Woorabinda Lake - Stirling South Australia

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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

  1. 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.

  2. 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.

  3. 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.

  4. 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.

  5. 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