Tuesday, July 11, 2017

Robotics - LiPo power and keeping it safe

Robotics experiments continuing. I have started by powering "Creative RAT" (Youtube Video Here) with a common 9V battery but it goes flat after only a few minutes of demo. Time for something rechargeable and better. Next up is to try changing to a 7.4 Volt "2S" (2 cell) LiPo battery as used in Radio Controlled model aircraft. These have an excellent power capacity but they also carry a risk of catching fire and melting down if overcharged or undercharged so we need to regard them as "prima donnas" needing much loving care and attention. My first 2 "Wild Scorpion" batteries cost 10 NZ dollars each but they need a support infrastructure!

Storage. In a special fireproof "LiPo bag". I am keeping this bag on a steel shelf by a brick wall.

Safe voltage range. 3.1 to 4.2 V.  Range centre and commonly quoted rating is 3.7 V per cell (7.4 V total - I will quote per cell from now on).

Max voltage when charging is 4.2 V per cell. For LiPo batteries we need to buy a special charger which "balance charges" each cell individually and gives an optimum charging sequence of voltage and current over time.

Low-cutoff voltage recommended by "HobbyWing" is 3.15V [1].  Others say 3V. The battery is at risk if it falls below that. This is a big issue for us because Quadcopters etc all have "ESC" (Electronic Speed Control) units with cutoffs. We are doing speed control differently so we need to provide the cutoff.

I am starting with a small unit that sounds an alarm at 3V for any cell. [2]
 This is a warning beeper rather than a cutout but when I get this I plan to investigate the power supply to the beeper to see if it can power a relay or trigger a MOSFET for cutoff. I am also calculating and sketching possible diy cutoff circuits.

One of the aims of this project is to find ways and means that can work well for getting teenagers interested in tech - as well as being do-able in places like School Science Labs and after school workshops. I am guessing that the special care demands of the full-on LiPo are a heavy responsibility "turn off" for these situations.

Point and shoot cameras use efficient rechargeable Lithium batteries, single cell, 3.7 V
They should have less critical care needs because the battery includes an electronic module which includes a cutout. Can these work as our robotics batteries?
[NOTE LATER - Tested 2 such batteries - one is working well, the other "died" under test load. Therefore it looks better to follow common practice for radio controlled model cars and use NiMH batteries.]
The HobbyWing FlyFun ESCs have selectable cutoff options of  2.85V, 3.15V and 3.3V. I plan to follow their "medium" setting of 3.15 V as a good recommendation.

BW4701 AOK 1-8S Lipo Battery Tester/ Low Voltage Buzzer Alarm
NZ 9 dollars from "HobbyHangar" in Hamilton, NZ.

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