Difference between revisions of "Batteries"

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*Very dangerous if abused. Protection circuitry is essential.
*Very dangerous if abused. Protection circuitry is essential.
As with primary lithium batteries, there are various chemistries and formulations with somewhat different characteristics.
As with primary lithium batteries, there are various chemistries and formulations with somewhat different characteristics. They come in standard shapes an 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.

Revision as of 21:52, 7 April 2017

All you need to know about batteries.


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.


  • Button 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 corrosions 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 comntain toxic materials. Always dispose of used batteries in a responsible way.

How batteries work

(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 battery by pushing a copper coin and a galvanised nail into a potato (but not touching each other). Touch one probe of a voltmeter on the coin and the other on the nail. It should show 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 as it does 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, where they find positively charged atoms in the potato juice less determined to dissolve than the zinc. The electrons neutralise these positively charged atoms, creating bubbles of oxygen.

N.B. Don't eat the potato, even cooked, when you finish the experiment!

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.

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.

  • 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.

  • 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.

  • 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 and reasonably cheap.
  • 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.
  • Can deliver a high current and accept a very fast charge.
  • Contains cadmium, which is toxic.
  • Repeated partial discharge causes a "memory effect".
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.
  • No memory effect.
  • Available as direct replacements for common sizes of non-rechargeable batteries.
  • Relatively high self-discharge rate.
  • Low self-discharge variants have reduced capacity.
  • May not work in all equipment designed to accept alkaline batteries on account of a lower voltage (1,2V as opposed to 1,5V for alkaline).

NiMH has replaced NiCd in all but specialist applications.

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 an 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.

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.

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 out of 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!

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, however, this is completely unnecessary as the don't suffer from a memory effect.

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 he 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 not fully charged but charged to around 60% only.

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

External links