Batteries

All you need to know about batteries.

Summary

There are various types of battery based on different chemistries. To get the best out of them and to avoid problems you need a basic understanding of their advantages and disadvantages, as well as a little about purchasing, maintaining and disposing of them.

Safety

• Coin cells must be kept away from small children. If swallowed, this is a medical emergency as death can result in just a few hours, from caustic chemicals generated by electrochemical action in the stomach.
• Lithium batteries can catch fire and burn violently if over-charged, shorted, punctured, or physically damaged in any way.
• Old batteries often leak, causing corrosion of the battery contacts, so preventing new batteries from working. Always remove spent batteries from equipment.
• Improperly disposed of batteries can damage the environment. Some types contain toxic materials. Always dispose of used batteries in a responsible way.

How batteries work

A potato battery.

(You can skip this section if you like, though a little more knowledge than you actually need is always helpful.)

You can make a very simple "potato battery" by pushing a copper coin (or a piece of stiff copper wire) and a galvanised nail into a potato (or a lemon). Don't let the coin and nail touch each other. Touch or connect one probe of a voltmeter onto the coin and the other onto the nail. It should read around 1 volt.

Both the copper of the coin and the zinc of the galvanised nail would like to dissolve in the potato juice, each atom leaving behind a couple of negatively charged electrons in order to do so. However, the zinc is more determined to dissolve than the copper, so it does so, shedding electrons and creating a positive charge in the potato juice. This positive charge discourages any copper atoms from dissolving.

Meanwhile, the electrons abandoned by the zinc atoms flow through the voltmeter to the copper coin. Here, they find positively charged atoms in the potato juice, less bothered about being dissolved than the zinc. The electrons neutralise these positively charged atoms, creating bubbles of oxygen.

N.B. It's not recommended that you eat the potato (even cooked) when you finish the experiment - the coin may not have been very clean and traces of zinc will have dissolved in the potato juice.

Copper, zinc and potato juice don’t make the best battery but all batteries, like the potato battery, consist of two electrodes of different metals (or a metal and carbon) immersed in an electrically conducting electrolyte. Different formulations give different voltages and other characteristics such as cost, weight and capacity.

Strictly, the potato isn’t a battery but a galvanic cell, usually just called a cell. Properly speaking, a battery consists of a number of cells connected together, each adding to the "push" of the one behind it. In this way, 6 cells each of 1.5V can be combined into a battery giving a total output of 9V.

In practice, everybody talks about a battery, even if, as in the case of the familiar AA battery (or rather, cell) it only consists of one cell. A 9V alkaline battery consists of 6 cells, each giving 1.5V.

The potato is an example of a primary cell. In operation, it consumes some of its constituents, in this case the zinc in the galvanised nail dissolves. The process is not reversible, in part at least, because the oxygen bubbles escape. A rechargeable battery is known as a secondary battery, and uses constituents which can be restored to their original state by driving an electric current back through it in the reverse direction. Even if the chemistry is completely reversed, the physical properties of the electrodes will generally degrade to some extent, limiting the number of discharge/recharge cycles that are possible.

Types of battery

Below is a summary of the main characteristics of the commonest types of singe use and rechargeable batteries.

Non-rechargeable (primary) batteries

Type	Advantages	Disadvantages	Comments
Zinc carbon and zinc chloride	Cheap. Available in standard shapes and sizes.	Short life and slow death.	Zinc chloride is a heavier duty version of zinc carbon. Alkaline batteries are preferred in almost all applications.
Alkaline	Good life at a reasonable cost. Available in standard shapes and sizes, and also as button cells as cheap alternatives to silver.		This is the most economic general purpose type.
Silver oxide	High capacity.	Expensive.	Normally only available in small sizes as button cells for watches and calculators, on account of the cost.
Zinc-air	Very high capacity.	Short life once the tab has been remove to activate it by letting the air in.	Used in hearing aids in the form of button cells.
Lithium	Very long shelf and service lives.	Relatively expensive.	Various different lithium-based chemistries have somewhat different characteristics. Mainly used in smoke alarms and cameras.

