Reanimating a Rechargeable Brio Toy Engine

A friend whose wife is a kindergarten teacher brought me a Brio steam engine that was donated to her. Although it is equipped with a tiny motor, headlights and a rechargeable battery, it didn't move at all. He asked if I could have 'a look at it'. I love toys, so I promised him I would.

Without any accompanying paperwork or other information, becoming acquainted with and identifying this product needed some web browsing. I found out that there are three, slightly different, versions of this loco that look almost identical. All of them are powered by a rechargeable, AA-size NiMH battery, rather than by an LR6 dry cell like the still earlier versions such as the one called 'Speedy'.

I'm sure that what I had here is the first version (order no. 33247) with the green wheels. There was a later version (33596) with red wheels that featured (I think) a red charge indicator LED, but wasn't much different otherwise. Both versions seem to have been sold with a matching charger. The most current version (33599) has red wheels, too, and is sold without a charger but has a small USB charging socket and comes with a matching USB cable. If I understand correctly, there are, BTW, also some green-coloured diesel engines available that are equipped with technically identical battery packs.

Unfortunately, this loco was donated without its matching charger. When asking Brio for information on it, or if there even might be one available as a spare part – you guessed it – they didn't even bother answering, which I had, in fact, more or less expected. So I decided to try it on my own. To do so I had to find out what was needed, and then dive into my junk box.

The label at the battery pack's bottom already gives quite a bit of information: It says 'Ni-MH Battery' and '1.2 V' (which means it contains only one single, rechargeable cell) with a capacity of 2000 mAh (meaning it can deliver, e.g., 2 A = 2000 mA during one hour, or 0.2 A = 200 mA during 10 hours). There also is the diagram of the socket for the charger's barrel plug, indicating that the center of the plug is positive; the charging input voltage is said to be 1.5 V (which is to be taken with a pinch of salt; we'll deal with that).

The charging current of a garden-variety, standard NiMH battery should be about 1/10th of its capacity specification, in mA, which in our case is about 200 mA. So, the battery needs to be charged with this current during approx. 10 hours for a full charge – in theory. Due to internal losses the charging time needs to be rather 12 to 14 h. On the other hand, (over-)charging with this moderate current during, say, a total of 24 h will not be harmful for the battery.

While surfing the www, I even managed to find a picture of the complete system (3rd picture above) consisting of (top to bottom) the charger with the to my understanding rather funny British mains plug, the loco, and the battery pack. When zooming in on the charger's type label my estimate from above was confirmed, the charger is said to deliver 1.5 V at 200 mA.

I doubt that the charger delivers exactly 1.5 V at 200 mA, but rather contains a transformer with a somewhat higher output voltage, a rectifier, and a series resistor used for limiting the current to the required 200 mA. The 1.2 V indication is the battery's its nominal voltage; a fully charged NiMH battery has a voltage of some 1.5 V anyway, and about 1 V when fully discharged.

So, which of my retired chargers to choose? I decided that the easiest way was using a smart phone charger that had outlived its smart phone for several years. These chargers connect to the smart phone with a small (mini or micro) USB plug and can deliver a stable, regulated voltage of more or less exactly 5 V DC, at a current high enough by far for the battery at hand.

Identifying the required series resistor is no rocket science - just resort to the ubiquitous Ohm's law, saying that the voltage U across the resistor (in Volts/V) equals its resistance R (in Ohms/Ω) multiplied by the current I through it (in Ampère/A), or U = R × I. This simply means that the voltage is proportional to both the resistance and the current.

The voltage U across the resistor equals the difference between the voltage of the fully discharged battery and the output voltage of the charger. This will be

U = 5 V – 1 V = 4 V,

and we want a current I of 0.2 A ( = 200 mA), so the resistor R will be

R = U / I = 4 V / 0.2 A = 20 Ω.

During charging the battery voltage will rise to a final voltage of about 1.5 V, somewhat reducing the charging current to about 175 mA. So it is safe to select the next-lower value from the E12 series, which is 18 Ω.

The power P (in Watts) dissipated by the resistor equals the voltage U across it, multiplied by the current I through it, i.e.

P = 4 V × 0.2 A = 0.8 W.

However I recommend a rather higher wattage of your series resistor in order to keep its temperature low; you can select 2 W, or even 5 W.

