Batteries in RC Yacht
Racing
Types of batteries
There has been a revolution in batteries
available for model use over the last 10 years or so.
The most commonly used cell prior to 2000, the NiCad,
containing the very poisonous cadmium has become almost
entirely unavailable and is no longer manufactured by the
major cell manufacturers. The lead acid battery type
(gel cells) which found favour in the power boat fraternity and was used
by some RC sailors really has no place in a racing yacht
simply because of its inefficient power to weight ratio -
its too heavy!
So what has replaced these standbys and
what is in the wings waiting to be used by RC sailors?
The most familiar cell being used today is the NiMh.
This cell has a nominal voltage of 1.2v the same as the
NiCad cells. The two main advantages over NiCad cells
are its low toxicity when disposed of and its much higher
capacity for the same size - possible up to 2 to 3 times as
much. Unfortunately it does suffer from a few
disadvantages tho. It is not very tolerant of low
temperatures and has trouble supplying sufficient current
for our uses below 5C. (Who would be daft enough to
sail then??). It also may not have quite the same life
as NiCad - no of times it can be recharged. Perhaps
its most disturbing property is its tendency to self
discharge. a cell may lose 20% of its charge in the
first day or two. While this can be dealt with by
charging as late as possible it is a nuisance. A new
generation of NiMh cells has been available for a few years
now (since ~2007) which addresses this issue. Various
brands such as 'Eneloop' and Sony claim a very low self
discharge rate and cells charged immediately after sailing
can be used with confidence a week or two later without
recharging. In fact they are sold in a partial charged
condition and claim significant charge remains even after a
year. If you are using NiMh cells there really
is no excuse for not using this type of cell.
The other type of cell which is being
increasingly used by modellers including sailors is the
Lithium Polymer (LiPo) cell. This is also of low
residual toxicity and has the highest energy density of
commonly available cells. This means less weight
(about 1/3 - 1/2)
for the same capacity. They are able to supply a high
current and maintain a constant voltage during discharge.
As well they maintain their charge for years! so charging
can be done anytime after use confident they have a full
charge even weeks later. They are nominally 3.7v per cell so only two cells are
needed in most systems. They are not without their
drawbacks tho as they MUST be charged with a LiPo dedicated
charger taking extreme care in selecting the appropriate
capacity and number of cells
. If allowed to discharge
below 2.5v per cell their capacity and ability to produce a
high current is severely affected. How easy is it to forget
to turn the supply off! Of course the other
disadvantage is they become extremely unstable if their
internal temperature rises above 150C and may explode or
certainly burn very vigorously as the anode material
decomposes releasing oxygen which causes the Lithium to
burn. As the outside case is a thick plastic material
this fire may spread to surrounding material such as house,
cars, sheds etc. BEWARE!! If you are going to use
these cells you need to develop a very rigid and careful
regime of charging and storing these cells. This
condition can be produced by severe overcharging, shorting
the cell so discharging it with a very high current or
mechanical damage to the cell. They are
the best tho!! The image compares the size of two
common brands of AA size NimH and a LiPo cell. A 6
cell pack of AA weighs about 180g whereas the LiPo on the
left weighs about70g
A newer version of this cell
called A123 is now available that has the cell encased in a
metal jacket and uses a less unstable anode material. They are quite stable and are
appearing in some portable hand tools. Their
manufacture claim long life and very fast charge rates (15
mins!!) as well as high discharge rates. They are
heavier, more expensive and require a slightly different
charger to LiPo as their voltage is slightly lower at 3.3v
nominal. These amazing properties are the result of
using nanotubes to suspend the reactant materials giving an
extremely high surface area for the materials to react.
Are there other cells around the corner?
This area is still changing rapidly and there will be even
more advances made in the not too distant future
What are the requirements of the cells in the yacht
Assuming a smart winch from our South
Australian manufacturer Rob Guyatt as the standard sail
winch, a supply voltage of 6-9 volts is needed which is
capable of supplying the stall current of the winch ~ 6 amps
or more. The radio receiver and steering servos only
need about 5/6 volts and probably less than 1 amp. The
smart winch takes care of the receiver and steering servo
with a voltage regulator built into its electronics so the
requirements simplify down to a battery pack of 6-9 volts
capable of supplying about 7 amps or so. This current
rating is not called upon all the time and may well average
out much lower but these peaks are needed at the time of
maximum pull on the sheet winch. In addition the pack
has to supply the amount of electricity for a reasonable
time. Typically an hour's sailing will use from
150 to 200 mAh of electricity i.e. an average current draw of 150 to
200 milliamps of current. A day's sailing of 5 hours
will consume 750 to 1000 mAh.
What cells can produce these outputs?
Really there are only a few feasible solutions
-
A 6 cell AA NiMh pack. These
have more than enough capacity usually rated at 2000mAh
or more. Each cell is 1.2V nominally so 6 cells
will produce 7.2V. What about 5 cells? While
producing 6V there is not much overhead for the
regulator in the smart winch to produce the 5V supply
for the receiver and steering servos. The
regulator drops about 1V anyway at the best of times.
So 5 cells is marginal. What about current?
The AA NiMh cell will just supply the 7 amps tho with a
substantial drop in voltage probably supplying not much
over 1V per cell at this current draw. So where is
that 5 cell pack now? With only 5V available by
this pack and dropping another 1V in the regulator
leaving 4 Volts to run the receiver and servo. This is
just on the point of dropping out the receiver on a 36
MHz set. Ever had the sail winch suddenly let go
under heavy use - probably lost radio contact as the
voltage dropped too far especially when the cells have
been used for a couple of hours.
