Page 33 - Hacking Roomba

14 Part I — Interfacing

Power

The first consideration for any robotic system is power. Roomba is powered by a custom
high-power rechargeable battery pack. This pack provides enough power to run the Roomba
for up to 100 minutes of normal operation. It can be re-charged in 3 hours using the 3-hour
charger.

Battery Pack Details
Internally this battery pack consists of 12 sub C size nickel metal-hydride (NiMh) cells.
Each cell puts out 1.2V so 12 cells wired in series give 14.4 VDC. The newer yellow battery
pack uses at least 3000 mAh cells. Some people have taken their packs apart and even
found 3200 mAh cells. (The original black Roomba battery pack used 2300 mAh cells.) The
mAh differences only affect run time and are otherwise the same. The batteries are good for
approximately 1000 charging cycles and do not suffer from any sort of negative memory
effect from partial discharge. Do-it-yourselfers can find companies like BatterySpace.com
that will sell compatible battery packs using up to 3600 mAh cells. These packs give 20 percent
longer run time over the yellow pack and 56 percent longer time over the original pack. Of
course, such a hack does void your warranty, but it is a way to save an otherwise old and
unused Roomba cleaner.
The main metric of batteries is ampere-hours, which are more commonly referred to as milliamp-
hours (mAh). This describes how much current can be drawn from the battery and for how long,
and thus how much power a given battery can provide. A 1000 mAh (1 Ah) battery can supply
either a 1000 mA (milliampere) circuit for one hour, a 5 mA circuit for 200 hours, or a 2500 mA
circuit for 24 minutes. For comparison, a typical LED flashlight might draw 30 mA, while a typical
AA battery can provide 1000–1800 mAh. The Roomba batteries have increased in capacity from
their original 2300 mAh to at least 3000 mAh, making the current models last 30 percent longer
on a charge than previous models.

Available Power
When turned on but sitting idle, the Roomba draws 150 to 250 mA, depending on the
Roomba model. During normal operation, a Roomba draws from 1500 mA to 2000 mA
of current. This variation in current consumption is due to the variety of floor types: Thick
carpets cause more current draw than hard floors. The battery pack can be maximally dis-
charged at a 4 Amp rate, limited by an internal polyswitch (a device that acts somewhat like
a fuse that can be reset). Without the polyswitch, a short circuit would damage the battery
and the unit.
The full voltage and power available from the pack is available through pins 1 and 2 on the
ROI connector. Any projects using power through the ROI can draw as much power as
they need. However, drawing too much will shorten the life of the battery, shorten the run
time of the unit, and perhaps confuse the system’s internal firmware. All projects in this
book will draw less than 1 Amp of current and most draw less than 100 mA. A 100 mA
project running of Roomba power would shorten the normal Roomba run time by maybe
5 percent.