Batteries and Cells

A battery stores energy as chemistry and releases it as electricity on demand. This article covers what happens inside one, why each chemistry has its own voltage, the common types, what capacity figures mean, how each type is charged, and how to keep lithium batteries from starting fires.

Article 9 treated a battery as a circuit element with internal resistance, capacity and short-circuit dangers. This article opens the case.

1. A short recap

• Current is moving charge, carried in wires by electrons; voltage is the push, the energy given to each unit of charge (recap).
• Metals are conductors because some of their electrons are free to move (recap).
• A real battery acts like a perfect voltage source, its emf, in series with a small internal resistance, so its terminal voltage sags under load (recap).
• Cells in series add their voltages; cells in parallel add their capacities. Never mix mismatched cells (recap).
• Capacity is given in milliamp-hours (mAh); multiply by voltage to get the energy in watt-hours (Wh) (recap).
• A short circuit across a battery can melt wires and start fires (recap). A switch breaking battery current must be rated for DC, which is harder to interrupt than AC (recap).

2. Cell or battery?

A cell is a single chemical unit: two electrodes and an electrolyte that together produce a voltage.^[2] Strictly, a battery is a set of cells connected to give more voltage or current than one cell can.^[2] In everyday speech an AA cell is called a battery too.

The voltage of a cell is set by its chemistry alone; its size makes no difference.^[2] To get more voltage, cells are connected in series and their voltages add.^[2]​^[3] A car battery has six lead-acid cells of 2.0 V each, giving 12 V.^[2] A rectangular 9 V battery holds six small cells in series,^[4] 6 × 1.5 V = 9 V (calculated) (Figure 1).

3. How a cell works

Most cells have four main parts (Figure 2):

• The anode: the electrode where, during discharge, a material gives up electrons. On a battery this is the negative terminal.^[5]​^[6]
• The cathode: the electrode where, during discharge, a material takes in electrons. It is the positive terminal.^[5]​^[6]
• The electrolyte: a liquid, paste or gel in which ions (atoms or molecules carrying an electric charge) move between the electrodes.^[2]​^[6]
• The separator: a thin porous layer that keeps the electrodes from touching and shorting, while ions pass through it.^[3]​^[6]

Metal current collectors carry electrons to the terminals. In an alkaline AA, the anode is a zinc-powder gel, the cathode is manganese dioxide with carbon, the electrolyte is potassium hydroxide, and the steel can and a brass pin are the collectors.^[7]

Redox, plainly. The chemistry is a redox reaction (reduction and oxidation). Oxidation means losing electrons; reduction means gaining them.^[6] In a cell the two halves happen in different places, oxidation at the anode and reduction at the cathode, so the freed electrons must travel out through your circuit to get from one to the other.^[5] That flow is the current that does the work. Inside, ions drift through the electrolyte to keep the charges balanced.^[5]​^[6] Break the outside circuit and the reaction almost stops, which is why a primary battery can be stored for several years.^[8]

4. Why the chemistry sets the voltage

Some materials hold on to their electrons more loosely than others. Chemists measure this as a standard electrode potential in volts, compared with a reference, the standard hydrogen electrode, set at exactly 0 V.^[9] Put in order, these values form the electrochemical series (Figure 3).

A cell's voltage is the difference between its two electrodes' potentials:^[9]

E_cell = E_cathode − E_anode

Zinc's potential is −0.76 V and copper's is +0.34 V,^[10] so a zinc-copper cell gives 0.34 − (−0.76) = 1.10 V (calculated). The lead-acid pair (lead dioxide at +1.69 V, lead at −0.35 V in sulfuric acid)^[10] gives 2.04 V (calculated), which matches the 2.0 V nominal lead-acid cell. Lithium sits at the very bottom of the list, at −3.04 V,^[10] which is part of why lithium cells reach 3 V or more.^[6]

Because chemistry fixes the voltage, every size of zinc-carbon cell from AAA to D gives the same voltage. A bigger cell holds more reacting material, so it can move more charge and lasts longer, but it does not push harder.^[3]

