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Some of the batteries I looked at are 6V 75Amps for 115min(from Advance auto). That would suggest to me that a discharge rate of over 0.52C may be bad for the batteries. Is this true? If so, to handle a 9kw load it would be necessary to have 20 batteries. 20 batteries would weigh about 900lbs, which is a lot of weight for a small car. Now some of this would just be replacing ICE components, but much of it would be extra weight added to the car. Now logically 4 people in the car should weigh somewhere around 600lbs and the ICE components should weigh a couple hundred pounds so the weight of the converted car with no people in it shouldn't be much heavier than a fully loaded ICE version, but I'm still concerned about the extra load on the suspension. The other thing is that the batteries are about 7"by10"by12". In a fairly small car where would I put 20 batteries this size? There will be some room under the hood, and possibly in the back but will there be enough? I'm going to do more research on what other people have done regarding battery location in EV conversions.
There's also the problem of system voltage. In order to achieve 48 volts I'll need 8 batteries per string, and if I need at least 20 batteries that means I'll need 3 strings of 8 batteries. Now 24 batteries is a lot of batteries, but it should also decrease the discharge rate leading to better cycle life. On the bright side, having 24 batteries in addition to improving the number of charge/discharge cycles should increase the range of the vehicle. I don't really need more than 25-30 miles, but if it's extra cold or windy the range could come in handy. Now going 45mph according to the calculator is going to draw 9.3kw with a motor a little less than 80% efficient (about 7.2kw/0.8). So if the battery pack is giving me 20.7kwh(24* 6V * 75Ah * 115min / 60min/h) it should be able to handle 45mph for 133min, about 2.2hrs. That means a discharge rate of 0.45C and a total theoretical range of 99mi with no starting or stopping, heat, ac, windows or anything. Obviously that won't be the case, but even if all the energy starting and stopping was 100% wasted, a 2600lb vehicle going 45mph has 66wh of kinetic energy.(however around 80% efficiency 66/0.8 about 80-85wh is put in) So even if you stop 100 times from 45mph abruptly with no coasting that's just 8kwh of energy meaning you still have about 12 left to drive on which gives a range of 60mi. Now driving like this would obviously be detrimental to both the car and battery pack with such large bursts of energy demanded from the battery pack and draining it near 0% capacity, and I don't expect to do anything like that any time soon, this is just a thought experiment. If it holds true though there may be a promising bit of range and battery life. If I could achieve something around 60mi of range and 500 charge cycles that would mean the battery pack would last about 30,000miles, which would not be too bad.
As a side note, how many charge cycles do you think are possible if you only use about 30% of the battery on a daily basis, and never exceed a discharge rate of over 1C?
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