.
Revision 0 (Rev. 0) Design
Figure 3 is a photograph of the top and bottom surfaces of
the Rev. 0 printed circuit board (PCB) assembly. The PCB
and assembly design is well suited for automated
manufacturing using state-of-the-art surface-mount
technology equipment. The assembly of the 25 deliverable
SOCI units was done at an electronics contract
manufacturer which follows modern industry standards for
quality and process control. The assembly process went
very smoothly and required minimal engineering support.
Figure 3. Rev. 0 BA-8150 Printed Circuit Assembly
Figures 4 and 5 show the SOCI printed circuit assembly
(PCA) mechanical integration with a prototype BA-8150
battery case. The first prototypes were integrated into the
case using a machined plastic front plate that modeled the
final plastics and incorporated SLE’s patent-pending SOCI
actuator mechanism.
FAN_PWM
BATT
GND
Figure 2. Fan Driver Circuit
Voltage Cutoff/Delay: Zinc-air exhibits a very steep drop in
string voltage when approaching end-of-life. Therefore,
the SOCI has a 10 volt threshold for voltage cutoff and
shows 0% remaining charge once cutoff has been exceeded
Figure 4. Rev. 0 PCA and Actuator Plate

3. somewhat different, this is not expected to be a
contributing factor.
Rev.1 Design Changes
Based on Rev.0 test results, the following improvements
were made to Rev. 1 of the SOCI design:
1.
2.
Figure 5. Rev. 0 PCA in BA-8150 Case
Rev. 0 Test Results
The zinc-air SOCI PCA was successfully lab-tested at SLE
prior to integration into the BA-8150 batteries. All
measurements and resulting calculations met expectations.
The SOCI accuracy performed to specification remaining
within +5% / -15% of actual capacity, and the average
current consumption measured 25uA.
However, during test and evaluation of the assembled
prototype BA-8150 batteries, an unexpected number of
Rev.0 SOCI units indicated 0% remaining charge along
with an inoperative fan when removed from the packaging.
This was discovered on batteries taken from newly opened
bags. Expectations were that all fresh BA-8150 batteries
would display 100% remaining charge, and fans would run
if a load was applied. Failing batteries did measure around
16 volts open circuit as expected.
The following design issues were identified and analyzed
as potential causes of the failures.
1.
2.
3.
A transistor in the Vcutoff measurement circuit was
located near the board edge and could sometimes be
broken by contact with the plastic battery case during
SOCI installation in the battery. If nonfunctional, the
SOCI will detect 0 volts and after a 60 second delay,
indicate 0% remaining charge and disable the fan. This
is the most likely failure mechanism.
The voltage regulator exhibits a low but non-zero
failure rate. If it is damaged, the SOCI will indicate 0%
remaining charge and the fan will not operate.
However, the large percentage of BA-8150 failures
seen does not indicate this as a likely root cause.
Connectors J1 and J2 could be swapped during SOCI
incorporation into the BA-8150 battery. These are the
load and battery 1 connections. When these are
reversed, the SOCI is prevented from counting
Coulombs. Therefore, regardless of battery usage, the
SOCI will always display 100% and disable the fan.
Since the failure observed and that from this case are
Clearances between components and the PCB edge
were increased to eliminate any mechanical
interference with the plastic case or card guide.
Connectors were color coded to help eliminate
assembly errors on the manufacturing floor.
Additionally, the component footprint for the SOCI ASIC
was reduced from the original LQFP-44 to the very
compact MLPQ-44. Figure 6 shows the size reduction
realized by changing the ASIC packaging.
Figure 6. LQFP and MLPQ Packages
A voltage detector/reset generator chip was also
incorporated, and the design was simplified by eliminating
an unused LM335AZ remote temperature sensor, LED
constant current sources and EEPROM series terminators.
Figure 7 shows the top and bottom surface components of
the Rev. 1 PCA.
Figure 7. Rev. 1 Zinc-Air SOCI PCA
Rev. 2 Design Changes
Since completion of Rev.1, another upgraded version of the
SOCI circuit has been designed. Rev.2 is complete with
schematic and Bill of Materials (BOM) but has not been
released for artwork or manufacturing.

4. The following features were added to Rev. 2:
Voltage protection: A zener diode over-voltage protection
circuit was added for the voltage regulator IC.
Fan Controller: The fan controller was modified to run
independently of the SOCI ASIC. In the event the SOCI is
non-functional, the fan should still provide sufficient
oxygen.
Coin cell power: The SOCI can optionally be coin cell
powered. This provides the ability to monitor storage time
and temperature of the zinc-air battery. Both are important
factors in determining storage loss. The 540 mAh CR2450
coin cell was chosen to meet a capacity of 2 years of
storage plus 4 months of service.
SOCI firmware changes are required to provide
calculations for capacity loss due to storage. However,
these modifications have not yet been made to the
embedded code.
Conclusions
Significant progress has been made in developing SOCI for
zinc-air batteries. The average operational current usage
for the SOCI solution was reduced from 35 uA to 25 uA – a
savings of 29% - during this effort. Firmware was
developed for a non-volatile accumulator, preserving SOC
data accurately over battery voltage dip or cutoff due to
being bagged or immersed in water. Algorithms for
automatic calibration of analog circuits were created, and a
robust fan driver circuit was designed and implemented
saving charge and extending battery life.
Furthermore, productive work was done in reducing cost
for state-of-charge by reducing BOM cost and subsequent
assembly labor costs, as well SOCI ASIC costs by working
with our ASIC supplier to move to a one-time
programmable (OTP) device. Beyond the OTP-based
ASIC, there is some potential to save additional cost by
designing a smaller and more rugged assembly around the
next generation 0.18 um technology ASIC.
Recommendations
The following recommendations are provided as potential
future R&D effort related to zinc-air SOCI.
• Fully implement a feature to monitor bagged storage
temperature in real time and adjust the percent of charge
remaining as necessary to maintain SOCI accuracy.
• Embed an RH sensor in the zinc-air battery and add an
indicator LED to warn of a pending desiccation
condition. Correct the SOC display as needed when
desiccation levels persist. This implies the battery case
will have a convenient means to add a small amount of
water to counter any detected desiccation.
• Develop a field-removable electronics pod for the BA8125 and BA-8150 zinc-air batteries. This small modular
assembly would snap-fit onto the battery and could be
easily moved from a depleted battery to a fresh one as
needed. It would house the battery connector, air mover,
and a portion of the SOCI circuitry. This would reduce
cost of ownership of the battery electronics by amortizing
it over multiple batteries.
• Develop a remote SOCI user interface since batteries are
often carried in pouches that prevent direct SOCI
observation.
• Work on development of the next-generation SOCI ASIC
in 0.18 mm technology with the primary goal of reducing
die size and unit cost.
Acknowledgements
SLE would like to extend our appreciation to Dr. Terrill
Atwater and U.S Army CERDEC for all of their support
and assistance throughout this development effort.
We also thank Mr. Frank Malo, Director of Research &
Development, of Electric Fuel Battery Corporation for
assistance in providing and characterizing the BA-8150/U
zinc-air batteries and cooperating with SLE during the
prototype integration and delivery effort.