5. Primary and secondary cells

Primary cells are single-use: their reaction cannot be practically reversed. Secondary cells are rechargeable: running current backward through them reverses the reaction.^[3] Primary cells keep their charge far longer in storage.^[8]

Never try to recharge a primary cell. Energizer warns not to recharge any battery unless it is marked "rechargeable", because charging a primary battery "could result in rupture or leakage".^[7]

6. The main chemistries

Single-use (primary) cells

• Zinc-carbon (also called carbon-zinc or "heavy duty"): a zinc anode, manganese dioxide cathode and a slightly acidic electrolyte of zinc chloride or ammonium chloride, 1.5 V.^[11]​^[12] Energizer describes it as an economical choice for light to moderate loads.^[11]
• Alkaline: the same zinc and manganese dioxide, but with an alkaline potassium hydroxide electrolyte, 1.5 V. It holds more energy than zinc-carbon and does better at high drain, in the cold and on the shelf.^[7]
• Lithium coin cells such as the CR2032: lithium metal with manganese dioxide, 3.0 V, about 235 mAh.^[13] They run car keys, calculators, small medical devices and the memory backup on computer motherboards.^[14]
• Lithium AA and AAA (lithium-iron disulfide, such as the Energizer L91): 1.5 V, so they replace alkaline cells directly. They weigh about a third less, work from −40 °C to 60 °C and handle high currents well.^[15]​^[16]
• Silver oxide button cells: zinc with silver oxide, 1.55 V, with a flatter discharge voltage than alkaline. Used in watches, calculators and instruments.^[17]​^[18]

"Lithium battery" and "lithium-ion battery" are different things. Single-use lithium batteries contain lithium metal; lithium-ion batteries do not.^[15]

Lead-acid

The oldest rechargeable (1859) uses lead and lead dioxide plates in sulfuric acid, at 2.0 V per cell.^[19] It is cheap and can deliver the high current a starter motor needs.^[3] Three main forms:^[19]

• Flooded: liquid acid; not spill-proof, and needs water topped up.
• AGM (absorbent glass mat): acid soaked into a fiberglass mat; spill-proof.
• Gel: acid thickened with silica; spill-proof.

Lead-acid is heavy, and lead is toxic.^[19] Discharging coats both plates with lead sulfate, and charging turns it back.^[3] Left discharged or repeatedly undercharged, the battery suffers sulfation: the sulfate stays and capacity falls.^[19]​^[20]

Nickel-cadmium and nickel-metal hydride

Both give 1.2 V per cell, a little less than an alkaline cell's 1.5 V.^[21] NiCd is robust and long-lived but contains toxic cadmium. NiMH holds more energy and is considered non-toxic.^[21] NiCd is not as common as it once was.^[22] Energizer's NH15 rechargeable AA, for example, is a 2,300 mAh NiMH cell.^[23]

Lithium-ion

The anode is usually graphite (a form of carbon) and the cathode a metal oxide; lithium ions shuttle to the cathode on discharge and back on charge.^[6] The cathode names the family:^[24]

• LCO (lithium cobalt oxide): 3.6 V nominal, high energy, used in phones, tablets and laptops.
• NMC (lithium nickel manganese cobalt oxide): 3.6 to 3.7 V, high capacity and power, used in e-bikes, medical devices and electric vehicles.
• LFP (lithium iron phosphate): 3.2 to 3.3 V, less energy for its weight, but a very flat voltage, long life and the highest thermal-runaway temperature of the three (270 °C, against 150 °C for LCO). Battery University calls it "one of safest Li-ions".^[24]

LiPo ("lithium-polymer") usually means lithium-ion in a flexible foil pouch; to the user it is "essentially the same as lithium-ion". Pouch cells can swell from gas buildup and may be less durable than a metal can.^[25]

A supercapacitor holds less energy than a battery of the same size, but delivers it faster and lasts longer (recap).

