A prismatic LiFePO4 battery. Image courtesy of EV Works Australia www.evworks.com.au

Lithium Iron Phosphate batteries are fast becoming the battery of choice for electric vehicle converters. Known as LiFePO4 in geek-speak, these batterys are tolerant, lightweight and require zero maintenance. They also have more than twice the capacity of lead acid batteries per kilogram.

There are things to be aware of though when using LiFePO4 batteries in your electric vehicle conversion, starting with a Battery Management System, or BMS. LiFePO4 batteries are very susceptible to damage from overcharging. This is perhaps the only area where lead acid batteries are superior in their tolerance. The way to solve this problem is to use a battery management system (BMS) for your battery pack. It’s not so much a luxury as a must-have.

Without a BMS you’ll most likely find yourself replacing batteries within 3 to 5 years – a very expensive learning curve. The complete Battery Management System(BMS) consists of 3 main elements:
1. A Charger programmed for your battery pack
2. Individual Balancing Modules
3. A Master Control Unit

Let’s explain each one. Firstly, an intelligent battery charger that has been tuned to your particular battery pack is strongly recommended. If you buy your charger from the same company that supplies your batteries, they should take care of the required tuning adjustments. Having a “tuned” battery charger means that it will be fully automatic & will shut itself down well before overcharging and/or damaging your batteries.
However, the charger can only charge the pack properly if all the individual batteries inside the pack are ‘equalized”. To ensure this happens, you’ll need equalizers (also known as balancing modules) on each battery.

The only way to effectively equalize a series of LiFePO4 batteries that would normally change their individual battery voltages is with individual Balancing Modules (one on each battery). These devices monitor each battery individually, constantly checking everything from its state of charge to its temperature. Every battery should have one, and the collective data is sent to a master control unit which monitors the pack collectively.

Lastly, a Master Control Unit which oversees the battery pack during charging is an essential. Each cell’s balancing module is wired into it, and it can control the charging and discharging to prevent overcharge and over-discharge. Those 3 items: a programmed charger, balancing modules, plus a master controller are what make up a battery management system. Battery management systems are an extra cost, but an essential cost. Expect to pay around $200 to $300 US for a master control unit, and around $10 to $20 US per individual equalizer.

An additional gadget I recommend is an in-car battery pack monitor, such as a PakTrakr or similar device. This will act as a “Fuel gauge” to monitor the total state of charge on your pack. This will be more accurate than simply reading the voltmeter and should be an essential purchase to extend the life of your pack. It’s also fun to watch how each battery acts while driving – it’ll show up any battery problems on the horizon well before they fail.