Batteries

Batteries are the most expensive and most critical part of an electric vehicle conversion.  They are the fuel tank and the top speed combined.  There are a variety of battery technologies and formats available and a lot of factors to consider when buying them.

Terminology

• A battery is comprised of cells.  Each cell is a separate physical unit and they are connected together to form the battery.  Cells are usually connected in series (one after the other, nose to tail) but can also be connected in parallel.
• Each cell has several ratings: the maximum and minimum volts (V) it can produce, it's amp-hour (Ah) capacity, and its rate of discharge (C).  A one amp-hour cell can theoretically deliver one amp for an hour, although that might severely drain the cell and possibly destroy the chemistry.  A one amp-hour cell rated at 1C can deliver at most one amp - a 2C cell of the same capacity could deliver two amps.
• Cells in series add their voltage.  Cells in parallel add their amp-hour capacity.

Battery types

The most common format for vehicle batteries is still Lead Acid - the standard car battery.  These are relatively inexpensive to buy, but their weight and relatively low charge capacity makes them a last resort when planning an EV conversion.

The newer technology is Lithium Ion, and most of these batteries are Lithium Iron Phosphate (LiFePO₄).  This chemistry is quite stable and avoids most of the problems of expansion when heating, explosion when overcharged and so on.

Lithium Ion batteries come in two main formats.  One is the Prismatic cell - usually a rectangular box with two terminals on the 'top'.  The other is Cylinder cells with a terminal at either end.  Both usually have some kind of screw on each terminal to bolt the electrical connection on.  Cylindrical cells usually range from 1 to 10 amp-hours, Prismatic cells are much higher (40 - 400 amp hours).  Both hold roughly equivalent charge per kilogram.  While the prismatic cells allow closer stacking with no wasted space, the smaller cylindrical cells allow a more flexible battery layout, which may be crucial in some vehicles.

Factors to consider

When planning a battery pack the following factors have to be balanced:

• The maximum and minimum voltage required.  The motor you choose will have a maximum voltage it is designed to handle.  In brushless DC and some other types of motor, voltage is directly equivalent to speed, so setting a minimum voltage you want your pack to produce will determine your minimum 'limp home' speed.
• The amp-hour capacity of the pack.  The larger the amp-hour size of the cells, the longer they last but the larger and heavier they are and the more they cost.  It is best to get cells of the same amp-hour capacity because when
• Each cell goes from 3.2 to 3.5 volts in regular operation and can go as high as 4.8V and as low as 2.5.  Beyond that you have cooked your cell.
• The larger the cumulative battery voltage, the less current it needs to supply to achieve a particular amount of power.  Since resistance is proportional to the square of the current, at higher voltage smaller cables can supply a larger amount of power without heating up too much.
• The batteries have to fit in the space available and be carried by the frame and motor.