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Battery Monitoring

Devboard Jumper Configurations

To use the battery monitoring functions, jumpers U14/17, U23, and U27 should be populated. Each pin on jumper U14/17 correspond to different functionality respectively and can be used individually, not all of them have to be populated at the same time. The description of each pin on jumper U14/17 are as follows:

Pin Name Signal Description
INT USB_PRESENT USB (or external) +5V is connected
RST VOLTS_TEST Battery Voltage Measurement: Control
PWM VOLTS Battery Voltage Measurement: Battery Voltage to ADC for Measurement
AN CHARGE_STAT Battery Charging Status (Charging/Not Charging)

NOTE: VOLTS_TEST and VOLTS both need to be populated at the same time if the battery voltage is to be measured.

Methodology for Battery Percentage Calculation

If VOLTS_TEST and VOLTS are enabled, the battery voltage could be measured and converted to its corresponding value in percentage (%) by using the battery’s charge/discharge curve. Note that every type of battery will have its own charge/discharge curve. The battery types currently implemented are the RCR123A 3.7V Rechargeable Lithium-Ion Battery and the CR123A 3V Non-Rechargeable Lithium Battery.

RCR123A 3.7V Rechargeable Lithium-Ion Battery

The typical 3.7V Lithium-Ion Battery voltage vs capacity table is shown as follows:

Capacity Voltage
100 4.20
95 4.15
90 4.11
85 4.08
80 4.02
75 3.98
70 3.95
65 3.91
60 3.87
55 3.85
50 3.84
45 3.82
40 3.80
35 3.79
30 3.77
25 3.75
20 3.73
15 3.71
10 3.69
5 3.61
0 3.27

The voltage vs capacity graph is then plotted using Excel and is shown as follows:

The curve is then split into a few regions where each region could be reasonably linearised with a line of best fit. The straight line equation of each linearised region is obtained and the battery percentage value could be calculated correspondingly.

CR123A 3V Non-Rechargeable Lithium Battery

The typical 3V Lithium Battery voltage vs capacity table is shown as follows:

Capacity Voltage
100 3.15
95 2.91
90 2.89
85 2.88
80 2.87
75 2.86
70 2.85
65 2.84
60 2.82
55 2.81
50 2.80
45 2.79
40 2.78
35 2.76
30 2.75
25 2.70
20 2.65
15 2.60
10 2.53
5 2.35
0 2.00

The voltage vs capacity graph is then plotted using Excel and is shown as follows:

The curve is then split into a few regions where each region could be reasonably linearised with a line of best fit. The straight line equation of each linearised region is obtained and the battery percentage value could be calculated correspondingly.

References