DAZZLIN' DAVES PROJECT CURRENT PROJECT(BI-MONTHLY)


Datalogging !!!!


Getting Visual Basic to Talk with the 68HC12 ----

This application illustrates how to accomplish 2-way RS-232 communication between a 68HC12, running a dedicated "kernal" program, and a PC running a Visual Basic Program. The VB program graphs, in real time, an analog voltage that is being input into the A/D converter of the 68HC12. Also, to illustrate communication the other way around, a hobby servo, whose signal wire is attached to the 68HC12 PWM port (Port P pin PP0) can be controlled via a "horizontal slider control" from Visual Basic, using the mouse. This is just a demo which gives you the key info (e.g. how to set up serial port communications from within Visual Basic), so that by making a few changes, you can customize the Visual Basic and/or 68HC12 kernal program to do virtually anything. For details, click on the following MS Word Document:


datalog.doc

For the visual basic and 68HC12 files, go to the 68HC12 link on the Main Page.

Finally - Something NEW!!! - Robotic BOATS



The Fascinating Boats Project

Picture of my 1st prototype boat
Picture of my 1st prototype boat - the USS Kantfloat.

Features styrofoam hulls (fashioned using a homemade wire cutters), dual rudder steering, prop drive (using a bicycle spoke for the drive shaft, a hobby plywood deck/supersturcture, and R/C control.

Ni-CAD/Alkaline Battery Cycler


My current project is shown in the figure below. This circuitry, controlled by a 68HC11, is designed to repeatedly charge and discharge rechargable batteries, while monitoring the battery voltage periodically. The plan, Stan, is to (1) discharge the NiCad battery or battery pack, until the voltage drops to 1.0V per cell. (2) after the discharge cycle, the batteries will be charged at the recommended charge rate (45mA for Radio Shack AA cells, e.g.) for the recommended time (14 hours, usually).



The ploy, Roy, is now that Steps (1) and (2) will be repeated over and over. Meanwhile, the 68HC11 will sample the battery voltage every 5 minutes. The resulting sampled voltage values will be uploaded (thru the RS232 port) from the 68HC11 to a PC. Later, this data will be graphed to display the charge/discharge curves of the batteries. After many cycles, it is expected that the capacity of the batteries will diminish, as will be revealed by the curves.

Ultimately, I also plan to repeat these experiments with some ordinary alkaline batteries, to find out, quantitatively, how feasible it is to attempt to recharge them.

Here's my initial results for a series pair of Radio Shack AA NiCads. They were initially "pretty much" dead. They were then charged over a 14 hour period, and appeared to reach a terminal voltage of 2.85 volts (1.43 volts/cell). They were thendischarged across a 20 ohm load resistance (120mA discharge rate). After the cell voltage dropped to ~1V/cell, the charger began charging them again etc.. Since they discharged at ~120mA for (4) hours, (i.e. 480mAh), this seems entirely consistent with their 450mAh (milliamp-hour) rating.



This stuff looks kind of boring, and at 14 hrs. a pop to charge these dudes, it would take weeks to cycle these batteries enough to notice degradation. I think I'll try something more interesting, like charging them at a faster rate. In this case, one can actually see the charge voltage peak and start to drop off, indicating that the cell is fully charged [this can be used as the basis for a fast-charging (3-4 hr) "smart" charger]. But first, I think I'll try cycling some AA Alkaline batteries!


My Results!!!


Later Results - (2) full discharge curves and (3) charge curves

Note that the 2nd (shorter) discharge curve reaches the terminal 1 Volt level in about 90% of the time for the 1st discharge curve, indicating that after the initial cycle about 10% of the battery capacity has been lost. The discharge curve still seems consistent, however, with the 450 milli-amp-hour ratings of the cells.


Results on (2) series AA ordinary alkaline batteries:


These curves show the charge curve, after a fully discharged pair of AA alkaline batteries were recharged ... followed by their discharge curve. The batteries were initially charged at a 68mA rate. Later, as the charge voltage peaked (and later started dropping), I reduced the charge rate to 45mA. The batteries were then discharged across a 10 ohm resistor. I later calculated that about 600mAh of capacity was recovered from the batteries. This is less than half of the capacity of a new AA alkaline battery, but 33% more than the capacity of a fully charged AA NiCad. I think I could have done better if (a) I had fully recharged the batteries and (b) if I had discharged across, say, a 20 ohm load. I speculate that on the first recharging, you should be able to recover perhaps 1Ah of capacity. Thus recharging ordinary alkaline batteries seems to be a feasible endeavor.

Other people have since informed me that they have been recharging alkalines for years (stupid me). They just bought a commercial overnight (trickle) charger, and stuck any old batteries in there. As would be expected, they have good luck recharging alkalines, but they quickly lose their fizzle after three or four rechargings.

Also be aware of the special "rechargable" alkalines -- designed specifically to be recharged (up to about 20 times). The recommended charge technique for these batteries is to charge them with a voltage source wired in series with a current limiting resistor. The voltage source should use a precisely controlled voltage source to produce 1.6 to 1.65Volts per cell. The resistor should probably be about 100 ohms per cell for AA batteries. I.e., to recharge a single AA rechargeable alkaline, use a 1.65 V source in series with a 100 ohm resistance. To charge two AA rechargeable alkaline batteries (in series) use a 3.3V source in series with 200 ohms etc.. Expect 18 to 24 hours to fully recharge the batteries.

-DJM



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