5 ms·

Hmm, why are you putting the Bosch BMS in series? Is it going to turn off unless it detects a load? I'd simply not connect it's phase wires, but it's possible t

Hmm, why are you putting the Bosch BMS in series? Is it going to turn off unless it detects a load? I'd simply not connect it's phase wires, but it's possible the Bosch magic won't let that work. Thankfully I didn't have to spot-weld. I used cells from modem batteries, so they were already spot welded - I was able to very rapidly solder to the spot welded tabs which already had some spots to run balance wire through, and the batteries were connected to each other 3 by 3. Then I was able to solder the "balance" wires which were quite thick at each cell to put them in series and it worked fine. The pack as it is now does not heat up at all. That's to be expected I guess because I'm charging it overnight quite slowly and I'm not really drawing more than 1C. I had multiple cells that I threw out - I all ran them through a balance charger and fast-charged then fast-discharged them. Any cell that got warm, out of balance, or didn't hold the expected capacity was thrown out. I ended up disposing of 25 or so cells, but I had order an extra 46 anyways so it didn't matter. It also helps that I let the cells aside after a full charge for a week or so simply because of the balancing process. Any cell that self-discharged I knew wasn't good. No FLIR yet but I'd love if :) If you think 70 cents per cell is good, batteryhookup has 35 cent cells at the same time... They were in worse condition at a pretty low capacity though so I passed on that.

