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Batteries

Stored Power.

It is well worth remembering that a battery is stored power and how it is used and recharged may not be uppermost in the mind of the designer or builder. If your battery pack is to be located within the loco make sure that it is easy to remove. Make sure that the wiring rating, (see elsewhere), is ABOVE what the motors will draw. A G3 battery pack and motors are a High Current Low Voltage system and some thought should be given to this fact. The heating effect of current is the SQUARE of the current multiplied by the resistance thus thick heavy cables and low resistance motors with fans etc are a design point. The question normally asked is how big a battery pack do I build? A typical G3 loco running on 12V will draw about 5A when pulling a train. Thus your battery pack needs to have the capacity to supply this for at least an hour, i.e. a rating on 5Ah at 12V.

There are basically only three types of batteries that can be considered for a G3 loco.

  • The Sealed Lead Acid (SLA).
  • The Nickel Metal Hydride (NiMH).
  • The Nickel Cadmium (NiCd).

SLA

Of them my personal preference is for the SLA. These are "bricks" with nice rectangular shapes and have plenty of weight to improve traction. They have a typical lifespan of about three years and can be used in any position. BUT they must be recharged in a vertical position due to the pressure vent being at the top. The lead plates are at the bottom stacked horizontally -thus is has an uneven weight distribution. Most of the 12V and 6V batteries have 1/4" or 4mm spade connections. If you use two 6V batteries to provide a 12V line then put a fuse between the batteries as well as the main fuse feeding the loco.

An SLA will deliver 45% of its Amp Hour rating in the first hour. This means that although your battery pack will be rated at 9Ah the amount of power for continuous running in the first hour is only 4.05Ah. Most people use a 12V line from their SLA battery pack. This is a good choice as most electronics and switching systems are designed for 12V. Although there are several advantages from a current feed rating there are few ESC and RC systems that use 24V lines. This is despite the fact that 24V is the de facto working voltage for Gauge 1 systems.

NiMH

The NiMH is a common cell found normally in the AA size for toys and clocks etc. They can be recharged quite quickly but the number of recharges is limited to about 500 times. The most usable system for our use are the Tamiya "race packs" which are two parallel tubes of AA cells producing around 7.2V and 14.4V. These plug into a Tamiya charger and that is as hard as it gets. If you wish to produce your own battery packs using NiMH cell then I would recommend that you buy "tagged" cells in the "C" or "D" sizes. Connecting the tags is best done with wires as wrestling with a group of cells can be difficult. There is a small vent at the top of the Positive end of the cell. This vents hydrogen in case it needs to. Ensure that there is plenty of air around the battery pack. The batteries will get hot during charging so sitting them in a cool area will help.

NiCd

Although these have had a lot of bad press due to the so called memory effect, a modern charger will back load the cell to the initial position before charging it. These charge extremely easily but the rate of charge is only 10% of the discharge rate thus the charging times are longer. They are best used in tagged cells of the "C" and "D" sizes. They have a good life span but are limited to around 1000 charges.

I have not included anything in LION cells as at the moment I feel these are too dangerous for modellers to build unless you have experience and qualifications.

I do and my advice is -don't.


Assembling your battery pack.

Depending on your personal loco there are two main systems to use. These are "The Linear supply" and "The Parallel supply".

The Linear supply is the one that most people will use in that the cells are connected in one linear chain from start to finish. The main problem with the linear supply is the internal resistance of the cells in the chain. If each cell has (say) an internal resistance of 1 Ohm then 12 in series has a resistance of 12 Ohms. Thus it can quite rapidly produce a situation where adding more cells actually reduces the amount of power your battery pack can deliver. The resistance of the cells limits the current despite the increase in Voltage. This is more noticeable using AA and AAA cells with their small plate size.

The Parallel supply removes the problem of long series resistances but at a higher level of space and complexity. It is based on the simple fact that two resistors of the same value in parallel produce a value that is half of that of each resistor. BUT that is not all the story... When current flows from the battery pack to the motor then the rate of flow is dependent on the output resistance of the supply and that of the motor. The chief advantage of the parallel supply is its advantage to supply "grunt" to the motor. Having a source that is half the output resistance allows it to produce twice the amount of power in a standard draw but 2 x 1.4 ( 2 x root 2) times this if required. The trade off is a faster drain on the battery pack and an increased difficulty in charging. The Tamiya Race Pack is an example of a parallel design.

A typical Linear supply would be a 12V SLA of 6 cells in series. A typical Parallel supply would be two 6V SLAs in parallel with a further two 6V SLAs in parallel coupled to it in series. If we assume that the internal resistance of each cell is 1 Ohm... This gives the Linear supply an internal resistance of 6 Ohms. The Parallel supply is ((3/2) + (3/2)) = 3 Ohms.


Fuse Rating

This is possibly the strangest thing to comprehend. The fuse is designed to blow. When it does so it prevents short circuits and burnt cables resulting in fires. There are several methods and positions for fusing to take note of. In a linear supply of two 6V SLAs it is advisable to fuse between the batteries as well as the main fuse. In a parallel supply the batteries should be fused between themselves as well. The next question is How do I know what rating of fuse to use? If we assume a 12V 5A supply then it would seem sensible to use a 5A fuse to the main buss bar. This will blow instantly the power is connected! Use instead a 10A fuse of the Slow Blow type. These have what seems to be a bulge in the centre of the wire. The wiring should be rated at 10A and the motor wired to take this type of current. Fuses wear "thin" and can blow with age. Carry a spare with you and if that blows then retire the model to the service bay for inspection.


Common Sealed Lead Acid battery sizes and capacities are listed below.

VoltageAhLWHKg
61.39725510.3
64.570481010.9
61015151941.8
61215151941.9
121.39743520.6
122.217834601
123.313467601.3
124.290701001.6
127.215165952.5
121215199954

Sizes of battery.

NiMH and NiCd cells are easily available in the following sizes:

AAA44.510.5
AA50.514.5
C5026.2
D61.534.2

It is recommended that if the loco requires a specific battery pack making up then TAGGED cells are purchased. These are easier to assemble and the heat from the soldering iron will not damage the cells. Fitting tags to untagged cells can be dangerous and requires specific equipment to spot weld them to the cells.

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Page last modified on July 21, 2017, at 08:07 AM