robinhood said:
I am planning to use these to power a 5kW motor in electric vehicle application.
What vehicle?
I'm using mine on a (large and unfortunately heavy) custom-built electric cargo & dog-hauler trike, the SB Cruiser linked in my signature.
I use Li-NMC packs and they're not very heavy, a 2kWh pack weighs around 20lbs
Mine are also NMC (28x EIG 20Ah pouch cells); I think just my cells weigh somewhere around 30+lbs, then the busbars and cell holders (also by EIG) add several pounds more.
Which cells are you using to get them so light?
To prevent the issue of overloading the other packs in case of failure of a single unit, is it better to use a master BMS that will control the power output of these four battery packs and cut-off in case there is a single pack failure? I am talking about a BMS in addition to the ones already present in each of the battery packs. I could run them in parallel through this parallel BMS. Or is it just an unnecessary addition that will increase my costs?
It's not really necessary. Adds significant cost (because now you need a BMS that can by itself handle all the current of all the packs, or that can run a contactor to switch the paralleled output) and complication (lots of extra balance wires running out of each pack to the master BMS, etc).
If you wanted to, you could use optocouplers (so there is no direct voltage / current path from any pack's sensitive BMS parts to any other, in case of failures or wiring shorts) and link the BMSs so that if any one of them triggers for an LVC, it tells all the others to turn off, too. I don't know exactly how complicated that would get, but probably not very.
Personally, I don't even use a BMS, because the cells I have stay balanced in my application (rarely discharged all the way down, only once so far in 2-3 years of use), and I would rather damage the pack than have it cut off while I'm in traffic and really have to get out of the way (or else get hit or run over).
You *can* use something like the Cycle Analyst from Grin Tech, which can limit current or power based on presets (and maybe from the aux input; can't remember), then use that optocoupler circuit to bring the BMS enable/disable monitor lines out to a circuit that changes the preset from "normal" to "limp home" mode once a pack shuts down, reducing current draw from the system to reduce load on the batteries and ensure you have range to get back home or at least to an outlet to charge, instead of just suddenly being stuck where you are.
A simpler way is that if the packs have an LED on the BMS board, you can wire that out to somewhere that you can see them while you ride, so when the LED indicates a pack has shutdown, you'll at least know that, and can yourself decide what to do. A bit like having the Add Fuel / Gas Tank Low light come on in an ICE car.
BTW, sorry I misunderstood the number of packs. I assumed from the original statement that you were using 1kWh packs, because you said you were running the first one at 1C on a 1kW motor, and then said you were going to run more (four) of the same packs at 1C on a 5kW motor. That led me to assume you were going to *add* four more to the original.
But if you're actually going to be running four 2kWh packs, then that will be much less than 1C per pack.
