Auto Manifesto

October 19, 2009

EV Battery Selection

Let's select batteries for a hypothetical car. This car is a conversion of your basic commuter. Let's say we want a minimum range of 60 miles for regular driving.

Key battery information needed from manufacturers is voltage (V), maximum amperage (A), amp hours (Ah), weight or mass (kg), and physical dimensions of each cell (l x w x h).

First what is the peak power target? For example, 50 horsepower (hp), 100 hp or 200 hp? Peak power is a lot more than the nominal power needed. A car may be capable of producing 100 hp but it only needs to use say 15 hp to cruise at 60 mph.

However, if it only had 15 hp it would not be able to go up hills at that speed or accelerate very well at all. So we need to know the maximum power that should be available at some point during its operation. Let's go with 60 hp for now. At this
point I'm switching to metric. One kilowatt (kW) equals 1.34 hp. So 60 hp is roughly 45 kW.

One more thing. We're going to go with a 96V system here. Voltage times current is wattage. And of course a kW is 1,000 W. So a 45 kW system at 96V will require about 470 Amps (45,000 W / 96 V).

By the way, peak power determines top speed while torque determines acceleration. That's a topic for another post but suffice it to say that we need enough torque to get to our top speed and enough power to maintain our desired speed.

Then comes how much energy the batteries need to provide, and how much such batteries weigh?. Energy is power times time.

Will the batteries provide enough power for the time (distance) needed? These are often quoted in terms of kilowatt hours (kWh).

The way to determine how much energy is needed is to estimate or model the expected vehicle drive cycle. This is exactly why it's impossible to give an exact figure for energy or fuel consumption unless very specific conditions are provided, why we
are all well aware of the line "your actual mileage may vary". The way to validate this is with actual vehicle testing.

In the above graph, the two hatched areas are roughly equivalent and illustrate why operating at full power reduces running time (and thus distance traveled) - even though speeds are higher because aero drag is much, much greater. How far a given amount of energy will drive a vehicle simply depends on how it's used.

For this example, let's assume the peak power needed is 60 hp, and our average power output is 30 hp (22.5 kW) for each 60 mile trip, which takes 1.5 hours. Thus the nominal amount of energy needed is 22.5 kw x 1.5 = 33.75 kWh.

But just because the batteries may hold an adequate amount of kWh it does not mean that's all that's needed. Primarily this is because:

1. Battery depth of discharge (DoD) should not approach 100% because that will dramatically reduce battery life (number of cycles it can be charged and discharged)

2. It leaves us with zero margin for error.

3. Does not factor in any inefficiencies (or regeneration for that matter) in the system, whether electrical or mechanical.

If for example we only want to discharge up to a maximum of 70% (30% charge remaining) to extend battery life then we need to divide the kWh by the target percent discharge. So we actually need a battery pack with 48 kWh (33.75 kWh divided by 70%).

One lithium ion battery manufacturer makes cells that have a nominal voltage of 3.2V, provides 90 Ah of energy, and weigh 3.2 kg each. So to spec our pack we need to remember that batteries in parallel add current, while batteries in series add voltage.

To reach close to 48 kWh we need 30 cells in series to get to 96V. Then we need 5 in parallel for each of those 30 cells to reach 450 Ah (we won't make the 48 kWh target). That means 3.2 V x 90 Ah x 30 x 5 = 43.2 kWh.

So 150 of these cells at 3.2 kg each means the battery pack, sans wiring and other hardware will weigh 480 kg (1,058 lbs).

Pretty hefty considering less than 3 gallons of gasoline weighing about 23 lbs would power the IC equivalent (20 mpg).

It's not a favorable comparison but we have to start somewhere on this path to vehicle electrification. And there are a lot of other variables to consider which I'll get into later.

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