5 ms·

Ah, if the cells going out of balance even slightly will brick the BMS, that is something else. In that case "lying" to the BMS is a good idea and at that point

Ah, if the cells going out of balance even slightly will brick the BMS, that is something else. In that case "lying" to the BMS is a good idea and at that point you might as well add your own balancer. AS for the battery fire, that is why I suggest using a second BMS and running the phase wires from there - a good BMS such as the one I use will cut off the power when there is an even slight imbalance, which is my cue to investigate it and avoid a battery fire :) - while that second BMS would handle the balancing. But paying for 6$ cells, my oh my, that does make it worth it :) I paid all in all 70 cents a cell and even then I was quite skittish :P

jacquesm 5y ago
Yes, second BMS is included as well, it has a whole bunch of safety features and will be in series with the Bosch one so that it gets first say in cutting out. Though likely the whole Bosch BMS would evaporate if that pack ever shorts out through the BMS :) But that one didn't have the ability to actively balance the pack (which I don't know if I'll ever need but better safe than sorry, once sealed I'd like to keep it sealed, there is a lot of rain here), and it didn't have a bluetooth hookup possibility. Especially in the beginning I'll be monitoring that thing like a hawk, I'm a pretty good welder but spotwelding with rickety gear by hand is a thing that I'm not 100% confident about and it won't take much to mess that up. 70 cts / cell?? Wow. Consider me somewhat jealous, that's an absolute bargain. But better keep a good eye on them then, a FLIR isn't a luxury, it is a good idea to keep an eye on the cell temperature using a FLIR while charging, bad cells will stand out immediately (as well as later on, when they are possibly warmer than their brothers and sisters around them which should be at ambient fairly soon after you stop charging, that's a good indication of a partial short in a cell). I've already spotted some bad apples like that which when checked out on a reference charger seemed to be perfectly ok. It also helps to spot bad welds, simply load up the pack and look at the interconnects, any bad welds will be higher temp because of their increased resistance.
- sudosysgen 5y ago
  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).

jacquesm 5y ago

Yes, second BMS is included as well, it has a whole bunch of safety features and will be in series with the Bosch one so that it gets first say in cutting out. Though likely the whole Bosch BMS would evaporate if that pack ever shorts out through the BMS :) But that one didn't have the ability to actively balance the pack (which I don't know if I'll ever need but better safe than sorry, once sealed I'd like to keep it sealed, there is a lot of rain here), and it didn't have a bluetooth hookup possibility. Especially in the beginning I'll be monitoring that thing like a hawk, I'm a pretty good welder but spotwelding with rickety gear by hand is a thing that I'm not 100% confident about and it won't take much to mess that up. 70 cts / cell?? Wow. Consider me somewhat jealous, that's an absolute bargain. But better keep a good eye on them then, a FLIR isn't a luxury, it is a good idea to keep an eye on the cell temperature using a FLIR while charging, bad cells will stand out immediately (as well as later on, when they are possibly warmer than their brothers and sisters around them which should be at ambient fairly soon after you stop charging, that's a good indication of a partial short in a cell). I've already spotted some bad apples like that which when checked out on a reference charger seemed to be perfectly ok. It also helps to spot bad welds, simply load up the pack and look at the interconnects, any bad welds will be higher temp because of their increased resistance.

sudosysgen 5y ago
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).