I started with an 18650 battery without any protection electronics, just for testing. It has worked fine so far. My initial idea was to buy an 18650 with a protection circuit or design it on the board myself. After some research, I found that not many toys use Li-ion chemistry, so I decided to explore alternative solutions as well.

What does Brio use? According to this post, at least some of their trains use a 2000mAh 1.2V NiMH battery. That’s not as easy to use in my case, so I measured and found that I could use a 3S NiMH battery as a safer alternative to Li-ion. However, I also discovered that there are not many easy solutions for designing a 3S charger. This TI article suggests that a Li-ion charger could be used for 3S NiMH. If I were to use NiMH, I would buy Eneloop AAA batteries. The price for 3 Eneloop batteries would be between 9-12€.

Instead of trying to find room for 3 AAA batteries, I decided to look for Li-ion batteries locally in a similar price range. I found that a CR123 or 16340 battery would be available, costing around 9€.

The NL169 was locally available and has similar specs to what 3 AAA batteries would have:

Specifications:

• Capacity: 950mAh 3.6V (3.42Wh) (vs. Eneloop 800mAh)
• Max Continuous Discharge: 2A (vs. Eneloop 2.4A)
• Diameter: 16.6mm±0.2mm / 0.65"±0.01" (vs. Eneloop 3x 10mm)
• Length: 34.1mm±0.3mm / 1.34"±0.01" (vs. Eneloop 45mm)
• Weight: 20g / 0.71 oz (vs. Eneloop 3x 13g)

Now that the NL169 has been chosen, I can start working on the PCB design.