Cycle-life figures assume each cycle empties the battery completely; shallower cycles generally last longer, and lead-acid lives longer if it is not discharged below 60% charge.^[19]

7. Sizes and names

Sizes are standardized, so any brand fits (Figure 4):

• AA: up to 14.5 mm across and 50.5 mm long.^[26] AAA: up to 10.5 mm by 44.5 mm.^[28]
• 9 V: a block up to 48.5 by 26.5 by 17.5 mm with snap terminals.^[29]
• 18650: a lithium-ion cell 18 mm across and 65 mm long (the final 0 means cylindrical), used in power tools, laptops and e-bikes.^[30]
• 21700: about 21 mm by 70 mm; Molicel's P42A measures at most 21.7 mm by 70.2 mm.^[31] It has about 50% more volume than an 18650 and up to roughly twice the capacity.^[30]
• CR2032: a coin cell 20 mm across and 3.2 mm thick.^[13]

Decoding the codes. International Electrotechnical Commission (IEC) names use letters for chemistry and shape and numbers for size.^[14] In CR2032, C marks a lithium chemistry (here lithium-manganese dioxide^[13]), R means round, 20 is the diameter in millimeters and 32 is the height in tenths of a millimeter.^[14] Datasheets show others: an alkaline AA is an LR6, a zinc-carbon AA is an R6, a lithium AA is an FR6, a NiMH AA is an HR6, a silver oxide button is an SR44, and a 9 V alkaline, with its six cells, is a 6LR61.^[26]​^[12]​^[16]​^[23]​^[18]​^[29]​^[4]

8. Capacity in practice

Article 9 introduced amp-hours and watt-hours (recap). One amp-hour is 3,600 coulombs of charge.^[2] A capacity figure means little without its conditions: the current, the temperature and the cutoff voltage at which the cell counts as empty. Energizer's AA alkaline example ran 25 hours at 100 mA down to 0.8 V, giving 2,500 mAh; counted only to 1.2 V, it gives less.^[7]

Watt-hours compare different voltages. A CR2032 holds 3.0 V × 0.235 Ah = 0.7 Wh (calculated). A Panasonic NCR18650B holds 3.6 V × 3.35 Ah = 12.1 Wh (calculated, using the datasheet's nominal voltage and typical capacity).^[32]

C-rate. Currents are often given as a multiple of capacity, the C-rate. A 1 Ah battery at 1C supplies 1 A for about an hour; at 0.5C, 0.5 A for two hours; at 2C, 2 A for 30 minutes, less in practice because of losses.^[33]

Heavier loads give less capacity. Article 9 showed an alkaline AA giving about half its capacity at 500 mA compared with 25 mA (recap). Primary cells are rated at small currents and look less impressive at 1C.^[4] For lead-acid this is Peukert's law: Victron's example is a 100 Ah battery, rated over 20 hours, that may give only 56 Ah if emptied in two. Lithium is affected less.^[34]

Cold reduces capacity. Cold slows the reactions and raises internal resistance, so high currents cause large voltage drops.^[7] A zinc-carbon AA is typically 0.5 Ω at 21 °C but 5.0 Ω at −21 °C;^[12] at 100 mA that loses 0.05 V warm but 0.5 V cold (calculated). Lithium AA cells do much better in the cold.^[7]​^[15]

Discharge curves differ by chemistry (Figure 5). Alkaline slopes steadily from about 1.5 V toward a 0.8 V cutoff.^[7] Lithium batteries give a nearly constant voltage as they discharge,^[3] near 3.6 V for common lithium-ion types, and LFP is flatter still, near 3.2 V.^[24] A flat curve keeps a device at full strength but makes it hard to judge the remaining charge from voltage.

