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1. Lead-Acid Batteries

1.1. Overview

Lead-acid batteries are the oldest form of rechargeable battery, invented in 1859.

1.2. General Characteristics

Specific Energy30-40Wh/kg
Specific Power180W/kg
Common Battery Voltages6V, 12V
Nominal Cell Voltage2.105V

1.3. Chemistries

1.3.1. VRLA (Valve Regulated Lead Acid)

VRLA batteries do not suffer the same amount of water loss as a traditional lead-acid battery and therefore are very low maintenance. They are sometimes called 'sealed lead-acid' batteries, but in reality they have a small pressure valve for safety. Because of the tight containment, VRLA’s can stored and used in any position since they do not spill their electrolyte when turned upside down. Being called 'Valve Regulated' does not describe their technology perfectly. The real reason they don’t suffer from much water loss is that the oxygen evolved at the positive plate largely recombined with the hydrogen at the negative plates, reproducing the water it originated from.

1.3.2. AGM (Absorbed Glass Mat) VRLA

A subcategory of a VRLA, the AGM battery contains thinly woven fibreglass mats in which electrolyte is absorbed. The glass mats are slightly 'starved', meaning a few percent of the electrolyte is wrung out after immersion before it is built into the battery, leading to this technology also being called 'starved mat'. The plates remain the same, except the

1.3.3. Gel Cell VRLA

As the name implies, a gel cell battery contains a gelled electrolyte. This is made by dissolving silica fume into sulphuric acid. The gel prevents evaporation and leakage of the electrolyte. The pressure valve keeps the internal pressure high enough to allow total recombination of the hydrogen and oxygen back into water.

1.4. Charging

It is recommended to charge a lead-acid at C/10 or less. There are three different ways to charge a lead-acid battery. The best chargers incorporate all three in a multi-stage configuration.

1.4.1. No-Repetitive Charging

Non-repetitive charging of lead-acids allows you to charge it to a higher voltage than with repetitive charging (float charging). The battery can be charged to 2.4V per cell. Charging should be terminated when the current (in Amps) drops to 0.02C (where C is the Amp-hour rating of the battery, but to all those concerned that the unit don’t match up, ignore the hour bit when doing this calculation).

lead acid charging showing voltage and current
Figure 1. Monitoring the voltage and current is important while charging a lead-acid battery.

The table below shows a list of common lead-acid battery voltages and the voltage you should charge them to, based on a charge voltage 2.4V per cell and an ambient temperature of 25C.

Nominal Voltage (V)Voltage To Charge To (V)
charging a lead acid
Figure 2. Charging a 12V, 6Ah lead-acid battery with a variable-voltage power supply.

1.4.2. Float Charging

Float charging is charging at a fixed voltage for an indefinite amount of time. Float charging has no time limit and when done properly does not damage the battery. It allows the battery to be fully charged up to its maximum. The voltage to charge at (called the float voltage) is dependent on temperature and lead-acid technology.

1.5. Lifetime

The lifetime of a lead-acid battery is affected by:

  • The charging/discharging regime the battery has experienced during its life

  • The depth of discharge

  • The exposure to long period’s with the battery being only partially charged (causing sulphation)

  • The average temperature of the battery over its life

1.6. Sulphation

Sulphation occurs when the battery is left in a partially charged state for an extended period of time.

1.7. Equalization

It’s an obvious statement that not every cell in a lead-acid battery would be exactly equal. Because of this, they lose/gain slightly different amounts of charge when in use. This fact becomes more predominant with time and more cycles of charging. At some point, the cells can become so imbalanced that the performance of the battery is severely affected.

To 're-equalize' the batteries, the standard practise is to fully charge the battery to 2.4V/cell, and then keep charging at 0.02C until the voltage does not rise anymore. This brings the charge of the weaker cells in the battery up to the same level as the stronger ones.

This equalization process should not be done often, and only when the performance of the battery is being significantly effected by a charge imbalance in the batteries cells (which is not the easiest thing to work out!).

2. Button Cells (Coin Cells)

2.1. Overview

Button cells maybe also be known as coin cells or watch batteries.

2.2. Polarity

The insulated top cap of a button cell is the negative terminal, the base is the positive terminal.

