User Manual
DA1469x State of Charge (SoC)
Functionality
UM-B-134
Abstract
This user manual describes the use of Dialog Semiconductor's State of Charge (SoC) software.
Besides the State of Charge calculation, an overview is given of the whole process of charging and
discharging a battery. The amount of charge- and discharge current has an impact on the State of
Charge and because of that the battery needs to be characterized (profiled) to minimize errors. The
battery profiling process is also described in this manual. There is also a brief description on the
influence of temperature and aging on a battery.
UM-B-134
DA1469x State of Charge Functionality
User Manual
Revision 1.0
17-Nov-2020
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© 2020 Dialog Semiconductor
Contents
Abstract ................................................................................................................................................ 1
1 Terms and Definitions ................................................................................................................... 4
2 References ..................................................................................................................................... 4
3 Introduction .................................................................................................................................... 5
3.1.1 State of Charge Estimation Concepts ................................................................... 6
3.1.2 Effect of Temperature on State of Charge ............................................................ 8
4 Charging Process for Li-ion Batteries ...................................................................................... 11
4.1 Phases of the Charging Process ........................................................................................ 11
4.1.1 Phase 1: Pre-Charging ........................................................................................ 11
4.1.2 Phase 2: Constant Current .................................................................................. 11
4.1.3 Phase 3: Constant Voltage .................................................................................. 11
5 SoC Process Overview ............................................................................................................... 12
5.1 Overall Process ................................................................................................................... 12
5.2 Battery profiling process...................................................................................................... 13
5.3 Resulting LUTs .................................................................................................................... 15
6 SoC Algorithm ............................................................................................................................. 16
6.1 Concept ............................................................................................................................... 16
6.2 SOC Calculation .................................................................................................................. 16
6.3 Various Considerations ....................................................................................................... 18
6.3.1 SoC Reporting Error ............................................................................................ 18
6.3.2 Accumulation Error .............................................................................................. 18
6.3.3 Half Battery Charging .......................................................................................... 18
6.3.4 Battery Aging ....................................................................................................... 18
7 Conclusions ................................................................................................................................. 18
Appendix A Battery Conditions and Parameters ........................................................................... 19
A.1 State of Charge (SoC) (%) .................................................................................................. 19
A.2 Depth of Discharge (DoD) (%) ............................................................................................ 19
A.3 Terminal Voltage (V) ........................................................................................................... 19
A.4 Open-Circuit Voltage (OCV) ............................................................................................... 19
A.5 Internal Resistance ............................................................................................................. 19
A.6 Nominal Voltage (V) ............................................................................................................ 19
A.7 Cutoff Voltage ..................................................................................................................... 19
A.8 Capacity or Nominal Capacity (Ah for a Specific C-Rate) .................................................. 19
A.9 Charge Voltage ................................................................................................................... 19
A.10 Float Voltage ....................................................................................................................... 20
A.11 (Recommended) Charge Current ....................................................................................... 20
Revision History ................................................................................................................................ 21
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User Manual
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Figures
Figure 1: Li-ion Battery at Different States of Charge ........................................................................... 5
Figure 2: Open Circuit Voltage versus State of Charge for Different Discharge Rates ........................ 5
Figure 3: OCV vs SoC where DVL= Discharge Voltage Limit, CVL = Charge Voltage Limit ................ 7
Figure 4: Effect of Temperature on Battery Capacity for Li-ion ............................................................. 9
Figure 5: LUTs for Low- and High Discharging Currents .................................................................... 10
Figure 6: Charging Phases for a Li-ion Battery ................................................................................... 11
Figure 7: Overview of the Process for Characterizing the SoC Algorithm .......................................... 12
Figure 8: Bench Setup for Battery Profiling ......................................................................................... 13
Figure 9: Battery Profiling Process ...................................................................................................... 14
Figure 10: SoC Algorithm Concept Overview...................................................................................... 16
Figure 11: SoC Graph ......................................................................................................................... 17
Tables
Table 1: Lookup Tables for OCV vs SoC for High Load-, Low Load- and Charge Currents ................ 8
Table 2: Generated Lookup Tables ..................................................................................................... 15
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User Manual
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1 Terms and Definitions
SoC State of Charge
OCVL Open Circuit Voltage for low discharge current
OCVH Open Circuit Voltage for high discharge current
2 References
[1] DA1469x Datasheet, Revision 3.2, Dialog Semiconductor
[2] UM-B-090, DA1469x Getting Started User Manual (PDF), Dialog Semiconductor
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User Manual
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3 Introduction
Batteries are the main source for storing electrical energy. For wearable systems running on a
rechargeable battery the user would like to know what the remaining amount of energy in the battery
is, to estimate when the device has to be re-charged.The percentage of stored energy is measured
with the parameter called State of Charge (SoC). The SoC is defined as the remaining energy in the
battery as a percentage of a fully charged battery. Normally the rated capacity, expressed in mAh is
used as the reference for SoC estimation. Figure 1 shows the State of Charge of a battery from full
state to empty state.
