Before optimizing power consumption, it helps to understand what a battery actually is and how it fails. This chapter covers the chemistry behind common cell types, the parameters you will encounter in datasheets, and the role of battery management systems (BMS) in protecting and measuring battery state.
Not all batteries are interchangeable. Each chemistry has a distinct voltage curve, energy density, cycle life, and failure mode.
The dominant chemistry for laptops, phones, and portable electronics. Nominal cell voltage is 3.6–3.7 V, with a full charge at 4.2 V and a minimum safe discharge at 2.5–3.0 V. Energy density is high (~250 Wh/kg), cycle life is typically 500–1000 full cycles to 80% capacity.
Li-Ion is sensitive to overcharge (thermal runaway risk) and deep discharge (irreversible capacity loss). This is why every Li-Ion device has a BMS.
Essentially Li-Ion in a flexible pouch rather than a rigid cylinder. The chemistry is similar but the form factor allows thinner, lighter, and custom-shaped packs. LiPo cells are more common in drones, wearables, and small IoT projects. They are slightly more sensitive to mechanical damage and puffing from overcharge.
Nominal cell voltage is 1.2 V. Commonly found in AA/AAA rechargeable batteries. Energy density is lower than Li-Ion (~100 Wh/kg), but NiMH is more tolerant of abuse, requires no BMS for basic use, and operates well in cold temperatures. A good choice for devices that need to run off standard AA cells.
Heavy and low energy density (~30–50 Wh/kg) but cheap, robust, and capable of high discharge currents. Common in UPS systems and automotive applications. Relevant in the home lab context for battery backup of NUCs and servers.
Capacity (mAh / Ah): How much charge the battery holds. A 1000 mAh battery can theoretically deliver 1000 mA for one hour, or 100 mA for ten hours. In practice, capacity drops at high discharge rates.
C-Rating: Expresses charge and discharge rate relative to capacity. A 1000 mAh battery charged at 1C is charged at 1000 mA. A 2C discharge is 2000 mA. Most Li-Ion/LiPo cells are rated for 1C charge and 1–2C continuous discharge; exceeding this reduces cycle life.
State of Charge (SoC): The current charge level expressed as a percentage of full capacity. SoC is not directly measurable — it must be estimated from voltage (imprecise) or by integrating current over time (coulomb counting, more accurate but requires a fuel gauge IC).
State of Health (SoH): The ratio of current full-charge capacity to the original rated capacity. A battery at 80% SoH can only hold 800 mAh of its original 1000 mAh. Most devices consider a battery at 80% SoH to be “end of life.”
Internal Resistance: Increases with age and at low temperatures. Higher internal resistance means more voltage drop under load and more heat generated during charge/discharge.
The voltage of a Li-Ion cell is not linear with SoC. It drops steeply at the bottom and top and is relatively flat in the middle (roughly 3.6–3.8 V from 20% to 80% SoC). This flat region makes voltage a poor proxy for SoC — a cell at 50% and a cell at 70% may differ by only 100 mV under load.
For embedded systems reading SoC via an ADC, this means you need to use a calibrated lookup table and account for the voltage drop across internal resistance under load. A cell that reads 3.7 V at rest may read 3.4 V when driving a WiFi radio burst — this does not mean the battery is nearly empty.
A BMS sits between the cell (or pack) and the load/charger. Its core responsibilities:
Protection: Disconnects the cell if voltage goes below the minimum (deep discharge protection), above the maximum (overcharge protection), or if current exceeds a threshold (overcurrent/short circuit protection).
Cell Balancing: In multi-cell packs, individual cells charge and discharge at slightly different rates due to manufacturing tolerances. Over time these differences compound. A BMS with balancing either drains excess charge from high cells (passive balancing, energy-wasting but simple) or transfers charge from high to low cells (active balancing, more efficient but complex).
Fuel Gauging: Higher-end BMS chips include coulomb counters that integrate charge/discharge current to track SoC accurately. Common fuel gauge ICs: MAX17048 (I2C, SoC via voltage model), BQ27441 (I2C, full coulomb counter), BQ34Z100 (for larger packs).
The single most impactful thing you can do for Li-Ion longevity is to avoid keeping the battery at 100% or 0% SoC. Electrochemical stress is highest at the extremes. Most battery researchers recommend keeping Li-Ion between 20% and 80% SoC for maximum cycle life.
Practically:
| ← Introduction | Table of Contents | Chapter 2: Measuring Power Consumption → |