can someone better with electronics than me help with some calculations?
I want to do a battery pack of 40 cells (20 + 20) at ~72v
how do I do calculations to figure out amps and how do I know which battery management system and which charger?
my (possibly wrong) calculations seem off:
each battery cell is 3.7v at 3400mah (3.4ah)
total should be at ~250 watts
total current should be ~6.8 amps? so anything rated at 7 or 8 amps would work?
20 batteries in series would be 3.7*20 = 74 volts, current would be the same at 3.4 AH. Battery voltage adds in series, current stays the same.
A second set in parallel would double the current capability to 6.8 amps, voltage would stay the same.
So you would have a 74 volt, 6.8 amp hour battery.
Watts = Volts x amps
74 volts * 6.8 amps = 503.2 watts
I think your going to need a bigger battery, Jim ended up with something around 40 amp hour battery. Your going to need something like 11 sets of cells in parallel. So 20 in series to get 74 volts. Then 11 sets of those in parallel to get more current. I think some of those BMS allows for 100 amps of current.
A 4000 watt motor can draw 55 amps at 72 volts. So when you initially accelerate from a stop you could easily be pulling 55 amps, depends if that number is peak or continuous. Peak you may draw around 100 amps.
4000 watts @ 72 volts = 55 amps,
Your current calculation is a bit off, the battery max current needs to be calculated using the maximum discharge of the cells, not the capacity, so if the cell can discharge at say 10 A, then the max current for that battery pack is 20 Amps.
This of course is very dependant on battery chemistry, Li-Ion is probably the best bet for this application as LiFePo or LTO are too bulky at any reasonable amperage.
Check the specs on the 18650s, a single 18650 can have a discharge current of 5A, 10A, 20A or 30A. There's a tradeoff between discharge current/cost and capacity, so it's something you want to look into.
But, if we take this battery: https://eu.nkon.nl/rechargeable/li-ion/18650-size/lg-inr18650-mj1-buttontop.html
It has a voltage of 3.6V nominal, 3500mAh capacity and a discharge current of 10A.
If you start with 20 cells in series, aka a 20S battery, you get 3500mAh at 72V nominal. Then you can stack say 4 of these in parallell, that I'll give you a 20S4P battery, with 14Ah at 72V. This pack has 4 parallell strings of 10A batteries, so the maximum discharge current is 40A, aka 72V * 40A = 2880 Watts.
You usually don't want to exceed the rated discharge current of the cells under any conditions, so a 20S6P or 20S8P battery would be better, as they could theoretically support a 4320W or 5760W motor respectively. Of course, if you went for 20A cells, you could cut the parallell strings in half, but that would also cut your battery capacity in half (or even less, as you'd be hard pressed to find 3500mAh cells at that discharge current).
Also make sure that your chosen BMS has balancing leads for every cell in series, aka. a 20S BMS, it's important for these larger battery packs if you want to have long life from the cells. The BMS only cares about how many cells you have in series, and the maximum battery current, so size accordingly.
This is of course from a guy with just an electrical background, not someone with any experience with DIY EVs, so real world experience may dictate otherwise.