That concern is valid. The cells will only withstand a limited rate of discharge, relative to their capacity. But this is a peak demand issue, and to determine what current is sufficient for your vehicle, you need to consider it in combination with the voltage.I guess I am thinking in terms of discharge rate (C rate?).
Do I assume a 1C rate, 2C rate, 3C rate, 20C rate?
Now we're looking at range from an energy viewpoint, which is good, but there's the first problem... While each module has a nominal capacity of about 1 kWh and so ten of them will have a nominal capacity of about 10 kWh, I doubt that's enough for 60 km or 40 mi range. That's less capacity than a typical modern plug-in hybrid, and they have trouble going 60 kilometres or 40 miles.This pack will give me 10kWh. I only need 60km range (40 miles) .
If you know how much torque you need, you can look that up on the AC-51 performance chart to see what current is required, then divide that by capacity to get the "C" rate. How much is too much? I don't know; what is the maximum discharge rate in an eGolf?I guess I am thinking in terms of discharge rate (C rate?).
My concern is when I am accelerating or going uphill. Can 75ah batteries handle that?
Do I assume a 1C rate, 2C rate, 3C rate, 20C rate? How do you calculate the min ah needed in a battery pack (not for range) but for the amps that getting pulled out?
I agree that they don't need high rates, but this has nothing to do with voltage. Current EVs usually run at about 360 volts (so 80 kW is 222 A @ 360 V, 74 A per cell with 3 cells in parallel), but if they ran at half the voltage they would run at twice the current using the same total pack size with half the number of cells in series and twice the number in parallel (so 444 A @ 180 V, still 74 A per cell with 6 cells in parallel), leading to exactly the same discharge current for the same cells, or the same "C" rate.Commercial EV manufacturers batteries are not required to discharge at high C rates because they run much higher voltages. There is no reason they should spend that kind of money on a high discharge rate battery. All they need is 5C or less for acceleration, and they cruise on Sub C rates of C/2 to C/5.
I agree, in general. Discharge rate is low - and not a problem - for most EVs because their major constraint is total energy capacity, not power capability. You can express this in current, but working it out in terms of power gives the same result.At the very minimum the battery must be fully capable of delivering the maximum Discharge Rate that will be encountered.
Of course no EV manufacture would do that because RANGE is the name of the game, and they design on minimum RANGE, and Discharge Rates take care of themselves because they will be much lower.
Great. That is the info I was looking for.As for discharge rate, only the battery manufacture can tell you that. Commercial EV manufacturers batteries are not required to discharge at high C rates because they run much higher voltages. There is no reason they should spend that kind of money on a high discharge rate battery. All they need is 5C or less for acceleration, and they cruise on Sub C rates of C/2 to C/5.
So to design there are two ways to go. At the very minimum the battery must be fully capable of delivering the maximum Discharge Rate that will be encountered. So if you have a system that draws 450 amps peak, and you have 75 AH cells, they must be capable of 6C discharge rate for about 20 to 30 seconds every 5 or 10 minutes.
Otherwise your batteries will not perform, and worse overheat and go into Thermal Runaway. Using that methos range is whatever it is, piss poor because you went with the minimum size battery capacity you could get away with.
Cool. Thanks.These cells can do 10C, but need cooling then, that's how they're used in the PHEV versions of the volkswagen GTE / audi e-tron
the e-golf version I'd not use more than 3C (short burst of more power shouldn't be a problem)
The 25Ah cells are from Panasonic.
newer e-golfs (i think from 2017?) have a 50% higher capacity with the same size of battery modules, these are Samsung SDI cells.
also the VW passat GTE uses Samsung SDI cells, same size but only 15% higher capacity than the panasonic 25Ah cells. Presumably due to the higher C-rating of these cells opposed to the e-golf version.
All these battery modules you can monitor with Tom de Bree's 'Simple BMS'
Yes, and both of those alternative configurations require twice as many modules, so the same power means half as much power per module, so half the "C" rate.My concern was whether at 144 v the 75 ah battery pack would have a sufficient discharge rate for long enough to power the truck up hill for example.
So assuming 450 amp draw while climbing, the batteries would have to be able to perform at a 6C rate (6x75) = 450 amps.
In the above example if the system was 288 v and using 75ah batteries then the draw would be at 3C. Correct?
If the system was 144 v and using 150ah batteries then the draw would be at 3C. Correct?
So as it turns out I will be going with 20 modules. Thanks everyone.[*]450 amps @ 144 volts = 65 kW; 2 modules in parallel and 10 module pairs in series (20 modules total) are required; 450 amps from two parallel modules is 3 times the 75 Ah module capacity
If you can fit 20 of the e-Golf modules in (so 20 kWh nominal capacity), that would certainly be more appropriate for both discharge rate and range than making do with only 10 modules.