Estimating battery size & cost

There are a lot of EV conversion spreadsheets etc out there on the internet, and I found most of them pretty confusing. On this page, I’ll try to explain how I figured it all goes together. The outcomes match the results of the Internet-spreadsheets to within a few percent, so it looks like my logic should work.

Battery-size vs. range

Since I started this project, I’ve found 2 basic rules that seem to cover most of the data-points out there.

The simplest rule I found back in 2018 was that you need 8 kWh of battery capacity to match 1 gallon of regular gas.

My “normal” car (a Ford Flex) takes 12.5 gallons to get me to work and back, so 250-260 miles. That’s 5 gallons for a one-way trip of 100 miles (a decent range for an EV conversion), which turns into 8 x 5 = 40 kWh in battery capacity.

The Cadillac with its original gas engine gets between 10 and 11 mpg. To go 100 miles would be 9-10 gallons. I’m making an assumption here that the original engine was not as fuel-efficient as say a 1998 or 2004 gas engine, and that with a slightly more modern drive-train I could do the same trip getting 14 or 15 mpg (more or less the same as a regular full-size pickup from the 90’s). That’s 6.8 gallons of gas to go 100 miles, which turns into 53 kWh of battery capacity.

The other way of calculating that I found is that a light-weight conversion (like an MG-B or Beetle) uses app. 260-300 Wh per mile, and that my car should come in at between 1.5x and 2x that. The front area of the car is 1.35x that of a beetle, it has more corners etc, so the drag is probably 1.5x. Add the 2x (or 3x) weight differential, and a 2:1 ratio seems OK. That’s 550 Wh/mile, at 100 miles, comes out to 55 kWh in battery capacity.

Between the two ways of calculating this, fair enough, and let’s split the difference for 54 kWh of battery just for the driving part.

That’s the good news.

The bad news is that you are not supposed to drain a battery down to zero. You have to leave something else the life expectancy turns to mush. The numbers I have found show that a lithium battery will give you 2,500 cycles if you consistently leave 20%, 1,000 cycles if you leave 10%, and 500 cycles if you leave 5%.

What does that mean? If you drive your electric car on the weekends and you run through 1 charge per weekend, 500 cycles will last you 10 years. For that project, having only having 5-10% spare capacity & wearing out the battery faster (in “only” 10-20 years) is probably fine.

In my case, I intend to drive this car every day to work, so 200 days a year, and I need to charge it twice a day (once to get to work, once to get home). That’s 400+ cycles a year, and if I want to get a battery life of 5+ years then I need the 2,500 cycles so I have to  allow for 20% spare capacity.

So, to make everything work for this project, I need 54 kWh to make the 100-mile trip, plus all of the accessories, and then plus 20% extra to keep my life-span OK. The car has quite a few accessories, and over the course of 1.5 hours of driving (100 miles at 65 mph), they add up to:

  • Lights, 0.2 kWhr on LED’s
  • Radio, 0.1 kWh
  • AC and heat, 4 kWh
  • Pumps to drive the power-steering, brakes,  1 kWh
  • Windows, seats, gadgets,  0.1 kWh

All in all, my minimum battery-pack capacity should be 54 kWh for driving + 5.5 kWh for accessories = 60 kWh, + 20% spare = 72kWh total battery capacity.

Battery cost

There are a LOT of different battery technologies and systems out there, and just as many web-sites listing all of the pro’s and con’s of every single one. I won’t repeat all that, I’ll just say that my conclusion was that for EV conversions that need at least 30 miles of range and at least 2-3 trips a month, lithium batteries are the preferred option. Between the weight, the size, the cleanliness of not venting nasty gases etc, I believe the down-side of the additional cost seriously outweighs the technical down-sides of the other battery types.

The first time I did the math to see if this project could go anywhere, a kWh of battery capacity was around $300-$320. That was in early 2016. When I started the project in earnest (spring 2018), that cost had dropped to $225-$250 per kWh. At the time of this update (spring 2020), Tesla batteries still hover around $250-$300 per kWh, but cheaper units are starting to show up at $180-$200 per kWh

Given the need for a really solid battery pack (who wants to be stranded 60 miles from home in the middle of pretty much nothing), and the range of 125 miles, the cost of the battery pack is 72 kWh (for 100 miles) x 1.25 (to get to 125 miles) x $300 = $27,000.

Battery technology, cost-saving opportunities

The discussion around battery technology is everywhere on the internet, and I’ll let it be. For me, there were 3 choices:

CALB lithium cells (or similar)   

or, when you turn them into a “pack”, it looks something like this:    

These cells are very elegant. They hold a lot of capacity for their size, and they are really well laid out for easy interconnects. The only down-side I had was the price in that some of the other alternatives were on sale and CALB was not. If all had been equal, I would more than likely have gone with CALB cells (or same thing from different vendors).

Tesla lithium modules   

or, when you turn them into a “pack, it comes out like:

These Tesla cells are made up of 330 Panasonic 18650 lithium cells each. The come from excess/obsolete inventory and as a result have some issues around reviving them and getting to capacity, but they are excellent value for money ($750-or-so for 3 kWh incl water-cooling jacket, monitoring plug etc). The major down-side (for me) is that they’re extremely awkward at 40″ long. I would need to find space for 30 of them, and there is only so much usable space to hide things that big.

DIY lithium batteries / 18650-type cells   

or, when you turn them into a “pack, it comes out like:  

I originally decided to go with the standard Lithium cells out there (referred to as the 18650, originally a Panasonic product, and be very weary of knock-offs….) and make my own batteries.

The first set that I put together did work as advertised, and at $177 per kWh it would have made my project a lot less costly. However, building your own batteries does have some significant downsides:

  • It takes forever
  • In a pack with parallel cells, each cell needs its own fuse (to avoid fire etc when one shorts out in the middle of a pack). Those fuses are not-cheap and a pain to install
  • Getting the pack mechanically strong enough so it can be mounted in a car and ensure that the terminals are not bearing any mechanical load takes some very fancy sheet-metal, and that gets expensive fast (laser-cutting, spot-welding, specialty gluing).

My 90 kWh battery would have used 11,000 (yes, eleven thousand) of these cells, times 3 soldered connections, and it’s a second job / a cottage industry just to build the battery modules.

So, after building the first pack at 10 kWh, I have decided to switch. I’ll re-configure the pack to become the 12V low-voltage battery, and I’m looking around for a more time-efficient battery. Once it’s all figured out I will up-date the page.