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AMYA BUILDING TECHNICAL & THEORETICAL ASPECTS OF RC YACHT RACING HOW TO SAIL FAST VICTOR MODEL YACHT CLUBS & CLASS INFO SAILS TOWER HOBBIES WORTH MARINE RULES QUIZ US1M CONSTRUCTION
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How not to build a Soling. Note spars both perpendicular to center line.
Battery Problems?
I often overhear our skippers bemoaning the fact that their batteries have lost their charge after they “just charged them”. After a while it sort of became obvious that a large number of our members really don’t understand what it takes to fully charge a battery pack. So I thought that I would take a shot at trying to explain what seems to work for me with regard to batteries.
Here are some definitions that will enter the discussion that follows.
Key Terminology when dealing with Batteries
Amp Hours (Ah) – Refers to the amperage – the strength of the electrical current expressed in amperes that the battery can hold. The higher the Ah, the longer the battery will last in-between charges.
Capacity – Measured in Amp Hours or Milliamp Hours and is the amount of time the battery can supply the necessary voltage.
Cell– One individual battery canister. Commonly arranged with other cells to form battery packs of different voltage and capacities.
Charge – With the use of a charger, charging a battery will insert energy into it.
Discharge – The process of taking energy out of a battery.
Life Cycle – The amount of times a battery can be charged and discharged before it no longer has any power.
Memory Effect – The Effect that represents the decrease in capacity and voltage in Ni-Cd batteries due to repetitive charging and incomplete discharging. This results in loss of run-time in-between charges.
Milliamp Hours (mAh) – Applies to how much energy the battery can store – the capacity of the battery. The higher the mAh, the longer the run-time in-between charges. One mAh is the equivalent to 1/1000 Amps. IE: 2.7 Ah = 2700 mAh
Self-Discharge – If batteries are fully charged and sit on the shelf for one-two months, they will Self-Discharge. By Self-Discharging the batteries will lose capacity on their own without being used or placed in a charger that discharges the batteries.
The trick is to understand the arithmetic of battery charging.
Typically we use either 4 or 5 cell AA (“Double A”) or AAA (“Triple A”) battery packs. The individual cells in either AA or AAA packs each put out 1.2 volts, which is the battery electromotive force (EMF). This is the force that drives the servomotors. When cells are connected in series the EMF is additive. A pack of four AA or AAA cells, connected in series, will supply 4 x 1.2 volts or 4.8 volts and a pack of 5 AA or AAA cells will supply 5 x 1.2 volts or 6 volts. A 6 volt battery pack will provide more force to the servos than a 4.8 battery pack therefore driving the servos faster.
AA and AAA cells are available in a variety of capacities, from about 700 milliamp hours to about 2000 milliamp hours (mah). The capacity of a battery pack and the load put on the battery pack, by the servos, determines how long it will take for the battery pack to discharge. If the servos draw an average current of 200 milliamps, then a 700 milliamp hour battery pack will be fully drained in approximately 700/200 or 3 1/2 hours. The primary load is the sail servo and the work that it has to do under various wind conditions. The higher the wind speed, the higher the load on the batteries.
The cell EMF does not determine how long it will take the battery pack to discharge. However, all batteries discharge over time without any load on them. Most of us now use rechargeable batteries. I’ve read that Nickel Metal Hydride (Ni-mh) batteries lose about 10% of their charge per day, however they can be repeatedly charged to their full capacity without requiring a full discharge. Nickel Cadmium batteries don’t self discharge as fast as Nickel Metal Hydrides but they have a memory and must be fully discharged in order to regain their full capacity.
Battery packs will last longer if they are not overheated during the charging process. A charging current of 100 milliamps will generally not cause overheating. The charging efficiency of nickel metal hydride batteries is typically 66%, meaning that you must put 1500 milliamp hours into the battery for every 1000 milliamp hours you get out.
A 1000 milliamp hour Ni-mh battery pack will fully recharge in 15 hours at a charging rate of 100 milliamps. The battery charger that is used must be capable of applying a higher EMF to the battery pack than the battery pack EMF or the pack will not fully charge, no matter how long it’s attached to the charger. The minimum voltage you need to get a full charge varies with temperature--At least 1.4 volts per cell at 70 degrees F. If you’re charging a 6.0 volt battery pack, which is 5 cells you need a charge voltage of 5 cells x 1.4 volts per cell or 7 volts being applied by the charger.
Modern cells have an oxygen recycling catalyst which prevents damage to the battery on overcharge, but this recycling cannot keep up if the charge rate is over 10% of the capacity. Even though continued charging at 10% of the cell capacity does not cause venting, it does warm the battery slightly. To preserve battery life the best practice is to use a timer to prevent overcharging to continue past 13 to 15 hours.
In a standby mode you might want to keep a nickel metal hydride battery topped up without damaging the battery. This can be done safely at a current of between 3/100 and 5/100 of the cell capacity, or 30-50 milliamps. The voltage required for this is dependent on ambient temperature as discussed above, so be sure to regulate the current in the charger or continuously check the batteries for excessive heating. An easier way is to get a better charger, select a low charging current and plug the charger into a wall timer. An inexpensive reliable charger that will provide the needed features is the Hobbico R/C Multi-Charger, Hobbico Stock #HCAP0100. It’s available from Tower Hobbies, (800-637-6050) as Tower Stock #LXL331 for $29.99. You’ll need a set of leads, also available at Tower, to connect your battery pack to the charger.
Hobbico Stock #HCAP0100
A review of the specifications for the Futaba FBC 19B4 (Same as Tower TOWM 6101) and the Associated LXFVD4 Chargers, (Small black units that plug directly into wall sockets), reveals that the voltage output provided by these units for receiver battery charging is 4.8V at 100 milliamps for the Futaba unit and 4.8 V at 60 milliamps for the Associated unit. Neither of these will adequately charge the typical 5 cell Ni-Mh battery packs that we use, (unless running out of power while you're racing your boat doesn't bother you).
Overcharging causes heating which damages the cells. Once the cells are damaged, there’s no way to predict what your results will be and you’ll continually be surprised when your newly charged batteries quit with your boat in the center of the lake.
Good Luck!
Bernie Kantor
Howard Polishing the plug.
First layer of glass going on.
Still adding layers.
Complete & Curing
Winner - Most Ugly Boat Contest
Now a swan.
Zipper, A work in progress.
For more pictures showing the building of this hull by John Bluhm, see the January 2005 issue (05-1) of The Dock Line. (Follow link in the upper left hand corner of this page.) As far as I know, John is still looking for help to convert it to a plug so he can pull at least 1 carbon fiber hull off of it. Can you help?