9. Charging

A charger is a DC supply that controls the voltage and current into a battery. A plug-in one first turns mains AC into low-voltage DC with building blocks from earlier articles: a transformer (recap), rectifier diodes (recap) and smoothing (recap). Chemistries differ in how hard to push and when to stop, so never use "chargers that are not designed for the specific battery type".^[7]

Lithium-ion: constant current, then constant voltage

Lithium-ion uses two stages, CC/CV (Figure 6):^[35]

1. Constant current. The charger pushes a fixed current, often 0.5C to 1C, and the cell voltage rises.^[35]
2. Constant voltage. At the limit, typically 4.20 V per cell within ±50 mV, the charger holds the voltage and the current falls as the cell fills.^[35]
3. Stop when the current falls to 3 to 5% of the amp-hour rating.^[35] (Panasonic's NCR18650B: 0.5C maximum, 4.20 V, 65 mA cutoff.^[32]) Lithium-ion "cannot accept overcharge" and gets no trickle charge.^[35]

LFP uses the same method with a 3.65 V limit and no float charge. Four LFP cells can replace a 12 V lead-acid battery's six, but lithium cannot tolerate a lead-acid charger's float stage unless the pack's BMS is designed to cope.^[36]

Lead-acid: bulk, absorption and float

Lead-acid chargers use three stages (Figure 7):^[20]​^[38]

• Bulk (constant current): maximum current until the voltage reaches the absorption level, leaving the battery about 80% charged.^[38]​^[20]
• Absorption (topping charge): the voltage is held at 2.30 to 2.45 V per cell while the current tapers.^[20]​^[38] Skipping it repeatedly causes sulfation.^[20]
• Float: a lower 2.25 to 2.30 V per cell, held to make up for self-discharge.^[19]​^[20] Staying at absorption voltage over 48 hours causes gassing, water loss and corrosion.^[20]

NiMH: watching for the voltage dip

As a fast-charged NiMH cell fills, its voltage peaks and dips very slightly. Chargers watch for this negative delta V (−ΔV), 5 mV per cell or less, along with temperature rise and timers, whichever comes first (Figure 8).^[39]

NiMH "dislikes overcharge", so a trickle charge left on should be tiny: about 0.05C per Battery University, under 0.025C per Energizer, which prefers very small trickles to limit overcharge harm.^[39]​^[40] A NiMH in an old NiCd charger would overheat.^[39]

Temperature while charging

The U.S. Fire Administration says not to charge lithium-ion below 0 °C (32 °F) or above 40 °C (105 °F).^[41] Sub-freezing charging is a stress that can lead to cell failure.^[42] Panasonic allows 0 to 45 °C for the NCR18650B, with reduced current below 10 °C.^[32]

10. Why lithium needs a BMS

Battery University calls protection circuits for lithium-ion packs "mandatory".^[43] A battery management system (BMS) is the electronics that watches the cells and disconnects the pack if limits are exceeded or a fault occurs.^[44] It guards against (Figure 9):

• Overcharge: no cell may exceed about 4.25 V on charge (for most Li-ion).^[45]
• Overdischarge: the pack is cut off when the weakest cell drops to 2.80 V or lower, so stronger cells cannot push it into reverse.^[45] Most chargers will not recharge a cell taken below 2.50 V.^[8]
• Overcurrent and short circuit: solid-state switches, transistors used as switches (recap), disconnect the pack.^[43] They play the same role as the power transistor inside a DC solid-state relay (recap).
• Temperature: the BMS flags high temperature.^[44] Many cells add a PTC device whose resistance rises when hot and a current interrupt device that cuts the circuit permanently on excess pressure or heat.^[43]
• Balancing: series cells never match perfectly, so the BMS usually bleeds charge from the highest cells through resistors while charging. Packs above 24 V should have it.^[45]

Home and solar systems. Battery University says LFP is used primarily for energy storage.^[46] Stationary batteries were among the first with supervisory systems monitoring each cell.^[44] The U.S. Consumer Product Safety Commission (CPSC) recommends that battery packs have "proper Battery Management Systems, including charge control, short-circuit protection and cell balancing", and that cells, packs, chargers and product be tested together as a system.^[47] A home battery feeds the house through an inverter that turns DC into AC (recap; solar systems). For any particular system, the maker's and installer's documentation is the authority.

