4 ms·

Your Aliexpress link sadly doesn't work :/ It's a good idea to take care to balance the cells. That said, 1A of balance current is very overkill. I actually r

Your Aliexpress link sadly doesn't work :/ It's a good idea to take care to balance the cells. That said, 1A of balance current is very overkill. I actually ran tests as I was curious. I cycled my battery 10 times at 1C. The parallels only went 0.03v out of balance, over 10 hours of discharge and 30 hours of charge. That's at the top end, so really I had what, 0.5Ah of imbalance, over 40 hours of non-stop use in the very worst scenrio. So the 40mA of balance current from my BMS is actually enough even in the worst situation. In reality you'll be doing at most one battery cycle a day, maximum two, so you'll actually be needing only 4 mA of balance current! Because of that I decided that the 40mA was all that I needed and I didn't add any really heavy duty balancing circuit. The one that came with my BMS was more than enough, even though like you I was quite skeptical at first. So I think you should keep it simple. If I were you I'd solder the balance wires of both BMS's as parallel and go on with my day knowing that whatever I'll throw at it, it will stay balanced. Plus there is less that can go wrong :). But then again, I'm not the one not to enjoy some overengineering The thing is that when you have as many cells in parallel as we do, for you probably 15?, the imperfections between each cell really do balance out and even with the pretty knackered cells I went with it will stay in balance.

jacquesm 5y ago
Link is now fixed. Yes, 1A is overkill, 100 mA would be plenty (and likely still too much), but the Bosch BMS has maybe 1 mA balancing capability and that's too little for a pack that size so I decided to throw in something beefier and this was the smallest one that had bluetooth out so I can monitor it, a pack that gets unbalanced too much is a good indication that something is wrong, for instance a weld that's not done well, or a cell that is on the blink. Those things are best caught early with homebrew packs of this size, you really don't want to have a battery fire. A good, new pack stays within a few mV over a whole charge/discharge cycle. The reason is that if you start off with a balanced pack the laggards during discharge will be the first ones to be topped off so it all balances out in the end modulo the difference in internal resistance, and this leads to the various cells dissipating a bit more or less heat (as well as the cells on the inside of the pack being better isolated). That's cumulative and over a longer number of cycles you'll see the pack drift more and more out of balance. So it's not so much a matter of me being skeptical as it is knowing the exact limits of the Bosch BMS and the fact that if the pack ever goes out of balance more than that that it will brick the BMS, and that's a nuisance, coupled with the fact that this particular balancer had some more useful features. 16 cells in parallel, and you're right that the cells tend to self balance to a large extent. But it's also a really good way to monitor the pack so it's dual duty for me, wear monitoring (which has safety implications) and balancing all in one for less than $100, which on the total cost of this project is quite doable. The cells I'm using are $6 a piece (Samsung E35s, 3450 mAh) and 160 of them is already quite the outlay.
- sudosysgen 5y ago
  Ah, if the cells going out of balance even slightly will brick the BMS, that is something else. In that case "lying" to the BMS is a good idea and at that point you might as well add your own balancer. AS for the battery fire, that is why I suggest using a second BMS and running the phase wires from there - a good BMS such as the one I use will cut off the power when there is an even slight imbalance, which is my cue to investigate it and avoid a battery fire :) - while that second BMS would handle the balancing. But paying for 6$ 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

Link is now fixed. Yes, 1A is overkill, 100 mA would be plenty (and likely still too much), but the Bosch BMS has maybe 1 mA balancing capability and that's too little for a pack that size so I decided to throw in something beefier and this was the smallest one that had bluetooth out so I can monitor it, a pack that gets unbalanced too much is a good indication that something is wrong, for instance a weld that's not done well, or a cell that is on the blink. Those things are best caught early with homebrew packs of this size, you really don't want to have a battery fire. A good, new pack stays within a few mV over a whole charge/discharge cycle. The reason is that if you start off with a balanced pack the laggards during discharge will be the first ones to be topped off so it all balances out in the end modulo the difference in internal resistance, and this leads to the various cells dissipating a bit more or less heat (as well as the cells on the inside of the pack being better isolated). That's cumulative and over a longer number of cycles you'll see the pack drift more and more out of balance. So it's not so much a matter of me being skeptical as it is knowing the exact limits of the Bosch BMS and the fact that if the pack ever goes out of balance more than that that it will brick the BMS, and that's a nuisance, coupled with the fact that this particular balancer had some more useful features. 16 cells in parallel, and you're right that the cells tend to self balance to a large extent. But it's also a really good way to monitor the pack so it's dual duty for me, wear monitoring (which has safety implications) and balancing all in one for less than $100, which on the total cost of this project is quite doable. The cells I'm using are $6 a piece (Samsung E35s, 3450 mAh) and 160 of them is already quite the outlay.

sudosysgen 5y ago
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.