bagmaker wrote:Is the vent a pressure type, ie, will it only vent if the internal pressure builds?
And will it allow gas to come in again when the pressures reverse?
Thinking along the lines of mounting, perhaps the practice of sideways constraint of the cell that has been advocated is a bad thing?
If each cell were allowed to expand and contract due to thermal variance- even when overcharged- then it wouldnt suck oxygen back in when it cooled down again.
again, just musing
The topic of my next post.Prismatic cells have plastic casings that can withstand 150*C before it lets go under the pressure from the boiling electrolyte inside, the pressure relief valve lets go at ????? but well before the plastic case ruptures. It's a one way valve, but once a of volume of gas at high pressure had blasted through it I doubt it's one way integrity would remain.
The important part to keep in mind, the plastic case becomes pliable when heated, so the pressure will stretch the plastic case in any direction it can and this heating/stretching can occur during extremes in voltage at either end of the charge cycle. If the internal temp exceeds 70*C (not the case temp) the first of the volatile liquids will start to gas and this increases the internal pressure. The plastic is soft enough at this temp to stretch and allow the cases to swell. Holding a cell above 3.45v or below 2.8v for an extended period will cause the internals and particularly the electrolyte to heat up, the further past these extremes the shorter the time damage will start to occur, these extremes are voltage, not current rates in or out of the cell, something many BMS manufacturers and other "knowledgeable" people simply don't understand for some reason or another. Just past these extremes and/or for very short periods reduces the heating period so natural cooling can occur so little or no damage is done.
What does happen is a loss of capacity if the case swells and this links back to something I mentioned in an earlier post, the electrolyte starvation in the cell. If the cases are allowed to swell the active plates are no longer held in contact with the separator material so the electrolyte can drain away to the lowest possible level within the cell. No electrolyte contact between the active plates results in no ion exchange, so now only the area of the plates at the bottom of the cell are in contact with the electrolyte. If that equals only the lower 10% of the cell then you will only get the use of 10% of the cell function. In other words, the 100Ah cell now has only 10Ah of it capacity functioning.
The good news is, return the case to it's original shape with the active plates again in contact with the separator so the electrolyte contacts all the plate surfaces and the capacity returns. Boil the electrolyte so it vents out of pressure relief valve and it's gone. Just like letting the smoke out of electronic equipment, you can't put it back in, when you have lost the electrolyte the cell will no longer function.
How to work with this issue..... that's for another day
T1 Terry
