Batteries

(For ride on electrically powered locomotives)

There are different types of batteries available:

There is no perfect battery (if only!) and all have advantages and disadvantages. So what battery to use. Single-use batteries are prohibitively expensive for locomotive use, and do not generally have sufficient capacity. The rechargeable lead-acid battery is the best choice for locomotive use, but there are still trade-offs.

Liquid electrolyte batteries (like a typical car battery) contain corrosive sulphuric acid, and the potential for spillage. Sealed gel type batteries are available, but may be a bit more expensive (for equivalent size/capacity), and are preferred.

Voltage
A battery is composed of a number of cells. Each cell is capable of producing 2V, and to get a higher voltage, a number of cells must be connected in series. This may be done within the battery (6V and 12V batteries) or externally with multiple batteries.

As the energy storage capacity of a battery increases, so does the physical size/weight, and to achieve the desired voltage and capacity, multiple batteries often must be used. In general, as capacity goes up, the voltage of the battery decreases, and large capacity batteries are only 6V, or even 2V, per battery. Connecting multiple batteries in series is necessary to achieve the desired (say) 24V.

Typical voltages used for model traction are 12V, 24V and 36V, and DC motors are readily available with these ratings.

Capacity
Amount of energy a battery can store. Expressed in ampere-hours, and is the product of amps x hours. Simplistically the battery can supply this many amps for so many hours (Note there is no mention of voltage, only current and time). e.g. a 100AH battery may theoretically provide 20 amps for 5 hours, or 15 amps for 6.67 hours, or 10 amps for 10 hours, and all at the battery voltage.

In practical terms, the current drain will vary widely and often during operation, and it is the average current which is important, and used in calculations. The higher the battery AH rating, the bigger the battery physically, the longer the running time, the heavier the battery, the higher the price, and the more difficulty in housing the battery within the model.

What type of battery?
Gel electrolyte lead-acid batteries are preferred, as there is no risk of spillage or corrosion.

Car batteries are relatively cheap, but are also designed for a different type of service. We typically want a battery we can charge, then use until it is nearly exhausted (flat), then charge it up again for next time. Car batteries are designed for EXTREMELY heavy current for short periods during starting, and then are charged continuously while the engine is running. They are also internally ruggedised to handle the harsh vibrations from the engine, or from rough roads etc. They are NOT designed for being frequently discharged to flat (deep discharge). While they can (and have been) be used for this purpose, the number of charge-discharge cycles is limited requiring more frequent replacement. Also, as they are ruggedised, they have a lower capacity (AH) than an equivalently sized deep discharge battery due to the heavier internal construction.

A battery designed for deep discharge use is strongly recommended. They are readily available and used in electric mobility scooters, golf carts, electric fork lifts and the like, and in small fishing boats which use an electric trolling motor. Talk to your local battery supplier, and see what is available for deep discharge use.

What size or capacity battery? (Or how long will it last?)
The first thing to do is to decide what operating voltage to use (commonly 24V). Determine by experiment, or by basic calculations, the AH rating required to achieve the desired running time. Then contact your battery supplier regarding what capacity and voltage batteries are readily available. Assume that you need a 24V/100AH battery, and you use say 4x 6V/100AH batteries to make up a 24V/100AH battery bank, as 12V/100AH batteries may not be available, or not available in a size that will fit inside your model. You then either use 4 batteries, or decide to use 2 batteries of smaller capacity (say 2x 12V/75AH batteries) with a slightly less running time.

To increase the capacity of a bank of batteries, some people connect batteries of the same nominal voltage in parallel. While this can and has been done, it usually not recommended as every battery in the bank is not identical and will have a slightly different voltage and state of discharge. It also complicates charging, and a more sophisticated charger may be required. In general, it is better to use more batteries of a lower voltage/higher capacity in series rather than using higher voltage batteries in parallel.

To calculate the size of battery needed, you need the estimated (or real) current drain of the motors. For example, assume you have a model which uses two motors, each of 24V 200W rating. (1HP = 746W, so these 2 motors produce about 0.55HP, enough for a model) Typical motors like this are rated at 24V 11A (maximum, or a bit higher when stalled) =264W, and efficiency is therefore around 75%. Assuming the average current consumption is approx 50% of the maximum (including time spent stationary waiting [zero amps]), you will require 2x 11A = 22A x 50% = 11A on average. An 80AH battery should give you approx 7 hrs running time.

If batteries are discharged, it is best to charge them as soon as practical afterwards, and avoid storing batteries in the discharged condition for extended periods. Doing this will reduce the number of charge-discharge cycles available and reduce the life of the battery.

Summary

• Only use 'deep discharge' batteries designed for this type of service. (Not car batteries)
• Gel type batteries preferred (no liquids), but others OK with handling precautions.
• Use the biggest batteries that will fit in your model.
• Preferable not to use parallel connection of batteries.
• Charge the batteries after use, and store in a fully charged condition.

For those of you who have difficulty with the various terms used in electricity, an analogy using water may be useful.

Voltage = pressure. Water can be under pressure and not do anything, e.g. just sitting in a pressurised pipe without any flow. A power source may have a voltage (expressed in volts (V)) across its terminals and no current flowing. It has the potential to do work.

Current = flow. For water, flow is expressed in say litres/second. For electricity, a current flow is measured in amperes (or Amps) (A).

Resistance = ease of flow. For water, small pipe presents a resistance for water to flow through, large pipes less. For electricity, a high resistance (expressed in ohms (Ω)) equals less current flow. A low resistance allows a higher current flow just the same way as a larger water pipe permits a larger water flow.

Capacity = amount stored in the reservoir (or battery). Litres or gallons for water, ampere-hours (AH) for batteries.

To get more water to flow, you must either make the pipe bigger, or use a higher pressure. To make more electricity flow, you must either make the wire bigger (lower resistance) or use a higher voltage. (Or a combination of both).

See the page on electrical calculations for the formulas relating to voltage, current, resistance, power etc.

Text and photos Copyright © 2006 All rights reserved.