Background. Last year we had a number of custom batteries made up in a flat 10s X 1p cell layout arrangement for 2 reasons. #1 is so that we'd have convenient low profile 18650 batteries for mounting on powered skateboard projects (see here http://endless-sphere.com/forums/viewtopic.php?p=904577#p904577), and #2 was to have a supply of BMS protected 36V battery packs that were <100 Wh so that we could take them when travelling on airplanes, then just plug them back in parallel on arrival to have a typical 300-400 Wh ebike pack (See here http://endless-sphere.com/forums/viewtopic.php?p=1021791#p1021791).
All good. We ended up submitting a number of these to a test agency to get UN38.3 certification for shipping approval thinking it could become a handy little product. But as an actual product I'm loath to supply just a bare naked battery and BMS covered in shrinkwrap, as that offers no protection against water, impacts and drops, or solutions for mounting to a bike. Making a padded fabric sleeve can go partway to addressing those deficiencies but it feels like just a stop-gap solution. And since it's a pretty niche item there wouldn't have close to the sales volumes to justify the very high expense of tooling up a custom plastic enclosure or extrusion profile.
Then after the success of some of our other potting experiments with controllers we wondered heck, why not just try potting the entire battery too? We have a CNC machine to make a nice mold so it would cost almost nothing in tooling capital. There was a pretty lively debate in the controller thread about both the pros and cons of encapsulating ebike electronics (have read here: http://endless-sphere.com/forums/viewtopic.php?p=1035601#p1035601), and a lot of the exact same arguments would come into play with batteries.
Having been involved in many many 100's of lithium ebike battery pack repairs over the years, the reasons for actual pack failures typically stem from:
1) Packs being allowed self discharge in storage to the point that individual cell voltages are dangerously low and not safely recoverable.
2) Water damage either corroding/disolving cell tabs or causing erratic BMS behavior.
3) Impact damage from batteries being dropped or involved in accidents or mounted in a way that results in high stress on some individual cells.
4) Vibration causing eventual fatigue failure of tab weld connections.
5) Various random/unexplained BMS circuit failures.
6) Various random cell or cell groups failling (typically developing a soft short).
Of these things, potting the battery would have prevented almost all cases of 2, 3, and 4. Item 6 was an issue in the past but is almost unheard of now that we're dealing only with brand name 18650's. That leaves just items 1 and 5, and both of those should be resolved by robust circuit design and BMS parameters, like ultra low sleep mode quiescent current.
The big downside of potting is of course that should you have a failure it is difficult if not impossible to repair, but hopefully those issues needing repair would be way less frequent. And more interestingly, with this arrangement of a <100 Wh battery pack, it would cost less to replace with a new 100Wh battery than you would spend just on the hazmat shipping alone to send a normal ebike sized battery back to a vendor/manufacturer for repair. I could see being really nervous about having single 500-1000 wh battery be potted and unserviceable, but if this was instead 5-10 individually potted 100Wh packs, then it's no big deal if one module falls out, you still have the other 4-9 batteries to keep you going.
All good. We ended up submitting a number of these to a test agency to get UN38.3 certification for shipping approval thinking it could become a handy little product. But as an actual product I'm loath to supply just a bare naked battery and BMS covered in shrinkwrap, as that offers no protection against water, impacts and drops, or solutions for mounting to a bike. Making a padded fabric sleeve can go partway to addressing those deficiencies but it feels like just a stop-gap solution. And since it's a pretty niche item there wouldn't have close to the sales volumes to justify the very high expense of tooling up a custom plastic enclosure or extrusion profile.
Then after the success of some of our other potting experiments with controllers we wondered heck, why not just try potting the entire battery too? We have a CNC machine to make a nice mold so it would cost almost nothing in tooling capital. There was a pretty lively debate in the controller thread about both the pros and cons of encapsulating ebike electronics (have read here: http://endless-sphere.com/forums/viewtopic.php?p=1035601#p1035601), and a lot of the exact same arguments would come into play with batteries.
Having been involved in many many 100's of lithium ebike battery pack repairs over the years, the reasons for actual pack failures typically stem from:
1) Packs being allowed self discharge in storage to the point that individual cell voltages are dangerously low and not safely recoverable.
2) Water damage either corroding/disolving cell tabs or causing erratic BMS behavior.
3) Impact damage from batteries being dropped or involved in accidents or mounted in a way that results in high stress on some individual cells.
4) Vibration causing eventual fatigue failure of tab weld connections.
5) Various random/unexplained BMS circuit failures.
6) Various random cell or cell groups failling (typically developing a soft short).
Of these things, potting the battery would have prevented almost all cases of 2, 3, and 4. Item 6 was an issue in the past but is almost unheard of now that we're dealing only with brand name 18650's. That leaves just items 1 and 5, and both of those should be resolved by robust circuit design and BMS parameters, like ultra low sleep mode quiescent current.
The big downside of potting is of course that should you have a failure it is difficult if not impossible to repair, but hopefully those issues needing repair would be way less frequent. And more interestingly, with this arrangement of a <100 Wh battery pack, it would cost less to replace with a new 100Wh battery than you would spend just on the hazmat shipping alone to send a normal ebike sized battery back to a vendor/manufacturer for repair. I could see being really nervous about having single 500-1000 wh battery be potted and unserviceable, but if this was instead 5-10 individually potted 100Wh packs, then it's no big deal if one module falls out, you still have the other 4-9 batteries to keep you going.