BATTERIES / CELLS |
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Once you have chosen your motor, you'll need a source of electricity. You could use a very long extension lead from you car's cigarette lighter but most of us use re-chargeable batteries. |
A battery or cell's positive end or pole (+ve) should be consistently wired RED and the negative pole (-ve) BLACK. Stick to this colour code and you'll have less problems. Use heavy grade stranded wire, as least as thick as the wire on the cell packs to prevent power losses and burn-outs. If two batteries packs are connected in series, then the output voltage is doubled. If they are connected in parallel, then the current is doubled. In the first case the motor spins faster for a short while, in the second the motor spins slower but longer. Never connect different voltage cells in parallel i.e., the two pack MUST be of equal voltage and ALWAYS the same type i.e. NiCads only or Gel-cells only.
Series
Parallel
For a proper
explanation, have a look over at ... |
NICADS are the most common re-chargeable batteries used in radio models. The name Nicads comes from Nickel Cadmium of which the batteries are made. These are very powerful and robust batteries that can explode if abused. To give you an idea of the power of nicads just 20 cells wired correctly will probably start your car! The main power cells are just a little smaller than standard batteries and called sub-C's and the ones used in radios are the same size as AA or pencells. All NiCad individual cells produce 1.2 volts without exception. The power cells are usually made up in to packs of 6 or 7 giving 7.2 and 8.4 volts respectively. The capacity of the cells is measured in Amp-hours (AH) which tells you how much current they can supply and for how long. (1200 mAH = 1.2 AH). In theory a 1.2 AH cell will supply 1.2 Amps for an hour, or 72 Amps for 1 minute, but cells are not 100% efficient and you can't ever actually get this and in the latter case you will certainly destroy the cell.
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MATCHED PACKS |
Battery packs made up of cells that all have exactly the same
electrical characteristics. That is, no single cell is
weaker or stronger than another. The whole
pack will therefore discharges at the same rate and time.
This means you get all the useful available power out of all
the cells as opposed to an unmatched cell pack where a weak cell
will hinder the rest. Matched cells were
mainly used by the racing fraternity but
now, even though still quite expensive they are a better
long term investment as a matched cell packs are less likely to fail
due to a cell failure. |
TRANSMITTER & RECEIVER CELLS |
The usual plastic receiver battery holders supplied with radio outfits tend to rust and split after a while. Nowadays I always solder the four AA size NiCad's together with a charging socket and on/off switch then cover the cells with heat shrink plastic. Get yourself a Transmitter and receiver battery charger and change the receiver charge plug to suit the type of socket you've fitted to your cells. I use DC coax plugs and sockets, the type used on portable radios and tape recorders. From time to time check the physical condition of the cells and Tx battery box because when they start to get old they grow a white "fungus" around the positive end which needs to be cleaned off and polishing.
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LEAD-ACID Batteries |
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LEAD-ACID Batteries are similar in construction to car batteries and are used to power scale boats and sometimes to a power IC engine starters. There are three type of lead-acid batteries, Gel, Wet and Cyclon cells. Wet cells, (similar to a small car battery) aren't used much these days as they have to used upright and give off hydrogen gas which is explosive. The liquids now take the form of a gel and are held in sealed cases to prevent any gas leakage, these are termed Gel-cells and are the most common form of modellers lead-acid battery. Cyclon cells are a sort of 'dry' gel-cell. Gel-cells have much greater capacity than nicads but are much heavier. This makes them ideal for scale models that you want run for long periods and may need a lot of ballast anyway. Gel-cells should not be used in fast boats with high a current drain as even these sealed gel-cells will vent hydrogen if strained too much which will also reduce their life. For this same reason gel-cells must never be charged in an air tight container such as watertight radio boxes and submarine pressure hulls. |
Unfortunately you can't use your NiCad charger on gel-cells, you need another charger. Gel-cells are robust but can be easily damaged by overcharging or discharging too quickly. At first I bought one of those chargers built into a large black mains plug, I was told "connect up the battery and leave it for 16 hours and your battery will be fully charged". BE CAREFUL, THIS IS NOT ENTIRELY CORRECT. I followed this advise only to find that I had overcharged two new gel-cells which totally destroyed them. A destroyed gel-cells makes a sizzling noise which means get you wallet out. I then did a little research into gel-cells to find out what went wrong. It amazed me to find that not one in several model shops could give me any useful information on charging gel-cells. I decided to get the information from other sources. ( I believe that the latest type of these charges are now automatic - but 'once bitten, twice shy'. Ask the shop keeper if the charge has automatic shut-off before parting with any money.) |
Information was forthcoming from two main sources, RS components and Maplin, who both supply automatic gel-cell chargers. RS supplies a detailed data sheet on all sorts of batteries (Sheet no 10574) and Maplin whom now sell a few different types, some in kit form, provide a explanation of how gel-cells should be charged it and how their charger does it. I won't bother you with all the details (as I don't fully understand all of them myself), but the data sheet is very specific that gel-cells must be charged with a carefully controlled constant voltage. Also they MUST be charged to their Amp-Hour (AH) capacity and no more, overcharging results in overheating which produces the afore mentioned hydrogen gas and hissing. A gel-cell initially charges at a high current then steadily drops off as it charges up. The battery is deemed to be fully charged when the current falls to a determined level. The Maplin charger cuts off at 180mA and 45mA on the RS, anywhere between the two should therefore be safe indicator.
Well all of this is just my opinion, but what the hell do I know! |
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SAFETY, PROTECTION & FUSES |
If you want to a safety device can be fitted somewhere in the main power circuit to prevent a burn up or melt down during a overload. Theses can be normal fuses, an electronic fuse and circuit limiters. Most people use car type an in-line fuse holder but circuit breaker can also be installed. Both will mean rescuing a dead boat off the water but they will save most of the electrics. Circuit breakers just have to be reset and away you go, a fuse will have to be replaced. An electronic current limiter will limit the maximum current in a circuit or shut the circuit down then reset automatically. You'll have to investigate these circuits for yourself, but many of the home electronic constructor magazines and books have circuits that can be adapted if you are of that persuasion. I have built quite a number electric boats and none of them now have any type of electronic protection because, a) I don't push the motor and batteries to their limit i.e. a maximum of 14 cells to any motor and I don't overload the motor with a propeller that will give me less than 5 minutes run time. & b) it's pretty silly to hold throttle open when you can see that the boat is struggling or the motor is jammed. If you want to go competition racing then you will have to push these limits but this series of articles are aimed at the fun and club racer. |
Another form of protection is by using proper connections between battery, controller and motor. Use straight runs of heavy grade stranded wire, (I use 25 Amp stranded wire as a minimum), between each component. Soldered the wire where possible and cover with heat shrink tubing. For pluggable connections eg. batteries the normal Tamiya connectors are good enough for sports boats but for higher power boats, gold professional plated plugs and sockets are available from the model car world. Check the metal inserts in the plugs from time to time for corrosion and tightness, they may need cleaning, re-soldering or squeezing back to shape with a needle nose pliers. Only use 'chocolate block' strip connectors (below) if you are experimenting at the lake side. When your happy with the set-up, remove the chocolate block connectors and solder it properly and cover with shrink wrap insulation.
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For more information ( and probably much more accurate ), click over too... Well all of this is just my opinion, but what the hell do I know! |