TL;DR (simplified): Lithium batteries are dirty. Do not store empty or fully charged (recharge every ~6 months to ~50%). Dispose correctly, always salvage the BMS. Alkaline batteries will leak corrosive acid: do not leave them in stored devices.

The magic of batteries is to only use them where really necessary. Always make your batteries easily replaceable w/o tools! (Even Galaxy S5 phone could exchange battery and is water proof - there is no excuse)
Batteries will fail sooner or later. Don't be a crummy and make people throw away your product - think about the environment. Make your product to set battery/charging characteristics for minimum wear!
This is a short and comprehensive summary of things good to know about batteries and charging.

Fun fact: In German, a rechargeable battery is typically called “Akku”, the short form of accumulator.

Dispose of batteries correctly! For most countries there are collection boxes in supermarkets. Discharge batteries and tape off the poles of pouch cells and prismatic batteries.
Never just throw it into trash - there are daily fires on dumping grounds and batteries are DIRTY, polluting air&water; resources are lost.
Only because a lithium battery seems to be 0V on measuring, does not mean the cell does not hold any charge: the BMS probably disconnects the cell to protect the battery from further draining.
Consider to cut off the BMS and discharge the cell itself. To fully discharge, connect a load to the cell, e.g. a light bulb of a bike or a DC-motor (PC fan).

Lithium batteries are dirty on obtaining. Most lithium batteries use cobalt, which comes from the Democratic Republic of Congo (DRC), where mining is linked to:

• Child labor & poor working conditions
• Environmental damage (toxic waste, unsafe disposal)
• Human rights concerns

Lithium itself also has an environmental strain too (water use, chemical pollution, land impact).

Always recycle and try to avoid in the first place. Try to think if battery is needed in your use-case or if you are fine with USB-C instead.

Just for clarification: Li-Ion refers to the broad chemistry family encompassing all rechargeable lithium-ion cells.
Li-Po is a type of Li-Ion cell that utilizes a pouch (polymer foil) casing.
All Li-Po are Li-Ion, but not all Li-Ion are Li-Po. The difference is packaging, not the fundamental chemistry.
Classic Li-Ion (cylindrical, prismatic) use a liquid electrolyte soaked into a separator. Li-Po (pouch cells) originally meant polymer electrolyte (gel/solid-like), but in practice modern Li-Po still use liquid or gelled electrolytes, just packaged in a flexible foil pouch (sensitive to puncture/swelling) instead of a metal can (more robust packaging).

Listed are some common Li-Ion chemistries:

• LiCoO₂ (LCO): cathode is lithium cobalt oxide, anode is graphite; lithium ions move between them. High energy, common in phones/laptops.
• LiFePO₄ (LFP): cathode is lithium iron phosphate, anode is graphite; no cobalt; very safe, long-lasting, slightly lower energy.
• LiNiMnCoO₂ (NMC): cathode is a mix of nickel, manganese, cobalt oxides, anode is graphite; balances high energy and safety.
• LiNiCoAlO₂ (NCA): cathode is nickel-cobalt-aluminum oxide, anode is graphite; high energy, used in EVs.

How lithium works: Lithium ions shuttle between the cathode and graphite anode during charge and discharge, storing and releasing energy.

Nominal Voltage is ~3.7 V per cell, 4.2 V per cell when fully charged.

First of all: Try not to quick charge if not necessary.

Terminology: “C” stands for the battery’s nominal capacity, and charging at 1C means supplying a current equal to the full capacity in one hour. For example, a 2000mAh battery charged at 1C would use 2000mA (2A) to fully charge in roughly one hour. The same applies to discharging.

Charging above 1C regularly accelerates capacity fade. “C-rate stress” refers to both charging and discharging.
Heat generated at high rates is the main cause of wear. Not all lithium batteries support >1C charging safely — always check manufacturer specs. During quick charging, especially at high currents, lithium can start to plate as metallic lithium crystals (known as lithium dendrites) on the anode surface. These dendrites:

• Grow like tiny needle-like structures.
• Can pierce the separator between anode and cathode.
• Cause internal short circuits, leading to overheating, swelling, or even fire.

That’s why fast charging increases safety risks and battery wear—because dendrite growth damages the battery’s internal structure.

Lithium batteries need chargers with special characteristics: A CC/CV charger with balance charges batteries in two stages, ensuring efficient and safe charging:

• CC (Constant Current): It supplies a steady current until the battery voltage reaches the set maximum (e.g., 4.2 V per cell).
• CV (Constant Voltage): Then it holds that voltage steady while the current gradually drops, finishing the charge safely.

What is trickle charging? Trickle charging is feeding a battery a very small, low-current charge (often ~0.05–0.1 C) to slowly raise its voltage before normal charging.
For Li-Pos, it’s sometimes used to carefully bring a cell from slightly under-voltage (~3.0 V) back into a safe range — but it’s risky and not recommended below ~2.5 V per cell because internal damage may already be present, causing swelling and fire hazards.
Never charge a swollen battery, no matter the voltage!