Rechargeable (secondary) batteries

Type - Nominal voltage	Advantages	Disadvantages	Comments
Lead Acid - 2V	Rugged, tolerant of continuous over-charge. Reasonably cheap through mass production.	Lead is toxic and cannot be disposed of in landfill.	At its best when mainly kept fully charged, hence widely used for (petrol/diesel) car batteries, uninterruptable power supplies, emergency lighting, security alarms, but also used in milk floats, golf buggies etc. Smaller sizes generally come as non-spillable sealed units.
Nickel Cadmium (NiCd) - 1.2V	Very rugged and shows little deterioration over many recharge cycles. Can deliver a high current and accept a very fast charge.	Contains cadmium, which is toxic. Repeated partial discharge causes a "memory effect". Lower voltage than alkaline and zinc types (1.2V as opposed to 1.5V for alkaline).	Mainly used nowadays in power tools and radio controlled model boats and cars.
Nickel Metal Hydride (NiMH) - 1.2V	Greater capacity than NiCd. No toxic cadmium. Less susceptible to the "memory effect". Available as direct replacements for common sizes of non-rechargeable batteries.	Relatively high self-discharge rate. Not very tolerant of being left on charge for extended periods. Low self-discharge variants have reduced capacity. Offers fewer charge cycles than NiCd. Lower voltage than alkaline and zinc types (1.2V as opposed to 1.5V for alkaline).	NiMH has replaced NiCd in all but specialist applications. As with NiCd, the voltage is typically 1.2V per cell as compared to 1.5V for the same size zinc-based batteries. This may be insufficient for devices designed for running on non-rechargeable batteries.
Lithium - 3.7V	Very high energy density.	Very dangerous if abused. Protection circuitry is essential.	As with primary lithium batteries, there are various chemistries and formulations with somewhat different characteristics. They come in standard shapes and sizes such as the cylindrical 18550 type commonly used in laptop batteries, but very often in sizes designed for a specific model of smart phone or camera.

Battery shapes and sizes

A comprehensive list of battery types and designations is given in the Wikipedia article List of battery sizes.

Domestic batteries

The majority of domestic batteries, both non-rechargeable and rechargeable, come in a few familiar sizes such as AA, the smaller AAA, and for applications needing greater capacity, the larger C and D. All these deliver 1.5V for the non-rechargeable or 1.2V for rechargeable. They are often used in pairs to give double the voltage or four together giving four times the voltage.

The 9V PP3 size with the snap-on connector on the top is also very common. These are made up internally of 6 non-rechargeable 1.5V cells or generally 7 rechargeable 1.2V cells giving 8.4V, which is near enough and probably better than a half-used non-rechargeable battery.

These battery sizes are used to power the vast majority of torches, alarm clocks, radios and battery powered toys.

Some rechargeable domestic devices and appliances such as hand-held vacuum cleaners and cordless phones contain 2, 4 or 6 standard size batteries permanently connected together with metal tapes spot-welded to their ends. The whole assembly is held together in a pack with a heat-shrink sleeve. Single "tagged" batteries can be obtained online and can be soldered together into a replacement pack; you can't easily or safely solder directly onto the terminals of a standard battery but you can solder wires onto the tags to connect the batteries together.

Coin cells

Coin or button cells are generally used in watches, smaller digital clocks, calculators and car key fobs, as well as in some novelties. For watches where the maximum life is needed these are silver oxide cells but novelties tend to use cheaper alkaline versions in the same sizes. Both types produce 1.5V. There are various different manufacturer-specific type designations.

Lithium coin cells are also common, generally coming in larger diameters and producing 3V. They are available in at least two diameters and a range of thickness indicated by the designation, for example type CR2032 is 2.0cm in diameter and 3.2mm thick, whereas CR1220 is 1.2cm in diameter and 2.0mm thick.

You might occasionally come across rechargeable coin cells. These are sometimes used in central heating controllers, normally kept in a fully charged state by the mains power, to keep the clock running and maintain settings in the event of a power cut. A digital camera may also contain one to allow it to keep track of the date and time while the main battery is removed for charging. The main battery recharges it when replaced.