11. Lithium safety

Lithium-ion batteries pack a lot of energy into a small space. The National Fire Protection Association (NFPA) explains that if it is released uncontrollably, the heat can turn parts of the battery into flammable and toxic gases.^[37]

Thermal runaway

Thermal runaway is a chain reaction in which heat makes a cell produce more heat. It can begin with a tiny internal short, such as microscopic metal particles gathering in one spot: current heats the spot and damages the separator, letting more current flow. The temperature can quickly reach 500 °C, and the cell catches fire or explodes.^[42] A burning cell can heat its neighbors until they fail too; that is why cylindrical cells in packs are often spaced apart.^[30] External protection circuits cannot stop a thermal runaway once it has started (Figure 10).^[42]

What causes it

NFPA lists four kinds of cause:^[37] physical damage (crushing, puncture, water); electrical abuse (overcharging, or a charger not designed for the battery); extreme temperatures; and manufacturing defects. The Federal Aviation Administration (FAA) adds that thermal runaway "can occur without warning".^[48] Very thin separators, used to pack in more energy, make cells more sensitive to impurities.^[42] Never use packs modified by unqualified people or rebuilt with used cells.^[49]

Warning signs

Stop using a battery if you notice any of these:^[37]​^[41]​^[50]

• Swelling or any change in shape.
• Excessive heat while charging or in use.
• An unusual odor.
• Popping or other odd noises.
• Change in color, or leaking.

Never charge, open or repair a damaged battery.^[37] Stop using one that swells enough to press on its compartment.^[30]

Safe charging and storage

• Buy tested products. Choose batteries and chargers listed by a nationally recognized testing laboratory, such as UL, and check saferproducts.gov for recalls.^[37]​^[51]​^[50]
• Use the right charger: the one supplied with the device, or one the maker recommends.^[37]​^[49]
• Plug chargers directly into a wall outlet, not extension cords or multi-plug adapters.^[51]
• Stay present while charging e-bikes, scooters and hoverboards; never charge them while asleep or away. The CPSC has seen many fatal fires at night while families slept.^[49]
• Charge on hard surfaces, not on a bed, couch or under a pillow.^[50] Unplug once full.^[37]​^[49]
• E-bikes and e-scooters are safest charged outdoors, away from buildings and out of direct sun, and never stored where they block a room's only exit.^[37]
• Store at room temperature, away from anything that can burn, out of direct sun and not in hot cars.^[41]
• Protect loose cells from short circuits (recap): keep their terminals from touching other metal, for example by keeping them in a case.^[48]

If a battery overheats or catches fire

1. Stop using or charging it immediately.^[50]
2. If it is safe to do so, move the device away from anything that can catch fire and call 9-1-1.^[50]
3. If there is fire or heavy smoke, get out ("Get out quickly", in the U.S. Fire Administration's words) and call 9-1-1 from outside.^[41] The smoke and gases are toxic, and cells can explode.^[37]

NFPA notes that firefighters should use water on lithium-ion fires: the lithium is in salts that do not react with water as lithium metal does.^[37]

Air travel and shipping

On U.S. flights, the FAA requires:^[48]

• Spare lithium batteries and power banks in carry-on bags only, terminals protected (packaging, a case or tape).
• Lithium-ion batteries of up to 100 Wh each. With airline approval, up to two spares of 101 to 160 Wh are allowed.
• Lithium-metal (single-use) batteries with at most 2 g of lithium each.
• No damaged or recalled batteries.
• Tell the crew immediately if a battery or device overheats, swells, smokes or burns.

To find watt-hours, multiply voltage by amp-hours.^[48] A power bank marked 3.6 V and 20,000 mAh holds 3.6 × 20 = 72 Wh (calculated), under the limit. Shipping is regulated too: since 2016 lithium batteries cannot travel as cargo on passenger aircraft, and lithium-ion shipped on its own must be at no more than 30% charge.^[52]

12. Coin cells and children

The CR2032 datasheet warns that swallowing one may lead to serious injury or death in as little as two hours, from chemical burns and possible perforation of the esophagus.^[13] If you suspect a child has swallowed one:^[1]​^[53]

• Go to the nearest emergency room immediately.
• Call the National Battery Ingestion Hotline at 800-498-8666 or Poison Help at 800-222-1222.
• Do not make the child vomit.
• For children 12 months or older who swallowed it within the last 12 hours, the CPSC, citing the National Capital Poison Center, says to give 10 mL (2 teaspoons) of honey every 10 minutes on the way, up to six doses. Do not delay going to get honey.

