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Plot Descriptions |
| Quick Note:
To figure out the approximate charge
time of a fully discharged pack, divide the capacity (mAH) by the charge
current (mA). In this case, 800mAH / 300 mA = 2.7 hours. This time
will vary depending on the actual charge current, the real pack
capacity and the type of charge termination. The red plot is the battery voltage profile. This is a very important profile that will be interpreted by a smart charger in order to terminate the charging properly. In a charge termination, we want to make sure that the battery is neither undercharged nor overcharged. Undercharging is bad for obvious reasons - reduced run time. Over charging is bad because it reduces the life of the battery in the long run. You will get less charge/discharge cycles out of a pack if it has been repeatedly overcharged. Over discharging is also bad but that's another discussion. Position 3 is the ideal charge termination point. State of the art chargers can detect this point based solely on the voltage inflections. Position 4 is where most "peak" chargers terminate. In actuality it is slightly after the peak (you don't know where the peak is until you've past it). Anything after the peak falls in the region 5 area. The deeper this voltage is from the peak, the deeper the pack has been overcharged. The blue plot is the battery pack temperature. Notice how at room ambient charging the NiCD battery temperature decreases (endothermic reaction) . Near the end of charge, the temperature begins to increase rapidly. The energy being pumped into the battery is no longer being stored and is now being dissipated as heat. Detecting this temperature rise (label 6) is another way to terminate charge. However, it does require a temperature sensor in the battery pack. A poor man's version of this detection is to charge the pack at room temperature and then stop charging when the pack begins to feel warm. Note that detecting the temperature rise (degrees/minute) is not the same thing as detecting the absolute temperature (degrees). Absolute temperature termination is not consistent because the final battery temperature at the end of charge is based on the starting temperature and room ambient. The green plot is the charge current (note: weird left scale, it's roughly 280mA). This is the current coming out of the stock 300 mA wall transformer charger. As you can see, the current is unregulated and varies as a function of the pack voltage and AC mains voltage. Label 1 shows how the current jumped due to the AC mains voltage jumping when I turned off and on the halogen lamp in my room (it was plugged into the same outlet as the charger). You can also see the pack voltage (label 2) jumping up due to the current increase. Key take away-> if I am to build a smart charger based on voltage termination, then I either have to regulate the current to be constant or I have to take a voltage reading with the current temporarily turned off (the later is preferred because it's simpler and cheaper). Otherwise, the battery voltage profile will be "corrupted" due fluctuations in the charge current. At label 7 the current was turned off. You can see the temperature and voltage drop accordingly. This is just a brief summary of fast charging. Other questions left unanswered are:
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