Exploring rechargeable batteries
Rechargeable batteries: they're used
everywhere, and there's many different brands and types. Almost every amateur
has their own opinions on the merits of different types and the best ways to
look after them. This month we examine the main types available and their
suitability for various equipment amateurs use.
How rechargeable batteries work
Batteries convert stored chemical energy
into electrical energy. This is achieved by causing electrons to flow whenever
there is a conductive path between the cell's electrodes.
Electrons flow as a result of a chemical
reaction between the cell's two electrodes that are separated by an
electrolyte. The cell becomes exhausted when the active materials inside the
cell are depleted and the chemical reactions slow. The voltage provided by a
cell depends on the electrode material, their surface area and material between
the electrodes (electrolyte). Current flow stops when the connection between
the electrodes is removed.
Rechargeable cells operate on the same
principle, except that the chemical reaction that occurs is reversed while
charging. When connected to an appropriate charger, cells convert electrical
energy back into potential chemical energy. The process is repeated every time
the cell is discharged and recharged.
Different cells use different electrode
materials and have different voltage outputs (1.2, 1.5, 2 and 3.6 volts for the
types discussed here). Higher voltages are possible by connecting cells in
series. A set of several cells connected together is called a battery. However,
because lay people do not distinguish between a 1.5 volt cell and a 9
volt battery (which comprises several cells), the term battery is widely
used for both batteries and cells.
The capacity of cells is expressed in
amp-hours (Ah) or milliamp-hours (mAh). The approximate time that a battery
will last per charge can be found by dividing the battery pack capacity
(normally written on the battery pack itself) by the average current
consumption of the device. Thus a 600 mAh battery pack can be expected to power
a receiver that takes 60mA for 10 hours.
Cells can be visualised as consisting of a
cell with a resistor in series. You won't find an actual resistor should you
split open a battery pack, but the effect is the same. Some battery types have
higher values of internal resistance than others. High internal resistance
doesn't matter if powering items that draw fairly low currents (eg a clock or
small receiver). However, if running something like a 5-watt handheld
transceiver, a battery with a high internal resistance will not deliver the
current asked of it.
Having explained some of the characteristics
important to all batteries, we will now look at each cell type in turn.
Nickel-cadmium (NiCad)
Nickel-cadmium cells are the most commonly
used rechargeable batteries in consumer applications. They come in similar
sizes to non-rechargeable cells, so they can directly replace non-rechargeable
alkaline or carbon-zinc cells. NiCads have a lower voltage output than
non-rechargeable cells (1.2 vs 1.5 volts). This difference is not important in
most cases.
NiCad battery packs have voltages of 2.4,
3.6, 4.8, 6, 7.2, 9, 10.8 volts, etc. This corresponds to 2, 3, 4, 5, 6, 7, 8
and 9 cells respectively.
NiCads perform best between 16 and 26
degrees Celsius. Their capacity is reduced at higher temperatures. Hydrogen gas
is created and there is a risk of explosion when cells are used below 0
degrees.
NiCad batteries have a low internal
resistance. This makes them good for equipment that draws large amounts of
current (eg portable transmitting gear). However low internal resistance means
that extremely high currents (as much as 30 amps for a C-sized cell!) will flow
if cells are short-circuited. Short-circuiting should be avoided as it can
cause heat build-up and cell damage.
Most portable transceivers come with NiCad
battery packs where the cells are welded to metal connecting straps. There is
good reason for this. In high-current applications, the unknown (and varying)
resistance between cells and battery holder contacts can result in erratic
operation. This is especially so when the transceiver is used in a salt-laden
environment. An encased battery pack overcomes these difficulties and provides
more reliable operation.
The normal charging rate is 10 per cent of a
battery's capacity for 14 hours. For example, if a battery pack has a 600 mAh
rating, its correct charging current is 60 mA. Because the charging process is
not 100% efficient, the charger needs to be left running for about 14 hours
instead of 10 hours. Higher charging currents are possible, but the charging
time needs to be proportionally reduced. NiCads can be left on a trickle
charger indefinitely if the charging current is reduced to 2% of the battery's
amp-hour rating. Avoid the build up of heat during charging for long battery
life.
NiCad batteries require a constant current
charger; ie one where the current provided to the battery is fixed over the
entire charging period. Such a charger can be something as simple as an
unregulated DC power supply with a series resistor to limit the charging
current into the cells. If the charger's voltage and the battery's desired
charging current is known, Ohm's Law can be used to calculate the correct
series resistor value. Because NiCads have a low internal resistance, proper
charging can occur with several cells in series.
For best life, do not discharge NiCads to
less than 1.0 volt per cell. When charging, NiCads should read 1.45 volts per
cell. If the cell voltage is higher during charging (eg 1.6 or 1.7 volts), the
cell is faulty and should be discarded.