Prevention. Since Reese's Law (2022) and the standard UL 4200A-2023, products with coin or button batteries must have battery compartments that need a tool such as a screwdriver or coin, or two independent movements at once, to open, and the batteries must come in child-resistant packaging.^[1] At home, the CPSC advises keeping products with unsecured compartments away from children, checking toys' compartments, and never letting children handle coin batteries.^[1]

13. Testing a battery

Open-circuit voltage is a rough guide only. Measuring a battery with nothing connected, its open-circuit voltage (OCV), "can be misleading" as a guide to remaining life. A better test measures the voltage while the battery drives a load for one or two seconds, the closed-circuit voltage.^[7] A worn battery can read nearly full yet collapse under load (recap), and flat-curve chemistries barely change voltage until nearly empty.

Internal resistance. Measure the open-circuit voltage, apply a known load briefly, measure again, and divide the drop by the current, the DC load method.^[54] For example, if a cell reads 1.55 V open and 1.45 V while supplying 0.5 A, its internal resistance is 0.10 V ÷ 0.5 A = 0.2 Ω (calculated, example values). A fresh Energizer AA alkaline has 150 to 300 mΩ,^[26] and resistance rises as a battery ages or runs down.

Capacity. The most dependable test is still charge, discharge, recharge.^[55] Discharge at a known constant current to the cutoff and multiply current by hours, as in Energizer's 100 mA × 25 h = 2,500 mAh.^[7]

14. Disposal and recycling

The U.S. Environmental Protection Agency (EPA) says:^[22]

• Lithium-ion, NiMH, NiCd and other rechargeables should not go in household trash or recycling bins. Take them to a battery recycler or household hazardous waste collection point.
• Lithium single-use and coin or button cells should not go in household trash or recycling bins either.
• Lead-acid batteries should go back to a battery retailer or a hazardous waste program.
• Alkaline and zinc-carbon batteries can go in household trash in most communities, but EPA recommends sending them to battery recyclers or checking with local authorities.
• Before recycling, tape the terminals of lithium batteries or put each one in its own plastic bag to prevent fires.

Batteries in the trash can start fires in collection trucks, landfills and recycling plants.^[56]​^[37] Many stores that sell batteries take them back,^[37] and The Battery Network (formerly Call2Recycle) has a drop-off locator.^[57] For a damaged lithium battery, ask the maker, the fire department or the local waste authority.^[22]​^[37]

The whole story in brief

• A cell turns redox into current: electrons through the outside circuit, ions through the electrolyte. A battery is cells joined together.
• Chemistry sets the voltage; size sets capacity. Never charge a primary cell.
• Capacity depends on current, temperature and cutoff. Compare in watt-hours.
• Lithium-ion: CC/CV, no trickle. Lead-acid: bulk, absorption, float. NiMH: −ΔV and a tiny trickle.
• Lithium packs need a BMS for overcharge, overdischarge, overcurrent, temperature and balancing.
• Stop using a battery that swells, heats, smells, pops or leaks. If one overheats, move it away from anything that burns if safe, get out and call 9-1-1.
• A swallowed coin cell means the emergency room immediately.
• Recycle rechargeable and lithium batteries; never trash them.

Coming up in this section: 14 LEDs and Optoelectronics.

Sources

All links were checked in October 2026. External links open in a new tab.

