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Application Note

Operating the EnerChip™ in High Temperature Environments

Introduction

EnerChip™ solid state rechargeable batteries are distinct from conventional rechargeable batteries. The

EnerChip can be used in environments typically ill-suited for batteries and other storage devices at risk of

leaking toxic and volatile solvents, catching fire, exploding in high temperature environments, or that are simply

too large to fit within space-constrained enclosures. The EnerChip CC combines the EnerChip with integrated

power management, for use in applications requiring backup or bridging power, or as the main power source.

Some applications demand occasional operation at temperatures above the +70°C rated specification of the

EnerChip. If such temperature excursions are not properly compensated for in the system design, the result

can be degraded performance or outright failure of the energy storage device. This Application Note provides

information on how to mitigate the detrimental effects of high temperature operation on the EnerChip in such

environments. The following topics are addressed:

•

Factors that influence the operating characteristics of EnerChips and the parameters that are affected

Effects of high temperature, bias voltage, and state of charge on operation and service life of EnerChips

Recommendations for mitigating detrimental high temperature effects on EnerChip performance

Configuring the EnerChip CC to offset temperature-induced effects using the integrated functions

Operating the EnerChip in medical sterilization and high temperature food processing environments

AN-1052

Figure 1 illustrates the benefits of proper EnerChip management in high temperature environments.

Figure 1: Relative Capacity Retention with and

without Proper EnerChip Management

Factors affecting parameters such as cell resistance, self-discharge, and capacity fade include:

•

Time

Temperature

State of charge (SoC)

Bias condition (i.e., continuous charging vs. floating terminal voltage)

Depth of discharge

With a fundamental understanding of how these factors interact to affect battery life and performance, it is

possible to deploy readily available circuit techniques to mitigate such effects. By controlling or compensating

for these factors with proper battery management, the service life of a battery that might otherwise not meet

the requirements of a given application can be greatly extended – even under operating conditions beyond the

specified rating of the battery. The data and analysis in this Application Note are presented in several forms,

describing the influence of these factors on the EnerChip and means to achieve desired performance and

service life objectives under the particular application operating conditions.

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AN-1052: Operating the EnerChip™ in High Temp. Environments

As a starting point, it’s important to understand how the state of charge of the EnerChip can be adjusted by

regulating the charge voltage and allowing the charging or discharging current to settle to a steady state value.

Figure 2 represents the relative state of charge as a function of this charge voltage.

Figure 2: State of Charge can be Adjusted by Varying the Charge Voltage

The EnerChip CC has a charge voltage temperature coefficient with a slope of --2.2mV/°C so as to lower the

charge voltage as the temperature increases. The charging source can also be enabled or disabled by way of

an external control line. The advantage of activating/deactivating the charge pump will become apparent in the

next section. As shown in Figure 3, a lower state of charge improves capacity retention at higher temperatures.

In applications where high temperature excursions are likely, the cumulative life-cycle capacity can be improved

by keeping the EnerChip at a lower state of charge, thereby retaining that capacity over a greater number of

charge-discharge cycles. The experimental results of Figure 3 were derived by charging the cells to 4.1V (100%

SoC) at room temperature and discharging each cell to establish the baseline discharge capacity. The cells

were then charged to either 60% or 100% SoC and subjected to the temperature shown in the charts. After the

prescribed time at temperature, the cells were discharged (top quad chart), then fully charged and discharged

again (bottom quad chart). Note the higher discharge capacity in the cycle 3 discharge vs. cycle 2 for the 100%

SoC cells, as the third discharge cycle includes the recoverable portion of the capacity that was lost during the

previous (cycle 2) charge-discharge cycle.

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AN-1052: Operating the EnerChip™ in High Temp. Environments

Figure 3: High Temperature Effects can be Offset by Adjusting the State of Charge

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AN-1052: Operating the EnerChip™ in High Temp. Environments

Figures 4 and 5 illustrate the effects of time at temperature on the capacity loss. Capacity loss from the cell

can be both recoverable and non-recoverable. Recoverable capacity loss is that portion that can be replaced

on the subsequent charge cycle and becomes available on the following discharge cycle. The non-recoverable

portion of capacity loss is just that: Once it is lost, it can not be recovered on subsequent charge-discharge

cycles.

As depicted in Figure 4, cell capacity loss is strongly dependent on temperature and state of charge.

By maintaining a lower state of charge on the cell, the deleterious effects of temperature can be offset

considerably. This is true for both recoverable and non-recoverable capacity loss.

Figure 4: EnerChip Capacity Loss as a Function of Temperature and State of Charge

Figure 5 presents the data in another form, with stand time being the dependent variable and all data points

representing stand time at 70°C. The loss rate over time at temperature is a logarithmic function. Again, note

the significant reduction in capacity loss (both recoverable and non-recoverable) by maintaining a lower state

of charge, especially over extended time at temperature. The data here was collected at several intervals,

including 6 months of continuous exposure to 70°C.

In a typical application, leaving the cell in this ‘floating’ condition is achieved by disabling the EnerChip CC

charge pump after charging the EnerChip to a full or partial state of charge, leaving the EnerChip terminal

voltage to float, as opposed to maintaining a constant bias on the cell as would be the condition when leaving

the charge pump enabled at all times. The effects on cell performance as a function of constantly biasing the

EnerChip - versus allowing the terminal voltage to float - are described next.

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AN-1052: Operating the EnerChip™ in High Temp. Environments

Figure 5: EnerChip Capacity Loss as a Function of Time and State of Charge at 70°C

Figure 6 represents the effects of bias conditions and state of charge on the EnerChip cell resistance. While

there is an advantage to holding cells in a partially - rather than fully - charged state, the impact of trickle

charging or continuously biasing cells at temperature is no more detrimental to the cell resistance than storing

cells in an unbiased (i.e., floating terminal voltage) state in the normal operating temperature range. A 3.98V

bias condition (~70% SoC) as shown in Figure 6 is the voltage the EnerChip is charged to in the EnerChip

CC products, all having integrated battery management functions with a built-in charge voltage temperature

coefficient.

Figure 7 represents the same operating conditions as Figure 6, but in Figure 7 the effects on cell capacity

loss due to bias condition, state of charge, and temperature are shown. In addition, Figure 7 includes an

extrapolation of the data from 70°C to 85°C to give an indication of how the cell is likely to perform when

operated above its specified operating temperature.

Unlike the effects on cell resistance, trickle charging or biasing cells at temperature is more detrimental to the

capacity loss of the cells than storing cells in an unbiased state. There is a significant advantage to holding

cells in a partially - rather than fully - charged state at higher temperatures. This condition is implemented

easily by disabling the EnerChip CC charge pump after the EnerChip is charged. The charge pump is disabled

by driving the input pin ENABLE low. Doing so also lowers the operating current of the EnerChip CC device, thus

reducing total system power consumption.

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AN-1052

AN-1052概述

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