Figure 1: Li-ion Battery at Different States of Charge
A precise estimation of a battery’s SoC is required to inform the user how long the application can
still work before the battery will need to be recharged. From a safety perspective, SoC makes sure
that battery is not overcharged once 100% is reached. With SoC estimation the user gets an idea of
how long the device can be used before the next charge cycle. A fully charged battery has an SoC of
100% and an empty battery corresponds to a SoC of 0%. SoC can be effectively used to predict how
well the battery system functions relative to its nominal (rated) and end (failed) states. SoC drops
faster for higher load profiles and the provided SoC algorithm takes this into account. SoC
measurements help to select an appropriate charge and discharge profile for a longer life of the
battery.
SoC is proportional to the terminal voltage (Open Circuit Voltage or OCV) of the battery. SoC
decreases along with the decreasing battery voltage. An example graph with the plots of the SoC
against the battery Open Circuit Voltage and the load current is given in Figure 2.
Figure 2: Open Circuit Voltage versus State of Charge for Different Discharge Rates
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Figure 2 is taken from a typical (Li-ion) battery, discharged at different C-rates. It is well evident that
the SoC is related to the battery voltage and the load/discharge current. The applied load/discharge
current is mentioned as 0.2C, 0.5C, 1C and 2C.
The capacity of a battery is commonly rated at 1C, meaning that a fully charged battery rated at
200 mAh should provide 200 mA for one hour. The same battery discharging at 0.5C should provide
100 mA for two hours, and at 2C the battery delivers 400 mA for 30 minutes.
The SoC and the voltage drop faster for higher discharge currents. The green line in Figure 2
corresponds to a discharge rate of 2C and is the highest. The black line corresponds to the lowest
discharge rate of 0.2C. The voltage drops much slower for 0.2C compared to 2C. Hence the SoC
drops faster for 2C compared to 0.2C. In Figure 2, the value 3.2 V corresponds to the Battery Empty
state. This is the cutoff voltage and the SoC corresponds to 0%.
Every brand and type of battery has its own charge and discharge characteristics. For the algorithm
to provide an accurate SoC estimation, the battery needs to be profiled to determine the relation
between voltage and SoC at various C-rates. Dialog Semiconductor can provide software to run the
battery profiling (or characterization) using lab equipment. This is described in Section 5.12.
3.1.1 State of Charge Estimation Concepts
SoC is the ratio of the currently stored charge Q to the total capacity C. The total capacity can be
found from the battery specification.
SoC = 1 corresponds to a fully charged battery and SoC = 0 corresponds to an empty battery.
Two states at interval tα and tβ are considered for a charge/discharge operation. Qα and Qβ are the
charges corresponding to tα and tβ. Hence the stored/drained charge would be Qα – Qβ.
As Qα changes to Qβ, the SoC changes from SOCα = SOC(tα) to SOCβ = SOC(tβ). By using equation
(1) for tα as well as for tβ and (2), the total capacity of the battery cell can be calculated with:
Equation (3) shows that two accurate SoC measurements and the integrated current between these
two values are enough to calculate the resulting capacity C at tβ.
The SoC and Open Circuit Voltage (OCV) are related and when the SoC changes from SOCα to
SOCβ, the OCV does from OCVα = OCV (tα) to OCVβ = OCV (tβ).
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Equation (4) can be written as follows:
The linear interpolation relationship of OCV vs SoC can be represented graphically as shown in
Figure 3:
Figure 3: OCV vs SoC where DVL= Discharge Voltage Limit, CVL = Charge Voltage Limit
From the above equation (4) the current capacity of a battery can be found from the amount of
charge, charged or discharged, State of Charge and the Open Circuit Voltage.
As stated in the introduction, for an accurate estimation of the state of charge, the battery needs to
be profiled (or characterized). The role of battery profiling is to charge and discharge the battery with
a known current and to measure the corresponding OCV voltage. Battery profiling starts by preparing
the battery to be at a fully charged level so that SoC corresponds to 100 percent. The battery
profiling tool will draw a certain current for a certain duration that equals to 5% drop in SoC value.
The measured voltage corresponding to this SoC values is recorded. The same is valid for charging
the battery. The battery profiling tool will now charge the battery with a certain current for a certain
duration that equals to 5% increase in SoC value.
Battery profiling generates lookup tables (lut) with SoC and voltages measured for high (hlut)- and
low (llut/OCV) load- and charge currents (clut). The lookup tables generated by the battery profiling
are inserted into the DA1469x SoC software, which can be downloaded from
www.dialog-semiconductor.com.
The SoC software can be added to a customer application and during run time of the application this
will be used to calculate the State of Charge at any point in time.
Details of the battery profiling process are described in Section 5.2 Battery profiling process.
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