Battery Test Systems

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ELECTROCHEMICAL

INSTRUMENTATION

Part I

B TTERY TEST

SYSTEMS


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Control Systems A typical control system incorporates not only

functions allocated to measurements of

variables and information transfer, butfunctions for energy conversion, utilizationand control employing controllers of theelectronic, mechanical, pneumatic,microprocessor, etc., type.

Exemplary control systems may be for position,level, speed, rpm control.

The electrochemical control systems areprimarily devoted to potential, current, power,temperature, density and other

electrochemical parameters’ control bymeans of electronic controllers.


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Control System Structure

Most electronic control systems include some kind of a feedback loop ----------------applying a corrective signal from the Output back to the Input.

The main purpose of this feedback signal is to correct the input signal in

such a way that an error signal be produced and amplified for the purposeof RESULTS to most faithful ly match the AIMS.


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Classical Feedback TheoryTwo distinct cases:

1. GCL = 1/β, when β A is too large

In this case the system closed

loop gain G is defined entirely

by the negative feedback

components and the outputsignal is precisely defined by

“β

” .

2. GCL = infinity, when β A = -1

In this case the system is totally

useless, loses control, the

closed loop gain G goes to

infinity and the output becomesdestructive.

∞⇒〈=ββ

⇒∞→β

β+==

CL

CL

CL

Gdeg1801AWhen

1GAWhen

A1A

UinUlG


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Advantages and Cost of Feedback

• Reduction of sensitivity to variations of the bipolaror FET transistors parameters due to changes ofambient temperature, d.c. supply voltages, radiation,

aging etc.• The application of feedback tends to reduce the

harmonic distortion and noise generated inside theinternal amplifier

• The above advantages however are achieved at thecost of risk of instability since in certain cases theamplif ier may become unstable demanding the

addition of suitable corrective circuit elements to theinternal amplifier and/or the feedback network so asto insure the overall system stability under allpossible operating conditions


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The Electronic Amplifier Ud - difference or error signal

Ur - feedback or return signal

A - amplifier open loop gain

β - feedback network transfer

function

Uin and Uout – Input andOutput signal

As the open loop gain A is increased in magnitude, the

error signal Ud becomes progressively smaller and the

feedback signal Ur approximates more closely the input

signal Uin, which is equivalent of saying that the closedloop gain G is effectively equal to 1/β.


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The Closed Loop Gain G

11

11

βA1

A

Uin

Uout

G

βUoutUinUrUinUdβUoutUr

Ud

UoutA

β

β

=

=

−

=

=

A


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The Electronic Amplifier Features

• The dominated speed of response is much higher incontrast to a typical industrial control system –

microseconds (or less) compared to seconds• The mutual influence among the internal amplifier

stages and the feedback network components is

always to be accounted for • The higher frequency operation at different power

levels should not result to the introduction ofnoticeable static or dynamic errors

• The transfer function is to be precisely analyticallydescribed to facilitate the judicious application ofstabilizing phase compensation techniques


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The Basic Operational Amplifier

Amplifier gain A – Very high, close to infinity

Input impedances Zin 1,2 – Very high, close to infinity

Output impedance Rout – Very small, close to zero

Bias currents Ib 1,2 – Small to negligible

Offset voltage Uoff – Small to negligible


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Inverter and Summing Amplifier

Uin1R

2R Uout −=

2U2R

3R 1U

1R

3R Uout −−=

The potential difference between

the two amplifier inputs is

negligible due to the high

amplifier gain, hence thesumming point Σ is virtually at

zero (ground) potential and is

frequently labeled as The“ Virtual Ground” of the amplifier.

The input resistances presentedto the input voltage sources U1

and U2 are correspondingly R1

and R2, hence these operational

circuits have final valued input

resistances.


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Voltage followers with/without Gain

Uin1R

2R Uout

+=

1 R

UinUout=

The simple voltage follower does

not change the sign of the input

voltage and the output voltage

equals the applied input voltage.Commonly termed “ Buffer

Amplifier” since its input

impedance presented to the input

voltage source is practicallyinfinite.

The same is valid for the follower

with gain with no sign inversion,

gain equal to (R1+R2)/R1 and

infinite input impedance presentedto the voltage source.


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Integrators and Differentiators

∫−= dtUin

RC

1Uout

???dt

dUinRCUout −=

The integrating operational configuration

should be cautiously utilized since no

matter how small the bias current and the

offset voltage are, they will be integratedover time and be presented as a

combined error term. By adding an

appropriate switching network this

operational circuit is often used for“ sample-and-hold” applications.

