Picking the right lithium battery for you
Now that lithium battery technology has been around for a while, the market is starting to get pretty crowded. I thought it was time to write an article on comparing and selecting the best lithium-ion battery for your RV. A quick web search will bring you hundreds of thousands of options and prices. How do you sort through that and select the best battery for the money? Read on. I will tell you what you need to compare to make the best decision for you.
Let me say that with all the cheap Chinese knockoff batteries coming to market, it is easy to find a low-priced model on Amazon and go with that. As with everything in life, not all batteries are the same. In fact, in many cases, it isn’t easy to dig out the actual specifications that mean something to the average person. Rest assured that there are some significant differences between the 350-dollar lithium battery versus the 950-dollar lithium battery, and I will show you how to compare the differences between them.
I may show products or specifications from manufacturers but do not take this as an endorsement. They are simply used to illustrate a concept I want you to understand. I am not sponsored or affiliated with any battery company. I just want to give you the tools to choose the best battery for your situation.
Defining the specifications you need to compare
This article will define the critical items (specifications) you need to pay attention to when comparing batteries. It is essential to compare apples to apples, and the following things are the most important to compare. The first thing to evaluate is battery capacity.
A battery’s capacity is how much energy the battery holds when fully charged. Most manufacturers list an RV deep cycle battery capacity in amp-hours. I must admit to being a little irritated when I see or hear people talking about a battery’s capacity in watt-hours. It is challenging to compare batteries with different capacity listings (watt-hours versus amp hours).
Here is a quick way to convert watts to amp hours using Ohms law:
A battery has an operating voltage of 12.8 volts (the most common value for lithium batteries) and a capacity of 1280 watt-hours. Using the formula from the Ohms law formula, circle 1280 watts/12.8 volts = 100 amp hours
What is an amp hour?
In simple terms, it is a battery’s ability to provide 1 amp of power at its rated voltage for 1 hour. An example:
A new 12-volt lithium RV battery rated at 100 amp hours will provide 100 amps of power at 12 volts DC for 1 hour when starting from fully charged. This is a bald-faced lie but bear with me for a couple more lines.
A practical example:
We’ll assume that you have 12 volt light bulb that requires 1 amp of power to turn on. If that light is connected to a 100 amp hour battery, it would theoretically stay lit for 100 hours. Pretty simple, eh? Well, there are caveats to this simple idea.
The lithium battery management system (BMS)
A lithium battery management system (BMS) is the custodian of a battery pack. It is an electronic device built into the battery that manages the battery’s functions. The battery management system is critical to the battery’s safe operation, performance, and lifespan. BMSs are the reason we can safely carry around lithium batteries and not have them explode or catch on fire!
It is challenging to compare the battery management systems between manufacturers. They often contain proprietary or patented technology that gives the manufacturer a competitive advantage, so they do not disclose much about it.
The thing to watch for is that the BMS automatically balances the charge across all cells. It dramatically extends battery life, and you should not purchase a lithium battery that does not have this feature!
All batteries lose storage capacity over time
I won’t spend much time on this, but a battery loses capacity as it is cycled over time. This means that you lose a little bit of storage capacity every time you charge and discharge a battery. The amount lost per cycle varies by battery. Another example:
A new 100 amp-hour battery, new out of the box and fully charged, will provide 100 amp-hours. A battery that has gone through 1000 charge and discharge cycles may only have a capacity of 95 amp hours, 2000 cycles, 90 amp hours, and so forth. This leads us to the next concept, depth of discharge and the number of cycles.
Speaking of storage, I have a detailed article on properly storing your RV for winter. You should review it for tips on storing your batteries.
Lithium Battery Depth of discharge (DOD)
Depth of discharge is how deeply discharged the battery gets before it is charged again. Manufacturers show the number of cycles the battery is rated for as a sales feature. The typical depth of discharge used to test the number of cycles a battery is capable of is 80%. So, to test a battery, the manufacturer will discharge 80% of the battery’s capacity, recharge it to full and discharge 80% of the capacity again and again. They repeat this charging and discharge cycle until battery capacity drops below some value. Depth of discharge is crucial because repeatedly discharging a battery by 80% or more shortens the battery’s life.
How deep do you discharge your lithium battery?
If you repeatedly totally discharge your battery (use 100% of capacity), you may only get 4000 cycles before the battery essentially becomes unusable. If you only discharge to 50% of capacity, you will get many more cycles, and your battery will last longer.
When comparing the number of cycles a battery is rated for, you need to know the depth of discharge the battery was tested at. If you can not identify this number quickly when looking at the lithium batteries datasheet, I recommend you drop that manufacturer from consideration. It should be clear that you need to compare battery cycle specifications using the same discharge depth. That is the only way to fairly compare the value of more cycles because more is better! But cycles are not the only thing that impacts battery life and capacity.
Peukert’s Law in Lithium Batteries
One more thing we need to address briefly is Peukert’s law
It changes everything about a battery’s amp-hour rating and, ultimately, how long your battery will last. A 100-amp hour battery should provide 1 amp of power for 100 hours, 5 amps for 20 hours, etc., right?
Not quite–and this is where the Peukert Effect applies. It is a complicated mathematical formula that explains how the discharge rate changes the battery’s actual capacity. If you discharge your battery at a high rate, the internal resistance within the battery creates a voltage sag that shortens how many amps it can deliver.
In simple English, the faster you discharge the battery, the less capacity it has. If you discharge at 100 amps, you may only get 50 minutes of run time. Discharging at 50 amps may get you closer to 2 hours of run time. Fortunately, the Peukert effect is small for most lithium batteries.
