ElectricGod
10 MW
I was thinking...dangerous I know!!!
Anyway, the idea is this. use three 12S or two 16S BMS in series to get to 32S or higher. They are ridiculously cheap compared to a 32 or 36S BMS. Most of the cost in a BMS is in the mosfets. Since a 36S BMS needs 200 volt mosfets that adds significantly to the cost. If you think of a battery pack that includes a BMS as a single unit, you have a positive and negative wire that you connect to the outside world. Why not put two or 3 complete packs in series? I'm going in a couple of directions here.
1. The first one is to make modular battery packs that are all identical...say 6S or 12S blocks.
2. Be able to use the same packs interchangeably in everything.
3. Make each pack removable by simply undoing a few screws or cover and pulling out the pack.
So this gets me back to running them in series. 12S is pretty limiting IMHO. I have a scooter that runs on 12S LION, but honestly I max out the power band too quickly. I also have a scooter that currently runs at 20S and motor torque keeps going and going up to a much higher RPM. So depending on if I need a setup that is 12S, 18S or 24S or whatever, I can string multiples of these smaller packs in series to get higher voltage. I would also run them in parallel as well. Obviously there would need to be a back plane of some kind where the individual packs can be connected together in series and parallel to get the voltage and current requirements I need. I'm thinking of how easy something like this could be implemented with say 10000mah turnigy LIPO packs. I would need to make an end cap that contained the power connections and then hide a 6S or 12S BMS inside the end cap too. The whole thing would plug in end first. This same idea could be implemented in 18650 packs as well.
So then the real question is this...
Why not use several small BMS in series? Lets say, each individual pack can deliver 15-20 amps. There's lots of 6S or 12S BMS that can do that...no big deal. What kind of issues am I likely to see by running multiple smaller BMS in series? They are cheaper than a single larger BMS since there are no high voltage/high amperage components.
I've been looking at the Mitsumi MM3474 LION control chip. It is capable of handling 3 to 5 cells depending on how you set its two select pins. It detects HVC, LVC and over load conditions. I've seen implementations of this chip where they are used in up to 5 in series on the same BMS. Each 3474 only ever sees a maximum of 5S or 20.5 volts. Since the 3474 can handle a maximum of 30 volts, this is quite workable. Each 5 cells/3474 is daisy chained to the next 5 cells/3474. I see no reason why you couldn't use this little controller at 200S since each one only ever sees it's 5 cells. The total voltage of the entire pack is divided among these groupings of cells.
So this gets me back to running BMS in series. Since it is commonly done on the same BMS at the control ship level but is using the same switching mosfets in common for the entire BMS, why can't a daisy chain several BMS in series and get the same result with several lower cell count BMS's ? What's really the difference between two 12S BMS in series and one 24S BMS?
The only thing that came to mind that might be an issue is the mosfets. If I used 12S BMS as my standard and lets say I have 3 BMS in series, then that's effectively 36S. Each BMS has the 148 volts divided equally into thirds so it should only ever see 12S or 49.2 volts. My thought was, what if I have a 3 X 3 arrangement. IE: Three sets of 12S packs in series of 3 and in parallel at their end points. The idea is I need 30,000mah and 148 volts made out of 10,000 mah turnigy packs. What happens when a single 12S pack shuts off? The other two strings of three 12S packs have to take up the load obviously. There's another implication too. Current through that 3 pack string with the down BMS is going to drop to close to zero, but the two remaining packs in the string will now have 74 volts across them. Will this fry the remaining BMS's? I think as long as nothing goes wrong that multiple BMS in series will be OK. It's when things go wrong that concerns me. Every EV I've ever built used multiple BMS on separate battery packs in parallel, but there were no BMS in series. It has worked well, when you need more current delivery than a single BMS can deliver.
What do you think? Is running BMS in series doable and OK?
Anyway, the idea is this. use three 12S or two 16S BMS in series to get to 32S or higher. They are ridiculously cheap compared to a 32 or 36S BMS. Most of the cost in a BMS is in the mosfets. Since a 36S BMS needs 200 volt mosfets that adds significantly to the cost. If you think of a battery pack that includes a BMS as a single unit, you have a positive and negative wire that you connect to the outside world. Why not put two or 3 complete packs in series? I'm going in a couple of directions here.
1. The first one is to make modular battery packs that are all identical...say 6S or 12S blocks.
2. Be able to use the same packs interchangeably in everything.
3. Make each pack removable by simply undoing a few screws or cover and pulling out the pack.
So this gets me back to running them in series. 12S is pretty limiting IMHO. I have a scooter that runs on 12S LION, but honestly I max out the power band too quickly. I also have a scooter that currently runs at 20S and motor torque keeps going and going up to a much higher RPM. So depending on if I need a setup that is 12S, 18S or 24S or whatever, I can string multiples of these smaller packs in series to get higher voltage. I would also run them in parallel as well. Obviously there would need to be a back plane of some kind where the individual packs can be connected together in series and parallel to get the voltage and current requirements I need. I'm thinking of how easy something like this could be implemented with say 10000mah turnigy LIPO packs. I would need to make an end cap that contained the power connections and then hide a 6S or 12S BMS inside the end cap too. The whole thing would plug in end first. This same idea could be implemented in 18650 packs as well.
So then the real question is this...
Why not use several small BMS in series? Lets say, each individual pack can deliver 15-20 amps. There's lots of 6S or 12S BMS that can do that...no big deal. What kind of issues am I likely to see by running multiple smaller BMS in series? They are cheaper than a single larger BMS since there are no high voltage/high amperage components.
I've been looking at the Mitsumi MM3474 LION control chip. It is capable of handling 3 to 5 cells depending on how you set its two select pins. It detects HVC, LVC and over load conditions. I've seen implementations of this chip where they are used in up to 5 in series on the same BMS. Each 3474 only ever sees a maximum of 5S or 20.5 volts. Since the 3474 can handle a maximum of 30 volts, this is quite workable. Each 5 cells/3474 is daisy chained to the next 5 cells/3474. I see no reason why you couldn't use this little controller at 200S since each one only ever sees it's 5 cells. The total voltage of the entire pack is divided among these groupings of cells.
So this gets me back to running BMS in series. Since it is commonly done on the same BMS at the control ship level but is using the same switching mosfets in common for the entire BMS, why can't a daisy chain several BMS in series and get the same result with several lower cell count BMS's ? What's really the difference between two 12S BMS in series and one 24S BMS?
The only thing that came to mind that might be an issue is the mosfets. If I used 12S BMS as my standard and lets say I have 3 BMS in series, then that's effectively 36S. Each BMS has the 148 volts divided equally into thirds so it should only ever see 12S or 49.2 volts. My thought was, what if I have a 3 X 3 arrangement. IE: Three sets of 12S packs in series of 3 and in parallel at their end points. The idea is I need 30,000mah and 148 volts made out of 10,000 mah turnigy packs. What happens when a single 12S pack shuts off? The other two strings of three 12S packs have to take up the load obviously. There's another implication too. Current through that 3 pack string with the down BMS is going to drop to close to zero, but the two remaining packs in the string will now have 74 volts across them. Will this fry the remaining BMS's? I think as long as nothing goes wrong that multiple BMS in series will be OK. It's when things go wrong that concerns me. Every EV I've ever built used multiple BMS on separate battery packs in parallel, but there were no BMS in series. It has worked well, when you need more current delivery than a single BMS can deliver.
What do you think? Is running BMS in series doable and OK?