2.3. Internal Resistance

The internal resistance of a coin cell battery starts of at a rather large 10Ω. This then rapidly increases as the batteries energy is consumed. Where high pulse currents are needed, coin cell batteries can be used in conjunction with high-valued capacitors.

A rule of thumb for coin cell batteries is that 10mA is the maximum current draw.

2.4. Technologies

2.4.1. Silver Cell

Silver cells have a very stable output voltage over the lifespan of the battery.

2.5. Common Types

CR2032. CR2025.

3. Lithium Thionyl Chloride Batteries

3.1. Overview

Lithium thionyl chloride batteries are good for long-term power applications such as wireless remote sensors, backup power for non-persistant memory ICs, and real-time clocks.

3.2. Voltage

Lithium thionyl chloride batteries have a nominal voltage of 3.6V.

3.3. Current Draw

This is where Lithium thionyl chloride batteries show their weakness. They are really bad at providing large pulse currents, due to their high internal resistance. This can make the batteries bad at powering pulse-current drawing things like cellular modems. You can add super-capacitors to provide the pulse current in some applications. Some manufacturers have begun selling a combined battery/supercap module which allows for higher pulse currents.

3.4. Leakage Current

Lithium thionyl chloride batteries have a really low leakage current.

3.5. Discharge Characteristics

They have a pretty flat voltage discharge characteristic.

3.6. Energy Density

They have an energy density of 970mWh/cm^3 (when the discharge current is 100uA).

3.7. Temperature Range

They can be used over a wide temperature range of \(-55^\circ C\)` to `\(+85^\circ C\).

3.8. Chemical Reaction

Lithium thionyl chloride batteries use liquid thionyl chloride (\(SOCl_2\) as the positive active material, and lithium (Li) as the negative active material.

Note that the following table describes the reactions for discharging, the reactions occur in the opposite direction when the battery is recharging.

Positive Reaction:$$2SOCl_2 + 4Li^+ + 4e^- -> 4LiCl + S + SO_2$$
Negative Reaction:$$Li -> Li^+ + e^-$$
Combined Reaction:$$2SOCl_2 + 4Li -> 4LiCl + S + SO_2$$

4. Zinc-Air Batteries

4.1. Overview

Zinc-air batteries are batteries characterised by their use of zinc and a reaction with atmospheric oxygen. There is both non-rechargeable (primary) and rechargeable (secondary) zinc-air batteries, although the primary batteries are far more common.

zinc air battery cut away annotated
Figure 3. A cut-away of a zinc-air button cell with annotations. Image from

4.2. Chemistry

A zinc-air battery is a specific type of metal-air battery.

The reactions at each part of the battery are shown below:

4.2.1. Anode

\[Zn + 4OH^- \to Zn(OH)_4^{2-} + 2e^-\]

This has a voltage potential of \(E_0 = -1.25V\).

4.2.2. Fluid

\[Zn(OH)_4^{2-} \to ZnO + H_2 O + 2OH^-\]

4.2.3. Cathode

\[\frac{1}{2}O_2 + H_2 O + 2e^- \to 2OH^-\]

This has a voltage potential of \(E_0 = 0.34V\).

4.2.4. Overall

\[2Zn + O_2 \to 2ZnO\]

This has a voltage potential of \(E_0 = 1.59V\).

4.3. Voltage

The theoretical voltage of a zinc-air cell is 1.65V (based on the chemistry). In reality, the voltage of a zinc-air cell is between 1.35-1.4V.

4.4. Energy Density

One of zinc-air’s unique properties is that they consume atmospheric oxygen as part of the chemical reaction. This means that they can have higher energy densities than other similar batteries because one of the reactants is not actually contained within the battery.

4.5. Storage

Zinc-air batteries can be stored without losing much energy as long as oxygen is kept away from the battery. Digital hearing aids come with a little tab that must be removed before use, this tab prevents air from entering the battery and reacting with the zinc. They can last for 3 years with little capacity loss.

4.6. Applications

Most digital hearing aids use zinc-air button cells. You remove a tab to allow air into the battery, starting the reaction that produces an electro-motive force (i.e. the voltage).




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