Some notes on batteries.

There are 2 types: primary cells (non-rechargeable) and secondary cells (rechargeable)

Lithium Ion[edit]

Lithium Ion batteries (Li-Ion, LIB) is a type of battery that uses lithium ions. Lithium Ion Polymer (LiPo) batteries are also included in this category, but are constructed with a polymer electrolyte.

Lithium-ion battery consists of 3 primary components: The positive and negative electrodes, and the electrolyte. The negative electrode is made with carbon, and is typically graphite. The positive electrode can be one of lithium cobalt oxide (layered oxide), lithium iron phosphate (a polyanion), or lithium manganese oxide (spinel).

While discharging, lithium ions carry the current from the negative to the positive electrode.

While charging, an external power source applies an over voltage in the same polarity. Most Lithium Ion batteries charge to 4.2 volts. Unless otherwise specified, batteries should be charged at 0.5C, where C is the rated capacity of the battery. (For example, a 3400mAh battery should be charged at 1700mA). Charge is terminated at 3% of the initial charge current. During charging, current within the battery flows from the positive to the negative electrode.

Type Discharged Voltage Charged Voltage
Lithium metal oxides (eg. LiCoO2) 2.7 - 3.0V 4.2V
lithium iron phosphate (eg LiFePO4) 1.8 - 2.0V 3.6 - 3.8V

Note: A 3.7V Lithium Ion battery can be charged up to 4.2V. 3.7V is the voltage the battery will output for the bulk of its discharge.


18650s are cylindrical lithium-ion rechargeable batteries that are 18mm in diameter an 65mm in length. These are most commonly found inside laptop batteries and are used in various applications such as ebikes.



The TP4056 can charge a battery at 1 amp and these modules are wired to do so with the 1.2K resistor connected to the PROG pin on the TP4056.

I ordered 10 TP4056 modules with output protection for about CAD$4. The modules without output are slightly cheaper.

Nickel Cadmium[edit]

Nickel Cadmium (NiCd, or NiCad) is a rechargeable battery that uses nickel oxide hydroxide and metallic cadmium as electrodes.

Nickel Metal Hydride[edit]

Nickel Metal Hydride (NiMH, Ni-MH) is a rechargeable battery similar to a Nickel Cadmium, rather than using cadmium in the negative electrode, it uses a hydrogen-absorbing alloy instead. This yields a higher energy density around 2-3x that of a nickel-cadmium and can approach that of a lithium-ion.

Each cell has a nominal voltage of 1.2V.

Lead Acid[edit]

A lead-acid battery is a rechargeable battery that uses lead (Pb, negative) and lead oxide (PbO2, positive) plates submerged in sulfuric acid (H2SO4). When discharged, both plates become lead sulfate (PbSO4) and the acid dilutes as it becomes primarily water.

The construction of a lead-acid battery typically requires a separator between the plates to prevent dendrites from forming between the plates and causing a short circuit.

A lead-acid cell has a nominal voltage of 2.1V. Charging a lead-acid requires at a minimum of 2.15V per cell. It is possible to charge a lead-acid with a higher voltage (eg. 2.5V) without damage so long as it is not near its fully charged state.

Overcharging a lead-acid battery will cause electrolysis of the water in the electrolyte, producing hydrogen and oxygen. This happens only when any of the lead sulfate or sulfuric acid has been reacted and unavailable.

As lead-acid batteries are cycled, its capacity will be reduced due to sulfation where the lead sulfate does not combine with the electrolyte. The result of this is a reduction in the amount of lead on the negative plate.

USB Power Banks[edit]

Most USB power banks use Lithium Ion batteries in the form of 18650. The rated capacity is usually based on the 3.7V of the internal battery rather than at the 5V output voltage.

A 20000mAh battery at 3.7V will provide 74000mWh (or 74Wh) of power (assuming 100% efficiency).

A Raspberry Pi using 0.5A at 5V will use 2.5W. Using the battery above, 74Wh/2.5W = 29h run time.