Li-ion vs LiPo vs LiFePO4 vs NiMH for Hardware Design

Quick answer

Choose a pouch LiPo for a thin wearable, or cylindrical Li-ion for a handheld IoT device with room for a supported battery pack. For an outdoor solar sensor that recharges frequently, consider LiFePO4 because of its cycling characteristics, provided the design includes temperature-qualified charging and a suitable regulator. For a high-current robot or tool, choose a documented power cell, such as a high-drain cylindrical Li-ion cell, and a battery pack designed for the required current. NiMH is a practical choice when compatibility with replaceable AA batteries is mandatory.

Below freezing, qualify discharge separately from charging. A cell's ability to discharge in cold conditions does not establish that charging below freezing is permitted.

The following battery categories describe different aspects of cell construction:

• Li-ion refers to rechargeable lithium-ion chemistry, commonly supplied in cylindrical cells such as 18650 and 21700.
• LiPo, or lithium-polymer, commonly describes lithium-ion cells supplied in flexible pouches. The pouch format does not, by itself, identify a particular cathode chemistry.
• LiFePO4, or lithium iron phosphate, is a specific lithium-ion cathode chemistry with a different voltage profile and charging requirements from conventional 4.2 V lithium-ion cells.
• NiMH, or nickel-metal hydride, is a separate rechargeable battery chemistry commonly available in AA and AAA formats.

Cell chemistry determines much of the electrical behavior, while packaging affects mechanical integration. Engineers should consider both before selecting a battery.

Comparison table: exact examples, not universal ranges

The comparison uses one example cell from each category: Molicel/E-One Moli Energy INR-18650-P28A, EEMB LP503450, Drypower IFR18650 E1600, and FDK HR-3UTG.

The values describe the selected cells rather than universal limits for each battery category. The P28A datasheet identifies the cell as lithium-ion but does not explicitly confirm an NMC or NCA cathode. Fields without confirmed numerical specifications are marked as omitted rather than filled with chemistry-wide estimates.

Table 1. Specifications of four example rechargeable battery cells

C-rate expresses charge or discharge current relative to the cell's rated capacity. A plotted discharge current does not necessarily establish the maximum continuous C-rate.

Cylindrical Li-ion cells are also available in the 21700 format. Engineers should evaluate the specifications, holders, tabs, and enclosure requirements of each selected cell rather than transferring an 18650 rating to a larger format. Neither format alone determines cathode chemistry or guaranteed power capability.

Choose cylindrical Li-ion if...

• A handheld IoT terminal accommodates a cylindrical pack and needs serviceable battery support.
• A portable instrument prioritizes stored energy over minimum thickness.
• A robot or tool has a qualified power pack with documented handling of motor stall current.
• A field logger benefits from a replaceable pack and traceable sourcing.

Obtain an explicit NMC or NCA declaration for procurement if the cathode chemistry matters to the design. Do not infer the cathode material from a model prefix.

Choose pouch LiPo if...

Pouch LiPo cells are useful when battery thickness and enclosure geometry limit the available space. The flexible outer package also requires attention to mechanical protection.

• A wearable needs a flat battery beside a rigid PCB.
• A tracker has an enclosure shaped around a custom pouch.
• A wireless accessory uses a protected factory pack with a temperature-monitoring lead.
• A compact robot has a verified high-current pouch with allowances for swelling and strain on its tabs.

A thin pouch intended for low-current electronics is not interchangeable with a high-current radio-control (RC) battery pack. Specify the actual discharge current and mechanical requirements.

Choose LiFePO4 if...

LiFePO4 cells have a different voltage profile from conventional 4.2 V lithium-ion cells and are often considered for applications that require frequent charge and discharge cycles.

• A solar sensor expects frequent recharge cycles and has adequate enclosure space.
• A remote instrument supports a converter matched to the LiFePO4 discharge-voltage profile.
• A reusable industrial accessory prioritizes cycling performance over minimum mass.
• A robot has a purpose-designed LiFePO4 pack with monitoring, balancing, and fault isolation.

The lower LiFePO4 voltage profile affects charger selection and the voltage regulator supplying the electronics. LiFePO4 cells still require suitable protection against electrical and temperature faults.

