Some Thoughts on using Batteries in Beam

As battery technology progresses and solar cell technology lags, batteries will become increasingly a part of Beam robotics. The reason is simple. A rechargeable battery is one step shy of a solar engine. Add a solar cell and voltage monitor (i.e. the venerable 1381 or the Maxim 8212), and you have a solar engine that performs on par with the capacitor-based ones in terms of firing/time ratio, with time being firing + charging.

To parse this thought out, let us look at the typical firing/time ratio of a solar engine. Most of the tests that I have performed on the standard Beam SEs, such as the MSE and the Fred SE, show about that the engine spends about 1/4 of the time firing, and 3/4 of the time charging. In a photopopper, this looks like near continuous motion as the time is in seconds. By comparison, a battery can be charged in around ~21 hours by solar power and then power a hypothetical Beam robot for ~13 hours. *

This hypothetical Beam robot will have close to a 1/3 ratio of firing/time or, if you prefer, activity/day. Given that experts typically give the 8-hour figure for human sleep requirements, the 1/3 ratio is the opposite our own activity cycles.

The problem is that this hypothetical Beam robot will be unlikely to find 21 hours of contiguous sunlight. Ideally, we would want to charge for around 6 1/2 hours for a run time of 13 hours (and thus reach the 2/3 activity/day ratio). Well, we can do a thing or two about this. As you pointed out, battery technology has made some strides in the past several years. Newer batteries can charge faster, and can even charge while sourcing (which, I have read, is one of the reasons that we used capacitors in the first place). The current difficulty is that solar cells cannot charge the battery anywhere near as quickly as the battery's ratings.

Researchers have made several advances in solar cell technology and efficiencies are rising. However, the technology has yet to make its way into small packages with high voltage outputs. Ever wonder why we still use the Panasonic Sunceram, now nearing a decade in age? The reason is that the Sunceram still provides some of the highest voltages, albeit at the cost of less overall power. Solarbotics appears to always be looking for better products, and I am certain that they will release a newer, higher powered solar cell when one becomes available.

While we wait for new solar cells to become commercially available, I think that building battery-powered robots is entirely warranted. After all, any discovery or design that you come up with can, at a later date, be retrofitted with a solar cell and a simple battery charger. And, as more power will bring with it the ability to run more motors and more complex Nervous Nets, using battery power may open up a new area for Beam evolution.

J Wolfgang Goerlich
March 2004

* Two Panasonic SC3733 in parallel, four 650mAh AAA NiMH Battery in series, 50 mA power consumption.