RV Solar Series: For a solar-charged RV battery, battery choice is critical to living with an RV solar battery charger. Battery design determines success or failure. The question comes down to Lead or Lithium batteries.
Originally published March 2019 Updated June 2020 and March 2024
The first question that the RV Solar Series needs to address about batteries is that there are two types. Some batteries are created to give a short burst of energy, and others are designed to deliver smaller amounts of energy over a more extended period. Engine-starting batteries do a poor job of delivering small amounts of electricity over long durations. Engine-starting batteries are damaged when used this way, causing them to die prematurely.

There are two types of deep-discharge batteries: Lead-Acid and Lithium.
Other types and chemistries of batteries are not suitable for long-term deep discharge.
Lead-Acid:
If you choose lead-acid batteries, you need to understand the different types. There are multiple types of lead-acid batteries. If this is your choice, get the right one. Engine-starting batteries won’t last long if they are used as deep-discharge batteries.
Lead-acid batteries have been the gold standard in battery design for 150 years. But they have their limits. If you see a battery with cold cranking amps or CCR followed by a number, it is not a deep-cycle battery. Instead, the design is an engine-starting battery. There is no such thing as a combination battery. Making a battery that excels both in engine starting and long slow discharge is impossible.
Starting batteries are great for starting an engine but horrible choices for operating the house system. Make this error (all because battery manufacturers lie to you). You will end up replacing your batteries way before they would have lasted if they had indeed been intended to operate the house side of the RV. See why understanding batteries is essential.

True deep-cycle lead-acid batteries can be flooded with Lead Acid, Absorbed Gas Mat (AGM), or Lithium Iron Phosphate. Both types of lead-acid batteries work the same way; both have liquid. With a flooded lead-acid battery, you can add distilled water to replace the water that evaporates. Both lead-acid batteries create energy in the same way. Sealed lead-acid AGM batteries have a longer life and never need filling.

Actual deep-cycle lead-acid batteries unwillingly give up their energy quickly. Engine-starting batteries give lots of energy quickly, followed by inactivity, and immediately need recharging. All lead-acid batteries need frequent recharging to 100%. Both engine-starting and deep-discharge batteries hate being overcharged or over-discharged, which will cut their expected life by two-thirds.
Correct battery choice is critical. Don’t put a starting battery where a deep-cycle battery is needed. Another consideration is the battery’s energy density, which is energy per pound.
Lithium
So, what is the answer if lead acid is not an option? The answer is lithium IRON phosphate. Notice the slight difference: Lithium IRON is not the same as lithium ION. The chemistry of lithium iron phosphate is the most stable of lithium-based batteries. Lithium iron phosphate avoids the fire problems that lithium-ion has experienced and is made so apparent on the news. Understanding this aspect of batteries is very critical.

