5 ms·

Hey, I've done something similar with a Lenovo battery. If you're willing to scrap a battery pack, it's definitely doable by reusing the electronics. You should

Hey, I've done something similar with a Lenovo battery. If you're willing to scrap a battery pack, it's definitely doable by reusing the electronics. You should add your own BMS though. It's fine to run them in parallel for the balance wires, but connect the phase wires to your own BMS. It should work :) I recommend Daly BMS's, they're not too expensive and they kept my 1.5kWh pack in balance and protected it from a few accidents.

jacquesm 5y ago
That's exactly the plan, with one very small modification, I'll be adding a 1A balancer that gets triggered on the 'charge' signal, and I'll replace the balance wires to the original BMS with a laddernetwork of resistors to fool the old BMS into thinking it is still seeing a nicely balanced pack. This is the balancer (not a BMS, just an active balancer): https://www.aliexpress.com/item/32945565819.html https://www.aliexpress.com/item/32945565819.html What's neat about this one is that it uses two supercaps as a chargepump so it can rebalance quite effectively without producing a lot of waste heat or wasting power (of course you waste some but a small fraction of a resistance based balancer, which just burns off the power in the cells that are highest voltage). I'm building a small test pack (20 cells) right now, and if that works properly I'll scale it up eight fold for full range. It will still plug into the standard charger and I'll have a switch that will allow the pack to be charged to 100% capacity in case it is needed for an even longer trip.
- sudosysgen 5y ago
  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

That's exactly the plan, with one very small modification, I'll be adding a 1A balancer that gets triggered on the 'charge' signal, and I'll replace the balance wires to the original BMS with a laddernetwork of resistors to fool the old BMS into thinking it is still seeing a nicely balanced pack. This is the balancer (not a BMS, just an active balancer): https://www.aliexpress.com/item/32945565819.html https://www.aliexpress.com/item/32945565819.html What's neat about this one is that it uses two supercaps as a chargepump so it can rebalance quite effectively without producing a lot of waste heat or wasting power (of course you waste some but a small fraction of a resistance based balancer, which just burns off the power in the cells that are highest voltage). I'm building a small test pack (20 cells) right now, and if that works properly I'll scale it up eight fold for full range. It will still plug into the standard charger and I'll have a switch that will allow the pack to be charged to 100% capacity in case it is needed for an even longer trip.

sudosysgen 5y ago
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