This is the power source and is essentially a store of electrical energy. It has two poles, or terminals, which are termed negative and positive. The battery has chemicals inside it which react to produce little particles of electrical charge called ‘free electrons’ and shove them all up at the negative pole. This is insulated internally from the positive pole, which has a deficiency of electrons and whose sole function in life is to grab them from the other end of the battery. If a metallic conductor is connected between the two poles then the electrons will flow along it from negative to positive. The conductor is usually in the form of a thin metallic rod called a wire, or a bundle of very thin rods called a cable. The flow of electrons is called a current**.
Strictly speaking, a battery consists only of one cell, which has just one pair of positive and negative poles. For most uses this is impractical as the voltage of a single cell is only between 1.2v and 3.7v, depending on the type of battery, so most model batteries are actually packs of cells which are either welded/soldered together in a chain or manufactured together inside the same case. A pack will therefore contain several individual cells, but only one overall pair of positive and negative terminals, i.e. the positive of the first cell in the chain and the negative of the last one. For the purposes of this article I will use the terms ‘battery’ and ‘pack’ as if they mean the same thing.
Any battery is identified by two very important properties. Its voltage puts a value on the electrical 'pressure' it exerts between the negative and positive poles, i.e. the higher the voltage then the more electrical pressure it can exert on a load. Increasing the voltage will, for example, makes a bulb glow brighter or a motor turn faster. The battery also has a capacity, which is a measure of how much electrical energy it holds and can supply before it becomes discharged, or ‘flat’. The rate of flow of electrical current along the conductor is measured in Amps; the more amps which a load draws from the battery then the quicker it will discharge. The value of a battery’s capacity is the arithmetical product of the current and the time for which it can be supplied and is quoted in Amp-Hours (AH), or Milliamp-Hours (mAH) which are 1000 times smaller. A battery which can supply one amp for one hour has a capacity of one amp-hour. To give a figure more typical of a model boat application, a 7.5AH battery will supply a current of 2.5 Amps for 3 hours (2.5 x 3 = 7.5). Note that this does not depend upon the voltage of the battery which, as I hope you have learned, is a different thing altogether, Figure 2
There is an arithmetical relationship between voltage and current in a circuit and it’s called Ohm’s Law and states that the voltage across a load divided by the current flowing through it is called its resistance, or R=V/I, where I is the usual symbol for current. For most practical purposes you won’t really need Ohm’s Law but it’s useful to be aware that it exists, if only to know what 'resistance' means. For model use there are two types of battery:
Dry batteries, which are not rechargeable and useful really only for low-current applications such as the transmitter
Rechargeable batteries, which we tend to use for everything else. Rechargeable batteries come in various different guises and to describe each fully would take a lot of space and probably bore you rigid. The main types used are Sealed Lead-Acid (SLA), Nickel-Metal Hydride (NiMH) and Lithium Polymer (LiPo). Their names refer to the chemicals inside them (electrolytes) which react to generate the electrical energy, and each type has its use in model boats. Nickel-Cadmium types (NiCad's or NiCd's) were popular at one time but have been removed from the market because of the adverse effect on the environment of discarded heavy-metals such as cadmium.
are heavy, the ‘L’ stands for lead after all, and cannot generate very high currents, so are used where slower motors are appropriate, and where they can contribute usefully to the total weight of ballast required to get the model down to its scale waterline, e.g. in tugs. The usual sizes for SLA batteries are 6v and 12v, both in many different capacities, although there are also some 2v cells around these days, Photo 1
Nickel Metal-Hydride (NiMH) batteries are generally made up in packs of joined-up cells, each cell having a nominal voltage of 1.2 volts and the cells coming in different case sizes. The smaller case sizes have the smallest capacity; typically around 850mAH for an AAA pack, while the largest cells, ‘F’ size, go up to 10000 mAH (or 10AH). Packs of these cells are much lighter than their equivalent capacity SLA types and thus suitable for faster models where low weight is more important. NiMH cells can supply a lot more current than SLA batteries and can also be fast-charged at the lakeside from a portable charger. The term 'nominal voltage' is used to indicate the voltage at which the pack will discharge for the majority of its discharge cycle. When fully charged, a NiMH cell can have as much as 1.55 volts across the poles. This will quickly settle down to about 1.2v and stay at that level until it is almost fully discharged. At that time the voltage starts to drop quickly and should not be allowed to go below 1v per cell or damage will occur.
Finally, Lithium Polymer (LiPo) batteries
are the newest kids on the block and are lighter and more powerful even than NiMH cells. Their nominal voltage is 3.7v per cell, BUT they do have to be carefully handled and monitored. Careless handling or overcharging/discharging can in the extreme case, cause them to catch fire and even destroy your model. For this reason it’s always advisable to use a speed controller which will monitor the battery voltage and cut off the power to the motor before it reaches a critically low value, e.g. the Mtroniks Tio range. You can also obtain a stand-alone device to do this, or a simple monitor which just sounds an alarm when the value is reached. That said, LiPo batteries are in use all over the world and instances of such accidents are becoming rare. As regards charging, if you purchase the correct type of balancing charger and follow the instructions, then you will have no problems. LiPo packs come in multiples of 3.7v and are quoted in the form ‘XS’ where X is the number of cells. Thus a ‘3S’ pack is 3 x 3.7 = 11.1 volts. The capacity is quoted in the usual way i.e. mAH, and the maximum current which can safely be delivered is given in the form ‘YC’, where Y is the value of the capacity (confusingly in Amp-Hours). As an example, a 1700 mAH LiPo pack has a corresponding ‘C’ value of 1.7 and so a pack rated at 20C can supply a maximum of 20 x 1.7 Amps = 34 Amps, Photo 2
The only other thing which I should emphasise about any type of rechargeable battery is to use the correct type of charger and always follow the charging instructions.
**Please note that unfortunately for some reason the 'conventional current' flow in circuit diagrams is always labelled as being from positive to negative, but it’s not important as long as you remember to connect the terminal or wire of any load which is marked with a + sign to the positive terminal of the battery etc.