Rechargeable lithium batteries

Smartphones, tablets and media players generally contain a rechargeable battery of a size and shape unique to the precise model of device, in order to make the device as compact and as thin as possible. Unfortunately there is no perceptible standardisation, even within one manufacturer's products.

However, rechargeable lithium batteries do come in standard sizes, and it is these that are generally used in laptop batteries and often also in the power banks designed to allow you to recharge your device on the go. Probably the commonest size is 18650, which is approximately 18mm in diameter and 65.0mm long.

Note that even though a laptop battery may contain standard cells fairly easily obtained, fitting new cells to a dead laptop battery is not recommended. For a start, it's usually (deliberately) difficult to open without major damage to the case, but also, there's a chip which keeps track of the battery's history which would be very difficult to reset.

Purchasing batteries

Domestic batteries

Zinc-carbon, zinc-chloride, alkaline and rechargeable NiMH batteries all come in the common C, D, AA, AAA and PP3 sizes and are widely available on the high street. Generally, you can expect to get what you pay for though a good quality battery may be just as good as a more expensive premium brand one.

Most can be found more cheaply online, especially if you buy them in multi-packs.

Very cheap batteries sold in discount stores are probably best avoided as they may be old stock or poor quality. In any case, check the best-before date. In the case of rechargeable NiMH batteries, check the capacity. It may well be worth paying extra for higher capacities ones.

NiCd batteries have been almost completely superseded by NiMH. Their only advantages are that they can supply a heavier current (useful for powerful toy cars or boats and for power tools) and that they have a lower self-discharge rate. On the other hand, they contain toxic cadmium and suffer from the memory effect.

Lithium batteries

Lithium versions of domestic batteries are available in a few sizes, and are mainly useful for smoke alarms where their very long life is an asset.

Buying replacement laptop, mobile phone and tablet batteries is something of a minefield. Check online and find out the range of prices for the battery you need. At the cheapest end, these are likely to be poor quality or used batteries pulled from equipment, and are best avoided.

Manufacturer's branded batteries are usually sold at a high price, which is not necessarily a guarantee that they are fresh. (An unused lithium battery can deteriorate markedly in 2 - 3 years just sitting on a shelf!)

An upper-middle price may well be best value, but before buying check the seller's guarantee and returns policy. A personal recommendation for a seller may well be worth following.

Coin cells

Watch batteries can be purchased much more cheaply than the cost of having them fitted on the high street, but you are likely to need special tools to fit them yourself. In the case of a watch with a snap-on back, even if you can get it off you can quite easily smash the glass trying to snap it back on.

There have been reports of large numbers of counterfeit coin cells being sold cheaply online. These bear the name of a reputable manufacturer and are hard to distinguish from the genuine article, but are of greatly inferior quality. Here again, you are unlikely to get value for money at the bottom of the market.

Battery care and maintenance

Never mix battery types or batteries of different ages in the same device. The strongest will force the weakest into deep discharge or reverse charge, possibly causing it to leak corrosive chemicals and in the case of rechargeable batteries, doing it permanent damage.

Non-rechargeable batteries

There is little to be said here except to reiterate the importance of removing spent batteries. Often, a little used radio or a battery powered toy that has lost its appeal is put aside, and a year or several years later the owner wonders why it no longer works. The old batteries have leaked and the electrolyte has corroded the battery terminals. Thoroughly cleaning them with switch cleaner and a stiff brush will generally restore the device to working order if the corrosion isn't too severe.

Instructions are commonly seen for making a "joule thief". This is the name of a circuit which extracts the last remaining charge from an effectively dead battery. Whilst instructive from an electronic point of view, the battery is likely to leak, possibly causing damage to its surroundings.

However, batteries which are no longer able to power a motorised toy or a radio may still be good for a quartz alarm clock, drawing much less current. This may be satisfying, but the small cost saving may seem less attractive if you miss your bus because the alarm failed to go off!