Choose NiMH if...

NiMH cells are useful in products designed around replaceable AA batteries, including products that also accept primary, nonrechargeable AA cells.

• A product must also run from ordinary AA cells in a replaceable holder.
• A remote control should accept rechargeable spare cells.
• A service instrument uses an approved external NiMH charging procedure.
• An educational product benefits from accessible batteries and mechanical polarity control.

If the holder accepts both rechargeable NiMH and disposable primary AA cells, the charging circuit must prevent attempts to recharge primary batteries. The circuit must also accommodate the voltage range and reverse-polarity risks of every permitted cell type.

Design considerations: schematic, BOM and PCB

Charger selection, profile and current

Conventional lithium-ion cells commonly use constant-current/constant-voltage (CC/CV) charging. The charger first controls the charging current and then holds the specified voltage as current decreases near full charge.

Microchip MCP73831T-2ATI/OT is the nominal 4.20 V ordering option, suitable only when the selected cell permits that charging voltage. Higher-voltage Li-ion variants require their own approved charger settings.

LiFePO4 also commonly uses CC/CV charging, but at a lower manufacturer-specified voltage. TI BQ24650RVAR supports LiFePO4 charging through an adjustable voltage-feedback divider. The manufacturer's application note illustrates a particular cell configuration, so the selected E1600 cell still requires its own charging settings.

NiMH charging uses a different method. A NiMH charger controls current and typically monitors the battery for a small voltage decrease near full charge, called negative delta voltage (−ΔV). Temperature monitoring and a timeout provide additional termination safeguards.

The TI BQ2002SNTR is a NiMH charging-controller example. TI associates the part with the BQ2002/F datasheet, although the exact reference circuit and pin connections still need verification for the intended implementation. TI currently lists BQ2002SNTR as a last-time-buy part, so a new design should also consider an available alternative.

Set charge current according to the cell's permitted charging rate, the available power supply, and thermal limits. MCP73831 uses the programming relationship:

IREG (mA) = 1000 / RPROG (kΩ)

The charger's maximum current capability does not establish the current permitted by the battery cell.

Load sharing and termination

Load sharing determines how an external power supply supports the system while charging the battery. If a load is connected directly to a basic charger's battery output, the charger's current measurement may include both battery-charging current and system current. The additional load current can interfere with charge termination.

The MCP73831 introduced above has no separate system-power output or external thermistor input. Disable the load during charging or use an external power-path circuit designed for simultaneous operation and charging.

For a charger with integrated power-path control, TI BQ24075RGTT provides a separate OUT connection for the system and BAT connection for the battery.

Another option is TI BQ25895RTWT, a switching charger with programmable regulation from 3.840 V to 4.608 V in 16 mV steps. The programmed voltage and its tolerance must match the selected cell.

When charging from USB-C, the USB supply negotiation should also be considered. Flux's guide to Programmable Power Supply (PPS) charging explains how USB Power Delivery can negotiate the voltage and current supplied by an upstream adapter.

The battery-charger ICs discussed here should not be assumed to perform USB-PD PPS negotiation without an appropriate USB controller.

Protection and return nodes

A battery charger controls the normal charging process, but the pack also needs protection against electrical faults. Depending on the cell and pack configuration, protection must address overcharge, overdischarge, excessive current, and short circuits.

Match the protection thresholds, tolerances, and delays to the cell, wiring, contacts, and switching transistors. The system should shut down normally before the battery reaches its emergency protection threshold, rather than depending on repeated protection trips.

For a single-cell lithium-ion protection circuit, TI BQ29700DSER controls two low-side N-channel MOSFETs that interrupt charging or discharging during faults.

The VSS pin references the cell's negative terminal, while V− senses the protected pack's negative terminal. DOUT and COUT control the discharge and charge switching devices.

CSD85302L is a dual-MOSFET candidate listed in the Flux library, but the exact pairing and connection topology need verification before use. LiFePO4 designs also require protection thresholds appropriate to their lower cell-voltage range.

