Choosing a battery type and size requires answering several questions:
- What voltage does your motor and controller require for optimum operation? The AC motor/controller we used operates between 144 and 336 volts. One conversion kit provider recommended a 144 V battery bank (twelve 12 V batteries in series), but another strongly recommended 156 V so the controller would not cut out when voltage sagged under load.
- How much weight can the donor car carry? Stay within the gross vehicle weight limit, including passengers and cargo. Exceeding the weight limit can damage the axle bearings.
- What is the weight distribution fore and aft? Heavily weighting a front-wheel-drive vehicle to the rear is probably not a good idea. Consider staying close to the stock vehicle weight distribution or about 50/50 fore and aft.
- How much room do you have? Many EV conversions have batteries mounted above the motor in the engine bay, obstructing the view of the motor and other drive components. For educational purposes, I wanted to be able to show off the electrical components, so I specified a smaller battery pack to meet the decreased space available and weight limits.
- What range do you want between charges? Although most people want as much range as possible, I purposefully sacrificed range to stay within the original gross weight and not have batteries in the engine bay. I also intend to upgrade to lithium ion batteries in the future, which will improve range.
- How much battery maintenance are you willing to do? Flooded lead-acid batteries are the least expensive type of battery but require vented battery boxes and regular maintenance, such as adding distilled water. Sealed lead-acid batteries do not require active venting or watering but require more careful charging to avoid loss of their irreplaceable electrolyte. They also need a battery management system (BMS) to ensure equal charging or regular checks of individual battery voltage to discover any state-of-charge imbalance between batteries. Nickel metal hydride or lithium ion batteries also generally require a BMS.
- How much money do you want to spend initially and in the long run? While lead-acid batteries may be cheaper initially, lithium ion may be cheaper over the long run because the battery bank will last longer.