Guides
Batteries and Charging Explained: Chemistries, Limits and Fixes
Compare alkaline, NiMH, lithium-ion and lead-acid batteries, learn how charging limits and heat age a cell, and troubleshoot slow, failed or swollen batteries.
What to take away
- Same size does not mean same chemistry. Energizer's E91 AA alkaline cell is rated at a nominal 1.5 V, while a typical AA NiMH cell is rated at 1.2 V, so a device may behave differently on each.
- Compare batteries in watt-hours (volts times amp-hours), not milliamp-hours alone. The FAA uses the same arithmetic to set the 100 Wh limit for spare lithium-ion batteries on aircraft.
- Heat and a full charge age lithium-ion cells. Battery University's tables show a cell stored at 25 C keeping about 96% of its capacity after a year at 40% charge, against about 80% at 100% charge.
- NFPA says not to charge lithium-ion batteries below 0 C (32 F) or above 40 C (105 F).
- Swelling, odor, a color change, a change in shape or heat that keeps rising means stop using the battery, per NFPA's list of warning signs.
- The EPA says no lithium-ion or other rechargeable battery belongs in household trash or curbside recycling.
The main consumer chemistries side by side
A battery stores energy as chemical potential energy. The U.S. Department of Energy explains that charging moves electrons from the cathode to the anode, and discharging reverses the flow. In a rechargeable cell the reversal is never complete, which is why every rechargeable battery ages.
| Type | Rechargeable | Nominal voltage per cell | Verified detail |
|---|---|---|---|
| Alkaline (AA) | No | 1.5 V | Energizer E91 datasheet: zinc-manganese dioxide, 12 years shelf life at 21 C |
| NiMH (AA) | Yes | 1.2 V | Battery University: requires a complex charge algorithm and is sensitive to overcharge |
| Lithium-ion, cobalt or NMC | Yes | 3.6 to 3.7 V | Battery University: usual range 3.0 to 4.2 V per cell |
| Lithium-ion, LFP | Yes | 3.2 to 3.3 V | Same source: 2.5 to 3.65 V range, 2,000 or more cycles |
| Lithium metal (coin, button) | No | Varies by cell | EPA: single use, never in trash or municipal recycling |
| Sealed lead-acid | Yes | See the battery label | Battery University: store fully charged, since low charge causes sulfation |
Editorial suggestion: three practical consequences follow.
- Do not put a single-use cell in a charger, and do not put NiMH cells in a lithium-ion charger unless the charger names both chemistries and the cell size.
- In a multi-cell holder, keep all cells the same type, age and charge state.
- A "universal" charger label proves nothing. Read the list of supported types.
The cells inside phones, laptops and power banks sit in packs with protection circuits. This is why the U.S. Consumer Product Safety Commission warns against using loose 18650 cells taken out of packs. Exposed terminals can short against keys or coins, and an unprotected cell in the wrong charger can go into thermal runaway.
Capacity, lifespan and cycles
Battery life is the runtime of one charge. Lifespan is how long the battery stays useful. Apple defines a charge cycle as using an amount equal to 100% of the battery's capacity, which can be spread over several partial charges. Apple's battery page says iPhone 14 and earlier batteries are designed to keep 80% of original capacity at 500 complete cycles, and iPhone 15 at 1,000 cycles, under ideal conditions.
The voltage a cell is charged to changes the trade-off. Battery University's Table 4 lists a cell charged to 4.20 V at 300 to 500 cycles, and the same cell charged to 4.00 V at 850 to 1,500 cycles but with about 73% of the stored energy. That is the reason phone makers now offer a limit below 100%.
Storage condition matters as much as use. The same source gives these estimates for capacity left after one year:
| Storage temperature | At 40% charge | At 100% charge |
|---|---|---|
| 0 C | 98% | 94% |
| 25 C | 96% | 80% |
| 40 C | 85% | 65% |
The site's article on batteries in hot and cold climates covers what to do in a car, a garage or a winter trip.
