LiFePO4 Care Guide: Looking after your lithium batteries

Ian Hooper, 8th May 2010

 

Introduction

LiFePO4 chemistry lithium cells have become the popular choice for EV conversions in recent years due to being one of the most robust and long-lasting battery chemistries available. They will last up to ten years or more if cared for correctly. Please take a moment to read these tips to ensure you get the longest service from your battery investment.

This guide applies to all ThunderSky and Sky Energy Lithium Iron Phosphate (LiFePO4) batteries.

Tip 1: Never use without a BMS!

The most common causes for premature failure of LiFePO4 cells are overcharging and over-discharging. Even a single occurrence can cause permanent damage to the cell, and such misuse usually voids the warranty. A Battery Management System (BMS) is required to ensure it is not possible for any cell in your pack to go outside its nominal operating voltage range, as well as to provide balancing of the cells.
In the case of LiFePO4 chemistry, the absolute maximum is 4.2V per cell, though it is recommended that you charge to 3.65V per cell. There is less than 1% extra capacity between 3.65V and 4.2V, and going above 3.65V may reduce the cell’s cycle life. Going above 4.2V may result in permanent damage to the cell.

At the other end of the scale, over-discharging can also cause cell damage. The BMS must provide a warning and/or shut off the drive system if any cells are approaching empty (less than 2.5V). Cells may suffer mild damage below 2.0V, but are usually recoverable. However, cells which get driven to negative voltages are usually damaged beyond recovery.

Tip 2: Clean your terminals before installation

The terminals on top of the batteries are made from aluminium and copper, which over time builds up an oxide layer when exposed to air. Before installing your cell interconnectors and BMS modules, clean the battery terminals thoroughly with a wire brush to remove oxidation. If using bare copper cell interconnectors, these should be cleaned too. Removing the oxide layer will greatly improve conduction and reduce heat buildup at the terminal. (In extreme cases, heat buildup on terminals due to poor conduction has been known to melt the plastic around the terminals and damage BMS modules!)

Tip 3: Use the right terminal mounting hardware

ThunderSky and Sky Energy cells using M8 terminals (90Ah and up) should use 20mm long bolts. Cells with M6 terminals (60Ah and under) should use 15mm bolts. If in doubt, measure the thread depth in your cells and ensure that the bolts will get close to but not hit the bottom of the hole. From top to bottom you should have a spring washer, flat washer, BMS module then the cell interconnector.

A week or so after installation, check that all your terminal bolts are still tight. Loose terminal bolts can cause high-resistance connections, robbing your EV of power and causing undue heat generation.

Tip 4: Install in tight boxes

Sometimes when LiFePO4 batteries are worked hard, pressure will build up inside the case which can cause the sides to swell up. This can be prevented by installing the cells in tight-fitting battery boxes, or fastening metal plates on the ends of cell rows with straps to hold them tight.

Tip 5: Charge frequently and shallower cycles

With lithium batteries, you will get longer cell life if you avoid very deep discharges. We recommend sticking to 70-80% DoD (Depth of Discharge) maximum except in emergencies.


 
 

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