Batteries

(For ride on electrically powered locomotives)

What is a battery?
"Battery" in electrical terms, is an electro-chemical device that converts chemical energy into electricity, by a galvanic cell. A galvanic cell is a fairly simple device consisting of two electrodes of different metals or metal compounds (an anode and a cathode), and an electrolyte (usually acid, but some are alkaline) solution. Each cell produces approximately 2 volts.

Battery vs Cell - While the term "battery" is often used, the basic electrochemical element being referred to is the cell. A battery consists of two or more cells electrically connected in series to form a battery unit. In common usage, the terms 'battery' and 'cell' are often used interchangeably, but a cell is just that - a cell, and a battery is 1 or more cells assembled into a complete unit.

A battery is a storage device - it does not make electricity, and the energy is stored in the chemical bath inside the cell. As the cell is charged the chemicals change and store energy, and release this energy as electrical current when a load is connected.

As each cell is capable of producing only around 2V, and to get a higher voltage, a number of cells must be connected in series. This may be done either internally within the battery (6V and 12V batteries), or externally with multiple batteries.

Every battery has some internal resistance, and as current flows through the battery, there is heat generated in this resistance which is why batteries get warm when being charged or discharged. The lower the internal resistance the better. Note that the internal resistance will increase as batteries are progressively discharged, as well as the cell voltage reducing.

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 usually 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 where the electrolyte is in a gel form, but may be a bit more expensive (for equivalent size/capacity), and are preferred due to their non-spill and maintenance-free properties. Both types may be used for model traction use.

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 are connected in series inside the battery. This gives the commonly seen 6V and 12V batteries. Multiple batteries can be then be connected in series to produce higher voltages.

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. Anything over 48V is regarded as 'High Voltage' by the electrical authorities, and any wiring is subject to and required to conform to the releveant national standards. It can still be used or course, but must comply with wiring and safety regulations.

Capacity
Amount of energy a battery can store. This expressed in ampere-hours (AH), 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, etc and all at the rated battery voltage. Note too that the actual capacity of a battery will vary depending on the current drain, usually a higher load = less capacity for any given battery.

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?
Batteries, although nominally all very similar, are usually designed with a particular application in mind. The common terminalogy is 'Starting, 'Deep Cycle' and 'Marine'.

Starting batteries are commonly used to start and run engines. Engine starters need very high starting currents for a very short time. Starting batteries have a large number of thin plates for maximum surface area. The plates are composed of a lead 'sponge', similar in appearance to a very fine foam sponge. This gives a very large surface area and hence large current capability, but if deep cycled, this sponge is consumed and falls to the bottom of the cells as a grey sludge, eventually shorting out the electrodes.

Deep cycle batteries are designed to be discharged down as much as 80% time after time, and have much thicker plates. The major difference between a true deep cycle battery and others is that the plates are SOLID - not sponge. This gives less surface area, thus less 'instant' power like starting batteries can produce. A deep cycle batery is regarded as 'flat' or discharged when at about 20% of its capacity.

Marine batteries are usually a 'hybrid', and fall between the starting and deep-cycle batteries. In the hybrid, the plates may be composed of lead sponge, but it is coarser and heavier than that used in starting batteries.

As an approximate guide, the typical lifetime expectations for batteries if used in deep cycle service are:
Starting: 3-12 months
Marine: 1-6 years
Golf cart: 2-7 years
Gelled deep cycle: 2-5 years
Industrial deep cycle): 8-20+ years
NiFe (alkaline): 5-35 years
NiCad: 1-20 years

Car batteries are classified at 'starting' batteries and while relatively cheap 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. Car batteries will generally fail after 30-150 deep cycles if deep cycled, while they may last for thousands of cycles in normal starting use (2-5% discharge) then kept float charged.

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.

Series connection of batteries is often used to achieve a higher total battery voltage. Batteries should have the same nominal capacity in AH. With series connection the battery voltage increases while its Amp-Hours, cranking performance and reserve capacity remain unchanged.

Parallel connection of batteries of like voltages and capacities is used to increase the capacity (AH) of the battery bank. The final voltage remains unchanged while the capacity of the bank is the sum of the capacities of the individual batteries of this connection. Amp-Hours, cranking performance and reserve capacity increases while voltage does not.

While parallel connection is used, it usually not recommended as every battery in the bank is not identical and will have a slightly different voltage and state of discharge. 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 achive the same total voltage/AH rating.

Calculating Capacity
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 motor of 24V 200W rating. (1HP = 746W, so these 2 motors produce 400W or 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 consume 528W total at maximum load and efficiency is therefore around 75% (400/528). 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. And if batteries are discharged to very low levels (say <20% capacity) the lifetime of the battery in charge/recharge cycles is reduced.

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.

[Updated: 02 Sep 2011] 790

Text and photos Copyright © 2006-2012 SMEX, All rights reserved.