Final thoughts on comparing cycle numbers between lithium batteries
The take-home message is this: It is difficult to compare different brands unless you can determine how the battery was tested. That information can be difficult or impossible to determine, depending on the manufacturer. Top manufacturers list the depth of discharge used to estimate the number of cycles, but if you cannot find this information about a battery you are considering purchasing, buyer, beware! It is easy to manipulate this data because a manufacturer may list their battery at 4000 cycles, but they only test to 50% discharge. Recall from above that deeper discharge cycles shorten battery life. Another thing that will impact lithium battery capacity and performance is temperature. You will find this information in the operating temperature rating specification.
Lithium battery operating temperature ratings
Operating temperature ratings may or may not be important to you. To decide if the operating temperature range is important if you don’t camp in freezing (32° F or 0° C) or scorching (110° F, 43° C) temperatures, you can generally disregard operating temperature ratings as long as they are reasonable.
For example, a battery may list an operating temperature range of, say -4° F (-20° C) to 135° F (57° C). This means that the battery will provide power anywhere inside that range. Just make sure that it is within the range of temperatures you expect to spend time camping in. Keep in mind that battery temperature impacts battery operation.
Battery temperature changes the chemical reactions inside lithium batteries, reducing performance, depth of discharge capacity, and lifespan expectations. This means that the maximum amount of energy, lifespan, etc., that the battery can provide decreases in low or high-temperature situations. By the way, this is true of all batteries (lead-acid, lithium, AGM, etc.) across the board.
Charging while below freezing damages lithium batteries
Lithium batteries do not like to operate in freezing temperatures. Lithium batteries are permanently damaged because of the plating of metallic lithium on the anode during sub-freezing charging. This is permanent and cannot be reversed. So the BMS will stop charging the battery when a lithium battery’s internal temperature drops below freezing (32° F or 0° C). Because of this, some manufacturers have started to incorporate heating elements in their batteries to extend the operating range of their batteries. If you expect to spend time camping in near or sub-freezing temperatures, you must select a lithium battery with self-heating capacity! Now that we have discussed the physical environment let’s look at the critical electrical specifications you need to compare.
I have an article here on my website loaded with valuable information about using the alternator of your vehicle to charge your RVs batteries.
Lithium Battery Continuous vs. Surge Capacity
The continuous specification value says that the battery can continuously provide that amount of power (current), for example, 100 amps. Keep in mind that discharging a battery at high rates for long periods is harmful and will decrease the battery’s lifespan. Theoretically, it will discharge 100 amps until the battery voltage drops below the low cutoff point, and the BMS goes into protection mode and turns the battery off.
Surge capacity is the battery’s ability to surge and provide more power (current) for a short (surge) amount of time. That specification should include a length of time measurement. For example, a 200 amps surge for 30 seconds means just what it says. The battery can provide 200 amps of current for 30 seconds. It is important because equipment like fans, fridge compressors, etc., require more current (surge) to get started.
Finally, it is easier than you think to need surge capacity when camping. It is easy to forget that you are on battery and start the microwave and other high-draw appliances, voice of experience. Keep in mind that when you are on an inverter using 110 power, the amount of 12-volt energy needed to make 1000 watts of AC power is close to 100 amps of DC power(depending on your inverters power conversion efficiency).
With the varying loads in a typical RV, it is not uncommon to have brief surges of high current, and good lithium batteries account for this fact by providing some surge current level. The higher the current and/or length of time indicates the battery’s strength. Again more is better!
Lithium Battery Charging Rate
The last important thing to consider is the charging rate, or how fast you can charge the battery. Manufacturers will usually list two values for charge rate, the recommended charging rate, and the maximum charging rate. These things seem simple but influence how long the battery will last. For more great tips on extending the lifespan of your batteries, check out this article: How to Extend the Lifespan of Your RVs House Batteries
The maximum charge rate is self-explanatory. It is the maximum amount of current (power) you can use when recharging the battery. However, like high rates of discharge, high rates of charge can damage the battery and shorten its life. When a manufacturer provides a recommended charge rate, that rate extends the battery’s life as much as possible. Charging at the recommended rate does little to no damage to the battery.
Determining the best battery depends on how you anticipate using the batteries most of the time.
You’re dry camping for the weekend and don’t want to run a generator for long periods of time. In this case, you want to select a battery with a high recommended charge rate. The higher the charge rate, the faster your batteries will charge. The ability to charge at high speed also depends on how much current your inverter/charger can deliver.
In the next scenario, let’s say you depend totally on solar. Your system produces, on average, 25 amps of power (300 watts) available for charging. You can get by with a battery with a lower recommended charge current. For example, your batteries have a 20-amp recommended charge rate, which is close enough to the 25 your system can provide, thereby minimizing lifespan shortening caused by high charge rates.
One final scenario, add to the example above that you occasionally camp with electrical hookups and would like to charge your batteries when on shore power. Again you need to balance charge capacity with your expected usage. If you are sitting in the campground using shore power, you are not using power from your batteries. You have all the time in the world to charge your batteries. Remember that the slower you charge or discharge your batteries, the longer they last.
So stop and think about how you will use and charge the battery, and select one that falls within those parameters. You can waste a lot of money picking the wrong battery.
Lithium Battery Warranty
Finally, warranties vary wildly and only cover manufacturing defects. In my opinion, they are much less critical when selecting a battery. Because of the widely varying environment, the batteries may be operating in and under, you will not find a manufacturer’s warranty covering consumer-grade lithium batteries’ capacity. A manufacturer can’t offer such a guarantee.
If you concentrate on the things I’ve listed above and answer the questions I’ve asked regarding your personal use and expectations, you’ll easily pick a battery that you’ll be happy with. Try not to focus on one thing like cost but look for a balance between cost, how you expect to use the lithium battery and the lifespan of the lithium battery.