Functional description

STC3115

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DocID023755 Rev 4

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 STC3115.

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.

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

battery state.

The REG_OCVTAB registers are used to adjust the internal OCV table to a given battery

type.

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 mΩ and Cnom is in mAh.

Example: Ri = 190 mΩ, Cnom =1650 mAh, REG_VM_CNF = 321

Voltage mode

algorithm

OCV adjustment registers

Voltage register

VM configuration

To SOC

management

OCV value

(VM)

Reference

OCV

curve

AD

converter

VIN

REG_VM_CNF

Ri Cnom

×

977.78

⁄

=