Batteries - useful information
Terms:
Galvanic cell – a cell in which electric current is obtained from reactions between an electrode and the electrolyte. Two electrodes immersed in the electrolyte (half-cells) form a galvanic cell. When the electrodes are connected by an external conductor, the electrons flow through the external electric circuit and their charge is balanced by the flow of negatively charged ions (anions) in the electrolyte solution.
Electrolyte – substances dissolved in an appropriate solvent, where they break down into free ions, as a result of which electric current can be conducted. Anion – a negatively charged ion. Cation – a positively charged ion.
Electrode, half-cell – a set comprising e.g.: metal (M) immersed in electrolyte solution. On an electrode in a galvanic cell, the oxidation (anode) or reduction (cathode) occurs. In an electric circuit, there are always two electrodes: anode and cathode, opposite as regards the charge sign.
Anode – in a galvanic cell and electric batteries, the anode is the negative electrode. In batteries, depending on their mode of operation, on the anode the charge is negative (electric current generation) or positive (the charging process).
Cathode – the electrode on the lower electric potential than the other one (anode). In a galvanic cell, the cathode is the positive electrode, and along with the anode it generates the electromotive force.
Electric battery – a type of the galvanic cell that can be used and charged with electric current many times. Each electric battery accumulates and later releases electric energy owing to reversible chemical reactions that occur in the electrolyte and on the contact point between the electrolyte and the electrodes.
Battery parameters:
Voltage and capacity – the main parameters of a battery are the nominal voltage, expressed in volts, and the nominal capacity. The battery capacity (also, the capacity of a disposable, non-rechargeable battery) is given in milliampere-hours [mAh] or in ampere-hours [Ah].
1 mA (milliampere) = 0.001 A (ampere).
The capacity of 2000 mAh means that the battery, when fully charged, can deliver current of 200 mA for 10 hours (200 mA x 10 h = 2000 mAh).
The capacity can also be given in watt-hours [Wh] or milliwatt-hours [mWh]. By dividing the value expressed in [Wh] by the battery voltage, we obtain the battery capacity in [Ah].
Charging and discharging current – expressed as a fraction of nominal capacity C. The C current (1C) is the so-called one-hour discharge - a battery discharged with such current should operate for one hour. For example, for a battery with capacity C = 2000 mAh, the charging current C/10 (0.1C) is the current of 200 mA (2000 mAh / 10 = 200 mA), current C/4 is 500 mA, etc.
Memory effect – loss of real battery capacity. This phenomenon was observed in NiCd (nickel-cadmium) batteries. A battery that is partially discharged and then charged, after a few such cycles behaved as if its capacity was equal to the recharged capacity, not the nominal one. This is a reversible phenomenon. The battery should be several times fully discharged and charged.
Popular battery types:
Nickel-cadmium (NiCd or Ni-Cd) battery - the electrodes are made of nickel hydroxide and cadmium hydroxide, and the electrolyte is composed of semiliquid or solid substances; their composition depends on the manufacturer, but the reaction is always strongly basic (alkaline).
Features:
- nominal voltage 1.2 V
Advantages:
- large current efficiency,
- resistance to overcharge and high operation temperature (40 degrees C)
- withstand about 1000 charging-discharging cycles.
Disadvantages:
- troublesome memory effect,
- quick self-discharge, about 1% a day (full after three months),
- maximum AA battery capacity about 2000 mAh,
- harmful to the environment (toxic cadmium compounds).
Recommendations:
- should be discharged and charged in full cycles,
- store charged,
- recommended charging with C/10 current (in case of a 2000 mAh battery - approx. 10 hours).
Application:
Today they are ousted by newer battery types (in particular due to lower capacity and toxicity): in standard battery sizes (R6, R14, etc.) production - by NiMH batteries, in applications such as phone, laptop or remotely controlled models - by Li-Ion or LiPo batteries.
