Gavin Shoebridge – an electric vehicle nut, a keen environmentalist

                Electric Car Conversion Blog By Gavin Shoebridge

February 10th, 2010 at 6:49 pm

Battery Pack Sizing for Electric Car Conversions

Calcars Plug-in HybridImage courtesy of www.calcars.org

Possibly the most confusing calculation when converting a gas clunker to run on electricity, is figuring out just how many batteries you’ll need to achieve your desired range. People often under-size their battery pack and end up disappointed with their conversion efforts.

Well, enough is enough. Let’s cut through the confusion once and for all. Let’s figure out roughly how many batteries you’ll need to drive your average electric car the distance you want. It’s actually a piece of cake:

Firstly, let’s assume a mid-sized, average home-converted electric car uses around 0.3 kWh (kilowatts per hour) of electricity per mile when driving at 35 mph. A kWh (kilowatt hour) is a unit of electricity – a bit like a “Gallon of electricity”. Easy so far, right?

This simply means to drive 30 miles at that above speed, you’ll need approximately 9 kWh (or 9 electric gallons) of battery capacity. That’s because there’s 0.3 kWh used per mile x 30 miles = 9 kWh. (0.3 x 30 = 9) Easy stuff!

Right, now if a small Deep Cycle lead-acid battery has about 1 kWh of energy stored in it, you’ll need 9 batteries to make 9 kWh. It’s a little over-simplified, but you get the idea.

You can easily figure out how many kWh are in a battery by multiplying it’s voltage by it’s Ah (amp hour) rating.
For example, a 12 volt battery with a rating of 80 Ah would be: 12 x 80 = 960 Wh.
To figure out it’s rating in kWh, you just divide the number by 1000 (960 Wh ÷1000 = 0.96 kWh). Not so scary after all huh?

This next bit is where people make mistakes. Deep Cycle lead acid batteries (ideal for electric cars) have one horrible problem: you can’t take them below half-full. If you do, you’ll damage them and shorten your battery life substantially. Trust me on this!

This means to have a usable 9 kWh of battery capacity available, you’ll actually need to double your battery pack to 18 kWh – all because you can’t go below half-full. It’s annoying, but a fact of life with lead batteries.

The next problem with doubling your pack, is the weight and size of the battery pack.
This is the same problem that every single electric car converter goes through – myself included. It’s also the reason why home-converters don’t build lead-acid electric cars with 100 miles of range: The battery pack would weigh a tonne – literally.

If you’re looking to build an electric car with more than 60 miles of range, then I recommend stepping up to Lithium Iron Phosphate (LiFePO4 in “geek speak”) batteries. There are many home conversions around with ranges exceeding 100 miles thanks to these small and lightweight batteries.

Now don’t forget that these calculations are heavily simplified using an average electric car of around 2600 lbs or so, and being driven only on roads (not highways). Driving at high speed and/or having a heavy vehicle reduces your range substantially. To combat this, I recommend several basic aerodynamic modifications to help your car slip through the air, rather than push into it, from wheel covers to grille-blockers and everything inbetween.

So that’s it. Figuiring out your battery pack size isn’t scary nor complicated. The next part is to figure out what size and voltage of lead batteries to use, and how you want them configured.
This will determine your car’s performance, as the higher the car’s total voltage, the less work the system has to do to get up to a particular speed. It’s the difference between having a road cruiser, or having something that eats gas cars for breakfast each time you get a green light!

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4
  • Batterykind
    9:50 pm on December 6th, 2011 1

    This website is a great resource! However, I found a minor error in the arithmetic above where it says “For example, a 12 volt battery with a rating of 80 Ah would be: 12 x 80 = 1200 Wh.” Shouldn’t that equal 960 Wh since 12 x 80 = 960?

    The subsequent kWh calculation should be 960 Wh / 1000 = 0.960 kWh.

    Thanks.

  • gavin
    10:11 am on December 12th, 2011 2

    You know, you’re absolutely right! I don’t know how that one slipped me by! Must have been a late night when I wrote it! Thanks for the update. :)

  • Michael Greig
    10:16 pm on January 27th, 2012 3

    Great site, thank you for all of the free information. You were one of the first to motivate me to start on my own conversion. If you’re interested, you can check it out at http://www.howtoelectriccar.com

    Keep up with the great content Gavin!

  • john
    12:27 pm on April 15th, 2014 4

    Hi, im hanging with 2 (for you) simple question about a little conversion trial im about to start for myself. I have an old sit lawn mower with a broken engine and I want to give it a go in converting it as it is in my perspective a nice smaller way to start out with. My 2 questions are:

    1: which electric motor do you advise me to get?
    2: how much battery charge do you advise?

    Could you help me out? You can email me and I thank you up front for your input.

 

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