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A collection of standard cells and batteries

A collection of standard cells and batteries. Most are available in a variety of chemical technologies. From left to right: 3R12 4.5v battery, D cell, C cell, AA cell, AAA cell, AAAA cell, A23 12v battery, PP3 9v battery, CR2032 coin cell and LR44 coin cell. For an exhaustive analysis of battery and cell formats, see this List of Battery Sizes.

Definitions: A Primary Cell is an electrochemical device consisting of two electrodes immersed in a container of solid or liquid electrolyte. A non-reversible chemical reaction causes a voltage to appear across the electrodes and a current to flow between them and through an externally connected load. A Secondary Cell permits the chemical reaction to be reversed by applying an external voltage, recharging the cell. A Battery is a group of cells connected in series to achieve a higher output voltage. Unfortunately, the terms Cell and Battery have become interchangeable in general use.

The choice of battery power supply for a mobile or portable electronics project may be the least interesting part of the design, but make the wrong choice and all that development work will have been wasted. I’m not talking about full-size electric vehicle (EV) batteries here, just those in the range from single ‘coin’ cells to standard 12V car batteries.

Think “Battery” before designing circuits

There’s quite a wide choice of energy capacity, voltage, and physical size/weight on the market. Making the optimal choice, particularly for a portable device, can determine the ultimate viability of the whole project – no matter how ‘clever’ it is. Here are some factors to consider:

Finished size and weight

  • Fits into a jacket pocket or wearable?
  • Fits into the average briefcase?
  • Compatible with airline cabin luggage regulations?
  • Needs to travel in the airplane baggage hold in a flight case?
  • Fits on a wooden pallet and requires a pump-truck/forklift to move it?

The size and weight of the device obviously determines just how ‘portable’ it can be. Early portable PCs were described as luggables, which indicated the presence of a carrying handle on the top of an otherwise desktop-bound machine. We had to wait for IC and battery technology to improve to the point where laptops became possible; truly portable, battery-operated PCs which would run Windows for more than 20 minutes without a mains supply.

A mobile device doesn’t need to be portable, a robot being an obvious example. It just needs a battery, preferably rechargeable, that enables it to achieve its mission goals measured in elapsed time or distance travelled. Battery size and weight is not unlimited though; a balance needs to be achieved between energy capacity and the power of the motors needed to move the robot around.

There is also a category of battery-based projects that needs to be moveable so that it can be set up in a remote location temporarily when needed. An example could be a wildlife monitor that uses a large, high-capacity battery, not to handle heavy instantaneous motor current as in the mobile robot, but to maintain continuous operation of a low-current data-logger over perhaps one year. In this situation, the installation is static while operating, so battery size is largely immaterial. Instead, the rate of self-discharge over a long period becomes the deciding factor.

Capacity in Ah

Battery capacity is generally defined as the amount of electric charge it can deliver at its rated voltage. For a given chemistry, the bigger the battery, the greater the capacity. Manufacturers normally quantify capacity in terms of amp-hours (Ah) which suggests, in theory, a 100Ah battery can sustain a load current of 100A for one hour while maintaining a terminal voltage within a specified percentage of its fully-charged no-load value. Or 50A for two hours. In practice, it’s nothing like that simple because various factors badly distort that nice linear relationship:

  • Secondary chemical reactions.
  • Ambient temperature: low temperatures reduce capacity in most cases.
  • Storage time since manufacture: charge leakage, electrode/electrolyte deterioration.
  • Nature of the load, e.g. constant dc or brief high-current pulses.

And for rechargeables:

  • State-of-charge while stored.
  • Number of discharge/recharge cycles endured so far.
  • Average depth of discharge in each cycle.
  • Recharge current.

Capacity in Wh and C-rate

To get the battery’s energy capacity, just multiply the Ah value by its nominal output voltage. This parameter in Watt-hours (Wh), or more likely kWh is used by manufacturers of really big batteries, such as those for electric vehicles, to describe their power storage capability. It’s used because at this level, the load is more likely to be specified in watts rather than amps.

As indicated above, rechargeable batteries add some extra factors to complicate the issue of charge capacity. The big one is the charging current needed to achieve maximum capacity, and for things like EV batteries, it’s usually given in the form of the ‘C-rate’ (C), defined as the charge current divided by the battery's capacity in Ah. So, for example, a 1Ah battery specified with C = 0.5 will use a charging current of 0.5A, increasing its state of charge by 50% in one hour. A separate value for C, this time for discharge current will tell you when maximum power is being delivered.

Issues of how long a battery can deliver usable power, and how long recharging takes, are of course of vital interest to potential owners of electric vehicles, but not so much when it comes to rather smaller consumer products such as an electric toothbrush or vacuum cleaner.

Lifespan

Lifespan is obvious for a primary cell: it’s the time taken for it to discharge and the voltage to drop below a defined threshold. For rechargeables, it’s the number of discharge/recharge cycles achieved before the battery ceases to function. Nothing lasts forever and some capacity is lost with every cycle.

Examples

Here are some pictures of battery formats commonly used today – and one or two that aren’t.

Coin cell next to a NiCd pack

Little and Large. A 3V Lithium coin or button primary cell, with a heavy-duty rechargeable 9.6V NiCd power pack for a radio-controlled model car. This type of coin cell has a very long shelf-life and is often used in very low-drain situations such as maintaining the data in a small volatile RAM chip.

