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I have some experience now, but I find that I now might need to go for a parallel string setup, which you typically try to avoid because of all the downsides to it.
I am using EV modules which are 3s and all covered up, meaning the only connections to the cells inside are the thin balancing/monitoring wires.
I have created a battery pack with that with an IC Gogogo BMS with active balancing, which is working nicely.
To be able to double that capacity, I can add another string. I cannot physically parallel the cells.
I am aware of all the downsides of parallel strings.
In another project I am using SimpBMS with paralleled modules, and each module will get a relatively low fuse on it. Match the voltages once, and typical load only 0.2C at max, I am not to worried.
But for this project, I would like to continue the modules without integrated slave BMS boards (so no SimpBMS) because of their suitable specs. I cannot find packaged reused EV cells at 15s.
What I could do is just create a second pack, also with the same nice active balancing BMS board connected at the - side of battery as is usually the case.
And then give each module a fuse of half the peak load (+some margin of course)
Since all modules would be from the same batch, and thus far they have been behaving very nicely and consistent, I am not expecting to much issues.
But I would be taking my chances, meaning that a fuse could blow or the BMS could keep blocking of reconnects because of the power surge between two batteries.
The BMS would be capable of handling the power, but it would trip one of its overcharging thresholds.
I am pretty confident to take that chance, and to have the customer come back only when one cell is actually failing. Which is the right time to come back.
I am being so confident because I know with that setup, the battery modules and the wiring are all protected properly. There would only be weird behavior for the user (like losing half the capacity without getting an alarm), which is annoying, but not unsafe.
However, I would much rather have an intelligent BMS that handles two strings at once, knowing when to reconnect them. And give each string a relay for charging/discharging and preferably precharge to handle any remaining power surges.
I know I could make it myself, meaning, I could write a controller which reads from Modbus or CAN of the two BMSs and disconnects and most importantly reconnects at the right moments (when they are matched again in SoC) with some big relays from an EV. But that would make the project a lot more expensive. So I would rather use of the shelf components/BMS.
I am using EV modules which are 3s and all covered up, meaning the only connections to the cells inside are the thin balancing/monitoring wires.
I have created a battery pack with that with an IC Gogogo BMS with active balancing, which is working nicely.
To be able to double that capacity, I can add another string. I cannot physically parallel the cells.
I am aware of all the downsides of parallel strings.
In another project I am using SimpBMS with paralleled modules, and each module will get a relatively low fuse on it. Match the voltages once, and typical load only 0.2C at max, I am not to worried.
But for this project, I would like to continue the modules without integrated slave BMS boards (so no SimpBMS) because of their suitable specs. I cannot find packaged reused EV cells at 15s.
What I could do is just create a second pack, also with the same nice active balancing BMS board connected at the - side of battery as is usually the case.
And then give each module a fuse of half the peak load (+some margin of course)
Since all modules would be from the same batch, and thus far they have been behaving very nicely and consistent, I am not expecting to much issues.
But I would be taking my chances, meaning that a fuse could blow or the BMS could keep blocking of reconnects because of the power surge between two batteries.
The BMS would be capable of handling the power, but it would trip one of its overcharging thresholds.
I am pretty confident to take that chance, and to have the customer come back only when one cell is actually failing. Which is the right time to come back.
I am being so confident because I know with that setup, the battery modules and the wiring are all protected properly. There would only be weird behavior for the user (like losing half the capacity without getting an alarm), which is annoying, but not unsafe.
However, I would much rather have an intelligent BMS that handles two strings at once, knowing when to reconnect them. And give each string a relay for charging/discharging and preferably precharge to handle any remaining power surges.
I know I could make it myself, meaning, I could write a controller which reads from Modbus or CAN of the two BMSs and disconnects and most importantly reconnects at the right moments (when they are matched again in SoC) with some big relays from an EV. But that would make the project a lot more expensive. So I would rather use of the shelf components/BMS.