Battery choice is a critical part of living with an RV solar battery charger. Battery design determines success or failure. The question comes down to batteries Lead or Lithium.
The first question that the RV Solar Series needs to address pertaining to batteries is that there are two types of batteries. Batteries that are created to give a short burst of energy and batteries that excel in delivering smaller amounts of energy over a longer period of time are called deep-discharge batteries.
There are two types of deep-discharge batteries to choose from: Lead- Acid or Lithium
Other types and chemistries of batteries are not suitable for long-term deep discharge.
If you choose lead-acid then you need to understand the different types of lead-acid batteries. There are multiple types of lead-acid batteries, if this is your choice, get the right one. Don’t get the wrong one, it won’t last very long.
Lead-Acid Batteries have been the gold standard in battery design for the last hundred and fifty 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. You need to understand that there is no such thing as a combination battery. It is not possible to make a battery that excels both in engine starting and long slow discharge.
Starting batteries make great batteries to start an engine and very poor batteries to operate the house system. If you make this error (all because battery manufacturers are lying to you) you will end up replacing your batteries way before they would have lasted if they were truly intended for operating the house side of the RV. See why understanding batteries is important.
True deep cycle lead-acid batteries can be either Flooded Lead Acid or Absorbed Gas Mat (AGM) or Lithium Iron Phosphate. Both types of lead-acid 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.
True deep-cycle lead-acid batteries give up their energy unwillingly. CCR batteries give large boosts followed by inactivity and frequent recharging. Both hate to be over-discharged and this will cut their expected life by two-thirds.
Correct battery choice is critical, don’t put a starting battery in a place a deep cycle battery is needed. The other consideration is the energy density of the battery which is a question of energy per pound.
So if lead acid is not an option, what is the answer? The answer is lithium IRON phosphate. Notice that a 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, that is to say, 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 take a recharge quicker, in fact possibly twice as fast as a lead acid. (This estimate of the charge rate and not something I have tested personally.)
- The biggest benefit is that lithium iron phosphate can be discharged and then 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.)
- Cost less than lead-acid. Before I get hate mail on this, please read on.
Batteries Lead or Lithium Compared
Weight and Capacity:
Compared to my battery, 150 amps of useable energy — a lithium battery weight would be 35 lbs, not 180 lbs. Or if I installed 180 lbs of battery, I could have 800 amp hours of battery, not 150 amps.
The recharge rate as outlined above it takes about half a day for my 700-watt, poorly aligned panels to produce 150 amps of electricity including the 100 amps my battery needs daily. (The problem with this is that my battery is unwilling to accept 100 amps in such a short time. This means that it takes all day to charge my battery.)
I could probably fully recharge more than 400 amps of lithium discharge in a single day. This means that I could probably go multiple days without direct sun and only recharge twice a week instead of every other day. But wait there is more…. (Just like info commercials.) Not only will lithium recharge faster but they don’t 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. In fact, if you never achieved a full charge, this would lengthen the life span of the battery.
Depending on your RV, your charger may not be up to the task of fast recharge 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.
For me, 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 didn’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 to use the electricity (or hurt the reservoir to take out the water). Of course, you want to recharge them as often as possible so you can have more reserves in storage.
Cycle life is also a huge benefit and where lithium really becomes a good decision. Here I have to introduce an apparent issue—lithium batteries are expensive. Installed in every battery (box) there 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%. This is which is way better than flooded-cell lead-acid batteries. Yet if my batteries were discharged to 18% remaining, the battery is only expected to last 500 cycles. Even if the battery was fully emptied it should last more than 300 cycles. Of course, all of this is misleading. Fully exhausted lead-acid batteries never fully recover even after the first ordeal.
Lithium iron phosphate batteries, on the other hand, are designed to be fully discharged, before recharge. They do not suffer from full discharge. The little computer inside the battery will never let the battery be damaged by over-discharge. Thus, I can use all the water in the lake without ever worrying about being able to refill it. Even when fully using all the available electricity in a lithium iron phosphate battery, it is expected to last 3000, possibly even 5000 discharge/recharge cycles.
The cycle life of lithium can be improved by not discharging totally. The cycle life of lithium is also improved by not charging fully and not 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.
In terms of understanding batteries, cost is a huge issue. My 300 amp AGM battery cost more than $850 (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).
Lithium-iron batteries would cost about $1350. (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 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 costs less than lead-acid batteries when calculated over the life of the unit even though the initial price is higher.
Flooded lead-acid batteries require maintenance every month. Both AGM and Lithium iron are 100% maintenance-free.
Understanding the true cost of batteries is not just a one-time purchase question.
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 do I do with my perfectly good, 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 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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