AGM Batteries have come a long way from the old lead acid battery that was in the family car growing up. AGM batteries are far superior to the old lead acid batteries too, they’re very resistant to vibration, they’re sealed, and maintenance free. They also offer a deeper cycling performance than a regular lead acid battery. One issue with AGM batteries though, is that once they’ve been depleted below a certain point, the ability for the battery to ever fully recharge is permanently reduced.
What about Lithium Batteries?
Lithium batteries have a higher upfront cost but last up to six times longer than AGM. They’re also lighter; an AGM battery can weigh around 34kg where a lithium battery only weighs around 16kg.
Depth of Discharge
All battery types have an amount of discharge known as the “depth of discharge” (or DoD for short.) This is how much they can safely handle before the life of the battery is permanently reduced.
As an example, a 100 Ah lead acid battery with a 25% DoD means you can only use one quarter of the battery’s rated capacity. Lead acid batteries both sealed and flooded typically have a DoD of 25% to 50%
Lithium batteries on the other hand can safely be discharged up to 80% of their DoD, or even up to 100%
Because of this, a lithium battery bank can be smaller than a lead acid battery bank to supply the same amount of usable energy.
Cycle Count – Lead Acid vs Lithium
A cycle count is when a battery goes down from 100% charge to any amount off 100 and is then fully recharged. If you were to cycle your battery every day for a year, that is of course 365 cycles.
Any amount of discharge is counted as a cycle, but the deeper the discharge, the less cycles you will get from your battery over time.
A standard sealed lead acid battery can have up to 1200 cycles at 25% DoD whereas a lithium battery can achieve a DoD of 80% or more for over 5000 cycles.
Lead Acid vs Lithium – Discharge Rate
Another consideration for choosing a battery type is just how fast each will draw power. If we again use the 100 Ah battery as an example, if you have a 10 Amp load, that will drain the battery entirely in 10 hours. This is known as a C/10 rate. Similarly, if you have a 20 Amp drain on the same battery, it will be totally depleted in 5 hours for a C/5 rating.
If you have a high current load in your system, or are charging it very quickly with a high current, such as your alternator or shore power, you need to consider the charge/discharge rate of the battery bank. If you need a higher rate than the batteries can handle, you would need to increase the battery bank by adding more batteries in parallel so that the batteries can share the current between themselves. This may result in needing a battery bank that has a higher Ah capacity than you need to power your loads, just to handle the high current.
Another consideration is that if lead acid batteries charge too slowly, they suffer from sulphation, shortening their life. Lithium batteries on the other hand, do not suffer from this issue.
Lead vs Lithium – Voltage Sag
If you’ve run battery power before, you may have noticed your voltage dropping when something drawing a high power load is operating. For example, a hair dryer or a microwave oven. Due to the voltage curve of lithium batteries and their ability to handle high current, loads like these will not cause the voltage to drop dramatically, eliminating the problem of voltage sag.
Lead vs Lithium – Overall Cost
If we simply compare Ah to Ah, there’s no question that Lithium batteries are more expensive.
But when you consider the cycle counts for each type of battery (1200 for the AGM and 5000 for the lithium), the lithium battery bank will last 4x longer than the AGM bank. You would need to buy four AGM battery banks to match the lifetime of a single lithium battery.