Some devices allow to set the maximum charging percentage, like laptops. For Android there is ACCA in f-droid store (requires root), which allows to set charging speed too.

chemical aging is sped up If lithium batteries are stored at 100% State of Charge (SoC), which is ~4.2V.
When the voltage is that high (around 4.2 V per cell), the electrolyte and cathode are under more stress, which accelerates:

• Electrolyte breakdown → creates gas and swelling.
• Lithium plating → reduces capacity and can increase short-circuit risk.
• Internal resistance growth → battery delivers less power over time.

That is why the “storage voltage” of ~3.7–3.85 V per cell is recommended (which is around ~50% SoC) — it minimizes stress while keeping enough charge to avoid deep discharge. Keep in a cool, dry place (ideally 15–25 °C). Self discharge of Li-Po is around ~2–3%/month (slightly higher than rigid Li-Ion (~1-2%)).
Check voltage every 6-9 months and recharge if voltage drops below 3.5V. Do not let it go below 3.3V!

Empty storing (aka forgetting the battery): Do not store lithium batteries empty. They die if voltage drops below 3.0V. Safe minimum under normal use is 3.3 V. Everything below 2.5V per cell is a danger zone (don’t attempt revival)! Do not attempt to trickle charge them for revival.
When voltage drops too low, copper from the anode can dissolve into the electrolyte. On recharge, this copper can plate back onto the cathode as tiny dendrites, which may pierce the separator and cause internal shorts — leading to swelling, overheating, or fire. Recycle deep discharged battery.

Always salvage the Battery Management System (BMS) board of a battery pack! You may still need it to re-cell (e.g. if no genuine aftermarket battery available).
Also, with the BMS it is possible to replace the battery with external power (for devices which would not turn on w/o battery) - nice for USB-C mods.

What is the purpose of a BMS?

• Protect the battery from overcharge, overdischarge, overcurrent, and short circuit.
• Balance cells in multi-cell packs so voltages stay equal.
• Monitor voltage, current, temperature, and sometimes state of charge (SoC).
• Control charging/discharging safely and communicate status to the device/MCU.
• DRM: authenticate as original battery (some systems refuse to work with 3rd party batteries) - this is why you need to keep the BMS board!

Due to the protection feature it can be that you measure 0V on an empty battery - the cell itself can still have a charge. This does not necessarily mean that the cell is dead, it still can have ~3V and be safely rechargeable. When disposing of the battery, cut off the BMS and discharge the cell itself.

For DIY purposes there are charger modules on breakout boards. Compared below are a few common ones.

Note: IP5306 needs heartbeat. Some modules need a fix for CD42 issue.
*Beware that only IP5306-I2C has I²C-bus (there is a plain IP5306 version too)! You can repurpose CD42 boards though. There is a repo with a lib for IP5306-I2C

IMO: For advanced products, see BQ-series of Texas Instruments: they have everything the heart desires.

Lithium Titanate Oxide (LTO) batteries use no cobalt.

https://en.wikipedia.org/wiki/Lithium-titanate_battery

Read about open source BMS on hackaday.com

Alkaline batteries cannot be recharged and pose an environmental hazard when discarded.
They have 1.5V when full. How to tell if an alkaline battery is full or empty without measuring it?
If you drop a battery upright onto a hard surface, an empty battery will bounce higher than a full one. This happens because the electrolyte inside an empty battery has dried and hardened.

It is similar to spinning an egg: a raw egg wobbles because the inside is liquid, while a cooked egg spins smoothly because it is solid.

Hardened electrolyte will cause leaks, which is why batteries have a “best before date” (BBD). The leaking battery acid is corrosive, damaging the contacts in your device. Do not leave batteries in stored away devices.
If your contacts are corroded, clean them and apply solder tin, so they stop corroding (bc electroplating is too complex).

Ni-MH (Nickel-Metal Hydride) batteries are rechargeable and behave quite differently from lithium batteries when they are depleted or overcharged. They have a long life span, as there is no significant memory effect

When Nickel batteries are fully charged, they have 1.2V, which sometimes is a problem when a device expects 1.5V from an alkaline battery (=bad product design). For that purpose there are Li-Po batteries in form factor of AA/AAA with 1.5V voltage regulator inside (and USB-C for charging).

When Ni-MH is depleted:

• A Ni-MH cell is “empty” at about 1.0 V under load (sometimes 0.9 V in low-drain devices).
• Going too far below ~0.9 V risks cell reversal in multi-cell packs — the weakest cell gets driven backwards, which damages its chemistry permanently.
• Deep discharge increases internal resistance and reduces capacity.

When overcharging:

• Ni-MH chemistry can tolerate mild overcharge much better than Li-ion.
• Once fully charged, extra energy goes into electrolyte decomposition (water → oxygen + hydrogen) at the positive electrode.
• Modern “low self-discharge” Ni-MH cells recombine these gases inside the cell, but the recombination process generates heat.
• If overcharge continues for long periods, the heat dries out the electrolyte, permanently reducing capacity and possibly causing venting.

Structure of Ni-MH:

• Cathode: Nickel oxyhydroxide (NiO(OH))
• Anode: Hydrogen-absorbing metal alloy (MH)
• Electrolyte: Aqueous potassium hydroxide (KOH)
• Key points: Higher capacity than Ni-Cd, environmentally friendlier, minor memory effect.

Ni-MH has replaced Ni-Cd (Nickel-Cadmium) batteries for home use. Ni-Cd are only found in specific industrial applications nowadays.