Battery Runtime and Capacity Estimation

Capacity is the total charge a cell can deliver. It is usually stated in milliamp-hours or amp-hours. The rating assumes a specific discharge rate and temperature. Draw current faster or run the cell colder, and usable capacity drops below the number on the label.

Renesas fuel gauge ICs track three related figures: Full Charge Capacity, Remaining Capacity, and Relative State of Charge. The R-BMS F Battery Fuel Gauge IC overview lists voltage, current, and temperature measurement alongside the calculation of these three figures (R01PF0239, Firmware section).

Divide usable capacity by average current draw. A 2000 mAh pack supplying 200 mA runs about 10 hours. The word “usable” matters. Cutoff voltage and self-discharge both reduce what you can actually pull from the cell.

Renesas builds this into the pack itself. The RAJ240055 datasheet describes an IC that estimates remaining capacity on-chip, using firmware stored in embedded flash (R01DS0399, Section 1.3, p. 1).

State of Charge is how full the battery is right now, shown as a percentage of full capacity. Think of it as a fuel gauge. Unlike a fuel tank, there is no way to see the level directly. It has to be worked out from other measurements.

Renesas treats SOC estimation as a core fuel gauge function. Power On-the-Go covers it alongside State of Health, which tracks how the pack ages (blog, Design Challenges and Advanced Battery Management sections).

Coulomb counting measures charge flowing in and out of a battery. Multiply current by elapsed time and you get charge in coulombs. Keep adding that up and you have a running tally of what the pack has gained and lost.

The RAA489206 datasheet describes the mechanism directly. A 32-bit timer records elapsed time, and multiplying that interval by the measured current gives a charge value in coulombs (R16DS0141, Section 5.3.7, p. 44).

Every current measurement carries small errors. Integration adds those errors up instead of averaging them out. Over long periods the estimate slowly drifts away from reality, so it needs periodic correction.

Battery monitors fix this by anchoring to known reference points. The RAA489206 sample code manual defines flags for a fully charged battery and for end-of-charge conditions, the kind of reference points a gauge can use (R16UZ0123, Section 3.1, Table 2, p. 7).

A charger needs to know when to stop. There are two common methods. Watch for cell voltage to cross a threshold, or watch for charge current to drop below a set level as the pack fills.

The RAA489206 sample code manual supports both, letting you detect end of charge by either voltage or current threshold (R16UZ0123, Table 3, p. 8; Table 4, p. 10).

State of Charge tells you how full the pack is today. State of Health tells you how much capacity the pack has lost since it was new. A battery can be fully charged and still hold far less energy than it did a year ago.

Renesas fuel gauge ICs measure capacity and impedance automatically to track this. Power On-the-Go explains how those measurements help decide when a pack needs replacing (blog, Advanced Battery Management section).

Internal resistance rises as a cell ages. Higher impedance means more voltage sag under load. The pack then hits its cutoff voltage sooner and delivers less runtime, even when the chemistry still holds charge.

Impedance measurement is built into the hardware. The RAJ240055 datasheet lists an impedance measurement circuit alongside voltage, current, and temperature detection (R01DS0399, Section 1.1, p. 1).

A capacity estimate is only as good as the current measurement behind it. That usually calls for a high-resolution converter, since current can range from microamps at rest to amps under load.

Renesas uses dedicated analog hardware for this. The RAJ240055 datasheet describes an IC that pairs a 15-bit sigma-delta converter for general measurement with an 18-bit current integration circuit for charge counting (R01DS0399, Section 1.1, p. 1; Sections 4.8.4 and 4.8.5, pp. 44 and 46).

In a series pack, the weakest cell sets the limit. Once any single cell reaches its cutoff, the whole pack has to stop. The other cells may still hold charge that never gets used. Balancing evens out the cells so more of the total capacity is reachable.

Renesas pairs balancing with state estimation. The E-Mobility solutions page outlines how precision sensing works with MCU-based algorithms for state estimation, cell balancing, and fault handling (E-Mobility page, Battery Intelligence section).

Three at minimum: voltage, current, and temperature. Voltage gives an approximate charge level. Current feeds the coulomb counter. Temperature affects both capacity and safety limits.

The R-BMS F overview lists these same three measurements alongside its Full Charge Capacity, Remaining Capacity, and Relative State of Charge calculations (R01PF0239, Firmware section).

Firmware lets you tune the algorithm for your chemistry and use case. Fixed-function hardware removes that development work and shortens time to market. The trade-off is less room to customize.

Renesas offers both paths. The R-BMS F ships with fixed firmware already built into the fuel gauge IC (R01PF0239, Firmware section). That takes firmware development off the schedule, while a dedicated tool still allows parameter adjustment (R01PF0239, Quick Design Safe System section).