NiCd batteries, however, are still used in industry, because they can operate in high temperatures without explosion risk.
Nickel-metal hydride (NiMH or Ni-MH) - a battery, in which the cathode is a nickel plate, and the anode is made of a metal alloy. The alloy reacts with hydrogen released during charging, thus creating metal hydride.
Features:
- nominal voltage 1.2 V.
Advantages:
- approximately 30% bigger capacity and energy density (in theory 50%, in practice - approx. 25%) than the equivalent NiCd,
- significantly smaller memory effect than in the equivalent NiCd,
- withstand on average above 500 (up to 1000) discharge-charge cycles,
- do not contain heavy metals and thus are environmentally friendly,
- are not heavy.
Disadvantages:
- more prone to damage as a result of overcharge and high discharging currents (in relation to NiCd),
- quicker self-discharge, loss of approx. 1.5% capacity a day in relation to NiCd.
Recommendations:
- NiMH should be stored not charged, in a low temperature, the long self-discharge process causes crystallization and thus reduction of nominal capacity
- recommended charging with C/10 current (in case of a 2000 mAh battery it will be about 10 hours).
Application:
Today, NiMH cells are commonly used in the production of standard battery sizes (R6, R14, etc.). They can power portable devices, such as: torches, CD/mini disk players, tape recorders, cordless and cellular phones, car batteries, UPS, clocks, remote controllers (for a TV set or a hi-fi system), cameras, etc.
Lithium-ion (Li-Ion) battery – a battery, in which one of the electrodes is made of porous carbon, the other one from metal oxides, and the electrolyte is composed of chemically complex lithium salts dissolved in a mix of organic solvents.
Features:
- nominal voltage approx. 3.6 V.
Advantages:
- large capacity (more than in NiMH and NiCd batteries),
- no memory effect,
- small weight,
- small self-discharge approx. 0.5% a day.
Disadvantages:
- discharge below 2.4 V may cause battery damage (oxidation of one of the electrodes - an irreversible process). - A Li-Ion battery charger must finish charging at a specified battery voltage, therefore it operates with high precision of this measurement,
- overcharge - even a small one - by 10 or 15% can have fatal results: loss of capacity, leaking or even explosion,
- the necessity to install circuits monitoring the battery voltage,
- lifetime is 400 – 500 charging-discharging cycles.
Recommendations:
- avoid deep discharge, and charge often - it increases the lifetime,
- charging can last several hours and depends on the battery type. Abide by the manufacturer's instruction, if possible, use the chargers recommended by the manufacturer,
- it is best to store them charged to 40% in low temperatures.
Application:
Li-Ion batteries power: phones, digital cameras, laptops, media players, remote-controlled models, etc.
Lithium-Ion Polymer (LiPo or Li-Poly) battery - a battery type constructed of metallic lithium alloys and conductive polymers. It operates in a way similar to a Li-Ion battery. Due to the presence of polymers, the new technology allows for the construction of flexible, very thin (as little as 1 mm) and flat cells.
New technologies
Today, new batteries, manufactured in an enhanced NiMH process, can be sold charged. In addition, such cells are able to withstand up to 1000 charge-discharge cycles without noticeable capacity loss. They are to a large extent resistant to the self-discharge process. After a year, they remain charged and ready to operate. Examples of such batteries include Eternal by Pentagram, Sanyo Eneloop, GP Recyko, Varta Ready2Use, Sony CycleEnergy, Panasonic Infinium..
Practice
When charging NiCd and NiMH batteries, avoid makeshift chargers. Such chargers can dramatically shorten the cells lifetime, and even make them explode, because it is easy to overcharge a battery and thus increase its temperature and gas pressure inside them.Microprocessor chargers are the best - they constantly measure the battery charge. In addition, such chargers discharge the batteries prior to their charging. It increases the lifetime of the batteries and prevents the memory effect.
Prepared based on the Wikipedia materials, Zbyma72age.