DocID023052 Rev 2

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AN4085

Discharging the EFL700A39 battery

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To prevent deep discharge damage, a battery voltage sense and disconnection circuit - with

a threshold voltage higher than 3.0 V - has to be put in place using a battery management

IC such as ST SPV1050 device. These ICs must have a very low leakage current drawn

from the battery specifically when the battery is disconnected.

The Table 1 provides some example of allowable elapse time in battery disconnect mode for

different settings of battery disconnect threshold voltage and different settings of leakage

current of the battery management IC. For each case of the Table 1, if the duration of the

battery disconnection is longer than these values, the voltage of the EnFilm can drop down

below 3.0 V with a risk of unrecoverable damage.

For example, referring to the product datasheet of the ST SPV1050 IC, the leakage is

specified below 1 nA as shown in the Table 2 and the cut-off voltage is adjustable from 2.1 V

up to 3.6 V (“2.1 V TO 3.6 V trimmable battery under voltage protection level”).

Choosing a 3.60 V cut-off voltage setup ensures, according to the Table 1, 17 months of

EFL700A39 micro-battery survival time. It means that once the battery is disconnected, a

source of energy like a harvester has to charge the EFL700A39 before 17 months (1.4

years).

3.2.3

Recommended maximum load current overtemperature

The Figure 5 provides the characteristics of the EFL700A39 internal resistance on the

battery operating temperature range. The battery internal resistance is decreasing when the

temperature is rising.

The Figure 6 shows the maximum load current available over the ambient temperature for

safe operating cut-off voltages of 3.0V (minimum), 3.2 V, 3. 4V and 3.6 V.

Table 1. EFL700A39 elapsed time from battery disconnect until 3.0 V maximum

discharge level for different leakage current at 30 °C temperature

Battery disconnect

threshold voltage

IC leakage current in battery disconnect mode

1nA

10nA

100nA

3.80V

3 years

9 months

36 days

3.60V

17 months

4 months

17 days

3.40V

8 months

2 months

8 days

3.20V

2 months

19 days

2 days

Table 2. ST SPV1050 datasheet extract for leakage current in battery disconnect mode

Sumbol

Parameter

Test condition

Min.

Typ.

Max. Unit

Static current consumption

Shutdown current

Shut down mode:

Before first startup or BATT_CONN

high

1nA