You'll often hear discussions about the
so-called 'memory effect' exhibited by NiCad cells. This refers to the claimed
tendency of cells not to deliver their rated voltage when placed in a charger
before being fully discharged. Belief in the existence of the 'memory effect'
is widespread amongst users of NiCad batteries. However, textbooks and data
from battery manufacturers make little or no mention of it. Believers say that
to prevent it batteries must be discharged to 1 volt per cell before charging.
Non-believers say that this discharging merely reduces cell life.
Evidence suggests that true 'memory effect'
is rare. It was first noticed in communications satellites where cells were
discharged to precisely the same discharge point every time. In casual amateur
use batteries are most unlikely to be discharged to the same point after every
use. Much of what is mistaken for the 'memory effect' is voltage depression,
which is caused by long, continuous overcharging, which causes crystals to grow
inside the cell. Fortunately both the 'memory effect' and voltage depression
can be overcome by subjecting the battery to one or more deep charge/discharge
cycles.
Another term you will hear is 'cell
reversal'. This can occur when a battery of cells is discharged below its safe
1.0 volt per cell. During this discharge, differences between individual cells
can lead to one cell becoming depleted before the rest. When this happens, the
current generated from the remaining active cells will 'charge' the weakest
cell, but in reverse polarity. This can lead to the release of gas and
permanent damage to the battery pack.
NiCads can short circuit due to the build up
of crystals inside the battery. The use of a fully-charged electrolytic
capacitor placed across the cell can effect a temporary cure. Over-discharging
of batteries invites short circuiting. Batteries should be stored charged. A
lifespan of 200 to 800 charges is typical for NiCad batteries.
Nickel metal hydride (NiMH)
Like NiCads, nickel-metal hydride cells
provide 1.2 volts per cell. Battery makers claim that NiMH cells do not suffer
from the 'memory effect' and can be recharged up to 1000 times.
NiMH cells are not quite as suitable as NiCads for
extreme current loads, but do offer a greater capacity in the same cell size. A
typical AA NiCad may have a 750 mAh, but a NiMH may provide 2400 mAh - three times the capacity.
If your style of portable operating involves going out for 3 or 4 hours and running around 5 watts output, NiMH cells are an excellent choice and are lighter than sealed lead acid.
Where to get them? 7.2 volt battery packs are often used for models. Two in series gives 14.4 volts, but you'll get over 16 volts immediately after charging. That's above what many commercial rigs are rated so use at your own risk. Still it's much easier to get higher RF power outputs from transistors with 15 - 20 volts than 12 volts, so provided you're happy to use a somewhat non-standard voltage then it may still be sensible to use them for homebrew rigs (provided heatsinking is adequate and transistor ratings are observed). Another source are high-quality battery packs discarded by critical commercial users (such as hospitals). They may still be 80% good and entirely adequate for amateur use. One of my favourite QRP battery packs is a used ex-medical 24 volt NIMH pack bought at a hamfest for a few dollars. I split it in two and made two 12 volt packs. It's small and keeps a five-watt radio going for hours, even with heavy operating.
NiCad chargers can be used to charge NiMH
batteries, but the charging time needs to be lengthened to take NiMH's
typically larger capacity into account. The main enemy of rechargeable cells is
heat. If cells get hot during charging, reduce the charging current to no more
than that recommended.
Rechargeable alkaline manganese
Unlike the preceding two battery types,
rechargeable alkaline manganese (RAM) cells give a full 1.5 volts each. They
are therefore suitable for applications where the substitution of 1.2 volt
NiCads for 1.5 volt dry cells results in degraded equipment performance.
RAM cells are cheaper to buy than NiCads.
They can be recharged between 50 and 750 times. They also have a greater
capacity than do NiCads - 1500 mAh is typical for size AA cells. RAM cells are
good for use with outdoor and solar equipment as they will work efficiently at
temperatures up to and exceeding 60 degrees Celsius.
RAM cells have a much higher internal
resistance than NiCads (0.2 ohms vs 0.02 ohms). This means that they cannot
supply high peak values of current. For this reason they are unsuitable for use
with standard amateur HTs. However, their high capacity and long shelf life (5
years) makes them suitable for low powered or emergency-use applications, such
as clocks and emergency torches.
Chargers intended for NiCad and NiMH cells
will not charge rechargeable alkalines. This is because rechargeable alkaline
cells require a constant voltage source of between 1.62 and 1.68 volts to charge.
RAM cells should be connected in parallel rather than in series when charging
several cells at a time. Unlike other rechargeable batteries, RAM cells are
pre-charged and do not require charging before first use. I have not had much
success with rechargeable alkalines and do not recommend them for amateur use.
Lithium ion
Lithium ion cells came onto the market in the
1990s. They offer higher cell
voltage (3.6 volts) and greater capacity for a given volume. This makes them
especially suitable for handheld equipment where long operating times are
important, such as mobile phones.
