The Big picture
|The car has taps at each 12 cell section of the pack, and we have been assuming all these years that it is specifically for watching for a drop out as well as looking at the tap to tap match. that is the drop out protection that is doing the recal activation, which we also assume is the recovery from a drop out. This means we expect the car to stop assist when a cell drops out, so if our assumptions are correct, which I have not seen any evidence to the contrary, using the car to cycle while not as deep a discharge as the discharger can achieve, it is totally safe if it in fact works as we assume it was designed.|
The detection of a cell drop out in the middle of the full 120 cells is 10 times more difficult, and would never work in the car, as we are constantly charging and discharging, with the associated delay in returning back to the steady. With a discharger under the chargers control, we have a steady state ~2A discharge, and the voltage drop over time is a good indicator of a drop out mainly because it is so steady, and when the cell drops out, we see a quick drop of ~0.6V to 0.8V, then the slope returns to the earlier slope. On a fully balanced pack, we will still see the weakest cell drop out first, but it could wait until the whole pack is starting its dive.The lower the drop out voltage the better the pack.
If a load is applied, the voltage immediately drops due to the IR, then it drops due to depleting charge. The more current the larger the voltage shift. On the charge side we again immediately see a voltage rise, that is proportional to the current.
A pack will increase in IR as the cells age.
A voltmeter, or Peters OBDII gauge, will show the battery voltage under the various real time loads and charge conditions, and a good approximation of pack overall health, is possible by watching how low the voltage drops under full assist conditions. A resting (no assist or regen) 155V pack, may drop to less than 120V during heavy assist, and then pop right back to nearly the same 155V on returning to the resting state. A better pack will not drop as far with the same load. Same on the regen. If the pack hits 180V+ during full regen,and it drops very low during assist, it is suffering from IR issues, and may not recover as well with cycling as a pack that has lower IR.
I believe the OBDII gauge has an IR measurement based on this voltage drop, but the measurement suffers from the delayed recovery of the voltage after a shot of assist or regen. The voltage stays elevated for a period of time after a burst of charge, and stays depressed after a burst of assist, so the voltage is bouncing all over the place, which makes this a difficult measurement to make with high accuracy, but a relative IR reading is still very useful in answering the big " how do I know the condition of my pack" question
A very slow discharge may not show a drop out as easily as a higher current faster one, as the difference between the rest of the pack discharge slope could be quite similar to the cell drop out slope and be difficult to pick up.
I begin the testing of our new drop out detect software this morning.
We now have several detection processes operating in parallel.
1. If the pack is dropping too quickly during the initial slope detect which starts at a voltage we set for the pack voltage, it indicates a very weak pack.
2. We compute an average slope based on the time it takes for the pack volts to drop 5 bits. this sets the detection target rate of drop, which works quite well on any fast drop, but a slower slope may in theory slip through.
3. We have a minimum discharge volts setpoint, that will stop a discharge if the target voltage is detected, so on a questionable pack, you can set this high, and ass the cycling progresses, and the pack gets better, it can be lowered.
4. The new dynamic slope detect system works in parallel, and it derives a running average of the last 50 data points, and a short term average of the last 4 datapoints.
this presents a look at the last 4 datapoints as they relate to the long term running average, when in the normal operation zone, the two averages will show the same value, but on the more rapid drop out, the short term average will reflect the faster discharge slope.
The sample timing is increased to give more samples so the slope angle can be more accurately determined.
So with your eyes as a drop out test, dont blink.
With the charger in control, we have 4 ways to assure we see and stop the discharge, so the cycling procedure is much safer.
The idea is to let the thing run 3 cycles unattended by humans, and we go back and see the results when it is finished.
Of course as with the charge, we show the reason for stopping the discharge, so one can better understand what is happening.