jacquesm 5y ago
The Bosch BMS is a pretty clever little device, it governs not only the charge discharge cycle but also keeps track of the state-of-charge, and reports the remaining capacity in % to the controller via a CANBUS link, which - annoyingly - also features as auhtorization to release the motor, using some challenge-response system that to date nobody has managed to crack (though some people spent a lot of time trying to do so). So you can output voltage from the pack all you want, without the proper authentication your motor will not turn. So that's why you really need that Bosch BMS. Then the bad news: the Bosch BMS is on the edge of what's safe, the FETs are underpowered (just a hair under rated maximum current and voltage), not cooled, there is a 40V Schottky diode there which regularly operates at or even slightly above it's ratings, there is a single chip DC-DC convertor which also isn't cooled properly and which tends to blow. So I don't particularly trust the Bosch BMS from a safety perspective, I've seen a whole bunch of these now (about 15) and a good number of them had blown up. And that's with the standard battery. The one that I'll be hooking up to it is four times more powerful. Any short circuit protection in the old BMS is going to be terribly undersized so I need something in series with the Bosch BMS to ensure that if there ever is a short (say in the motor) that it's the battery that gets disconnected well before the wiring goes and I simply do not trust the Bosch BMS not to fail 'closed' in which case the whole thing will turn into a giant fuse. That's the kind of scenario that I can really do without. On more than one pack that I've opened I've found evidence of small fires, typically BMS balancing wires that shorted out (there are no fuses there, obviously) and that could have ended quite badly. The wires from the pack to the motor are substantially thicker so then the question is what cuts out first: the BMS or the wires. I simply want to play that as safe as possible and aim for the disconnect to happen fast enough that the wiring doesn't light up. Another option is a very fast fuse in line with the battery terminals, that's definitely a possibility as well (and easier to mount and less chance of it in turn breaking than yet another BMS in series). Anyway, work in progress and to be continued, nice to find someone on HN who has a lot of knowledge about this! Waiting for a while to see if there is a self discharge is a very good method to spot flawed cells before you assemble a pack but such defects can also develop at a later date, so please do keep a good eye on your pack, especially if - as I suspect - you are using reclaimed batteries, you can't be 100% sure that they have never been deep discharged. Safety is one of the main reasons why I decided to go with brand new high capacity brand name cells, yes, it's a lot of money, but even so, the whole thing will cost about as much as a 600 Wh commercially available pack for this bike and have more than 3 times the capacity (and a better form factor to boot, and will last a lot longer).
- sudosysgen 5y ago
  So the real danger from cells that have been deep discharged is dendritic growth as we both know. Yes dendrites could develop later on. But the process of dendritic growth is quite slow and gradual. Because of that, before the cells start getting too hot and threaten thermal runaway, what will happen is that the pack will run out of balance faster than the 40mA my BMS can balance. And shortly after my BMS will trip protection and I will be safe. This is actually why BMSs have a lower balance voltage - 1mA is of course not enough but they tend not to exceed 50mA so they will catch damaged cells very early removing all the risk. As for deep discharge, you might enjoy this paper : https://www.nature.com/articles/srep30248 https://www.nature.com/articles/srep30248 which also includes a short literature review. The TL;DR is that damage is relatively limited during deep discharge even to 0v. There is plausibly possible capacity degradation but it not very different from accelerated wear and tear, though the authors did not detect any. What is really dangerous though is discharging a battery pack to 0v - when that happens it's easy for some cells to gain a negative state of charge, which is when the nasty stuff that actually causes fires and damage happen. Copper dissolution according to this paper in more modern batteries does not even really happen until -12% state of charge. Indeed in modern higher capacity batteries they found no detectable damage at all until -12% soc. Because of that, I am very confident from my setup - the risk of internal short circuit is incredibly low given that I know that the cells were always in balance, and any slow-forming internal short circuit will trip the protection much before there is any fire risk. It's very interestinf though from reading the paper that the deep discharge risks essentially disappeared at 0v from pure coincidence as capacities increased though, I'd have expected somewhat the reverse :)
  - jacquesm 5y ago
    Interesting information. Agreed that in an unbalanced pack deep discharge is really dangerous, but, and this is where that copper in solution comes back to bite: the bulk of the fires happen during charging, which is when the copper comes out of solution, I'm not quite sure what 'modern' means in this context but all of the batteries that I've seen so far (Samsung, LG, Sony) caution in their MSDS against deep discharge both from a functional risk as well as from a safety perspective and there are a number of pretty knowledgeable people online who are on the record with their reputations at stake cautioning against this. There is some distance between 'paper in nature' versus 'people building high capacity Li-Ion packs for industrial applications with currently available materials', I'd rather follow the latter than the former when it comes to safety! Agreed that your setup is pretty safe though, especially if you have already cycled the pack many times and see no deviation between the cells. I've figured out the layout, it - unfortunately - won't be possible to put the pack in the frame triangle, there are too many details that will become messy, space is constrained, mounting options are limited and wiring the pack would be a nightmare with a lot of odd groupings of cells which would have a lot of potential for mistakes, so I'm going with the 'boring' option of making a rear carrier based box that will house the battery. A port at the bottom will connect a cable with a custom made plug from an old pack to the port on the bike, which can then still do double duty to charge the pack. From a center-of-gravity perspective it's not ideal, but it does allow boosting the pack to a full 200 cells in 10S20P configuration, which should be more than enough to do my runs and have room to spare. Can't wait to get it tested :)

jacquesm 5y ago

The Bosch BMS is a pretty clever little device, it governs not only the charge discharge cycle but also keeps track of the state-of-charge, and reports the remaining capacity in % to the controller via a CANBUS link, which - annoyingly - also features as auhtorization to release the motor, using some challenge-response system that to date nobody has managed to crack (though some people spent a lot of time trying to do so). So you can output voltage from the pack all you want, without the proper authentication your motor will not turn. So that's why you really need that Bosch BMS. Then the bad news: the Bosch BMS is on the edge of what's safe, the FETs are underpowered (just a hair under rated maximum current and voltage), not cooled, there is a 40V Schottky diode there which regularly operates at or even slightly above it's ratings, there is a single chip DC-DC convertor which also isn't cooled properly and which tends to blow. So I don't particularly trust the Bosch BMS from a safety perspective, I've seen a whole bunch of these now (about 15) and a good number of them had blown up. And that's with the standard battery. The one that I'll be hooking up to it is four times more powerful. Any short circuit protection in the old BMS is going to be terribly undersized so I need something in series with the Bosch BMS to ensure that if there ever is a short (say in the motor) that it's the battery that gets disconnected well before the wiring goes and I simply do not trust the Bosch BMS not to fail 'closed' in which case the whole thing will turn into a giant fuse. That's the kind of scenario that I can really do without. On more than one pack that I've opened I've found evidence of small fires, typically BMS balancing wires that shorted out (there are no fuses there, obviously) and that could have ended quite badly. The wires from the pack to the motor are substantially thicker so then the question is what cuts out first: the BMS or the wires. I simply want to play that as safe as possible and aim for the disconnect to happen fast enough that the wiring doesn't light up. Another option is a very fast fuse in line with the battery terminals, that's definitely a possibility as well (and easier to mount and less chance of it in turn breaking than yet another BMS in series). Anyway, work in progress and to be continued, nice to find someone on HN who has a lot of knowledge about this! Waiting for a while to see if there is a self discharge is a very good method to spot flawed cells before you assemble a pack but such defects can also develop at a later date, so please do keep a good eye on your pack, especially if - as I suspect - you are using reclaimed batteries, you can't be 100% sure that they have never been deep discharged. Safety is one of the main reasons why I decided to go with brand new high capacity brand name cells, yes, it's a lot of money, but even so, the whole thing will cost about as much as a 600 Wh commercially available pack for this bike and have more than 3 times the capacity (and a better form factor to boot, and will last a lot longer).