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32. Panasonic (Sanyo), NCR18650B lithium-ion cell datasheet (PDF, hosted by BatterySpace): https://www.batteryspace.com/prod-specs/NCR18650B.pdf
33. Battery University, "BU-402: What Is C-rate?": https://batteryuniversity.com/article/bu-402-what-is-c-rate
34. Victron Energy, SmartShunt manual, "Battery capacity and Peukert exponent": https://www.victronenergy.com/media/pg/SmartShunt/en/battery-capacity-and-peukert-exponent.html
35. Battery University, "BU-409: Charging Lithium-ion": https://batteryuniversity.com/article/bu-409-charging-lithium-ion
36. Battery University, "BU-409b: Charging Lithium Iron Phosphate": https://batteryuniversity.com/article/bu-409b-charging-lithium-iron-phosphate
37. National Fire Protection Association (NFPA), "Lithium-Ion Battery Safety": https://www.nfpa.org/education-and-research/home-fire-safety/lithium-ion-batteries
38. Victron Energy, Smart IP43 Charger manual, "Operation" (charge algorithm): https://www.victronenergy.com/media/pg/Smart_IP43_Charger_120-240V/en/operation.html
39. Battery University, "BU-408: Charging Nickel-metal-hydride": https://batteryuniversity.com/article/bu-408-charging-nickel-metal-hydride
40. Energizer, Nickel Metal Hydride (NiMH) Handbook and Application Manual (PDF): https://data.energizer.com/pdfs/nickelmetalhydride_appman.pdf
41. U.S. Fire Administration (FEMA), "Battery Fire Safety": https://www.usfa.fema.gov/prevention/home-fires/prevent-fires/batteries/
42. Battery University, "BU-304a: Safety Concerns with Li-ion": https://batteryuniversity.com/article/bu-304a-safety-concerns-with-li-ion
43. Battery University, "BU-304: Why are Protection Circuits Needed?": https://batteryuniversity.com/article/bu-304-why-are-protection-circuits-needed
44. Battery University, "BU-908: Battery Management System (BMS)": https://batteryuniversity.com/article/bu-908-battery-management-system-bms
45. Battery University, "BU-803a: Cell Matching and Balancing": https://batteryuniversity.com/article/bu-803a-cell-matching-and-balancing
46. Battery University, "BU-205: Types of Lithium-ion": https://batteryuniversity.com/article/bu-205-types-of-lithium-ion
47. U.S. Consumer Product Safety Commission, "Batteries" (voluntary standards topic page): https://www.cpsc.gov/Regulations-Laws--Standards/Voluntary-Standards/Topics/Batteries
48. Federal Aviation Administration, "PackSafe: Lithium Batteries": https://www.faa.gov/hazmat/packsafe/lithium-batteries
49. U.S. Consumer Product Safety Commission, "Micromobility: E-Bikes, E-Scooters and Hoverboards": https://www.cpsc.gov/Safety-Education/Safety-Education-Centers/Micromobility-Information-Center
50. FDNY Smart, "Lithium-Ion Batteries" safety sheet (PDF): https://www.fdnysmart.org/wp-content/uploads/2025/05/2024-09-20_FDNY-Lithium-Ion-batteries-6-8-objective-1.pdf
51. U.S. Fire Administration, "Lithium-Ion Battery Safety for Electronic Devices" handout (PDF): https://www.usfa.fema.gov/downloads/pdf/publications/lithium-ion-battery-safety-handout.pdf
52. Battery University, "BU-704a: Shipping Lithium-based Batteries by Air": https://batteryuniversity.com/article/bu-704a-shipping-lithium-based-batteries-by-air
53. National Capital Poison Center (Poison Control), "Swallowed a button battery? Battery in the nose or ear?": https://www.poison.org/battery
54. Battery University, "BU-902: How to Measure Internal Resistance": https://batteryuniversity.com/article/bu-902-how-to-measure-internal-resistance
55. Battery University, "BU-904: How to Measure Capacity": https://batteryuniversity.com/article/bu-904-how-to-measure-capacity
56. U.S. Environmental Protection Agency, "Lithium-Ion Battery Recycling": https://www.epa.gov/hw/lithium-ion-battery-recycling
57. The Battery Network (formerly Call2Recycle), home page with drop-off locator: https://batterynetwork.org/