SLUAAR1 july 2023 BQ24630 , BQ25170 , BQ25180 , BQ25300 , BQ25620 , BQ25730 , BQ25798

1 Introduction

When choosing between batteries there are many tradeoffs to consider: cost, size, weight, energy density, cycle life, stability, etc. In general, Lithium Iron Phosphate (LiFePO4) batteries are preferred over more traditional Lithium Ion (Li-ion) batteries because of their good thermal stability, low risk of thermal runaway, long cycle life, and high discharge current.

However, LiFePO4 batteries have a lower energy density and lower charge voltage, so they typically have to take up more area compared to a Li-ion battery. Furthermore, due to the lower charge voltage, a LiFePO4 battery may need a boost converter when a Li-ion may not. Table 1-1 shows a general comparison between typical low power multi-chemistry charging designs.

Later sections will focus on the differences between LiFePO4 and Li-ion’s charge profiles and thermal performance and the design implications these differences cause. We will then provide potential charger designs that are suitable for LiFePO4 batteries.

Table 1-1 Tradeoffs Between Typical Low Power Multi-Chemistry Batteries (1)

	Li-Ion	LiFePO4	Ni-MH	SuperCap
Energy Density	High 150-180 Wh/kg	Medium 90-120 Wh/kg	Low 60-120 Wh/kg	Low 4.5Wh/kg
V(nom)/cell	3.6 V	3.2 V	1.2 V	2.7 V
V(charging)	3.9 V-4.2 V	3.5 V-3.65 V	1.4 V-1.6 V	2.7 V
Area	Low	Medium	High	High
Price	High	Medium	Low	Medium
Benefits	High Energy density High voltage/cell at 3.6 V can lead to single cell use and space savings Long cycle life 500 cycles	High current rating 3°C Long cycle life 2000 cycles Good thermal stability Safer Li-Ions: enhanced safety/tolerance if abused Tolerant to full charge conditions	Reliable and Durable Safe: Overcharge and discharge do not create high temperatures More cost effective	Safe: No volatile chemicals, Overcharge and discharge do not create high temperatures Long life, no wear out mechanism 500k cycles
Limitations	Fragile: requires protection circuit for safe operation Peak voltage limited during charge Temperature needs to be monitored	Lower voltage of 3.2 V/cell Higher self-discharge which can cause balancing issues with aging	Quick self-discharge, needs to be charged more frequently Lower voltage 1.2V/cell require multi-cell packs and larger designs	Voltage can have large change with discharge, SOC. Low cell voltage can require series cell and possible balancing circuit.
Charge Temp	0°C to 45°C	0°C to 45°C	0°C to 40°C	-40°C to 65°C
Discharge Temp	-20°C to 60°C	-20°C to 60°C	0°C to 50°C -20°C to +85°C Possible	-40°C to 65°C