You may sometimes see schemes or circuits for recharging single use zinc or alkaline batteries. This is not recommended as leakage is likely. In the past, alkaline battery variants have been offered which are designed to be recharged using a special charger, but results were poor compared to NiMH batteries. Best results depended on recharging while they were still above 50% capacity, and even then a maximum of around only 50 recharges was achievable. The only case were they might have been useful was in equipment which required the full 1.5V rather than 1.2V provided by NiMH batteries.

Domestic rechargeable batteries

In the case of NiCd batteries, it's important to fully discharge and recharge them from time to time to avoid the "memory effect". This became known some years ago as the "Dustbuster effect" after the hand-held vacuum cleaners of that name. These were often used briefly to pick up crumbs or to clean out a dusty corner, causing a partial discharge only. When this happens repeatedly, the unused chemical becomes hard and unreactive, so unable to deliver the full charge of the battery.

Some people still recommend periodically completely discharging and recharging batteries even of the NiMH type. Whilst not completely memory free as sometimes advertised, the benefit for NiMH types has to be weighed against the wear it imposes on the battery. Particularly if you commonly leave a battery in the charger for an extended period, a full discharge cycle, maybe every 3 - 6 months, is likely to benefit it.

Rechargeable lithium batteries

Rechargeable lithium batteries need a certain amount of tender loving care to get the best out of them, and abused, can be highly dangerous.

Some lithium batteries have a built-in protection circuit which protects them from over-charge or deep discharge. This may not be obvious and is often completely hidden, so you should never rely on it. If the voltage falls below a threshold this circuit will completely disable the battery, and it will read 0V on a voltmeter and cannot normally be recharged. Although it may be possible to recover it, it may have suffered irreversible damage and could even be dangerous to use.

In the case of a laptop battery a separate protection circuit within the battery pack will disable the entire pack if one cell falls below the threshold.

Lithium batteries are best kept cool for longest life. So if you normally use your laptop on mains power, you can prolong the battery life by removing it. Bear in mind though that in the even of a power cut or accidentally unplugging the adapter your laptop will shut down instantly, with a small but nevertheless non-zero chance of disk corruption.

If you prefer to leave your battery in even though mainly using your laptop on mains power, the battery settings may offer a battery life preservation mode which prevents the battery being charged to 100%. Holding it continuously at full charge is less than optimal for a lithium battery. You can set it back to full charge mode to top it up an hour or two before needing to unplug it.

Even though there is no benefit to a lithium battery in fully discharging and recharging it, it may still be worth doing so just occasionally. This is because most devices using them contain a "gas gauge" circuit which keeps track of the state of charge by continually measuring the amount charge going in or coming out. This can never be perfectly accurate and may occasionally need to be recalibrated by a full discharge. Only do this if your battery seems to be losing capacity, or at most every 3 - 6 months otherwise.

If you are going to store a lithium battery for a while, for example if you bought a spare camera battery, it will store best charged to around 60% only rather than fully charged.

Bear in mind that there are restrictions on sending lithium batteries through the post. These don't normally apply to a lithium battery within a device such as a phone or a camera. If you do need to send a lithium battery on its own, you will need to find a carrier which will accept it.

Disposing of batteries

Any outlet (in the UK, at least) which sells more than a small number of batteries is obliged by law to accept spent batteries for recycling. Your local council may also accept them on their weekly rounds or at your nearest recycling point. All domestic zinc, alkaline and nickel-based rechargeable batteries can be responsibly disposed of in this way. Such batteries are recycled in bulk.

Local councils will also accept all types of lead-acid batteries such as car batteries and batteries from mobility scooters and security systems. Much of the lead can be recycled.

Lithium batteries are more problematical. Your local council may not specifically say that it will accept them, but since you have no other choice you should give them the problem rather than disposing them with general waste. Unfortunately the lithium itself costs more to recycle than to dig up out of the ground and refine, and deposits are not likely to run out any time soon. Also, there is a fire risk in transporting and storing lithium batteries in bulk before disposal, and they are not easy to disassemble safely. However, they do contain metals other than lithium which are worth recycling.

External links

• The Battery University contains a wealth of information about all types of battery.
• Wikipedia contains a comprehensive list of battery sizes