When weighing up a lithium battery vs AGM battery, what’s the difference and which is better? We explain the facts you need to know in this comparative guide.
Right now, in a variety of deep cycle applications, there’s a bit of a battery war raging.
The biggest complaint about lithium deep cycle batteries in any application is the price point. However, did you know that during 2020 the prices for lithium battery technology took a price drop of 13% from 2019?
This, along with the drop in cost for photovoltaic cells is pushing solar to new heights. The question about solar often comes to deep cycle battery storage. The solar industry is not the only industry majorly affected by this price-drop, however.
Many people are starting to wonder, what’s the difference between the tried-and-true AGM deep cycle batteries and lithium batteries?
Keep reading to get a direct answer on the lithium battery vs AGM battery debate.
Depth of Discharge and Useable Energy
One of the most important things about batteries is how far down you can allow the batteries to drain before they lose cycle life and stop delivering max load voltage. The aspect of deep-cycle batteries to safely and reliably charge and discharge allows us to have reusable batteries in devices like mobile phones.
Lead-acid batteries have their use-cases, just as lithium batteries do. Currently, they’re routinely used in:
- Four-wheel drive (4WD) vehicles
- Recreational Vehicle (RVs) or a caravan vehicle
The accepted “depth of discharge” or DOD for an AGM battery is approximately 50%. Lithium-iron-phosphate batteries have a DOD of 80%. AGM batteries can deliver their high output of 12V until about 70% DOD, but dramatically reduce their lifespan in the process.
Going below the DOD will excessively wear the cells of either lithium or AGM. Lithium, though, having a higher DOD will be less likely to receive damage to its cycle life. Useable energy per charge is also increased with lithium batteries for this reason.
How does DOD affect useable energy? Useable energy is not how much energy the battery stores, but rather a representation of how much energy can be discharged from the total battery system scale.
LFP (also known as LiFePO4 or lithium-iron-phosphate) battery cells are internally configured in series. Each cell has a typical voltage of 3.3V. This means each LFP battery has eight cells, equalling 26.4V total and a 180 Ah rating.
To calculate the kilowatt hours of the battery system, you multiply the voltage with the Ampere hours. This gives you 4.75 kWh for the battery’s capacity.
To get the useable energy of the lithium battery system, multiply the capacity by the DOD, 80%. This gives you 3.8 kWh that you can use on your RV or boat.
A comparable AGM system consists of six cells giving 220 Ah and linking two 220 Ah batteries in series to increase the voltage to 24V. The available energy in this system seems to be a much higher 5.28 kWh.
What happens when you multiply by 50%? The resulting 2.64 kWh ends up being 69.47% of the total useable energy of the lithium battery system.
To catch up to lithium batteries, after doing a bit of math you’ll find that you need to have twice the Ah rating in an AGM system than in a lithium one to catch up.
When it comes to battery weight, lithium wins, hands down. Lead is one of the heavier naturally occurring elements while lithium is one of the lightest. After our last section, you might be wondering how heavy the lead-acid battery system is starting to get.
Since the normal rating is made to accommodate 20 hours of load, let’s look at a typical system today. The high load requirements have pushed voltage requirements to 48V DC, or direct current.
The capacity likewise has also been pushed to a 320Ah capacity usage. This comes out to 400 Ah for lithium and 800 Ah for the AGM system.
The resulting average weight of cells and components brings lithium to a hefty 336 kg. A lead-acid system on the other hand comes in at a gargantuan 1360 kg! This is four times the weight of the lithium system.
Remember that every kilogram you bring onboard requires more fuel to push, a bigger engine, or both.
Charge Efficiency and Cycle Life
When it comes to charge efficiency, lithium deep cycle battery technology is hard to beat. Lead-acid battery systems, whether AGM or wet cell, all suffer from a lot of internal resistance, especially in the final 20% of the charge.
Increasing the charge rate doesn’t fix this, and actually may damage the battery, as it needs adequate cooling to prevent thermal runaways from occurring. Because of this, it’s recommended to charge AGM batteries at 0.2C or 20% the capacity. This means, for our 800 Ah example from before, a charge rate of 160A.
In comparison, a 200Ah lithium battery has a recommended charge rate of 0.5C or 100A or less. You can charge up to 500A but it will eat into the cycle life of your battery.
Speaking of cycle life, how many cycles you get from your batteries will depend on if you follow recommendations or not.
Lithium deep cycle batteries now have built-in battery management systems and benefit by getting between 2000 and 5000 cycles. A lead-acid battery like an AGM battery gets between 400 and 1500 cycles.
What is a cycle? A cycle is a battery going from a full charge to full discharge and back to a full charge. It doesn’t matter if you only used 20% of the battery before recharging, it still counts as 20% of a full charge to the battery.
Batteries that charge or discharge at high heat will reduce the operational lifetime of the battery. Ambient temperatures also play a role.
Baintech lithium deep cycle batteries use lithium iron phosphate cells that last more than 5,000 cycles at 50% DoD and 2,000 cycles at 80% DoD.
This equates to +10 years of life expectancy compared to 2-3 years of a lead acid AGM.
Lithium Deep Cycle Battery vs AGM Deep Cycle Battery: Who Wins?
Lithium batteries tend to run at four times the upfront cost of a comparable AGM system, it’s true. However, with a lithium battery you have:
- About 1/4 the weight of AGM
- Almost 30% more usable energy
- 30% deeper DOD
- At least three times longer cycle life
Really, the only consideration is whether you feel you can invest in lithium battery technology and its future now or switch over down the road.