As an example of what Lithium ion battery
packs can do, a typical lithium ion battery pack is 55x45x20mm but provides 7.2
volts with a 1100 mAh capacity. Lithium ion batteries are more expensive than
older battery types and came into amateur use through their inclusion in handheld
transceivers such as Yaesu's VX-1R and VX-5R models.
Lithium polymer (LiPo)
Lithium polymer cells are the most recent of the
battery types discussed here to come onto the market. They're particularly favoured
by model aircraft enthusiasts for whom light weight combined with high current capacity
is essential. Most amateurs don't need such exacting requirements. Overheating and
even fire are risks with poor handling due to their low internal resistance and potential
to deliver extremely high currents.
Sealed lead acid
Sealed lead acid batteries (or 'gel cells')
are less popular than NiCads in handheld equipment, but find widespread use as
back up batteries in security systems and for amateur portable operation.
Per-cell voltage is 2.3 volts when charged, and 1.8 volts when discharged. This
equates to 13.8 and 10.8 volts respectively for a battery of six cells. For
best use of the full battery charge, equipment intended to operate with '12
volt' sealed lead acid batteries should operate well (if not at full power) at
voltages of 10.8 volts or less.
Gel cells are cheap, rugged and reliable and
should last several years at least. If you want a battery to run a QRP HF
station or a VHF/UHF handheld for several hours, they are the ideal choice.
They are also widely used with small solar systems.
Sealed lead acid batteries can either be
used on a cyclic charge regime (battery connected to charger for a specific
time) or continuous float use, where the battery is across the charger any time
it's not in use. Cyclic chargers should charge at 2.4 or 2.5 volts per cell and
be current limited to prevent overcharge. In contrast continuous float charging
(or trickle charging) requires a charging voltage of only 2.3 volts per cell
(13.8 volts for a '12 volt' battery). With both types of use the charger
voltage is held constant. Connect batteries in parallel if charging two or more
from the one charger.
Chargers for sealed lead acid batteries are
available commercially or can be made at home. Special gel cell charger ICs
exist to provide the necessary voltage and current regulation. Alternatively
chargers can be made from the more common regulator chips such as the 723 or
LM317. These chargers can be used to directly trickle charge the smaller '12
volt' gel batteries. No damage is done if the charger remains on, even when the
battery is fully charged. This is because as the battery voltage approaches 13.8,
the charging current will fall to negligible levels.
Sealed lead acid batteries should not be
charged at voltages higher than those indicated as safe above. This is because
high charging voltages (eg 2.6 volts per cell) will endanger the battery due to
the production of excess gas. At a 13.8 volt charging voltage the production of
gas is low, and the battery should give years of service. Charging current
should not exceed 20 per cent of the rated amp hour capacity of cells. If using
a high current 13.8 volt power supply as a charger, some form of current
limiting is desirable to stay within the battery's limits.
Sources of batteries
The explosion of portable lightweight battery powered devices has been great news for the amateur
seeking batteries for their station with a big range now available. Note though that battery quality
varies. People have taken apart battery cases only to find a very small low capacity battery delivering much less
than the claimed amp hour rating is inside. Be on the lookout for that especially if a battery pack's low price seems too good to be true.
The items below could be a starting point when looking for the ideal battery or power accessory for your equipment.
Disclosure: I receive a small commission from items purchased through links on this site.
Items were chosen for likely usefulness and a satisfaction rating of 4/5 or better.
Conclusion
This article has examined the
characteristics of all major types of rechargeable batteries used by amateurs.
We learned that NiCads and Lithium Polymer were best for high current
applications. Sealed Lead acid and nickel metal hydride were good for casual
amateur portable use. Rechargeable alkaline wasn't recommended for transceivers
though may be OK for receivers. The charging of batteries
varies too - Rechargeable alkaline and sealed lead acid required a constant
voltage, but nickel cadmium and nickel metal hydride cells needed a constant
current to charge properly. In all cases over-charging, through excessive
voltages, currents or charging periods can cause heating, gas build-up and
possible cell damage. However, if you treat your batteries well, you should
have many years of successful operation from them, whichever type you choose.
Acknowledgments
I wish to acknowledge the people and
organisations who have contributed to the writing of this article. These
include:
* The late Bill Trenwith VK3ATW for suggestions on the manuscript and
imparting of knowledge gained through many years as a mechanics teacher,
model engineer and radio amateur.
* Peter Wegner from Coorey & Co, distributors of BIG rechargeable
alkaline cells.
* Danielle Cvetkovic from Invensys Energy Systems Pty Ltd for
material on Hawker sealed lead acid batteries.
* Adeal Pty Ltd for information on Varta's range of NiCad and NiMH
cells.
References
1. Hawker P G3VA, Technical Topics Scrapbook
1990-1994, RSGB, pages 1, 16, 142
2. ARRL Handbook 1988, ARRL, pages 6-25,
27-32
3. Gruber N WA1SVF, QST November 1994, ARRL,
page 70.
An earlier version of this article appeared in Amateur Radio December 1999 with updates made in 2017.
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