Nowadays, low-power technology means that a coin cell can power a whole embedded MCU system as part of the Internet of Things. Remote wireless sensors are designed to take readings of say, rainfall and temperature and send the results to a base-station at regular, but widely spaced intervals. The unit ‘sleeps’ using very little power between transmissions yielding an average load current of little more than the cell’s own internal ‘leakage’. In theory, the sensor could run for years before the cell needed replacing. There is a snag though, and that’s down to the Brown-Out detector in the MCU which senses when the power supply voltage has dropped below a threshold, signalling that the cell has reached the end of its life. This can happen even with as much as 50% of nominal capacity remaining. The internal resistance of the cell increases with age, so a long time before the cell reaches its theoretical lifespan, the current surge caused by the MCU waking up and transmitting its data brings the terminal voltage down below the Brown-Out threshold. A solution is to place a large capacitor across the power-rails as a reservoir to supply the surge current, which ‘recharges’ from the cell once the MCU has shut-down again.

The big yellow ‘power-pack’ is at the opposite end of the scale to the tiny coin cell. It consists of eight AA-size 1.2V NiCd cells arranged in series to yield 9.6V with a capacity of around 1.5Ah. Single NiCd cells are no longer available because of their poisonous Cadmium content, but power-packs like this are still around. This type of cell chemistry is known for its ‘memory’ effect which appears if the cell is repeatedly partially discharged before being recharged. Eventually, the remaining capacity after each discharge becomes permanently inaccessible. But, this cell technology has a very high capacity per unit volume (high energy density) and is extremely robust, able to withstand incredible mistreatment without catching fire or exploding. This power pack is designed for radio-control model car racing where charging times are very short - of the order of minutes – with the same going for discharge. There’s so much current (amps not milliamps) flowing in both directions, these battery packs can get really hot. Of course, such brutal treatment takes its toll resulting in a very short lifespan. In this competitive environment, lifespan is not an issue: all that matters is that high energy capacity which can be drained very quickly.

The power underneath a mobile robot

The power underneath a mobile robot. Nothing exotic here; just five AA 1.5V alkaline primary cells yielding a power rail of 7.5V. Alternatively, five 1.2V NiMH rechargeable cells will provide 6V. There are low-drop-out (LDO) regulators on the PCB providing 3.3V and 5V rails for electronics. A switch selects the power source for the two servomotors depending on which battery technology is being used. The maximum servo supply voltage is 6V, so when alkaline cells are fitted, the servo power is taken from the 5V regulator. When NiMH cells are used, the switch will allow the servos to receive the full 6V directly from the battery pack.

Nickel-Metal Hydride technology replaced Nickel-Cadmium as the environmentally-safer alternative. It has a higher energy density so equivalent size cells have higher capacity; 2800mAh NiMH AA cells are available now. Although it doesn’t exhibit any permanent memory effect, it’s not as robust as the NiCd. Overcharging can cause permanent damage, so a ‘smart’ or ‘delta-V’ charger is recommended which monitors the rising terminal voltage and halts charging when there is no further increase. Care must also be taken not to over-discharge the cell below 1.0V, as again, permanent damage may result. I found that some manufacturers’ products had a tendency to lose most of their capacity after a few months of inactivity. This was some years ago, and it may be that the problem of poor shelf-life has been solved.

Larger mobile robots will need bigger batteries. Although Li-Ion technology could handle the power, for development work it might be better to use good old-fashioned Sealed Lead-Acid (SLA) batteries as they are a lot less ‘fragile’ (see below).

Li-Ion cell powering an MCU based instrument

Lithium-Ion chemistry has an even higher energy density than NiMH and has replaced the latter in high-drain consumer product applications such as smartphones. For the same reason, it has displaced Lead-Acid as the energy source of choice for electric vehicles. The Li-Ion cell in the picture (125-1266) is only a few millimetres thick but has a useful capacity of 2000mAh at 3.7V. With that kind of power available, it can keep an embedded MCU-based portable instrument going for hours on one charge. In this example, it slips nicely between the Clicker 2 dsPIC33E board (144-8343) and a same-size expansion board plugged in underneath.

The big problem with Li-Ion chemistry is that unless charging and discharging are tightly controlled, internal short-circuits will develop causing batteries to rapidly overheat and ultimately catch fire. Boeing experienced this when they used Li-Ion for the first time on their new 777 airliner. It’s not unknown for electric car drivers to make a hurried exit when smoke appears in the cabin. And for a while, pictures of smouldering smartphones and laptops were all over social media. Things seem to have settled down now, but the days of driving amps into NiCds until they practically glowed or doing the same to vented Lead-Acid batteries until they hissed like a boiling kettle are long gone. And (don’t try this at home), zapping dead NiCd cells with high-voltages to break internal short-circuits and restore capacity…..

experimental battery state-of-charge tester from 1982

An experimental battery state-of-charge tester from 1982 based on an embedded Z80 MPU programmed in Forth. It needed a lot of power supply current while remaining portable and useable for at least an hour.

Inside the instrument

Inside the instrument: a large circuit board covered in big chips. To supply that lot, four D-size cells were used in two circular battery holders – the white tubes above the circuit board. The Clicker 2 could replace the whole main board with the exception of a few discrete components in the test input. The mains supply socket would not be required; its function of external supply and battery charger input provided by a USB socket instead.

Conclusion

Selecting the right sort of battery for a particular portable or mobile project is a far from simple task. It deserves as much research and careful selection as the processor chip in an embedded application.

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Engineer, PhD, lecturer, freelance technical writer, blogger & tweeter interested in robots, AI, planetary explorers and all things electronic. STEM ambassador. Designed, built and programmed my first microcomputer in 1976. Still learning, still building, still coding today.
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