The benefits of lithium iron phosphate batteries
- Weight is less than 1/3 of that of lead acid.
- Capacity is double that of lead-acid batteries; one lithium battery holds twice the useable amps of a lead-acid battery.
- Lithium iron phosphate batteries are much more willing to hold the voltage at a high level longer and to recharge quicker, possibly twice as fast as lead-acid batteries.
- The most significant benefit is that lithium iron phosphate can be discharged and recharged many more times than a lead-acid battery. (Depending on how you recharge them, they may last four times longer than AGM lead-acid batteries.)
- It costs less than lead-acid. Before I get hate mail on this, please read on.
Batteries Lead or Lithium Compared
Weight and Capacity:
My 300 amp hours (rated capacity) lithium battery would weigh 44 lbs, not 180 lbs, compared to my lead-acid battery’s usable capacity. If I installed a lithium battery of 180 lbs, I could have more than 800 amp hours of usable battery capacity, not 150 amps.
Recharge Rate
As outlined above, the recharge rate takes about half a day for my poorly aligned panels to produce enough electricity (numbers based on my previous RV with 700 watts of solar panels). The problem is that a lead-acid battery is unwilling to accept the electricity quickly. If I had lead-acid batteries, it would (and has in the past) take all day to charge them.)
Don’t have to recharge lithium batteries every day.
I could probably fully recharge more than 300 amps of lithium in a single day. If I had batteries, I could probably go multiple days without direct sun and only recharge every third day instead of every day. But wait, there is more…. (Just like info commercials.) Not only will lithium recharge faster, but it will also not have to be fully recharged every day to avoid battery damage. This is huge. I can’t express how wonderful this is. You do not need to top off lithium iron batteries to full. This would lengthen the battery’s life span if you never achieved a full charge.
Charger
Depending on your RV, your charger may not be up to fast charging when recharging from the generator. (Mine is not.) This is not a factor when hooked to shore power. A slower charge is better for the battery than a quick recharge.
Since I had to recharge my battery every day to avoid battery damage, there was no reason to install more solar panels. If the sun doesn’t come out, I still have to run the generator to get back up to full charge before I start a discharge cycle again. Lithium Iron Phosphate batteries don’t care if they are recharged every day. Like the water in the reservoir, it does not hurt the battery when using the electricity (or hurt the reservoir when taking out the water). Of course, you want to recharge them as often as possible to have more reserves in storage.
Cycle life
Cycle life is also a considerable benefit, and lithium is a good decision. Here, I have to introduce an apparent issue—lithium batteries are more expensive. Installed in every battery (box) is a small computer (called a battery management system) that regulates everything about the battery. This battery management system computer will not allow you to abuse a lithium battery.
Depth of discharge:
Depth of discharge and recharge relates directly to battery life. My sealed AGM battery is expected to last 1000 cycles when discharged to 50%, which is way better than flooded-cell lead-acid batteries. Yet, if my batteries were discharged to 18% remaining, they are expected to last only 500 cycles. The battery should last more than 300 cycles even if fully emptied. Of course, all of this is misleading. Fully exhausted lead-acid batteries never fully recover, even after the first ordeal.
On the other hand, lithium iron phosphate batteries are designed to be fully discharged before recharge. They do not suffer from complete discharge. The little computer inside the battery (BMS) will never allow the battery to be damaged by over-discharge. Thus, I can use all the electricity without ever worrying about being able to refill it. Even when entirely using all the available electricity in a lithium iron phosphate battery, it is expected to last 3000, possibly even 5000 discharge/recharge cycles.
Lithium’s cycle life can be improved by not discharging or charging fully or charging too fast. RVs with solar are ideal for this life-extending treatment because full discharge is not anticipated. Solar is also a variable charge rate, given clouds and other shading issues.
Cost:
Cost is a considerable issue in understanding batteries. My 300 amp AGM battery costs more than $850 (2019 dollars, including sales tax). Assuming I discharged my battery to a full discharge (150 amps has yet to happen), I could easily calculate the price at AGM at 85 cents per cycle (1000 maximum cycles).
The same lithium-ion batteries would cost about $1350 (again, 2019 prices). (be 1/3 the weight and less than half the size) would last 3000 cycles and would calculate at 45 cents per cycle. A correctly treated lithium battery may last for 5000 or more cycles.
What about flooded lead acid? They are way cheaper than AGM or Lithium.
Flooded lead acid (usually called golf cart batteries) at 50% depth of discharge may deliver 300-500 cycles and cost around 80 cents per cycle at 500 cycles, and assuming 300 cycles, they would cost $1.33 per cycle.
Lithium batteries are expensive until you realize how long they will last.
Lithium iron batteries, although the initial price is higher, cost less than lead-acid batteries when calculated over the unit’s life.
Flooded lead-acid batteries require maintenance every month. Both AGM and Lithium iron are 100% maintenance-free.
Understanding the actual cost of batteries is not just a one-time purchase question.
It’s an obvious choice unless you have one and want the other.
Lithium batteries have so many advantages over lead-acid batteries that the choice is obvious. What isn’t so obvious is what I do with my perfect AGM batteries. Do I get out of them and spend money on lithium, or do I milk them along, getting my money out of them, knowing all the time that I could be living large on lithium? I’m still waiting for this decision. Does anyone want some gently used AGMs at a bargain price?
You might even say that I have been living below my standards, unnecessarily limiting my solar abilities because I didn’t purchase Lithium Iron Phosphate batteries before I purchased my solar panels. This is true, but it also complicates things because my RV inverter isn’t big enough to recharge Lithium batteries efficiently or distribute the power to my desired destinations.
Update: June 27, 2020, We are now powering our RV using a major electrical remodel. We don’t have solar yet, but the battery remodel has changed our life (yes, it is lithium). Here is the story. Hybrid Mongrel Battery and Massive Electric Bucket
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I absolutely love your blog and find that you already answer my questions before I knew what they were. Again, awesome website!
Typically, Lithium batteries do not charge and discharge in COLD weather. How do you overcome this? Are lithium designed for RV’s different then stand alone solar systems?
Tom, I call this battery myth number 10 in my list of ten lithium battery myths. Here is a link to that post.
https://foxrvtravel.com/ten-lithium-battery-myths-and-answers/
As for me, even though our lithium batteries are installed in our lower compartment I have never seen less than 40 degrees even when the outside temperature was 17 degrees.
Really the big myth is that lead-acid batteries work in cold weather… they suffer greatly as well.
AGM = Absorbed Glass Mat. related to the construction using glass. besides that discrepancy your solar/battery blog is great. definitely shows how you can run RV on solar without having a huge system.
I visit this website every day.