Lithium vs. AGM vs. Gel vs. Flooded – Solar Battery Comparison

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There are two types of solar batteries, lithium and lead acid. Lead acid has two variants, flooded lead acid (FLA) and sealed lead acid (SLA). SLA batteries are available in two kinds, AGM and gel. Each has its own pros and cons. Your budget, lifestyle and storage power requirements determines which battery is appropriate.

Lithium batteries offer the best performance with zero maintenance, but they’re the most expensive. Flooded lead acid batteries are the most cost efficient but need regular maintenance. AGM and gel batteries require zero maintenance and offer better performance than FLAs, but can’t provide as much power as lithium batteries.

What is a Flooded Lead Acid Battery?

FLA battery plates are immersed in water, hence the term flooded. The water evaporates when the battery runs so it needs to be refilled every 1 to 3 months. FLAs also release gases so it must be installed in a well ventilated place.

FLAs are the most commonly used in homes, camping and RVs. However it needs regular maintenance and failure to do leads to performance deterioration. Improper maintenance or lack of it voids the warranty, so it can be hassle.

Because FLAs require ventilation you can’t just install it anywhere. That can be a problem if you want to set up a battery bank comprised of FLAs. Not only do they need space, but ventilation too. However, FLAs last longer than SLA batteries.

What is a Sealed Lead Acid Battery?

The biggest difference between flooded and sealed lead acid batteries is SLAs have an encased interior compartment. Unlike FLAs there is no need to add water. SLA batteries also have a valve to facilitate the release of gas, which is why they’re also called VRLA (valve regulated lead acid). There are two kinds of SLAs: AGM and gel.

AGM Batteries

AGM (Absorbent Glass Matt) are vibration resistant and suitable for homes, vans and RVs. Unlike FLAs which require upright mounting, AGM batteries can be mounted in any direction. No maintenance is needed, which makes AGM batteries ideal for remote, hard to reach locations.

If you have to install a battery where maintenance is difficult, AGM is ideal. These batteries cost more than FLA and have a shorter life cycle, but work very well in cold place and are more versatile.

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Gel Batteries

Gel batteries have similar properties to AGM, but AGM batteries have superior charge and discharge rates. These batteries however, perform better in hot weather. it is more efficient than AGM when it comes to slow, deep discharges.

Gel batteries cost more than AGM, require specific settings to run, and don’t carry as much charge either. But if you’re looking for batteries that can run in very hot temperatures, the extra price of gel may be worth it.

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What is a Lithium Ion Battery?

A lithium is the best battery available for solar. It needs no maintenance and has a longer life cycle than FLA or SLA batteries. Lithium ion batteries have a deeper depth of discharge and does not require any ventilation.

Performance wise lithium ion batteries are superior to FLA and SLA. The one disadvantage is the cost. Lithium batteries are higher than any FLA, AGM or gel battery. While the upfront cost is higher, lithium will pay for itself with high end, long term performance gains. More about lithium battery benefits can be found here.

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Solar Battery Comparison Chart

The following comparison is based on average / typical performance of these batteries. However, real life situations will affect results. How the battery is used, temperature, solar power system design, discharge depth and other factors will determine the battery’s performance.

LithiumFLAAGMGel
Depth of Discharge90%-100%50%50%-80%75%-80%
Life Cycle300-1200200-300500-1000500-1000
Efficiency95%80%85%85%
Weight 180-200 lbs.120 lbs130 lbs.130 lbs.
Average Cost$6500$400$500$600-$700

Now let’s take a closer look at what the figures on the chart mean.

Cost

Lithium batteries are the most expensive. Though the price has gone down, it’s still going to be higher compared to FLA or SLA batteries. it is a considerable investment especially when you add the cost of the solar panels and other components. if you are going to buy lithium batteries, think of the long term.

FLA is the most affordable, hence their popularity. But we have pointed their shortcomings and limitations. SLA batteries may be a nice compromise between the two, but again AGM and gel have their own pros and cons.

Depth of Discharge

Depth of discharge (DOD) refers to how much of the battery capacity can be used before a recharge is needed. With FLA the recommended DOD is 50%. Once you use 50% of the capacity it needs to be recharged. Gel batteries have a DOD of 50%-30% depending on the manufacturer. AGM batteries are even better at 80%.

Lithium batteries have the best discharge rate at 90%-100%. You can fully discharge a lithium battery and use it until the level drops to 0%. That’s a huge plus if you have a large solar power system and don’t want frequent recharging. Also, the depth discharge is closely related to the life cycle as you will see.

Life Cycle

When you use a battery to run appliances and recharge it, that is one life cycle. All batteries are measured in life cycles, not years. A five year old battery with 100 life cycles is in better form than a one year battery that has had 500 life cycles.

Life cycles are connected to depth of discharge. The deeper the DOD, the shorter the life cycle will be. That’s why FLA batteries should be recharged at 50%. While lithium, AGM and gel batteries can be discharged at lower rates, it’s advisable to recharge before the level drops to 0%. Recharging a lithium or SLA battery at the 30%-50% increases the life cycle by several hundred of thousand.

Efficiency

Solar panels are not 100% efficient and neither are solar batteries. Lithium batteries are the best at 90%-95%. That means 95% of solar panel energy is stored in the battery. With FLA and SLA, only 85% of the energy is stored.

This is an important factor to remember. The stated capacity of some batteries may not be 100% accurate. Just like solar panels, it refers to the maximum solar energy that can be stored, but the actual amount may vary. If you want to get the best storage capacity for your system, check the efficiency rating.

Which Solar Battery Should I Buy?

No single battery can answer the needs of every solar power user. There are different situations and circumstances, and you have to decide where you best fit. Here are some scenarios that can help you make the right decision.

Living off the Grid

if you want to live off the grid, be it at a house, cabin or RV, lithium batteries are your best bet. You get the best performance, the best DOD, life cycle and charge rate. You’ll pay more that’s true, but over the long term you’ll recoup that investment.

Can you use FLA batteries for off grid living? Only if you’re willing to put it in the time needed for maintenance. Appliances like refrigerators need a lot of batteries to run, and it might be troublesome to have to keep an eye on the DOD level as you go about your day to day activities. But if you don’t mind this, FLA will be much cheaper than lithium.

Solar Powered Vacation Residence

if you’ve got a vacation home that runs on solar, go with AGM or gel batteries. They cost less than lithium and won’t deteriorate even with inactivity. You can visit your home a few times a year and the batteries will run fine even without maintenance.

FLA batteries are not practical in this case. The low price is attractive, but if you’re only going to be there on an irregular basis, how can you do the upkeep? It is better to spend a little more on AGM or gel and have assurance the batteries won’t deteriorate.

Emergency Backup System

SLA batteries make the most sense here. Lithium batteries are too expensive to be used as backup, i.e. you will only use these during power outages and other emergencies. FLA batteries require too much maintenance, so AGM or gel is the way to go.

Commercial / Business / Industrial

For large establishments and industrial complexes, lithium batteries are preferable. SLA are suitable if solar usage is partial or for basic tasks. FLAs are not ideal in these instances due to the maintenance requirements.

Conclusion

It’s easy to say that lithium battery is the best option, and from a performance standpoint it is. But there are other factors to consider as we have shown here. If you want the best battery and cost isn’t an issue, lithium batteries are the answer. But in other situations it’s not worth spending that money when FLA or SLA will do. In the end it’s your call.