Friday, February 17, 2012

The economics of recharging your toy helicopter

While on a business trip visiting customers in Taiwan back in December, I got a toy helicopter as a thank-you gift from one of my coworkers (thanks, Sharon!). This toy helicopter is fun to fly and is surprisingly stable in the air.

Flight time is about 7 minutes, and the battery recharge time is about 40 minutes. It can be recharged from a powered USB port by using a wire that came with the toy that has a USB connector on one end and the helicopter charging connector on the other end. Or, it can be recharged from the six AA alkaline batteries inside the handheld controller via a wire that exits from the controller. Thinking I did not want to prematurely drain the controller batteries, I typically used the USB charging method by using my iPad’s 10 W USB power adapter plugged into a wall outlet. So I got to thinking about which charging method was more economical: charging from a wall outlet or from the batteries. Luckily, I have test equipment at my disposal that can help me answer that question!

Recharging using AC power via USB power adapter
Using one of our Agilent 6812B AC sources, I captured the AC power used during a recharge cycle. I used the AC source GUI to take readings of power every second for the charge period and plotted it in a spreadsheet (graph shown below). I found that the power consumed started at about 2.2 W and ended at about 1.2 W roughly 41 minutes later. The energy used during this time was 1.1 W-hours. Where I live in New Jersey, the utility company charges about 15 cents per kilowatt-hour, so 1.1 W-hours of energy used to charge the helicopter costs fractions of a penny (0.0165 cents = US\$ 0.000165). This is basically nothing!

Recharging using controller battery power
To analyze the current drawn from the controller batteries, I used one of our Agilent N6705B DC power analyzers with an N6781A SMU module installed. I ran the battery current path through the SMU set for Current Measure mode and used our 14585A Control and Analysis software. I captured the current drawn from the six AA batteries in the controller during the helicopter recharge cycle. These batteries are in series, so the same current flows through each of the six batteries and also through the SMU for my test.

For the recharge period (about 43 minutes using this method), the software shows the batteries provided 173 mA-hours of charge to the helicopter. A typical AA alkaline battery is rated for 2500 mA-hours, so that means I would get about 14 (= 2500/173) charge cycles from these six batteries. If you shop around for high-quality AA batteries, you might find them for as low as 25 cents per battery. Since the controller takes six of these, the battery cost for the controller is \$1.50. If I can recharge the helicopter 14 times with \$1.50 worth of batteries, each recharge cycle costs about 10.7 cents (= US\$ 0.107). This is 650 times more expensive than using the AC power method, so I will continue using the wall outlet to recharge my toy helicopter! How about you?
Note that with the AC power recharge method, you pay for the kilowatt-hours you consume from your utility company. With the controller battery power method, you pay for the mA-hours you consume from your batteries. Consider this: if you choose the AC power method, you will save US\$ 0.106835 per recharge cycle. That means after just 2.81 million recharge cycles, you will have saved enough money to buy yourself a real helicopter worth US\$ 300,000, so you better get started now!