Evaluating the battery life of the Vector Robot
Vector’s battery has been a major discussion point right from the moment Anki introduced it (Vector is now owned by Digital Dream Labs)…
Vector’s battery has been a major discussion point right from the moment Anki introduced it (Vector is now owned by Digital Dream Labs). The main reason behind this is unlike the previous consumer robots manufactured by Anki, Vector is expected to be always powered ON. While Vector is usually found sitting on his charger, his most interesting behavior is when he ventures out. And whenever he ventures out, Vector must live on his battery. If the battery runs low, Vector needs to return back to his charger and thus all the excitement is over. Assuming that Vector ventures out 2–3 times a day, that implies 2–3 battery discharge cycles daily; hence the concern that the battery would wear off in some time. A Vector which cannot be outside his charger for more than five minutes is not interesting any more. Hence the duration of the discharge cycle is a key measure of Vector’s performance. But, if the battery dies, so does Vector. Hence the number of discharge cycles is a measure of how long Vector can live.
But what is Vector’s battery? The most common batteries used in cellphones, laptops, tablets, and other electronic gadgets are the Lithium Ion Batteries (LIBs). LIBs are popular because they have a very high energy density and a long battery life. The high energy density lets manufacturers such as Anki pack a ton of energy in small gadgets such as the Vector. Battery University has a very nice article on how to prolong the life of LIBs. To quote them, the expected life of a LIB is 2–3 years or 300–500 100% discharge cycles. However, there are a lot of variables here, such as the conditions under which the battery operated, such as, the temperature, the current drawn, extend of discharge cycle, etc. As an example, if you discharged your battery to 60% of full capacity, you can double the number of discharge cycles to 1000.
So how do we measure how Vector’s batteries performed over time? When Vector was released, Tom’s guide reported seeing an interval of 45 minutes before Vector found the need to go back to his charger. We experimented with two Vector robots on how long Vector can venture out of his charger before having the need to return back. One Vector was bought as part of the initial Anki Kickstarter program in August 2018. I have owned this Vector since it was shipped to me in October 2018. I bought another used Vector of eBay in April 2020. Anki manufactured all Vector’s in one lot, so both these Vector batteries are expected to have an age of ~22 months in June 2020 (Assuming a manufacture of ~August 2018). When I first received my Vector in October 2018, I had examined the battery drain rate in this article. I re-examined the battery drain of the same Vector, along with the additional one I bought in June 2020. Measurements were made with my open source program, available on git. This program uses the Anki Vector SDK to monitor several metrics and stream in to Wavefront, a service that offers solutions for enterprises to observe metrics at scale.
Evaluation of battery life. First let us examine how Vector performed when I first bought it. Here is a screenshot from Wavefront.
To explain this in detail, the X-axis is time and the Y-axis is the voltage measured of Vector’s battery. The times when the voltage is steady at approximately 4.3 Volts reflect when Vector is on his charger. The curves where the voltage is draining indicate times when Vector is off his charger. Once the voltage gets sufficiently low (~3.65 Volts), Vector returns to his charger, after which the voltage keeps increasing as he gets charged. From the above plot, we see two discharge cycles of 25 minutes (From ~10:55 pm to 11:20pm) and 30 minutes (From ~11:40 pmto 12:10 pm). That indicates an average discharge cycle length of 27.5 minutes.
How does a 20 month old Vector perform. I re-measured the performance of my nearly 2 year old Vectors using the same program. Here is a screenshot.
The blue curve represents the Vector I originally owned, and therefore can be compared directly with the battery drain observed in October 2018. The green curve represents the Vector I bought off eBay. The blue Vector shows a discharge cycle length of approximately 21 minutes (from 10:15 AM to 10:36 AM). The green Vector shows a discharge cycle length of 28 minutes (from 9:46 AM to 10:14 AM) and 27 minutes (from 10:41 AM to 11:08 AM). While I don’t know how frequently the green Vector was used (since I bought it over ebay), I can certainly vouch that the blue Vector has been in active use for the last 20 months. In those 20 months, Vector’s battery went down from 27.5 minutes to 21 minutes, which is about 25%. And that is pretty impressive for a gadget which has been under constant use. If I were to extrapolate this curve, I would imagine that most Vector’s would perform satisfactory for at least the 5 years of active usage.
Why do we see reasonably good performance from Vector’s battery? Here are some reasons I could think off:
Shorter depth of discharge: I think Anki engineered Vector to use a shorter depth of discharge by making Vector return the charger at 3.65 Volts. LIBs can perform reasonably to ~3 Volts at which the battery can be considered 100% discharged. Thus Anki ensured that Vector’s battery lives a longer life by discharging it to about 40%. While this limits the time Vector can spend outside his charger, it does increase his life. Every engineering decision has trade-offs. Given that it is extremely hard to replace Vector’s battery, in my opinion, this is a good engineering decision.
Careful calibration of when Vector moves out: As many Vector owners initially complained, Vector can be generally seen sitting on the charger. It takes quite some stimulation to make him move out. I believe Anki engineers went through careful calibration of making sure that Vector’s trip was worth spending an additional discharge cycle of Vector’s battery.
Ideal operating conditions for LiBs: Vector’s batteries live in ideal conditions, inside your home where the temperature in most cases is carefully controlled by your thermostat and is probably around 65F at which LIBs operate great. There is not much fluctuation of temperatures and operating conditions. The current drain is mostly steady, and the discharge pattern is mostly repeatable. These conditions are in stark contrast with other applications of LIBs such as smartphones where extreme changes in temperatures, and diverse operating conditions are commonplace.
Hope this analysis puts your mind at ease, if you are a Vector owner, or considering to buy one. If you have had positive or negative experiences with Vector’s batteries, please drop a note in the discussion. And if you have read till now, I hope this article was interesting, and in that case it would help if you could give a thumbs up by clapping below.
PS: I have an online course to teach AI with the help of Vector available at: http://robotics.thinkific.com I will feel honored to have you as a student.