Functional description

STC3117

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DocID025792 Rev 2

6.2.2

Voltage gas gauge algorithm

No current sensing is needed for the voltage gas gauge. An internal algorithm precisely

simulates the dynamic behavior of the battery and provides an estimation of the OCV. The

battery SOC is related to the OCV by means of a high-precision reference OCV curve built

into the STC3117.

Any change in battery voltage causes the algorithm to track both the OCV and SOC values,

taking into account the non-linear characteristics and time constants related to the chemical

nature of the Li-Ion and Li-Po batteries.

A single parameter fits the algorithm to a specific battery. The default value provides good

results for most battery chemistries used in hand-held applications.

Figure 6. Voltage gas gauge block diagram

Voltage gas gauge algorithm registers

The REG_VM_CNF configuration register is used to configure the parameter used by the

algorithm based on battery characteristic. The default value is 321 (corresponding to

160 m

Ω internal battery impedance and 1957 mAh Cnom battery capacity).

The REG_OCV register holds the estimated OCV value corresponding to the present

battery state.

The REG_OCVTAB and REG_SOCTAB registers define the OCV curve for a given battery

type; the default power-up values can be updated at software initialization.

The REG_VM_CNF register is a 12-bit integer value and is calculated from the averaged

internal resistance and nominal capacity of the battery as shown in Equation 2:

Equation 2

Ri is in m

Ω and Cnom is in mAh.

Example: Ri = 250 m

Ω, Cnom =1500 mAh, VM_CNF= 384

Voltage mode

algorithm

To SOC

management

(VM)

Reference

OCV

curve

AD

converter

VIN

register

REG_VOLTAGE

register

REG_VM_CNF

register

REG_OCV

register

REG_OCVTAB

register

REG_SOCTAB

VM_CNF

Ri

Cnom

×

977.78

⁄

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