Capacity – Estimating range<> battery size

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-capacity/size based on my range requirement

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.

The Cadillac with its original gas engine gets between 10 mpg (or, using the then-brand-new feature of Cruise Control, 11 mpg!), so going 100 miles would be 9-10 gallons. I’m making an assumption here that the original engine/transmission was not as fuel-efficient as say a 1998 or 2004 edition gasoline engine, and that with a slightly more modern drive-train in the same car I could do the same trip getting 13 or 14 mpg (more or less the same as a 1990’s regular full-size pickup or Ford Excursion). That’s ~7 gallons of gas to go 100 miles, which per simple rule #1 turns into ~7 * 8 = 55 kWh of battery capacity.

The other way of estimating that I found is to use multiples-of-Beetle-power. It’s a little empirical, but if it works then who am I to complain.

Supposedly, a light-weight conversion project (like an MG-B or Beetle) needs app. 250-260Wh per mile. The Caddy has a front area that’s about 35% larger than a Beetle’s, and front area is the biggest factor to go into “drag”. Drag adds a negative factor squared, so that would mean that just off-setting for drag, the Caddy would need 1.35 * 1.35 * 255W = 465W per mile. Then I added a weight penalty for the pieces of the commute that are “city streets” (seems fair :-), and 1 Cadillac = 2.2 Beetles. This gets me an estimate of 550-560W per mile for the Caddy, so 55 kW for the battery per 100 miles.

With both ways of estimating battery size essentially agreeing, I feel pretty confident that I’m not way off.

My commute is 125 miles and that takes just under 2 hours. For all of those 2 hours that I spend driving, I need the accessories to work. The biggest / most constant ones are

  • On-board computer, power-drives etc, 250 W for 2 hours on end, 0.5 kWh
  • Pumps for power-steering and brakes, 0.5 kWh
  • Lights (LED), wipers, windows etc, 0.4 kWh
  • Radio and gadgetry, 0.1 kWh
  • AC/heat, this is an unknown until I get that far into things, but let’s go with 5 kWh

So, for my commute, I would need 125 miles * 550 Wh/mile to drive and 6.5 kWh to be comfortable. The car is going to need 75 kWh in battery capacity to do what I need doing…

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 5,000 cycles if you consistently leave 20%, 3,000 cycles if you leave 10%, and 2,500 cycles if you leave 1%.

What does that mean? To me, it means that if you drive your electric car on the weekends and you run through 1 charge per weekend, then 5,000 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 200-250 work-days a year, and I need to charge it twice a day (once to get to work, once to get home). That’s 400-500 cycles a year, and if I want to get a battery life of 10 years then I need the 5,000 cycles so I have to allow for 20% spare capacity. It’s not bad, in a pinch I can push things to 90% discharge and not do damage, but in normal-normal use 20% seems right.

Starting at 75 kWh and adding the 20% Depth of Discharge limiter puts the minimum required size of the battery at 90 kWh to get me from A to B in comfort.

This is also where I repeat the caveat that all of what I did here is Hopeful Math used to get me an guess at battery size. No more no less, none of this is proven, and until we’re rolling down the road I really have no way of (dis-)proving any of it. I’m just going with it because I have to go with something….

Battery cost

There are a LOT of different battery technologies /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 of it, I’ll just say that my conclusion was that / IMO for EV conversions that need at least 30 miles of range and at least 2-3 trips a month, some form of lithium battery is the preferred option. Between the weight, the size, the cleanliness (like not venting nasty rust-inducing gases etc) I believe the down-side of the additional battery cost is a small price to pay for the technical down-sides of the other battery types.

The price of lithium batteries has been dropping, as they should be, and that means that the price of the battery pack is not necessarily what I paid for the thing.

The first time I did the math in early 2016, to see if this project could even go anywhere, a kWh of battery capacity was around $300-$320.

When I started defining the project in earnest (spring 2018), that cost had dropped to ~$200 – $220 per kWh. By the time I bought the major components (early 2019), Tesla batteries still hovered around $180 per kWh, but cheaper units were starting to show up at $150-$160 per kWh.

Given the need for a really solid battery pack (after all, who wants to be stranded 60 miles from home in the middle of pretty much nothing), the cost of a 90 kWh battery pack came in at almost $18,000 + taxes + Shipping & Handling (it’s big, it’s heavy, it’s dangerous) so just over $25k. Oomph.

Battery technology, cost-saving opportunities

The discussion around battery technology is everywhere on the internet, and I’ll let it be. For me, when I started buying the major bits for the projects, there were 3 choices:

Option 1 – CALB lithium cells (or similar)   

   or in pack form   

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 cost per kWh, not that they’re not worth it but that I can’t spend that much and still do the rest of the project. If money were not this tight, I would more than likely have gone with CALB cells (or same thing from different vendors).

Option 2 – (Used) Tesla lithium modules   

   or in pack form   

For this project, I ended up with Tesla cells from excess/obsolete inventory. As a result I’ll probably have some issues around reviving them and getting to capacity. I first ran across these in a work-project and was pretty impressed with how they were built, and when I found an offer for a batch of these for ~$250 per kWh I jumped. That price gets capacity, proper fusing, temperature sensors built in as well as water cooling. 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 “straight” space to hide things that big.

Nowadays, there are a LOT of used battery packs around, from Tesla cars, from the Nissan Leaf and the Chrysler Pacifica, and in the end it’s all just more of “this”. The pricing is OK and there’s competition, so the money math is what it is just like the battery size math is a given. Just consider how you’re going to fit X cubic feet of battery into Y cubic feet of car space. Donor cars were not designed/built to fit anything big and boxy, they were built to wrap around a combustion engine with exhaust pipes and a gas tank. It’s either working with a series of small, odd-shaped cavities, or a loss of rear seats etc…. 

Option 3 – DIY lithium batteries / 18650-type cells   

  or in pack form

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. Each 18650 roughly holds 12 Wh, so my 90 kWh battery would have used 7,600 (yes, seven and a half thousand) of these cells. Not that bad, then again.

The first set that I put together did work as advertised, and at “only” $177 per kWh it would have made my project a lot less costly (saving me $12,000). Also, when doing this, you can shape the pack to virtually anything which means I can put cells in awkward places etc. They’ll fit around things like power steering, brake booster and frame elements. If you can pull this off, it makes for an easy (easier) design overall.

However, building your own batteries also does have some significant downsides:

  • It takes forever. There’s thousands of these cells to make a pack (in my case, almost 8,000) and each one has 2 ends that need at least 3 welds each. That’s a LOT of welding.
  • 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)
  • Adding a water-cooling (-preheating) system into that pack, and the whole enterprise becomes ridiculous

All in all, my test-pack worked, but I have still decided to switch. I’ll re-use the pack I’ve made to become the 12V low-voltage battery, and I’m switching to pre-made packs for the major drive-battery