The distinction between cell negative and protected pack negative matters when the protection circuit disconnects the battery. Connect the charger and system returns to protected pack negative. A USB ground or other system connection wired directly to bare cell negative may bypass the protection switches. A fuel gauge connected on the cell side needs an interface that does not bypass the disconnect.

NTC placement, regulator and fuel gauge

A negative temperature coefficient (NTC) thermistor changes resistance as temperature changes, allowing the charging circuit to monitor battery temperature. Thermally couple the NTC to the cell rather than placing it beside a hot charger IC.

The BQ24075 TS temperature-sense input supports the specified battery-pack thermistor network. Validate the temperature thresholds, hysteresis, and response to an open or shorted thermistor lead against the cell's approved charging range.

After the battery and charger, the circuit needs a regulator to provide a stable supply voltage to the electronics.

For a 3.3 V supply rail, a low-dropout (LDO) regulator needs its input voltage to exceed the output by the regulator's specified dropout voltage. As battery voltage falls toward 3.3 V, the regulator may lose the ability to maintain the output. If the battery voltage can fall above and below the required rail, use a buck-boost converter, which can step voltage down or up.

Check the minimum input voltage under load, startup behavior, and the regulator's own quiescent current.

A fuel gauge estimates the battery's state of charge (SoC), meaning the amount of charge available relative to its capacity. Voltage-based estimates depend on chemistry, temperature, and load because battery voltage changes under different operating conditions.

Another method, coulomb counting, measures current flowing into and out of the battery over time. The calculation requires control of measurement offset and an appropriate capacity reference.

ADI/Maxim MAX17048G+T10 uses the ModelGauge algorithm without a current-sense resistor. The device is specified for single-cell Li-ion batteries.

For MAX17048, VDD senses battery positive, and CELL is not internally connected.

Connectors, copper and physical placement

Use a keyed battery connector to reduce the risk of reverse insertion, and verify the polarity supplied by the battery manufacturer. Identical connector housings do not guarantee identical wire assignments.

Provide strain relief and coordinate fuse selection with the expected fault current and the pack's protection thresholds.

The copper traces, vias, contacts, and current-sense resistors must also carry the expected current without excessive heating or voltage drop. Thermal-relief spokes, which connect pads to copper planes through narrow sections, can restrict current flow in high-current paths. Review the actual copper geometry rather than applying a single trace-width rule.

Keep switching-current loops compact. Route sensitive current-sense connections separately from the main current path, using Kelvin connections to avoid including voltage drops across high-current copper in the measurement.

Cylindrical cells need holders or mechanical supports that do not stress welded tabs or abrade insulation. Pouch cells need manufacturer-approved clearance for expansion and protection from screws, sharp PCB edges, and hot components.

Never compress a swollen pouch back into service.

Safety and cold operation

Lithium-ion battery abuse can cause heating, venting, and thermal runaway. Swelling is a reason to isolate the battery and follow the supplier's replacement instructions.

LiFePO4 cells also require protection against misuse, and NiMH cells can overheat, vent, or leak. Battery chemistry does not eliminate the need for thermal qualification and electrical fault protection.

The P28A datasheet distinguishes charging from discharging by specifying different temperature ranges for each operation. For equipment used below freezing, test voltage sag and usable capacity at the actual discharge current, and inhibit charging outside the approved cell temperature range.

The other example cells must also be checked against their specific charging limits. A permissible cold-discharge temperature does not establish permission to charge at the same temperature.

Four minimal reference-circuit starting points

The following examples describe circuit architectures rather than completed, electrically verified bills of materials (BOMs). Start from the relevant manufacturer's application circuit and verify the connections before implementation. Flux's guide to starting a PCB from a template provides additional guidance on adapting reference schematics.

Cylindrical Li-ion

Use an approved, protected P28A pack with the MCP73831 charging circuit.

Connect the pack's positive terminal to the MCP73831 VBAT pin, the regulated charging input to VDD, and VSS to protected pack negative.

The PROG pin sets the charging current through an external resistor, and STAT provides charging-status indication. Include at least 4.7 µF of bypass capacitance on the input and battery nodes.

Disable the system load during charging or add a suitable power-path circuit. The design also needs independent cell-temperature monitoring to inhibit charging outside the approved range.