sudosysgen 5y ago
So the real danger from cells that have been deep discharged is dendritic growth as we both know. Yes dendrites could develop later on. But the process of dendritic growth is quite slow and gradual. Because of that, before the cells start getting too hot and threaten thermal runaway, what will happen is that the pack will run out of balance faster than the 40mA my BMS can balance. And shortly after my BMS will trip protection and I will be safe. This is actually why BMSs have a lower balance voltage - 1mA is of course not enough but they tend not to exceed 50mA so they will catch damaged cells very early removing all the risk. As for deep discharge, you might enjoy this paper : https://www.nature.com/articles/srep30248 https://www.nature.com/articles/srep30248 which also includes a short literature review. The TL;DR is that damage is relatively limited during deep discharge even to 0v. There is plausibly possible capacity degradation but it not very different from accelerated wear and tear, though the authors did not detect any. What is really dangerous though is discharging a battery pack to 0v - when that happens it's easy for some cells to gain a negative state of charge, which is when the nasty stuff that actually causes fires and damage happen. Copper dissolution according to this paper in more modern batteries does not even really happen until -12% state of charge. Indeed in modern higher capacity batteries they found no detectable damage at all until -12% soc. Because of that, I am very confident from my setup - the risk of internal short circuit is incredibly low given that I know that the cells were always in balance, and any slow-forming internal short circuit will trip the protection much before there is any fire risk. It's very interestinf though from reading the paper that the deep discharge risks essentially disappeared at 0v from pure coincidence as capacities increased though, I'd have expected somewhat the reverse :)
- jacquesm 5y ago
  Interesting information. Agreed that in an unbalanced pack deep discharge is really dangerous, but, and this is where that copper in solution comes back to bite: the bulk of the fires happen during charging, which is when the copper comes out of solution, I'm not quite sure what 'modern' means in this context but all of the batteries that I've seen so far (Samsung, LG, Sony) caution in their MSDS against deep discharge both from a functional risk as well as from a safety perspective and there are a number of pretty knowledgeable people online who are on the record with their reputations at stake cautioning against this. There is some distance between 'paper in nature' versus 'people building high capacity Li-Ion packs for industrial applications with currently available materials', I'd rather follow the latter than the former when it comes to safety! Agreed that your setup is pretty safe though, especially if you have already cycled the pack many times and see no deviation between the cells. I've figured out the layout, it - unfortunately - won't be possible to put the pack in the frame triangle, there are too many details that will become messy, space is constrained, mounting options are limited and wiring the pack would be a nightmare with a lot of odd groupings of cells which would have a lot of potential for mistakes, so I'm going with the 'boring' option of making a rear carrier based box that will house the battery. A port at the bottom will connect a cable with a custom made plug from an old pack to the port on the bike, which can then still do double duty to charge the pack. From a center-of-gravity perspective it's not ideal, but it does allow boosting the pack to a full 200 cells in 10S20P configuration, which should be more than enough to do my runs and have room to spare. Can't wait to get it tested :)