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Power Bank

Power banks are commonplace and with our increasing use of battery powered equipment: everything from mobile phones to battery powered headphones, portable speakers, MP3 players can be charged via a power bank. They are effectively a portable charger. All they need is a USB charging interface.

Power banks come in a variety of shapes and sizes and to suit many different people and their needs.

Power bank definition

Power banks can be defined as portable batteries that use circuitry to control any power in and power out. They can charged up using a USB charger when power is available, and then used to charge battery powered items like mobile phones and a host of other devices that would normally use a USB charger.

The name power bank can be likened to a financial bank where funds can be deposited, stored, and withdrawn when needed. These items are also often referred to as portable chargers, as they can charge items like mobile phones without the need to be connected to the mains during charging, although they will need to be charged, and this normally requires a mains charger.

Power bank types

There are a few different types of power bank portable charger that can be bought. Obviously the size is one of the main criteria, but there are some other categories that can be considered.

  • Universal or standard power bank:   These are the normal power bank portable chargers which are available in the stores and online. They are charged from the normal USB sources like USB chargers.
  • Solar power bank:   As the name indicates, these solar power banks can use sunlight to charge up. To do this they have photovoltaic panels. These are really only able to trickle-charge the internal battery when placed in sunlight because the solar cells are relatively small, but nevertheless this can be a very useful function.
    • As the solar charging is slow, they can also be charged from a USB charger as well. The solar charging is a useful back-up, especially if you are travelling away from mains power.
  • Power bank lifetime

    There are two main forms of lifetime that are associated with power banks.

    • Charge discharge cycles:   Any rechargeable battery will gradually wear out. Normally the lifetime of a battery is quoted in terms of the number of charge discharge cycles it can undergo before its performance falls by a given degree. Some cheaper power banks may only have a life of 500 or so charge discharge cycles, but better ones will have lifetimes of many more charge discharge cycles.
    • Self discharge time:   All battery cells, whether rechargeable or primary have a certain level of self discharge. For rechargeable batteries these days with their own control circuitry, a small amount of power is required to keep these circuits alive. As a result there is only a finite time that a battery will remain charged.

      A good power bank can hold charge for up to 6 months with only a small loss of charge, but lower quality ones may only retain a useful charge for about a month. These figures are for room temperature, but storing them outside these temperatures considerably reduces their performance.

    Power bank battery technology

    All power banks use rechargeable batteries based around lithium technology. Lithium-Ion and Lithium-Polymer batteries are most commonly used for power banks but don’t be surprised if other types start to hit the market before long. Battery technology is key to many new developments: everything from mobile phones to electric vehicles, and as a result it is quite likely there will be some spin-offs into power banks.

    The two technologies that are currently used have slightly different properties:

    • Lithium-ion:   Lithium-Ion batteries have a higher energy density, i.e. they can store more electrical charge in a given size or volume, and are cheaper to manufacture, but they can have issues with ageing.
    • Lithium-polymer :   Lithium-polymer power banks do not suffer from ageing to the same extent so are a better choice. However they are more costly to manufacture and as a result they may not suit all budgets. Sometimes it may be that it is best to spend less, especially if they are likely to be accidentally lost or left in places.

Both forms of power bank work well, but it is a balance between cost and performance.

Power bank portable chargers are particularly useful as they enable battery powered items to be charged on the go. As it is not always possible to reach a mains power point every time a mobile phone or other battery powered item needs charging, these power banks are have now become an established product and they are very useful, especially when travelling.

Power bank operation basics

When looking at how a power bank works, it is essentially a battery into which power is passed typically from a mains powered USB charger. This is stored and then passed out to the device under charge as required.

To facilitate this operation, the power bank not only consists of a battery itself, but some sophisticated electronics which manages all of these operations.

Power bank battery technology

Whilst the management of the battery charging and discharging is key to the way the power bank works, so too is the battery technology employed.

Basic power bank usage

Power banks are generally very easy to use, and typically conform to some simple conventions, especially with respect to the connectors. Normally power bank connectors have separate functions.

  • Micro USB (sometimes mini-USB):   Most commonly a power bank will use a micro-USB for being charged. This enables the standard USB A to micro-USB leads to be used to charge the power bank. Often the same lead used to charge a mobile phone or other device can be used.
  • USB Type A connector:   The larger type A USB connector is used to enable the power bank to charge other devices. This means that standard charging cables supplied with most phones and other devices can be used. Normal USB chargers have a Type A connector from which the charge is supplied.

When using the power bank, it is normally only necessary to connect a powered USB micro connector to it for it to receive charge. Depending on the power bank capacity, its charge level, and the charger, the ambient temperature, etc., it can take quite a while to complete its charge. As an example, a 1500mAh rated power bank should take very roughly about the same time as a typical smartphone to charge. For larger power banks, this time can be considerably increased – it may take two three, four . . . times as long.

Power banks have electronic battery management and this includes a safety cut-off to prevent overcharging and overheating. However, whenever possible, it is best to remove the power bank from charger when it is full – at least avoid leaving it connected long-term after its full.

However when using the power bank to charge electronic devices, the leads need to be connected, and it is normally necessary to use a button on the power bank to enable the charging. This is required because a sort circuit on the output of the power bank could generate a lot of heat and possibly cause the power bank to catch fire or explode. The prevent keys in pockets and other metallic items causing an accidental short circuit, the output normally has to be enabled before use.

Often the power bank will have a simple LED indicator showing the level of charge it has when charging is enabled, or when it is being charged. These indicators often turn off after a short while to preserve the power bank charge.

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