JackFlorey wrote: ↑
Nov 07 2021 5:51pm
ebike4healthandfitness wrote: ↑
Nov 07 2021 5:06pm
JackFlorey wrote: ↑
Nov 07 2021 2:38pm
However, the same battery, designed to provide 40 amps at 36 volts (10s2p), will have to supply twice the current per cell if it is reconfigured to provide 40 amps at 72 volts (20s1p.)
True due to Kirchoff's Law.
At the top of a 20s1p stack, there is a single cell connected to the output lead (might be a BMS in line as well, but that doesn't affect current sharing.) All the current - 40 amps - will come from that 1 cell.
At the top of a 10s2p stack, there are two cells connected to the output lead. Kirchoff says that the sum of currents into each node must be zero. So 40 amps is leaving the stack; that means each 10s stack is supplying half the current - or 20 amps per cell. (Assuming matched cells and same charge level in each string of course.)
The thread has devolved into a horror genre.
https://www.goodreads.com/book/show/335 ... -just-read
Point is, when you ride at SAME speed, at same PHASE CURRENT, using SAME MOTOR, it means you are using SAME POWER.
And since power is volts times amps, and assuming that power is 1440w, than *controller* will draw, 20 amps from 72v or 40A 36 battery (and do the voltage/current conversion)!
And since our batteries in question are 20Ah and 40Ah respectively, both will have a nice 1C discharge rate per cell. *sigh*
Again, this is pretty obvious once you play with simulator.
In fact, 72v battery is slightly better, because your battery wires will stay colder AND it will allow you to achieve higher speeds BUT, of course, you'll need not 20A, but say, 40A current capacity because you'll be using much more POWER pushing air out of the way (and no amount of switches will help unless you'll add an *other* battery in series, but why not just wire it in and be happy about it?), but so far as climbing steep slopes are concerned, they will behave the same.
Of course, higher voltage controllers are more expencive, fets they use are a bit lossier usually, and chance of being electrocuted skyrocket when you move from 36v to 72v, but those are entirely different concerns.