I dug in my junk box and didn't find an 18 Ω resistor, but two recycled 10 Ω/5 W resistors instead. Connecting them in series will result in 20 Ω/10 W, a slightly lower charge current and a slightly longer charge duration - which we can live with. The rather simple circuit diagrams of the two versions, with two and one resistor(s), respectively, are shown in the 4th picture.

I installed my resistors according to the upper circuit diagram on a narrow piece of matrix proto board, as shown in the 5th picture.

I also found a barrel plug with 2.5 mm dia., matching the socket in the 'chimney' of the battery pack.

I cut the USB cable from the charger and connected it to one end of my 'PCB', the cable with the barrel plug to the PCB's other end. Pictures no. 6 and 7 show how this hardware looks in the real world. You need, of course, to strictly observe the polarity throughout the whole system – i.e. the positive wire from the charger (normally red or white) via the resistor connects to the hollow center contact of the barrel plug, the negative wire from the charger (usually black) via the second resistor, if used, to the outer contact of the barrel plug. If in doubt, use your multimeter to check for correct polarity.

The cables to and from the 'PCB' need to be sturdily fixed since kids – if allowed at all – won't be too careful with the system, and the resistor(s) on the PCB and the PCB itself need to be somewhat insulated. Not because there are dangerous voltages but for avoiding damage due to a possible short circuit. I used a piece of heat shrinking tube over the PCB and filled both ends with a generous amount of hot glue as a kind of strain relief for the cables. I glued this assembly to the top of the charger with some hot glue, too. I forgot to take a picture of the PCB before applying the heat shrinking tube and the hot glue, so I removed the heat shrinking tube but not the (cooled/hardened) hot glue portions on both sides for taking the 6th and 7th pictures. The completely insulated and assembled system is shown in the 8th picture above.

Now I was ready to charge the loco for later testing. Since the battery was fully discharged I gave it a 24 hours charge, checking the temperature of the complete assembly from time to time. It fortunately never got more than lukewarm which I considered a success.

If the loco runs ok after charging the battery, be glad. If not, there is one more step...

If you thought now that the engine with the freshly charged battery worked perfectly, think again...

There are two sets of contacts that can deteriorate over time due to dust, pollution, corrosion, and what not. Corrosion on a contact will lead to increased resistance and reduced motor voltage – which is totally undesirable, specially at these low voltages.

The first of these contacts is located below the front of the engine. It interrupts the motor current when the red button shown in the 1st picture is pressed from below (e.g. by a special 'stop' track piece). Corrosion on this contact – the circular, bowl-shaped part connecting the blue and red wires – made the engine stop prematurely. I measured a resistance fluctuating between 0.5 Ω and some 200 Ω. After carefully treating these contact points with rather fine sand paper the resistance remained at a constant 0.5 Ω. The 1st picture, btw, also shows the engine's drive train with the cover removed. Pictures 2 and 3 show the mentioned, bowl-shaped contact.

The second set of contacts is located under the roof of the driver's cab. There is a red plastic slider with some spring contacts attached. These slide on a small piece of PCB with tin (or nickel) plated contact tracks. The 4th picture shows these tracks on the PCB and the red contact slider (below). The upper red part is the latch holding the battery pack in place once inserted. Dismantling the driver's cab is not really easy, but using some patience, a small Phillips screwdriver, and some words unfit for reproduction in print it can be done. Relax, reassembly will be worse. Clean all the contacts and PCB tracks using some methylated spirit/rubbing alcohol or benzine on a paper towel. Don't use any abrasive tools on the PCB tracks in order not to harm the plating. You may want to carefully bend the slider contact springs upwards for increased contact pressure. After having reassembled the driver's cab there was, in my case at least, no real improvement, and I had to move the slide switch to and fro several (i.e. many) times for some additional 'contact cleaning by friction'.

After this procedure, the loco ran much better indeed. Or should I rather say, it creeped along much better? See for yourself in the 'Running Loco.mp4' video how it, uhm, ran on my desk.

Of course this is quite a bit of work for a rather cheaply made and unreliable toy – sorry, Brio, for not mincing my words. And you'll never be finished – be aware of the fact that you will repeat this process very soon, as the contacts will deteriorate again.