The Complete Guide to Lithium vs Lead Acid Batteries

Curious about whether you need a lithium battery or a traditional sealed lead acid (SLA) battery? More importantly, what’s the actual difference between these two types of batteries? Understanding their strengths and weaknesses is key to making an informed choice.

In this discussion, when we mention lithium, we’re specifically talking about Lithium Iron Phosphate (LiFePO4) batteries. Meanwhile, SLA refers to lead-acid batteries. Let's explore the performance differences between these two battery types. Check now

Cyclic Performance: Lithium vs SLA

The most notable difference between lithium iron phosphate and lead acid batteries is that lithium battery capacity remains stable regardless of the discharge rate. The chart below illustrates the actual capacity as a percentage of the rated capacity versus the discharge rate, denoted by C (C equals the discharge current divided by the capacity rating). At high discharge rates, such as .8C, a lead acid battery's capacity drops to only 60% of its rated capacity.

In cyclic applications where discharge rates regularly exceed 0.1C, a lower-rated lithium battery can often outperform an equivalent lead acid battery. While the initial cost of lithium is higher, the lower capacity requirements and longer lifespan make lithium more cost-effective in the long run.

Moreover, lithium batteries offer significantly better cyclic performance. Under most conditions, they provide ten times the cycle life of an SLA, which reduces the cost per cycle and the need for frequent replacements.

Constant Power Delivery: Lithium vs Lead Acid

Another advantage of lithium batteries is their ability to deliver consistent power throughout their entire discharge cycle. In contrast, SLA batteries start strong but lose power as they discharge. This consistency is depicted in the voltage vs. state of charge graph below.

For instance, a lithium-powered flashlight will maintain its brightness from start to finish, only dimming when the battery is entirely drained, thus eliminating the gradual fade seen with SLA batteries.

Charging Times: Lithium vs SLA

SLA batteries are notoriously slow to charge, often necessitating extra batteries to ensure continuous operation in cyclic applications. They also need to be kept on a float charge in standby applications.

Lithium batteries, on the other hand, charge four times faster than SLA batteries. This efficiency means fewer batteries are needed, and they recover quickly after events such as power outages. Additionally, there’s no need for a float charge during storage.

High Temperature Battery Performance

When it comes to high-temperature applications, lithium batteries outperform SLA batteries significantly. At 55°C, lithium batteries have twice the cycle life compared to an SLA at room temperature. This makes lithium a superior choice for high-temperature conditions.

Cold Temperature Battery Performance

Cold temperatures pose challenges for all battery chemistries, affecting both charging and discharging capabilities. Lithium batteries cannot charge below 32°F, whereas SLAs can accept low current charges even at low temperatures. However, lithium batteries have a higher discharge capacity in cold temperatures compared to SLAs.

For instance, at 0°F, lithium batteries discharge at 70% of their rated capacity, while SLA batteries discharge at 45%. Thus, although lithium batteries may face charging limitations in the cold, they don't require as much over-design for discharge as SLA batteries do.

Battery Installation

When installing lead-acid batteries, it's crucial to avoid inverting them to prevent potential venting issues. While SLA batteries are designed to minimize leakage, their vents do allow some gas release.

In contrast, lithium batteries feature cells that are individually sealed and cannot leak, offering flexible installation orientations without risk of leakage.

Battery Weight Comparison

On average, lithium batteries are 55% lighter than SLA batteries. This weight difference is particularly important for mobile applications, such as motorcycles and electric vehicles, as well as in installations where weight and performance are critical factors, like robotics.

SLA vs Lithium Battery Storage

Lithium batteries should not be stored at 100% State of Charge (SOC), whereas SLA batteries should be. This is due to the higher self-discharge rate of SLA batteries—five times greater than that of lithium batteries. Consequently, lead-acid batteries often require a trickle charger during storage to maintain their charge and lifespan.

Series & Parallel Battery Installation

Important note: When installing batteries in series or parallel, they should be matched in terms of capacity, voltage, resistance, state of charge, and chemistry. SLA and lithium batteries should not be used in the same string.

Since SLA batteries are "dumb" compared to lithium batteries (which include circuit boards for monitoring and protection), they can handle more batteries in a single string. Lithiums are limited by their circuit board components, which have current and voltage limitations. Thus, for most lithium battery strings, the maximum length is six or fewer batteries, although more can be achieved with additional engineering.

In conclusion, while lithium batteries generally offer superior performance across various metrics, SLAs shouldn't be discounted entirely. They do hold advantages in certain applications, such as long strings, high discharge rates, and cold temperature charging.