At this point I will have to explain those confusing terms parallel and series. There are two very simple rules here.
Loads connected 'in parallel' go alongside each other in side-by-side (parallel) cables which are then connected at each end, like two sidings along a single railway track. The voltage across each load will equal the total voltage across the circuit, Figure 6
Loads connected 'in series' are connected one after the other along the same cable i.e. like a series of programmes on TV. The voltages across each load will add up to the total voltage across the circuit, Figure 7
As I said earlier, a load can be anything which consumes electrical energy but typically will be a motor of some sort, or lights or a sound effect unit. You can fit such loads in series into a circuit where their total operating voltage equals that of the whole circuit. As an example; two 3 volt bulbs wired in series will require a total of 6 volts across the pair to operate them. This is a good way to combine bulbs of a different voltage, e.g. two 3 volt bulbs and a 6 volt bulb all in series adds up nicely to 12 volts. If you wire two 3v bulbs in parallel however, they will require only a 3v battery across the circuit as each will receive 3 volts. You can combine loads in circuits which involve both series and parallel wiring if you like. Please see the illustration of a typical lighting circuit in a modern tug model, Figure 8
. Here the supply voltage is 12v and it’s supplying both a series circuit comprising of 2 x 3v bulbs and a parallel circuit of 2 x 6v bulbs. Note that if one of the bulbs in a series circuit fails then the whole circuit becomes dead, whereas a failed bulb in a parallel circuit has no effect on the other bulbs. You’ve quite likely already discovered this when hanging Christmas tree lights?
The terms 'series' and 'parallel' are also applied to connecting batteries, but beware! While you can connect two batteries with different voltages and capacities in series, you must NEVER connect two batteries of a different voltage in parallel or one will discharge into the other, with potentially serious damage to both batteries and possibly the model. As before, the total voltage of a series pair will be the total of the two battery voltages, Figure 9
, while the voltage of a parallel pair will be the same as the two individual batteries, Figure 10
. The capacity of a parallel pair will be twice that of the individual batteries. In practice you should always make sure that the two batteries in the pair are the same type (e.g. SLA or NiMH) and the same capacity (e.g. 7AH), irrespective of whether you are wiring them in series or parallel.
Ideally you should disconnect pairs of batteries from each other when charging and charge them separately as there’s always the risk that one will charge at a different rate and be the weaker of the two if you charge them together as a pair. If you can’t, or don’t want to remove them from the model, then you can use a double-pole changeover switch to connect them to charging sockets in the model. The connections for series and parallel connections are shown in the diagrams Figure 11
& Figure 12
respectively. Note that the switch in the parallel set-up acts also as the On/Off switch for the whole circuit, while in the series circuit you need a separate On/Off switch (in the positive wire) to do that job.