How charging is controlled
The wall adapter converts power, the cable carries it, and the device decides what the battery gets. On USB, that negotiation is USB Power Delivery. The USB-IF says revision 3.1 allows up to 240 W, using fixed voltages of 28 V, 36 V and 48 V for 140 W, 180 W and 240 W. Earlier, 100 W was the ceiling at 20 V. A cable that looks identical can still be rated for far less, so read USB-C cable types and speeds before buying.
Charging has stages. Battery University notes that lithium-ion cells charge to a set voltage ceiling, commonly 4.20 V per cell, while NiMH needs a more complex charge algorithm and does not absorb overcharge well. The device and charger handle this. Your job is to use equipment built for the chemistry.
Manufacturers also add software limits:
- Apple's Optimized Battery Charging can delay charging past 80% when it expects a long charge. On iPhone 15 and later, a Charge Limit between 80% and 100% can be set in 5% steps.
- Apple's charging guide says charging that stops at 80% is often intentional, and that a warm battery can also pause charging above 80% until it cools.
- Editorial suggestionif you charge overnight at a desk, turn on the vendor's limit feature and ignore the percentage unless you need full range for a trip.
Fast charging and heat
Fast charging is real, with limits. Apple's iPhone 17 specifications quote about 50% in 20 minutes with a 40 W or higher USB-C adapter, sold separately. The claim applies only to that kind of setup. Battery University says most consumer cells should be charged at 1C or less and advises avoiding so-called ultra-fast chargers that claim a full charge in under an hour.
Heat is the cost. NFPA's tip sheet lists these rules: use only the battery and cord designed for the device, do not charge on a bed or sofa, do not keep charging after full, and stay inside the 0 C to 40 C range. Editorial suggestion: cases and heavy phone use during charging also add heat.
Troubleshooting: no charge, slow charge, shutdowns and swelling
Change one thing at a time. Apple's won't-charge steps give the order: try a different outlet, inspect the cable and adapter for damage, clear debris from the port, charge for 30 minutes, then force restart and charge another 30 minutes. If it still fails, get service. Google's Pixel guidance adds that a fully drained phone can show no response for up to 25 minutes on charge. It recommends a 30 W Google adapter or a PPS adapter rated 30 W or more, with other USB PD adapters at 15 W or more.
Slow charging. Check the adapter rating, the cable rating and whether the phone is in heavy use. A warm battery also reduces its own intake.
Fast drain or sudden shutdown. Look at the battery usage list first. Apple's drain guide tells users to wait a few days after an update, then check Battery Health, where "Service" appears when replacement is advised. A worn battery can show charge but fail under load.
Swelling, hissing, smoke or a strong smell. Stop charging and using the device. The EPA's lithium-ion FAQ says to contact the manufacturer, retailer or local household hazardous waste program for a swollen battery, to keep it away from flammable material (it names a bucket of sand or kitty litter), and to call 911 if there is imminent fire risk. Do not puncture or press it. Phone-specific steps are in common smartphone problems and fixes.
If the question is replace or service, see the battery replacement and right-to-repair rules for Massachusetts and New York.
Storing, flying with and disposing of batteries
- Air travel.The FAA's PackSafe page says spare lithium-ion and lithium-metal batteries, including power banks, go in carry-on only, with terminals protected from short circuit. The limit is 100 Wh per lithium-ion battery, or up to two spares of 101 to 160 Wh with airline approval. TSA repeats the rule for screening. To find Wh, multiply volts by amp-hours.
- Long storage.Follow the pack maker's instructions. For lithium-ion, the Battery University figures above favor a part charge and cool storage.
- Disposal.The EPA's household battery page says to tape terminals or bag each battery separately, and to keep button cells away from children as a swallowing hazard. Use a drop-off point listed through Call2Recycle, which now operates as The Battery Network, or a local household hazardous waste site. The site's recycling guide covers device take-back.
Sources
- [PackSafeLithium Batteries (FAA)](https://www.faa.gov/hazmat/packsafe/lithium-batteries)
- USB Power Delivery (USB Implementers Forum)
- How to prolong lithium-based batteries (Battery University)
