Longevity

The major consideration for a project like this is how to get it to move and how to make sure it keeps moving. Anyone who runs these distances knows the effort level involved, although a modern electric motor should be somewhat more efficient.

Our specifications imply that we should be able to travel a little over 13km (or 8mi) over an hour. Now it's fairly difficult to estimate what kind of power any particular stroller / kid / juicebox cache combination will require, especially when you factor in wind. A cycling-specific power calculator estimates the power required for the least-efficient bike with a 4ft tall, 60lb rider at 8.3mph (our 4:30/km pace) at 26 watts. Given that the calculator seems to rely on drag coefficient (and a stroller is typically less streamlined than a bike), it's likely that this estimate is somewhat on the low side. Most electric bicycles (albeit for adults) are rated at least 250 watts. For the sake of initial calculations, let's assume an average power of 35 watts with the expectation that power requirements may be somewhat higher once we can actually conduct empirical testing.

Battery

Now that we have an idea of what kind of power is required we can work on motor selection and battery design. Since we're concerned with efficiency here, a brushless (BLDC) motor is a good choice. Motors respond to power, and power (here) is comprised of volts * amps. So first let's choose a good voltage.

First we should pick something that's readily available. There are lots of types of batteries, from your reliable potato-flavoured science fair project to massive lead-acid or compact lithium-ion chemistries. Which one we pick will dictate convenient voltage levels.

• Potato battery would require an additional trailer, although is potentially delicious
• Lead acid has an excellent discharge rate, but is heavy, and has poor longevity
• Lithium Ion and related chemistries are compact, last a long time, and are not much more expensive than lead-acid.

Lithium Ion batteries are also the most configurable in terms of voltage, discharge rate, and capacity. For this project we'll use 18650 cells. In a future post I'll go over the implications of choosing this battery type and how we'll move from there.