Lead-acid

Nickel-metal Hydride (NiMH) types

These days I use AA size 2400 mAh Nickel-metal Hydride (NiMH) cells - any number I need for the required voltage, from 4 to 10 cells. Cells are soldered in series to give the required voltage - that means soldering the positive terminal of one cell to the negative of the next cell.
I have never needed more than 10 cells (12V) for realistic speed.
No load cell voltage is around 1.35V fully charged and 1.20V when about 'empty'. Voltages on load are less than this - there seems to be about 0.1 ohms internal resistance, so taking 1A load will reduce voltage from no load by about 0.1V.
These are very economical - less than $2/cell. I just solder the normal nipple types, being as fast as possible with the heat on the positive end so as not to damage the cell venting system. This is a picture of 4 cells connected up in series. I use masking tape to hold them together.
The problem with NiMH cells is that they self discharge within a few weeks. Although you can buy Long shelf types (LS).

Lithium types

They can be dangerous so a few precautions are needed. (But then so are lead acid!).

If the package is punctured and the Lithium exposed to air, it catches fire.
They MUST be charged using a special LiPo charger to avoid overcharging.
If they are overcharged beyond 4.2V/cell, they WILL catch fire, so the charger must limit the charge to this.
If they are discharged below 2.7V/cell they are ruined, so whatever circuit you are using must have an undervoltage shutoff.

Batteries are always placed over the drive wheels for best adhesion.

As overload protection, I use loudspeaker 'Polyswitches' as circuit breakers for the batteries. These are small solid state disks (20mm dia disks like a coin) and come in different amp settings from about 1 to 5 amps. Mount them right at the battery terminal to protect against short circuits.

NiMH Charging

I NEVER use fast charging but always charge overnight at the 10 hour rate (C/10). Every few months I do a very slow charge at C/100 for a couple of days to equalise all cells.
All the literature says you should use constant-current charging for NiMH, but that needs timers or fancy voltage monitoring. So for the last 5 years I've been using constant voltage charging, as used for lead-acid types. I set the charger at 1.40V/cell and connect it to battery. If the battery is flat, it will charge at around C/3 for a short time but the current will reduce to almost zero over time as it takes charge.
I've built a triple charger with long leads around the ceiling of the train room, so they can drop down to wherever the loco is for charging. It has 3 outputs from a single transformer:
11.4V approx for 8 NiMH or NiCd cells
14.0V approx for 10 cells or 12V lead-acid
a variable voltage 5-18V output for other voltages.
Here's the circuit:

I gave up using charging sockets and now just put 2 metal pins/nails/lugs or whatever, whereever they fit, and use alligator clips on the charging leads.

My locos rarely leave the track.

Controlling the speed

There are a number of ways to control the speed of battery powered trains.

For Manual control, the simplest way is to just choose the number of cells for the speed you need and use that number. This gives one speed only. I have found this a very useful system. Just a forward/off/reverse switch. (called a DPDT centre-off).
Here's the wiring you need.

Some people are tempted to use more cells and use a switch to select a different number of cells for different speeds. NEVER, NEVER do this !!! Each cell will have a different amount of charge and when you charge the whole battery, some cells will be overcharged and destroyed.

Diodes

You can use diodes in a track powered loco to increase the voltage where a loco starts to move, or to 'match' the speeds of different locos. (switch is optional). Note that you have to use two 'arms' of diodes conducting in opposite directions as shown in the circuit.

Pulse Width Modulation or PWM

It's possible to use simple PWM controllers to manually control the speed. For example the simple Critter control from G Scale Graphics.

Or you get PWM with radio control like this one fromHobbyking. link Be careful when using these motor controllers intended for model planes. Most model planes use brushLESS motors and need a different controller. We need BRUSHED type controllers.

PWM got a bad reputation in the old days, with motor burnouts attributed to the PWM in HO and N scales.
Older PWM controllers operating at low frequencies (50-100 HZ) had that effect on some motors. These low frequencies also caused an audible hum in the motor, especially at low speeds. Newer PWM systems run 1000s of Hz. No hum, and no ill effects. It’s true that PWM will always cause more heating in a motor than pure DC. The parameters that determine how much more heating are:
· the frequency of the PWM,
· the input voltage to the PWM controller,
· the duty cycle. ie the motor volts you are running at divided by the supply volts.
· the inductance of the motor. (Inductance is just a sort of measure of how strong the motor’s magnetic field is.)

When running on DC the motor heating is the same for low or high speeds (volts). But with PWM, the heating gets worse for lower speeds. So generally speaking, if you have a low frequency PWM, high input voltage, low inductance and are running at low speeds, you can expect problems.
Track power PWM will be worse than battery locos using PWM because with track power the input voltage will be higher and you will spend more time running at higher lower duty cycle. So if you’ve got a high voltage track power (say over 25V) and you run at slow speeds (say 6V on the motor) for long periods, you could have motor burnout problems. But battery power PWM will never be a problem.
I’ve been building PWM controllers for 40 years (N, HO and G) and haven’t had any failures yet. Our G size motors have enough inductance coupled with the modern higher frequency PWM controllers to reduce the extra heating to manageable levels.

Motor Power usage and Battery run time

There are many claims that using resistors or diodes to control speed, will REDUCE the run time you get from batteries. In actual fact, reducing the voltage using diodes or resistors, will NOT reduce your running time from a single charge very much at all.
BUT, using a PWM system will INCREASE your run time if you run at a lower voltage, compared to what you get at full volts.
It’s not that you LOSE runtime by using a resistor/diodes but you GAIN runtime by using a PWM controller and running slower.

WHY IS THIS SO???

Well a train motor is pretty much a ‘constant current’ type of load. If you take a given train on a given bit of track, it will take ABOUT the same current irrespective of voltage. Motors don’t act like resistors where doubling the voltage will double the current. The current drawn is a function of the load – train drag, gradient and loco efficiency. The voltage just sets the speed. That doesn’t mean the current never varies – it will if you make your train heavier or send it up a steep grade. If you don’t believe me do your own tests. The current may increase a little with voltage, but not much!

So lets imagine a given train taking 1A on a given piece of track powered by a 2Ah, 10V battery. It will run for 2 hours (2Ah/1A) and the battery will have supplied 20 watt-hours of energy to the train.
Now lets put a 2 ohm resistor or 3 diodes in series with the motor. It will still take 1A (maybe a tad less). So we’ll have 2V across the resistor and 8V across the motor. It will go slower (around 80% of original speed), BUT the battery is still supplying 1A and the train will still run for 2 hours. Sure energy is wasted in the resistor – 4 watt-hours and the motor only gets 16 watt-hours. The energy used by the train is less because it doesn’t travel as far, but it still runs for 2 hours!

Now lets use a PWM controller to reduce the battery voltage to 8V. PWM (pulse width modulation - means that the supply to the motor is rapidly turned on and off, maybe 1000 times a second or more. The ratio of on to off time determines the average voltage that the motor receives.)
In this case the motor is turned on for 80% of the time and off for 20%. During the off time the momentum of the train keeps it moving and we don’t notice it’s going on-off-on-off……
So while the motor is on it takes 1A again, but the average current taken from the battery is only 0.8A, and the average voltage on the motor is 8V. It goes slower. BUT now we can run our train for 2.5 hours on a battery charge! 2Ah/0.8A=2.5 hours. It will cover the same distance as a full voltage train.

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