-
A 6 cell AAA pack is
much lighter and might seem a good solution but they
will struggle to supply the current. In light
winds and smaller sails on say a IOM it might just be
feasible but otherwise its very marginal. As their
capacity is about 900 mAh they will struggle to last a full day and will
suffer as their output reduces during the last half of
their discharge cycle. They would need changing
every couple of hours to be safe.
-
A 2 cell LiPo pack. This is
really quite the best option. Cells can be
purchased with the required capacity say 1200 mAh or
1500 mAh. There is nominally 7.4v (3,7V per cell)
and the ability to supply more than the needed current.
A typical 1200 mAh pack could probably supply 15 amps or
more if needed with very little voltage drop. The
sail winch will move that much more quickly and with far
greater authority than with the NiMh pack. It
probably weighs less than half the AA option.
-
A 2 cell pack of A123
cells of 1100 mAh capacity will readily supply both the
voltage and current for a good day's sailing.
-
Lead acid gel cell -
yuk! I believe all lead should be in the keel bulb
and in a racing yacht have no place in the hull.
However they do have an appropriate voltage and can
source the required current BUT they weigh a ton!
Battery Care
Basically this means
keeping them dry and keeping them charged. Care should
also be taken in the choice of connectors and wire used in
the yacht and inspecting them regularly for wear.
There really is no excuse for water in a yacht! Every
effort should be made to track down water ingress and fix
it. One of my pet hates is seeing the rudder shaft set
below deck. At speed the tube and shaft will allow
water to get in readily. It is far easier to seal the
exit for the rudder push rod through the deck (silicon
grease works a treat) and have the rudder tube sealed to the
outside of the hull.. If any moisture is present on
the electrical circuits of the boats and a battery is
connected then there will be corrosion caused by
electrolysis. This is most serious on the positive
side of the circuit causing oxidation as the battery draws
electrons out of any available material. Quite
commonly this is simply the copper wire connected to the
positive side of the battery. The product of the
oxidation is copper oxide - a black non conducting, brittle
material. As moisture will wick along the strands of
copper wire, the corrosion will progress unseen all the way
from the battery to the devices it supplies. This is often
called the 'black wire' syndrome and recovery is well nigh
impossible. The copper material of the wire has been
reduced in diameter so it can no longer carry its designed
current and the wire becomes brittle and easily breaks.
Attempting to resolder the wire is almost a waste of time as
the black oxide coating rejects the solder. In the
worse scenario, this corrosion proceeds to the connecting
wires joining the battery pack and to the cells themselves.
Discard them and rewire your yacht after fixing all leaks!!
Plugs that connect battery packs to the wiring are also a
source of electrical problems. Again keep them dry and
coat them with silicon grease or similar. At least
here the action of unplugging and plugging the connectors
mechanically cleans the contacts each time they are used.
Clearly yachts sailing in sea or salty water are at even
greater risk than sailing in fresh water as corrosion rates
will sour as the electrolytic corrosion proceeds at a much
faster rate thanks to the conducting ability of the sea
water. What if you accidentally do get your system
wet? First and foremost do not continue sailing
if you wish to recover your equipment. Disconnect the
battery immediately as every second while it remains
connected causes rapid corrosion. As soon as possible
rinse the devices with distilled or demineralised water or
even tank water then rinse a few times with small amounts of
an alcohol such as methylated spirits or propanol.
These mix with the water left on your devices and remove it
then evaporate much more readily than the water that was
there. Dry with a hair drier and leave in a warm dry
environment for a day or so before trying your device.
Good luck! Be careful drying a LiPo battery with a
hair drier - use a low setting and keep the temperature down
to just warm. LiPo cells have an aluminium strip
exiting the cell to which a wire is then soldered. The
aluminium will corrode very readily so extra care should be
taken with them.
Charging Batteries
I am amazed at the
charging regimes some modellers use to care for their
batteries. The most common fault in charging batteries
is to overcharge them. With lead acid batteries this
will make them gas and loose electrolyte, with NiCad and
NiMh there is excess capacity built into the anode material
so that gassing at the cathode (hydrogen) can migrate to the
anode and be consumed by converting it back to water.
This works fairly well by there will always be some loss of
gas and moisture from the cell. The result is less
capacity and less ability to supply the required current.
LiPo and similar cells will self destruct if they are
overcharged and the LiPo almost explosively so use a charger
especially designed for LiPo and monitor the charging at all
times! DO NOT leave LiPo batteries unattended while
charging and DO double check the settings on the charger
before charging them.
We spend considerable
money on our hobby (passion?) and purchasing one of the
automatic chargers available from model shops is money well
spent - I would be saying essential. They have means
of automatically detecting when a battery is charged and
turning off the charger. This not only ensures the
battery is fully charged but also that it doesn't get
overcharged. With LiPo cells such a purchase is
not even an option - it is essential! As well the chargers have a
display which show the amount of electricity in mAh which
goes into the battery. It's very easy to see how much
electricity your system uses each day. This gives
immediate warning of a problem if any variation from the
norm occurs. Remember also that manufacturers of
radios etc often supply a charger. This is most often
the very simplest of chargers with no ability to control the
amount of charge supplied to the battery. You are
required to guess the time needed (or simply overcharge them
by leaving the charger connected for far too long).
Use them at your peril!
While NiMh will stand
being discharged completely and in fact may benefit from an
occasional deep discharge, lead acid or gel cells and LiPo
cells will suffer badly by being discharged to too low a
voltage (less than 2v per cell for LiPo) resulting in a
lowering of capacity and a reduction of the current it can
supply. Use a capacity that ensures the cell only
half discharges during a day's sailing and take care it
is not left turned on to go flat.
Please feel free to e-mail
me with your ideas, discussion points or suggestions
Ben Morris
STIRLING MARBLEHEAD YACHT RACING CLUB
Inc.