The “ pure” differentiator circuit is quite a

controversial one since, for reasons thatwil l be discussed later on, it has not only

a tendency toward instabili ty, but its gain

at high frequencies is very high and there

is a risk for the amplifier generated highfrequency output noise to obscure the

noise-free differentiated signal.


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Differential Amplifier and I/E Converter

1)Uin2(Uin1R

2R

Uout −=

IinR Uout

The simple differential amplifier finds numerousapplications when voltage differences are

measured. Its closed loop gain however

depends primarily on the resistors’ mach down

to less than 0.1%. The main advantage of thisoperational conf iguration is that it heavily

suppresses common mode signals.

The I/E current-to-voltage converter “ virtualground” summing point Σ allows for the precise

direct conversion of the “ short circuited” current

source signal to an output voltage signal without

introducing any current shunts. Frequently used

for electrochemical sensors conditioning and

electrochemical impedance measurements but

the inevitable electrochemical capacitance

applied to the amplifier input transforms it to a

differentiator-like circuit with all the associatedrisks of instability and obscuring noise

generation.


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Typical Operational AmplifiersGAIN OFFSET

voltage

BIAScurrent

INPUTimpedance

SPEED POWER output

Low

Offset

High Very

low [nV]

Medium Medium High Low

Low

inputcurrent

Medium Medium Very low

[fA]

Very

High

[Gohms]

Low Low

Fast Medium Medium High Low Very

High[>Mhz]

Medium

HighPower

Medium Medium Medium Medium Medium VeryHigh

[A]


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The Ideal/Real Voltage Source


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The Ideal/Real Current Source


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E Source

1R 2R 1R EE REF0

+=


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E Source Errors

dTSdIrdESdE tu00inu0 ++=


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I Source

2R 1R 1R

R EI

S

REFRl

+=


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I Source Errors

dTSdER

1

dESdI ti00

imi0 ++=


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E/I Source

S

REF0

REF0

R

EI

1R

2R EE

=

−=


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E/I Source Characteristics

The “α

” angle reflects the

influence of the final valued

internal resistance of the

E source, while the “β” angle

illustrates the combined Isource output resistance

participation

The automatic “ crossover”is user selectable.


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Thermal Feedback

1R

2R 1R ESZ

R 4

EdE REFthth

l

2

in[thfmax]0

+

±=


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High Power/Voltage E/I Source

α

β

α

β−

R E

R E

R EI

R IR

2R Ir

1R

2R EE

0

0

0

S

REF0

S000REF0

onCompensati R

E

onCompensati Ir


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The Bitrode Power Module


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The Output Stage – the Bitrode Module

The electrical and thermal design ofthe output power stage is an art i tself.

Various approaches for reduction of

the power dissipated by the active

components, mostly bipolar or FET

transistors, are employed.

The series-parallel operation of active

(transistors) and passive (resistors)

elements helps to dynamicallydistribute the power dissipated by the

semiconductor components and

extend their operational l ife,

enhancing their reliability.

Special cases are the high

voltage/power output stages where the

active components Safe Operating

Area should carefully be consideredsince the overvoltage destruction is a

nanosecond process.


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Constant Power Source

ms

REF2

0

K R 1.R

2R 1R EP

+=

Po

This power control ler utilizes an

analogue multiplier for fast and

precise control of the output power

El t i L d


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Electronic Load

Constant Resistance

1R

2R 1R

R I

E

R Sc

+≈≈

∋

The equivalent of an inf initely variablehigh power rheostat (0 to 100 ohms)

with close to 0.1% precision at

nominal power of up to 2000 watts (or

more)


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E/I/P Source


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E/I/P Source + I/P/R Sink+Elimit


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Universal Battery Test Unit

OPERATIONAL MODES

-

CC charge/discharge- CV charge/discharge

- CP charge/discharge

-

CR discharge- FV protection

- Ref Electrode control

-Cell pressure control

- Battery temperature limit

- Gas evolution control

- Heat sink temperature limit


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Battery Life Cycling Tester – Bitrode Corp.


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High Power Test Unit – Bitrode Corp.

S C


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Battery Test System – Digatron Corp.

The Solartron 1470 Battery and Fuel Cell


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The Solartron 1470 Battery and Fuel Cell

Test Unit

Documents

Battery Test Systems