Pouch LiPo

Use an approved, protected pouch pack with TI BQ24075, a single-cell charging IC that includes a separate system-power output.

Connect the battery's positive terminal to BAT, the external charging supply to IN, and the system regulator or load to OUT. Return VSS and the thermal pad to protected pack negative.

Connect the cell-mounted thermistor to TS, the temperature-sense input, using the specified resistor network. Add the required charge-current and input-current programming components, with defined enable states.

An optional MAX17048 fuel gauge uses VDD at pack positive, GND at pack negative, and suitable I²C pull-up resistors. Verify the fuel gauge's behavior when the protection circuit disconnects or reconnects the battery.

LiFePO4 solar

Start with TI BQ24650RVAR's switching-charger reference circuit and an approved, protected LiFePO4 pack.

The charger uses a resistor divider connected to its voltage-feedback pin (VFB) to set the final charging voltage. The relationship is:

VBAT = 2.1 V × (1 + Rtop/Rbottom)

Select Rtop and Rbottom according to the approved charging voltage and include resistor tolerances in the calculation.

Connect SRP and SRN, the current-sense inputs, across the charging-current shunt resistor using Kelvin connections. Implement the TS temperature-sense network and configure MPPSET, the input-voltage setting used for solar-panel power-point control, for the selected panel.

Include the required external switching components and a defined method for powering the system while the battery charges.

NiMH AA product

Use TI BQ2002SNTR's manufacturer reference circuit, qualified NiMH cells, and a keyed AA holder.

The circuit requires battery-voltage sensing, temperature monitoring at the cells, and an external controlled-current charging stage. It must also prevent charging when the holder contains nonrechargeable primary AA batteries.

Detailed connections should follow the exact BQ2002SNTR datasheet and reference circuit rather than an assumed pin assignment. Because the part is currently listed as last-time-buy, verify availability or select a supported alternative. An approved external NiMH charger is another option when onboard charging is unnecessary.

Shipping and certification

Lithium battery transport testing is addressed by the UN Manual of Tests and Criteria, eighth revised edition (2023), subsection 38.3, together with Amendment 1 (2025).

A cell test report does not automatically cover every assembled battery pack.

For portable battery safety, IEC 62133-2:2017+AMD1:2021 addresses lithium systems. Nickel systems are covered by IEC 62133-1, whose 2026 edition replaced the 2017 edition.

Confirm the applicable editions, product requirements, and shipping regulations with the test laboratory. Product safety certification and transport testing address different obligations.

FAQ

Lithium ion vs lithium polymer: what is the difference?

Li-ion vs LiPo: which should I use for IoT?

LiFePO4 vs lithium ion: is LiFePO4 safer?

LiPo vs LiFePO4: which fits a solar sensor?

NiMH vs lithium ion: which works with AA batteries?

18650 vs LiPo: which handles more current?

Sources and library verification

Example cells

• Molicel INR-18650-P28A datasheet, version 1.3.
• EEMB LP503450 product specifications. The original specification reference is ZJQM-RD-SPC-H0750, dated July 1, 2021.
• Drypower IFR18650 E1600 datasheet, April 2021.
• FDK HR-3UTG datasheet, revision date not printed.

Charging, protection, and fuel-gauge ICs

• Microchip MCP73831/2 datasheet, DS20001984H, June 2020.
• TI BQ24075 product and datasheet, SLUS810N, October 2021.
• TI BQ25895 product and datasheet, revision C, October 2022.
• TI BQ24650 product and datasheet, SLUSA75B, January 2020.
• TI BQ24650 LiFePO4 charging application report, SLUA565, August 2010.
• TI BQ2970 battery protection family, SLUSBU9I, August 2024.
• TI CSD85302L product information, SLPS561, November 2015.
• Analog Devices MAX17048, datasheet revision 7, November 2016.
• TI BQ2002 and BQ2002SNTR, BQ2002/F datasheet SLUS131A, April 2009.

Flux battery-management reference designs provide starting architectures, but cell ratings, protection thresholds, and power-return connections must be checked for the actual pack.