JaroMato wrote: ↑
Jan 17 2022 2:32am
so this is what i got.
voltage out of the main +/- wire 78.4V
voltage read from the red positive voltage wire.
I tried to get lower voltage numbers. They were all in this orientation. How can i determine what they are in the 3voltage range?
If you do the measurement as I described, moving the meter leads onto just each pair of balance connections, you will get the cell group voltage numbers directly. But since you have the numbers, you don't need to repeat the test.
So when i add 3.267 + 3.267 = 6.534 and than continuing up adding 3.267 to each preceding number none were really far out of spec.
If you want to find out what the values are for the numbers you actually have, you don't add, you subtract. Subtract the second-highest value from the highest, and that gives you the voltage of the cell group the highest number respresents. Continue doing this with each pair of numbers to get each cell group's actual voltage. The very lowest voltage will be the actual voltage of that cell group.
If you find that all of them are within a hundredth of a volt or so, then they may be well-balanced, however because 3.2v is the average voltage a LiFePO4 cell will read for most of it's discharge curve, it's not possible to say if they are balanced or not. They'd need to all be closer to full or empty to find that out. But it would mean that the BMS, if it is working, isn't shutting it's output off because of a low cell group.
If you find any that are significantly different from others, then that group is likely to be problematic for one reason or another. More investigation and pictures of things would be needed.
If you are measuring at the BMS PCB pads for the balance leads, and getting the voltages correctly there, then it does at least mean the wiring from cells to BMS is intact.
Note that cells could be leaking but still appear to operate normally, at least for a while, until electrolyte leakage drains enough from the cell to prevent it from doing so. If the problem leaves teh cell "open circuit", then other cells in parallel take up the load, leaving that group with less capacity and less current delivery ability, so they sag more in voltage than the rest under load. If they sag enough, the BMS detects LVC and shuts the pack off to prevent damage and fires.
Some BMSs renable the pack as soon as the laod goes away and the voltage goes back up, but some stay off until reset by connecting the charger.
If yours does reset by connecting the charger it means the BMS is likely operating correctly, but that there is likley a cell problem.
I could try plugging the battery in and charging it to full. However i suspect the bike will still not work. I really cannot remember why the last BMS was replaced.
If there is something wrong with the cells, recharging it could cause problems (rarely, dramatic ones, but it happens).
FWIW, the brown spot could be anything from coffee or soda/etc stain from anyone that previously handled it, to excess solder flux that never got cleaned off, to "goo" (electrolyte) from inside a cell or cells, or corrosion from an external or internal source, or even from an exploded FET.
If the cells are intact and working correctly, and the balance wires are as well, then the BMS could have blown the discharge FETs open, so that it can't output any current to the rest of the bike. This usually happens simply because of repeated overheating of the BMS inside the pack heatshrink and battery casing, or because more current was drawn from the battery for a longer time than it is able to handle (which also causes overheating), or because a voltage spike from regen braking damaged them.
If you can get good clear well-lit pics of the entire BMS on both sides (without the cardstock in the way of the bottom), with the aluminum cover on it unscrewed and removed (if it is removable and not epoxied in place), it may be helpful to diagnose potential problems it has. It may let us be able to describe tests you can perform on it to verify if it's failed or not.