Everything for Electronics
Posted in: Developing Perspectives (August 2012)

How Much to Invest?

By Bryan Bergeron

I have a general rule when it comes to tools, electronics, and accessories: I buy the best that I can afford at the time. There’s nothing more frustrating than having to buy another widget because I didn’t spend the extra dollar to get a feature that I didn’t know I needed. This defensive purchasing sounds simple, but in practice, it’s a difficult rule to follow.

Let me give an example. I’m preparing a series of articles for our sister publication, SERVO, on autonomous aerial vehicles. Thanks to open source hardware autopilots and ground control software, it’s possible to build a fully autonomous quadcopter (a helicopter with four motors) capable of carrying a 2 lb payload for about $1,000.

The ground control software — often referred to as waypoint software — is linked to GoogleEarth, enabling just about anyone to plan a graphical route for the UAV and have the payload delivered there autonomously. That is, you literally direct a drone to travel two blocks down Main Street, take a left at the corner, and then land at the doorstep of the fifth house on the right. Some think this is a great scheme for pizza delivery. Others think cruise missile.

Regardless of what the technology and capabilities bring to mind, it all depends on reliable battery power. So, as part of building and testing various quadcopter systems, I had to identify and purchase the ‘best’ batteries and charger that I could find and stay within my budget.

Simply going for the most expensive chargers and batteries and ignoring my budget wasn’t an option — even if I had unlimited funds — because a huge, high voltage, high capacity lithium battery wasn’t compatible with the airframes I was testing. No, I had to do my homework.

It turns out that lithium batteries generally offer the best energy density, weight, and cost combination, compared with the other common battery chemistries on the market. However, when you get into it, you’ll discover that there are various formulations of lithium batteries — including LiPo, LiLo, and LiFe. I won’t go into details here (as you can tell, I’m trying to entice you to take a look at SERVO), but each formulation has different costs and capabilities.

Then, there’s the physical packaging. Some lithium batteries are — because of safety regulations — encased in hard, heavy plastic. I discovered this type of battery on the Parrot AR Drone 2.0 (the ‘low end’ vehicle in my study) and immediately ordered a replacement battery with the same weight and dimensions, but that provided double the capacity. The new battery is encased in shrinkwrap.

The tradeoff is that lithium cells are known to catch fire and explode when damaged in a crash — a tradeoff that I’m willing to make. There are lightweight, fireproof battery wraps available, if you want to lower the risk of fire somewhat.

The problem with the battery upgrade for the Parrot AR Drone was that the original charger couldn’t handle the load, so I was forced to buy a new charger. The ‘best’ charger (in this case) had to handle not only the relatively minor demands of the AR Drone battery, but those of the batteries capable of powering the meatier motors and electronics of drones from DIYDrones (www.diydrones.com) and Parallax (www.parallax.com).

What’s more, because it’s illegal to fly a potentially lethal drone over a crowded city like Boston, I have to travel an hour outside of the city to test the drones. That means a supply of batteries. It also means that the ‘best’ charger for me had to include the ability to charge/discharge several battery packs simultaneously. So, I was forced to consider $100+, 1,000W battery chargers and multiple, $40+ LiPo batteries.

Then, there’s the ‘best’ power supply to power the charger — which has to be powerful enough to meet the demands of charging a half dozen batteries simultaneously.

Repeat the above exercise for the R/C transmitter and receiver (check out the Spektrum DX7) and onboard camera system (see the GoPro HD Hero2), and we’re talking a lot of research and a significant investment. Not the most expensive or capable components I could find, but I expect the investment in batteries, charger, camera, transmitter, and receiver will outlive the review.

For example, had I gone with a simple upgrade charger and battery pack for the Parrot drone, I would have had to buy yet another charger for the higher capacity batteries on the other drones.

I hope this helps you with your purchase planning. Do your best to look ahead to your next few system upgrades — whether it’s test equipment or a microcontroller development kit — to determine the best investment for you. NV